U.S. patent application number 11/998714 was filed with the patent office on 2008-04-17 for docsis 2.0 scdma capable sniffers which can capture legacy docsis bursts as well.
Invention is credited to Yehuda Azenko, Selim Shlomo Rakib.
Application Number | 20080089399 11/998714 |
Document ID | / |
Family ID | 32989015 |
Filed Date | 2008-04-17 |
United States Patent
Application |
20080089399 |
Kind Code |
A1 |
Azenko; Yehuda ; et
al. |
April 17, 2008 |
DOCSIS 2.0 SCDMA capable sniffers which can capture legacy DOCSIS
bursts as well
Abstract
A DOCSIS 2.0 compatible sniffer which can receive legacy DOCSIS
1.x TDMA bursts as well as DOCSIS 2.0 SCDMA bursts. One embodiment
uses two cable modems in the sniffer, one to capture downstream
data bursts and the other to capture downstream messages and to
recover the downstream symbol clock and generate an upstream
reference clock which is phase coherent with the recovered
downstream symbol clock. The reference clock is used by a cable
modem termination system to capture upstream SCDMA DOCSIS 2.0
bursts. DOCSIS 1.x TDMA bursts may also be captured. Other
embodiments use a DOCSIS 2.0 compatible modem in the sniffer to
lock onto a downstream, register as a cable modem in the system and
capture downstream messages in order to derive the correct timing
to generate control signals to control burst capture circuitry in
the sniffer to capture DOCSIS 1.x and 2.0 upstream bursts. The
digital samples of each burst can be sent digitally to the CMTS
under test after some fixed delay or can be sent to the CMTS under
test as an analog RF signal in repeater embodiments. This allows
the CMTS under test to be simplified by pushing the burst capture
circuitry out to the optical node.
Inventors: |
Azenko; Yehuda; (Cupertino,
CA) ; Rakib; Selim Shlomo; (Cupertino, CA) |
Correspondence
Address: |
THELEN REID BROWN RAYSMAN & STEINER LLP
P. O. BOX 640640
SAN JOSE
CA
95164-0640
US
|
Family ID: |
32989015 |
Appl. No.: |
11/998714 |
Filed: |
November 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10397508 |
Mar 25, 2003 |
|
|
|
11998714 |
Nov 30, 2007 |
|
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Current U.S.
Class: |
375/222 |
Current CPC
Class: |
H04L 43/18 20130101;
H04L 12/2801 20130101 |
Class at
Publication: |
375/222 |
International
Class: |
H04L 5/16 20060101
H04L005/16 |
Claims
1. A sniffer capable of capturing DOCSIS bursts transmitted between
one or more cable modems (CMs) under test and a cable modem
termination system (CMTS) under test via a hybrid fiber coaxial
cable system, comprising: a sniffer upstream receiver capable of
receiving DOCSIS upstream bursts; a sniffer cable modem coupled a
hybrid fiber coaxial cable (HFC) system by an analog signal path
capable of receiving downstream DOCSIS data bursts and messages
transmitted by said CMTS; a self ranging circuit coupling said HFC
cable system to said sniffer upstream receiver, comprising: an RF
demodulator means for amplifying, filtering and digitizing received
radio frequency signals carrying upstream DOCSIS bursts; a delay
and upstream equalization filter circuit; a sniffer clock generator
for generating a reference clock signal for said sniffer upstream
receiver; and a computer coupled to said sniffer cable modem, said
sniffer upstream receiver, and said self ranging circuit, for
controlling said sniffer cable modem, said sniffer upstream
receiver and said self ranging circuit and for receiving data from
downstream DOCSIS bursts captured by said sniffer cable modem and
for receiving data from upstream DOCSIS bursts captured by said
sniffer upstream receiver.
2. The apparatus of claim 1 wherein said sniffer cable modem is
structured and programmed to receive and send to said computer only
downstream DOCSIS messages, and further comprising a second sniffer
cable modem which is coupled to said computer and which is
structured and programmed to receive downstream DOCSIS data bursts
and transmit data therefrom to said computer.
3. The apparatus of claim 1 wherein said sniffer upstream receiver
is capable of receiving DOCSIS 2.0 synchronous code division
multiplexed upstream bursts.
4. A sniffer capable of capturing DOCSIS 1.0, DOCSIS 1.1 and DOCSIS
2.0 bursts transmitted by one or more cable modems (CMs) under test
to a cable modem termination system (CMTS) under test via a hybrid
fiber coaxial cable system (HFC), comprising: a port at which
commands are received indicating which DOCSIS 1.0, 1.1 and 2.0
upstream and downstream bursts to capture; a linecard circuit
coupled to said port for capturing the designated upstream bursts
and outputting them at said port; a cable modem coupled to said HFC
and to linecard circuit for recovering a downstream symbol clock
and recovering said designated downstream bursts and sending said
captured downstream bursts to said linecard for output at said
port, and for generating a reference clock signal which is phase
coherent with said recovered downstream symbol clock when said
sniffer is assigned to capture an upstream DOCSIS 2.0 synchronous
code division multiplexed burst; and a clock generator for
receiving said reference clock signal and generating an upstream
clock reference for said linecard circuit which is phase coherent
with said reference clock signal when said sniffer is assigned to
capture an upstream DOCSIS 2.0 synchronous code division
multiplexed burst.
5. The apparatus of claim 4 wherein said linecard circuit includes
a computer programmed to receive downstream messages captured by
said cable modem and use the content thereof along with
measurements and filter coefficients derived by said linecard
circuit from captured upstream station maintenance bursts to
generate suitable control signals to keep said linecard in
synchronism with upstream bursts transmitted by any cable modem
under test so as to be able to capture said upstream bursts if said
bursts have been designated for capture.
6. A process to capture upstream DOCSIS 2.0 bursts in a sniffer,
comprising: (1) recovering a downstream symbol clock and using it
to capture downstream messages transmitted from a cable modem
termination system under test, and, if DOCSIS 2.0 synchronous code
division multiplexed bursts are to be captured, generating a local
upstream reference clock from said recovered downstream symbol
clock which is locked to a master clock in said cable modem
termination system under test or at least phase coherent with said
recovered downstream symbol clock; (2) using said local upstream
reference clock to synchronize a timebase including a local
upstream minislot counter in a sniffer to an upstream channel upon
which upstream bursts to be captured will be transmitted; (3)
performing a self ranging process using captured upstream initial
and periodic station maintenance bursts and captured downstream
ranging response message data to get a timebase into
synchronization with an upstream on which bursts to be captured
will be transmitted; (4) using said upstream minislot counter and
data in captured downstream messages to capture upstream bursts,
possibly including DOCSIS 2.0 synchronous code division multiplexed
bursts from a cable modem under test.
7. The process of claim 6 wherein downstream bursts are captured
using first and second DOCSIS 2.0 compatible cable modems in a
sniffer which have been modified to capture bursts and messages
directed to other cable modems, and using said first cable modem to
capture only downstream data bursts and using said second cable
modem to capture only downstream messages.
8. The process of claim 6 wherein the step of recovering a
downstream symbol clock and generating said local upstream
reference clock from said recovered downstream symbol clock which
is locked to a master clock in a cable modem termination system
under test is performed by recovering said downstream symbol clock
and using said recovered downstream symbol clock as a reference
signal for a phase lock loop which generates said local clock and
removes phase noise from said recovered downstream symbol
clock.
9. An sniffer capable of capturing upstream DOCSIS 1.0, 1.1 and 2.0
bursts comprising: burst capture circuitry coupled to a hybrid
fiber coaxial cable (HFC) system upstream medium which couples a
plurality of cable modems (CM) under test to a cable modem
termination system (CMTS) under test for exchange of DOCSIS 1.0,
1.1 and 2.0 bursts therebetween; and a cable modem coupled to said
DOCSIS compatible cable system upstream and downstream mediums and
coupled to said burst capture circuitry, and functioning to
register with a cable modem termination system (CMTS) under test as
another cable modem in the system, recover a downstream symbol
clock and use it to capture downstream messages transmitted by said
CMTS under test and extract data therefrom needed to time and
control said burst capture circuitry to capture upstream DOCSIS
1.0, 1.1 and 2.0 bursts transmitted from cable modems under
test.
10. The apparatus of claim 9 further comprising means to divert
upstream bursts from said CMs under test away from said CMTS and
into said burst capture circuitry, and means for transmitting each
captured burst stored in said burst capture circuitry to said CMTS
a fixed time later after capture of said burst.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/397,508, filed on Mar. 25, 2003, entitled "DOCSIS 2.0
SCDMA Capable Sniffers Which Can Capture Legacy DOCSIS Bursts As
Well".
BACKGROUND OF THE INVENTION
[0002] In cable modem systems, there is a need to be able to debug
by capturing message and burst traffic in the upstream and
downstream directions. In DOCSIS systems, there is a need to be
able to capture all the data bursts, MAP and UCD messages,
timestamp data, ranging request messages, ranging bursts, ranging
response messages and all the other DOCSIS messages.
[0003] Sniffers exist in the prior art. A DOCSIS traffic sniffer
manufactured by Filtronic-Sigtek, Inc. (model number ST260B) can
captures data bursts and message traffic in DOCSIS 1.0 and 1.1
systems. However, DOCSIS 2.0 systems include the ability to send
spread spectrum bursts, and the Filtronic sniffer cannot capture
and demodulate spread spectrum bursts and time division multiplexed
bursts at symbol rates faster than symbol rates of DOCSIS 1.0 and
1.1 systems. This device does not appear to be covered in any
patents or published patent applications.
[0004] Conventional sniffers today are too complicated. It would be
advantageous to have a simple DOCSIS 2.0 sniffer which can use a
conventional DOCSIS 2.0 cable modem that has been modified to
provide timing and control signals for a burst capture circuit that
captures upstream bursts. It would also be advantageous to divert
upstream bursts away from a CMTS and capture them in such a sniffer
and provide such a sniffer with the capability to repeat captured
bursts upstream so that only bursts and upstream messages are
transmitted upstream and nothing else is transmitted. This could
replace the digital return prior art where everything going
upstream including empty minislots is digitized and sent upstream
as digital samples. Both digital return systems in the prior art
and a digital upstream sniffer/repeater allow the CMTS to not have
any burst acquisition circuitry and only use digital signal
processing circuitry to process the digital samples. This
simplifies the CMTS. However, a digital sniffer/repeater which
transmits upstream only bursts and messages allows greater
consolidation of upstream traffic from different optical nodes and
allows the CMTS to be used more efficiently.
[0005] Accordingly, a need has arisen for a sniffer which can
capture all types of DOCSIS bursts including SCDMA DOCSIS 2.0
bursts in both the upstream and downstream. There has also risen a
need for a simpler DOCSIS 2.0 upstream only sniffer and a simpler
DOCSIS 2.0 upstream only sniffer with repeater capability.
SUMMARY OF THE INVENTION
[0006] Two different classes of sniffers are described herein. The
first class uses one or more DOCSIS cable modems in the sniffer to
capture DOCSIS 1.0, 1.1 and 2.0 downstream bursts from the CMTS and
to capture downstream messages from the CMTS to implement the
DOCSIS protocols. The captured downstream messages include ranging
invitations, ranging response messages which include time,
frequency and power offsets and upstream equalization coefficients,
and all the other downstream DOCSIS messages. Captured data from
bursts and messages are transmitted by a local area network or
other data path to a line card which includes a computer programmed
to pass the data and messages to a personal computer coupled to the
sniffer and to use data in the messages to do a self-ranging
process and to control the upstream burst capture process.
[0007] The cable modem (CM) in the sniffer also recovers the
downstream symbol clock and uses it to recover downstream bursts.
If an upstream DOCSIS 2.0 burst is to be captured, the CM in the
sniffer uses the recovered downstream clock to provide a phase
coherent reference clock signal to a master clock generator. The
master clock generator smoothes out jitter in the reference clock
signal and multiplies the reference clock signal so as to generate
a high speed reference clock signal. This high speed reference
clock signal is coupled to a linecard which includes a CMTS
receiver. The reference clock signal is used in the linecard to
generate a 10.24 MHz local clock signal which is synchronized with
the 10.24 master clock signal in the CMTS under test which drives
the master timestamp counter. The 10.24 MHz local clock signal on
the linecard is used to drive a local timestamp counter in the
linecard and this keeps the linecard timestamp counter in
synchronization with a master timestamp counter in the CMTS under
test so that upstream minislot boundaries can be determined. The
master timestamp counter counts ticks of the master clock in the
CMTS under test.
[0008] An upstream minislot counter in the CMTS under test also
counts the same 10.24 MHz master clock signal, and functions to
count off the boundaries in time of the upstream minislots. The
sniffer linecard also has an upstream minislot counter which is
kept synchronized to the minislot counter in the CMTS by counting
the 10.24 MHz local clock signal and using captured DOCSIS 2.0
downstream timestamp snapshot messages to establish the correct
relationship between the sniffer local timestamp counter and a
local minislot counter and frame counter.
[0009] The local timestamp counter in the linecard is kept in
synchronization with the master timestamp counter in DOCSIS 1.x and
DOCSIS 2.0 advanced TDMA burst capture mode using timestamp samples
which are included in captured downstream sync messages and
captured timestamp snapshot messages in DOCSIS 2.0 systems. The
local timestamp counter counts a free running 10.24 MHz clock, and
differences between the timestamp count and the timestamp samples
are used to generate error signals to adjust the free running clock
frequency to minimize the errors.
[0010] In SCDMA mode in DOCSIS 2.0 systems, an upstream symbol
clock is generated from the recovered downstream clock in captured
downstream bursts. The upstream symbol clock in the sniffer CM is
output at a reference clock output and is generated at a frequency
which is M/N of the recovered downstream clock where the M and N
factors are included in captured downstream messages when upstream
DOCSIS 2.0 SCDMA bursts are to be captured. This DOCSIS 2.0
upstream symbol clock is phase coherent to the recovered 10.24 MHz
downstream symbol clock in that it is locked into phase coherency
with the recovered downstream symbol clock. Phase coherency means
that for every M cycles of one clock, N cycles of the other clock
will occur with simultaneous zero crossing at the end of the M
cycle period, where M and N are both integers and both can be one.
This upstream reference symbol clock is used by the sniffer
linecard to capture DOCSIS 2.0 SCDMA bursts. In DOCSIS 1.x and 2.0
advanced TDMA bursts, the above described processing by the sniffer
CM does not occur, and the CMTS receiver in the sniffer recovers
the upstream symbol clock from the preamble and the data of the
captured burst and uses that to recover the data in the data
portion of the burst.
[0011] The CMTS receiver in the sniffer and an RF demodulator
section and a field programmable gate array which includes a
programmable delay element and an a programmable digital
equalization filter, all are controlled by a programmed computer on
a linecard in the sniffer.
[0012] The downstream messages transferred from the cable modems in
the sniffer include the UCD messages that define burst and channel
parameters such as symbol rate, frequency, modulation type,
multiplexing type, etc. of each upstream logical channel. The
downstream messages captured by the cable modems in the sniffer
also include the MAP messages which define which upstream minislots
will contain bursts from each cable modem under test. The captured
downstream messages also include ranging response messages, each
such message including the ranging corrections sent by the CMTS
under test to a cable modem under test in response to an upstream
station maintenance burst sent by the CM. The linecard captures
upstream initial and periodic station maintenance bursts from cable
modems under test and makes its own ranging correction calculations
on each such burst. The ranging corrections in each ranging
response message are subtracted from the ranging offset corrections
(and upstream equalization coefficients in the preferred
embodiment) calculated by the CMTS in the sniffer to derive
difference values for the time, phase, frequency and power offsets
(and difference values for the upstream equalization coefficients
in the preferred embodiment). These difference values are
calculated by the computer in the linecard and used to generate
control signals to make corrections to circuitry of the line card
in the RF demodulator and the FPGA delay and equalization filter
coefficients in a process called self-ranging to get the linecard
into synchronization with the upstream transmissions from a CM
under test whose burst is to be captured. The CMTS in the sniffer
then uses the locally generated upstream symbol clock, the upstream
minislot count from its local timebase and data from captured UCD
and MAP messages to control the circuitry in the sniffer to capture
both DOCSIS 1.x and 2.0 bursts.
[0013] A second class of sniffers and sniffer/repeaters is
characterized by: (1) the use of a slightly modified DOCSIS 2.0
cable modem in the sniffer which registers with the CMTS as one of
the CMs in the system so as to capture the system timing of at
least one upstream; and, (2) a burst capture line of circuitry
which tunes, filters, amplifies and digitizes upstream bursts to be
captured and stores samples of each burst in a buffer. In some
embodiments, the burst capture circuitry includes a programmable
passband filter for precise passband tuning based upon symbol rate
and decimation circuitry to reduce the number of samples of lower
symbol rate captured bursts.
[0014] Timing for the burst capture circuitry is supplied by the
cable modem in the sniffer which synchronizes to the upstream on
which the bursts to be captured will be transmitted and also
captures the downstream UCD and MAP messages that define the timing
and burst characteristics of every burst to be captured. Since the
sniffer's cable modem is locked onto the same downstream as all the
other cable modems in the system, it also has its local upstream
timestamp, minislot and frame counters synchronized to the
corresponding counters in the CMTS under test for DOCSIS 2.0 SCDMA
bursts. This is done by recovering the downstream symbol clock and
generating an upstream symbol clock at a frequency which is M/N the
frequency of the recovered downstream clock and which is phase
coherent with the master 10.24 MHz master clock in the CMTS under
test. Captured downstream sync and timestamp snapshot messages are
used to initialize the local timestamp counter in the CM and to
establish the correct relationship between the local timestamp
counter and the local minislot and frame counters for 2.0 SCDMA
burst capture.
[0015] In DOCSIS 1.x and DOCSIS 2.0 advanced TDMA burst capture
mode, the timestamp, minislot and frame counters are kept in
synchronization using the same timestamp samples from the sync
messages as in the case of all the other cable modems in the
system. Every cable modem in the system uses the same free running
upstream clock frequency to drive the local timestamp counter in
DOCSIS 1.x and DOCSIS 2.0 advanced TDMA burst transmission mode.
Every CM in the system on a given upstream in 1.x mode, including
the CM under test, use the same sync messages in the linked
downstream to keep their local upstream timestamp counters
synchronized with the master timestamp counter in the CMTS.
Therefore, each cable modem in the system will be using the same
timestamp counter counts to determine the boundaries of the
upstream minislots at the location of that cable modem. Therefore,
the cable modem in the sniffer will be able to tell when each
upstream minislot is occurring at its location and can use the
copies of the captured downstream MAP data to determine which
minislots to enable the burst capture circuitry and can use the
pertinent UCD message data to control the burst capture circuitry
to capture the burst. In some embodiments, the symbol rate data in
the UCD message data can be used to control a digital passband
filter in the burst capture circuitry to have the proper passband
bandwidth to tune out RF signals outside the bandwidth of the burst
to be captured.
[0016] The only modifications that need to be made to the cable
modem's software in this second genus of sniffers is to use the MAP
and UCD data to generate suitable control signals to control the
burst capture circuitry to capture upstream bursts at the proper
times. In some embodiment, modifications must also be made to
receive user input defining which bursts to capture. In other
embodiments, no such modification to receive user input need be
made since the sniffer captures all upstream bursts.
[0017] In alternative embodiments within this genus, the captured
bursts are transmitted upstream from the buffer in the sniffer to a
CMTS that has no burst capture circuitry. This embodiment for a
sniffer/repeater can be advantageously used to replace the prior
art digital return data paths by capturing only the upstream bursts
and digitizing them instead of digitizing every minislot including
empty one like the digital return technology. In this way, more
efficient use of the CMTS can be made, and more optical nodes can
share one CMTS receiver. This is because only the bursts are sent
upstream so the CMTS can pipeline process burst after burst and
need waste no time on processing samples of empty minislots.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a block diagram of a system in which a DOCSIS 2.0
sniffer with two internal cable modems is employed to capture both
downstream traffic where one cable modem captures all downstream
message traffic and the other cable modem captures downstream data
bursts, and a linecard is used to capture upstream bursts from the
CM under test.
[0019] FIG. 2 is a block diagram of an alternative embodiment of a
sniffer capable of capturing both upstream and downstream bursts
and using only a single cable modem in the sniffer to capture both
downstream bursts and downstream messages.
[0020] FIG. 3 is a flowchart of the process to keep any cable
modem's local timestamp counter synchronized with the master
upstream timestamp counter in the CMTS.
[0021] FIG. 4 is block diagram of a sniffer using a cable modem to
determine the upstream timing and to control a burst capture
circuit to capture upstream bursts of other cable modems.
[0022] FIG. 5 is block diagram of a sniffer using a cable modem to
determine the upstream timing and to control a burst capture
circuit to capture upstream bursts of other cable modems and having
the capability to repeat captured bursts upstream.
[0023] FIG. 6 is block diagram of an alternative embodiment of a
sniffer using a cable modem to determine the upstream timing and to
control a burst capture circuit to capture upstream bursts of other
cable modems and having the capability to repeat captured bursts
upstream.
[0024] FIG. 7 is a flowchart of the new part of the process carried
out by the cable modem in the sniffer of FIG. 4.
[0025] FIG. 8 is a diagram of the prior art digital return
structure from the diplexer at the optical node to the CMTS.
[0026] FIG. 9 is a block diagram of a sniffer based digital return
structure from the diplexer at the optical node to the CMTS which
captures only the bursts and does not sample empty minislots like
the structure in FIG. 8.
[0027] FIGS. 10A, 10B, 10C and 10D are a flowchart of the process
for controlling the sniffer of FIG. 1 or 2 to receive designated
downstream and upstream bursts on a downstream and an upstream (or
logical channel) designated by a control computer.
[0028] FIGS. 11A through 11C illustrate the process the DOCSIS 2.0
upstream and downstream sniffers of FIGS. 1 and 2 go through to
establish synchronization with a designated downstream and a
designated upstream.
DETAILED DESCRIPTION
[0029] FIG. 1 is a block diagram of a system in which a DOCSIS 2.0
sniffer with two internal cable modems. Different parts of the
sniffer are employed to capture both upstream and downstream
traffic. A cable modem termination system (CMTS) 10 is one of the
units under test. It transmits DOCSIS 1.0, 1.1 or 2.0 data bursts
and messages downstream on a downstream medium 12. The CMTS 10 also
receives upstream DOCSIS 1.0, 1.1 or 2.0 data bursts and messages
from a cable modem 18 via upstream medium 14. The upstream and
downstream mediums are coupled by a diplexer 16 to a plurality of
cable modems (CM) of which CM 18 is typical.
[0030] A DOCSIS 2.0 sniffer 20 is coupled to the downstream and
upstream mediums by taps 24 and 22 via splitters (not shown). The
upstream tap 22 is coupled to a linecard 26 which includes an RF
demodulator 28, a field programmable gate array (FPGA) 30 including
a programmable delay circuit and a programmable digital
equalization filter and a clock management circuit (block 30 can be
implemented as an FPGA, an ASIC or in random logic), a CMTS
receiver 32 and a programmed computer 34. The RF demodulator 28 can
have the structure of any RF demodulator in a CMTS front end which
can be controlled by CPU 34 to tune the appropriate upstream
logical channel, convert that frequency to a lower intermediate
(IF) frequency, amplify the received signal by a programmable gain,
filter out unwanted signals and digitize the IF signal. Several
suitable structures are disclosed in U.S. patent application Ser.
No. 09/792,815 filed Feb. 23, 2001, which is hereby incorporated by
reference. The FPGA 30 imposes a programmable delay controlled by
CPU 34, and filters the signal in a digital equalization filter
which has its coefficients controlled by CPU 34. The CMTS 32 can be
any CMTS receiver, and CPU 34 can be any programmable computer
which is programmed to control the linecard in accordance with the
teachings herein.
[0031] The linecard 26 can be structured as any CMTS receiver
upstream line card with some software modifications and some
modifications to the FPGA. Only one channel of the conventional
linecard need be used. Those modifications to the conventional
linecard hardware and software are as follows:
[0032] (1) receiving the TLVs (data elements) of UCD and MAP
messages captured by the cable modem 40 or 42 in the sniffer and
converting them to the proper format and enabling searching MAP
messages for the minislot numbers of upstream bursts to be captured
and determining the burst parameters from the UCD messages of
bursts to be captured, and capturing upstream DOCSIS bursts
designated by a control computer and transmitting at least the data
from said captured bursts to said control computer via a port on
the sniffer--a TLV is a tuple with the first byte indicating the
type of the data element, the second byte indicating the length,
and the third part indicating the value;
[0033] (2) in mixed mode when two or more logical channels are
being received in the upstream direction, combining the MAP
messages for each logical channel into one MAP message for use by a
Jasper type CMTS receiver 32;
[0034] (3) differentially encoding preamble symbols of DOCSIS 1.x
differentially encoded bursts before they are loaded into a Jasper
type CMTS receiver 32 (Jasper type CMTS receivers use a different
preamble pattern than the CMs under test in differentially encoded
bursts, but this modification can be eliminated if CMTS receives
that use the same type preambles as the CMs under test for
differentially encoded and non differentially encoded bursts);
[0035] (4) receiving the ranging corrections directed to each CM
under test from the CMs in the sniffer and using the corrections
sent to the CM under test along with the ranging corrections
calculated in the CMTS receiver in the sniffer from captured
upstream training bursts to get the CMTS receiver in the sniffer in
time synchronization with the selected upstream;
[0036] (5) sending control messages to the cable modems in the
sniffer to control which downstream bursts the CMs capture via the
local area network segment coupling the linecard to the CMs 40 and
42 or via any other data path;
[0037] (6) performing all the other functions of the self ranging
processes described below in the self ranging section including
power corrections and setting of upstream equalization coefficients
in the sniffer equalization filter so as to correct for the
possibly different position on upstream HFC medium 14 from the CMTS
10 under test so as to enable proper reception of upstream bursts
from CMs which have implemented ranging corrections based upon
ranging response messages sent from CMTS 10 under test;
[0038] (7) interfacing via a LAN segment or any other data path
with the personal computer 37 to send captured data and messages to
it and to receive control commands therefrom indicating what
upstream and downstream bursts to capture and/or any other control
information needed to control the sniffer operations;
[0039] Any linecard with a CMTS receiver which has been modified in
any way to have at least the functionality of items 1, 4, 5, 6 and
7 above and which is capable of receiving at least upstream DOCSIS
2.0 bursts will be referred to in the claims as a "sniffer upstream
receiver" which is generally synonymous with a conventional
upstream linecard modified according to the teachings herein. In
the preferred embodiment, the CMTS receiver has the flexibility to
receive any of the different burst types specified in the DOCSIS
1.x or 2.0 specifications. There are 15 different SCDMA and TDMA
burst types at various symbol rates and modulation types that are
predefined in the preferred embodiment, and each has a different
IUC. Each burst has burst parameters, and the channel upon which
the burst is transmitted has channel characteristics. The burst
parameters and channel characteristics for each channel and each
burst and the minislot numbers assigned to each upstream burst are
controlled by the CMTS under test 10 and are sent downstream in UCD
and MAP messages which are captured by cable modem 40 and
transferred to computer 34. The MAP messages and burst parameters
and channel characteristics define what type of bursts are going to
be received during each upstream minislot.
[0040] The transmission characteristics of each logical channel are
separated into three portions: 1) channel parameters; 2) burst
profile attributes, and 3) user unique parameters. The channel
parameters include: a) the symbol rate which can be any one of 6
different rates from 160 ksym/sec to 5.12 Msym/sec in octave steps;
b) the center frequency; and c) the 1536-bit preamble superstring;
and d) the SCDMA channel parameters. These characteristics are
shared by all users on a given channel. User unique parameters may
vary from user to user even when on the same channel and same burst
type and include such things as power level. The power level of
each CM is controlled by the CMTS so that bursts arrive at the CMTS
at a nominal power level defined by the CMTS. Each CM must generate
each burst at the appropriate time so that the beginning of the
burst arrives at the CMTS at the assigned first minislot boundary
specified in the MAP message. The burst profile attributes, in the
preferred embodiment, include: modulation (QPSK, 64 QAM, 128 QAM
etc.), differential encoding on or off; Trellis Code Modulation
(TCM) encoding on or off; preamble length, preamble value offset;
preamble type (QPSK 0 or QPSK1), RS error correction T from 0 to 16
where 0 is no FEC bits to 16 for the maximum where the number of
codeword parity bytes is 2.times.T, RS codeword length (fixed or
shortened), scrambler seed, max burst length in minislots,
guardtime from 5 to 255 symbols for TDMA channels and 1 for SCDMA
channels, last codeword length, scrambler on or off, byte
interleaver depth, byte interleaver block size, SCDMA on or off,
codes per subframe, and SCDMA interleaver step size. In other
embodiments, any smaller set of the above defined programmable
burst parameters may be used so long as the receiver can receive
both TDMA bursts and SCDMA bursts.
[0041] The burst parameters and the MAP messages and channel
characteristics are stored by the computer 34 in a burst parameter
memory in CMTS receiver 32 via data path 36 by a MAC process
running in computer 34.
[0042] The RF demodulator section 28 is a conventional RF
demodulator section which is controlled by computer 34 to tune to
an upstream channel specified in control signals or data on bus 36.
The RF demodulator 28 also: filters out unwanted signals outside
the bandwidth of interest; amplifies the signal by a programmable
gain amount in accordance with control signals on bus 36; converts
the RF signal of the band to which the RF demodulator is tuned to
an intermediate frequency (IF) using a programmable frequency local
oscillator signal the frequency of which is controlled by said
computer 34 via control signals on bus 36, and converts the IF
signal to digital samples using a sample clock which has a
frequency high enough to meet the Nyquist criteria for the highest
sample clock of any type burst which may be received. Any RF
demodulator circuit, implemented in either analog or digital
circuitry, which can fulfill these functions will suffice to
practice the invention. All CMTS receivers which are commercially
available use RF demodulator sections which perform these
functions, and any one of them may be used.
[0043] One example of an RF demodulator section 28 is a
programmable gain amplifier which amplifies the received radio
frequency signals by a programmable gain amount. The output of this
filter is filtered in a bandpass filter having a broad passband
covering the entire upstream band of frequencies. The filtered
output signal is mixed with a local oscillator signal having a
programmable frequency to mix the RF signal down to an IF frequency
at the center frequency of a more narrow passband bandpass filter
with sharp rolloff skirts (typically a surface acoustic wave
filter). The IF signal is then filtered in the narrow passband
filter having a passband bandwidth which may be programmable in
some embodiments to the bandwidth of the actual burst to be
received based upon its symbol rate or which may be set at the
bandwidth of the highest symbol rate burst to be received. The
filtered signal is then digitized in an analog-to-digital converter
which, in some embodiments, does IF sampling.
[0044] Another example of an RF demodulator section 28 is a
programmable gain amplifier which amplifies the received radio
frequency signal by a programmable gain controlled by computer 34.
A bank of bandpass filters, each with a different center frequency
and a bandwidth equal to the highest symbol rate channel to be
received share an input coupled to the output of the programmable
gain amplifier. A multiplexer having one input coupled to the
output of each bandpass filter and an output coupled to an
analog-to-digital converter is controlled by the computer 34 to
pick the RF channel to be received. The A/D converter digitizes the
selected filtered signal and the digitized sample stream is
duplicated and mixed digitally with two quadrature local oscillator
signals of the same frequency but 90 degrees out of phase (or some
odd harmonic of 90 degrees) and represented in digital form. The
two quadrature sample stream are then decimated and filtered in a
decimation and filter circuit which decimates and passband filters
with a bandwidth set according to the symbol rate of the burst to
be received so that excess samples are removed and the passband
bandwidth matches the bandwidth of the burst to be received to
eliminate more noise.
[0045] Another example of an RF demodulator is a low resolution
synthesizer which amplifies the received RF signal by a
programmable gain and down converts the RF to an IF frequency of
approximately 5.12 MHz with the local oscillator frequency
controlled by computer 34. An A/D converter then digitizes the IF,
and a numerically controlled digital synthesizer then down converts
the digital IF to an exact IF frequency of 5.12 MHz. The computer
34 controls the local oscillator frequency using the frequency
offset measurement developed during the self ranging process.
[0046] Another example of an RF demodulator is a programmable gain
amplifier applying a programmable gain controlled by computer 34
and broadband passband filter and analog mixer with the local
oscillator frequency generated by the synthesizer controlled by
computer 34 to generate an analog IF signal. The analog IF signal
is then filtered by a SAW filter with a narrow passband at a center
frequency of the IF and a bandwidth set to match the bandwidth of
the burst to be received. Another down converter fed by a fixed
frequency local oscillator then lowers the IF frequency further. A
low phase noise bandpass filter then filters the new IF frequency
signal, and an A/D converter digitizes the result.
Function of the Sniffer CMTS Receiver
[0047] Any CMTS receiver design will suffice for CMTS receiver 32
especially one which supports any preamble pattern for upstream
bursts. A short summary of the overall function of CMTS 32 is that
it functions to receive the digital samples output by FPGA 30 and
process them in the signal processing section of the CMTS receiver
32 to make measurements on training bursts and output those ranging
corrections to CPU 34, and to recover the data of data bursts and
upstream messages and output the recovered data, ranging and
upstream messages to CPU 34 for transfer to computer 37.
[0048] U.S. patent application Ser. No. 09/792,815, filed Feb. 23,
2001 details structures for various CMTS receiver embodiments, any
one of which may be used for the CMTS 32 in the sniffer. CMTS
receivers of the type detailed in U.S. patent application Ser. No.
09/792,815 will be referred to herein as Jasper type CMTS
receives.
[0049] In one embodiment, a programmable digital filter in field
programmable gate array 30 functions to add a programmable transfer
function which imposes a delay and gain offset correction to get
the CMTS receiver 32 in synchronization to receive upstream bursts.
The amount of correction is controlled by filter coefficients which
are controlled by CPU 34. In other embodiments, each of the time,
phase, frequency and gain offsets is corrected by a separate
circuit or a rotational amplifier is used to make gain and phase
offset corrections, and a programmable delay circuit is used to
make the timing offset correction and a programmable digital
equalization filter is controlled to do upstream equalization. The
digital equalization filter in FPGA 30 is a 24-tap feedforward
filter with programmable taps in the preferred embodiment.
[0050] When the CMTS 32 in the sniffer is located at the same
position as the CMTS 10 and receives training bursts from the CMs
under test and makes the same time offset measurements as the CMTS
under test 10, the measurements will be the same as those made by
the CMTS under test 10 if CMTS 10 is working properly. Generally,
the CMTS 32 and the CMTS 10 have offset measurements and develop
equalization coefficients from training bursts which not the same.
In this case, the self-ranging process described below is carried
out to develop the proper corrections and equalization coefficients
so as to achieve upstream synchronization (a state where upstream
bursts can be successfully captured by the sniffer).
[0051] The CMTS receiver 32 transmits the captured upstream bursts
to the CPU 34 on data path 31 along with the ranging measurements
(time, phase, amplitude, gain and frequency offsets and
equalization coefficients) for each CM under test which sent a
training burst. The CMTS 32 also adaptively develops equalization
coefficients for each CM from the preamble and possibly data of
that CM's captured training burst.
[0052] CPU 34 functions to receive these measurements,
coefficients, data, and messages and generate the proper control
signals to control the system. It also transmits captured bursts
and messages to computer 37, and may also transmit the ranging
measurements made by the linecard for display and/or analysis. The
CPU 34 also receives commands from the computer 37 and generates
suitable control signals to control the linecard 26, cable modems
in the sniffer 40 and 42 and the clock generator 48.
[0053] Data path 38 could be an Ethernet LAN connection, a USB or
SCSI port, or any other known or proprietary bus or LAN and
communication/bus protocol. The PC 37 is programmed to display
and/or process the captured data, measurements, messages,
coefficients, etc. and provide reports or just display the captured
items.
Linecard Timebase
[0054] The linecard 26 needs to have a timebase which will keep it
synchronized with the system upstream of interest so that the CMTS
receiver 32 in the sniffer can capture designated upstream bursts.
Therefore, the CMTS 32 must have a local timestamp counter which is
synchronized with the master timestamp counter in the CMTS 10 under
test because the master timestamp counter in the CMTS 10 is used to
determine the upstream minislot boundaries for the upstream
transmission grants to the CMs in the downstream MAP messages. So
the linecard in the sniffer needs to have its local timestamp
counter and minislot counter and frame counter (these three
counters along with the upstream symbol clock are the most
important clocks in the linecard timebase) synchronized with
corresponding counters in the CMTS under test and in the cable
modem under test so that the sniffer can determine when upstream
minislot in which bursts to be captured are occurring at the tap
points of the sniffer.
[0055] The basic idea of timebase synchronization is to get the
timebase in the sniffer to a condition that an upstream burst in a
particular minislot or group of minislots can be captured. This
requires several things of the timebase. First, one must know when
particular minislots are occurring at the sniffer upstream tap.
Second, an upstream symbol clock in the sniffer must be in sync
with the upstream symbol clock of the CM which transmitted the
burst. In other words, when a cable modem under test has been told
to transmit an upstream burst during a particular minislot, the
timestamp and minislot counters in the sniffer (which will be
coupled to the HFC upstream medium between the CM under test and
the CMTS) must be in a state such that they can be used to
determine when that particular minislot's signals are propagating
past the sniffer's upstream tap point so that the upstream burst
capture circuitry can be turned on to capture the burst. Then, if
the upstream symbol clock in the sniffer linecard timebase is
synchronized with the upstream symbol clock in the CM which
transmitted the burst, the upstream symbol clock in the sniffer can
be used to successfully recover the data in the burst.
[0056] Accordingly, the downstream timestamp samples in the
downstream synchronization (hereafter sync or synch) messages and
timestamp snapshot messages need to be sent to the linecard 26 for
use in synchronizing the local timestamp counter in the CMTS
receiver 32 in the sniffer to the master timestamp counter in the
CMTS 10 under test at least in DOCSIS 2.0 burst capture mode, as
will be described further below.
[0057] The CMTS receiver 32 in the sniffer also needs to generate a
10.24 MHz clock which is synchronized with the 10.24 MHz master
clock signal used by CMTS 10 to increment the master timestamp
counter in 2.0 SCDMA burst capture mode or which is frequency
adjusted using timestamp samples in 1.x and 2.0 advanced TDMA burst
capture mode so that the CMTS 32 can increment its local timestamp
counter in synchronizm with the incrementation of the master
timestamp counter in the CMTS 10.
[0058] In some embodiments, the A/D sampling clock in the RF
section 28 also needs to be synchronized with the sampling clock in
the burst acquisition circuitry of the CMTS 10 under test.
Therefore, the linecard 26 must have a master clock synchronized
with the master clock in the CMTS 10 under test at least for 2.0
SCDMA burst capture at least for these embodiments.
[0059] Generation of a 10.24 MHz master clock for the CMTS 32
starts by using DOCSIS 2.0 cable modem 40 to recover the downstream
symbol clock. Once the downstream symbol clock is recovered, the CM
40 in the sniffer receives the downstream messages. These messages
include: UCD, MAP, SCDMA upstream ratio numerator, SCDMA upstream
ratio denominator, SCDMA timestamp snapshot and sync messages
containing timestamp samples for TDMA channels, and ranging
response messages. The ranging response messages contain ranging
corrections of power, time, phase and carrier frequency offset and
upstream equalization coefficients sent to the modems. The captured
timestamp samples are sent to the linecard 26 as are the UCD and
MAP messages. The ranging correction offsets and the US
equalization coefficients for each particular CM are also sent to
the linecard.
[0060] The CM 40 communicates with the linecard 26 to send these
captured messages to CPU 34 via a local area network link 82. Data
path 82 is also used to supply any control data needed by CM 40
from computer 34 such as data that defines which downstream RF
carrier to tune and which downstream bursts are to be captured.
[0061] The captured MAP, UCD and other downstream messages must
reach the linecard 26 early enough for the CPU 34 to use the data
therein to control the linecard's CMTS receiver 32 and other burst
capture circuitry in time to receive the designated upstream bursts
from the CM under test. In order to help with the timing, two CMs
are used to receive downstream data. CM 40 receives only downstream
messages and sends them to the linecard via LAN link 82, and CM 42
captures only downstream data bursts and sends them to the linecard
via LAN connection 80.
[0062] The delay in sending the timestamp samples to the linecard
from the sniffer cable modem needs to be compensated. The exact
timebase offset to use for this compensation can be found from the
ranging offset corrections sent downstream by the CMTS 10 in
response to an initial maintenance burst IUC3 from a cable modem
close to the sniffer. In SCDMA mode, the timestamp snapshot is also
sent from the CM 40 to the linecard 26 for use in establishing the
correct relationship between the local timestamp counter and the
local minislot and frame counters.
[0063] To generate a master clock for the CMTS receiver 32 in the
sniffer, the CM 40 is also used to recover the downstream symbol
clock in the normal manner and generate a phase coherent upstream
clock reference to enable capture of DOCSIS 2.0 SCDMA bursts. In
DOCSIS 2.0 SCDMA bursts, the upstream reference clock must be phase
coherent with the downstream symbol clock. In DOCSIS 2.0 cable
modems, and in CM 40 in the sniffer in 2.0 SCDMA burst capture
mode, there is circuitry that operates to recover the downstream
symbol clock and generate an upstream reference clock from it. This
upstream reference clock is output on line 44 and is 2.56 MHz in
the preferred embodiment, although in alternative embodiments, a
10.24 MHz or 20.48 MHz reference clock can be output on line 44
from the sniffer cable modem. In some embodiments, the CM 40 also
generates an upstream symbol clock in 2.0 SCDMA burst capture mode
which phase coherent with the recovered downstream symbol clock and
has a frequency at a ratio of M/N where M and N are integers
included in the UCD message. In embodiments where multiple
upstreams are in use, the UCD message pertaining to the upstream on
which bursts are to be captured is used. This M/N upstream symbol
clock is the clock the linecard needs to capture 2.0 SCDMA bursts.
Since this clock is generated in CM 40, it must either be provided
on a separate output line (not shown) to the linecard, or the
linecard must regenerate this clock frequency from the clock
reference signal on line 46 which is also phase coherent with the
recovered downstream symbol clock in 2.0 SCDMA burst capture mode.
In some embodiments, CM 40 sends samples of this clock via LAN
segment 82 to CPU 34 for use in keeping a PLL that regenerates the
upstream symbol clock on the linecard synchronized.
[0064] Typically, a phase locked loop (PLL) is used to generate the
M/N upstream clock. This PLL locks onto the downstream clock and
generates an upstream clock which is phase locked to the downstream
clock. An example of circuitry in a cable modem to perform this
function is given in U.S. Pat. No. 6,243,369 which is hereby
incorporated by reference. In some embodiments, a first PLL
recovers and tracks the downstream clock and provides it as a
reference frequency to another PLL which generates an upstream
clock at a frequency M/N times the downstream symbol clock
frequency.
[0065] In 2.0 SCDMA burst capture mode (which is the only mode
where the clock reference on line 44 is generated), the upstream
clock reference signal on line 44 is a 10.24 MHz clock signal
divided by 4. This clock reference is output by CM 40 on line 44 at
the upstream RF output (in a special test mode) as a 2.56 MHz
reference clock signal to the clock generator 48. The 2.56 MHz
reference signal on line 44 is phase coherent with the recovered
downstream symbol clock, and is actually alternating ones and zeros
at 5.12 MHz. In some embodiments, it is taken from the upstream
input to the tuner of CM 40, and the upstream input to the tuner in
CM 40 is disconnected. This is the main clock reference signal and
is selected by multiplexer 50 most of the time. The other clock
reference signals that can be selected for use by the clock
generator 48 are mainly for test purposes. In some embodiments, the
clock reference on line 44 derived by the cable modem is the only
clock reference signal for the PLL 58 and even this signal is not
required if only 1.x or 2.0 advanced TDMA bursts are the only
bursts to be sniffed. In the preferred embodiment, the PLL 58 has
its reference clock input coupled through a multiplexer to either
the 2.56 MHz reference clock on line 44 or a 10.24 MHz reference
clock on line 52 from an external sources such as a CMTS under
test.
[0066] Any clock generator 48 which receives at least the recovered
downstream clock reference signal on line 44 and which smoothes it
out using a PLL or using any other means to remove or reduce the
jitter and generates an upstream clock reference signal on line 46
will suffice the practice the invention and will be referred to in
the claims as a "sniffer clock generator".
[0067] For TDMA bursts, DOCSIS 2.0, 1.1 and 1.0 cable modems, and
CM 40 in particular, have a free running 10.24 MHz upstream clock
which is counted by a local timestamp counter in the CM. Each cable
modems however must keep its local timestamp counter synchronized
with a master upstream timestamp counter in the CMTS which is
counting the master clock because the local timestamp counter and
local minislot counter which are both counting the same master
clock, are used to determine when the upstream minislots assigned
to each CM are occurring. To keep the CM timestamp, minislot and
frame counters in sync with the CMTS corresponding counters in 1.x
and 2.0 advanced TDMA burst capture mode, the CMTS periodically
sends timestamp counter samples downstream in sync and timestamp
snapshot messages. The timestamp snapshot messages contain samples
of the CMTS master upstream timestamp and minislot and frame
counters taken at the time of the message. The CMs receive these
timestamp sample messages in TDMA mode, and use the timestamp
samples in them to make corrections to their local timestamp and
minislot counters as will be detailed below. This process of
maintaining synchronization of the local upstream timestamp counter
in each CM in the system using the sync messages is illustrated in
the flowchart of FIG. 3. A more comprehensive process flow which
includes an alternative embodiment of the process of FIG. 3 is
given for DOCSIS 2.0 SCDMA burst capture and DOCSIS 1.x and 2.0
advanced TDMA burst capture mode in FIGS. 10 and 11, respectively
(each of which is comprised of multiple sheets).
[0068] The CM 40 in the sniffer also performs this process, but it
is not necessary since neither CM 40 or CM 42 will send any
upstream bursts so knowledge by these CMs of the upstream minislot
boundaries is not necessary in some embodiments. However, the CMTS
receiver 32 in the sniffer does need to know the upstream minislot
boundaries so that it can use the MAP and UCD data to capture
upstream bursts from the CM under test. Accordingly, CM 40, which
recovers the timestamp samples from the CMTS under test along with
all the other downstream messages such as ranging corrections, MAP
messages and UCD messages, sends these timestamp samples and all
the other captured downstream messages to the sniffer linecard. The
linecard in the sniffer uses the timestamp samples received from CM
40 to make initial settings and corrections in an upstream
timestamp, minislot and frame counters, depending upon the
mode.
[0069] Each upstream minislot has a number and its boundaries in
time are defined by the master timestamp counter in the CMTS under
test. There will always be an offset at any instant of time between
the timestamp counter in the CMTS 32 and the timestamp counter in
the CMTS 10. This offset is caused by the propagation time of the
timestamp sample messages sent downstream. Even if the local
timestamp counter in the sniffer is set to the timestamp count in
the first timestamp sample received after power up and counts a
clock at the same frequency as the master clock, that timestamp
sample would have been take some time earlier, so the actual
timestamp count of the master timestamp counter will be ahead of
the local timestamp counter in the sniffer. This offset should be
constant however, and any difference is corrected by using the
timestamp samples and ranging corrections to correct the count of
the local timestamp counter or to change the local 10.24 MHz clock
frequency so as to maintain a constant offset which is set to
achieve precise minislot boundary and frame synchronization.
[0070] This tracking and constant offset between the local
timestamp counter in CMTS 32 and the master timestamp counter will
be referred to in the claims as either synchronization or "tracking
the changes" of the master timestamp counter. This tracking of
changes and correction using timestamp samples in downstream sync
messages is done in DOCSIS 1.x and DOCSIS 2.0 advanced TDMA burst
capture mode only. In DOCSIS 2.0 burst capture mode, the 10.24 MHz
clock in the linecard is not free running as it is in DOCSIS 1.x or
2.0 advanced TDMA burst capture mode. Instead, it is locked to the
10.24 MHz master clock in the CMTS by virtue of it being generated
from the 20.48 MHz reference clock on line 46 which is locked into
phase coherency with the downstream symbol clock recovered by CM
40. The downstream symbol clock is itself locked into phase
coherency with the 10.24 MHz master clock in the CMTS under test,
so there should never be any drift between the local timestamp
counter in the sniffer and the upstream timestamp counter in the
CMTS under test.
[0071] Since the recovered symbol clock in SCDMA mode is more
accurate, the sniffer CM should be set in the SCDMA mode when the
CMTS under test is in SCDMA or mixed mode with a first SCMDA
logical channel which is time division multiplexed with one or more
other logical channels carrying other types of bursts such as TDMA
or SCDMA at a different symbol rate than the symbol rate of the
SCDMA burst in the first logical channel.
[0072] To receive 2.0 SCMDA bursts, the CMTS 10 and the CMTS 32
each use their own upstream phase locked symbol clocks to receive
the bursts because the upstream symbol clock in the CM that sends
each SCDMA burst is locked to or phase coherent with the downstream
symbol clock generated by the CMTS as is the upstream symbol clock
in the CMTS 10. The CMTS upstream symbol clock is generated from
its master clock in both CMTS 32 and CMTS 10.
[0073] In 1.x and 2.0 ATDMA mode, each CM under test and the CMTS
32 in the sniffer has a free running clock which is counted by its
timestamp counter. The process to keep the local timestamp counter
synchronized with the master timestamp counter in 1.x and 2.0 ATDMA
burst mode capture in the CMTS 32 and the CM 40 in the sniffer and
in every other CM in the system is illustrated in the flowchart of
FIG. 3.
[0074] The first step in getting each cable modem's local upstream
timestamp counter in synchronization with the master upstream
timestamp counter in the CMTS 10 is symbolized by block 68 in FIG.
3. There, the first timestamp received from CMTS 10 after a CM
powers up is set into the cable modem's local timestamp counter
(this assumes the CM has already recovered the downstream symbol
clock and is receiving downstream messages).
[0075] In step 70, the local free running 10.24 MHz clock is
counted by the local timestamp counter in the CM. Since the master
upstream timestamp counter in the CMTS is also counting a master
10.24 MHz clock, the counts at the CMTS and the CMs should remain
relatively close except there will be a fixed offset caused by
propagation delay and some drift caused by frequency drift of the
clocks. In step 72, another timestamp sample of the count in the
upstream master timestamp counter at the CMTS is received in a sync
message. The newly received timestamp sample is compared in step 74
to the local timestamp counter in the CM in step 74. Step 76
represents the branch condition on the comparison. If there is a
difference, step 78 is performed to reduce or eliminate the
difference. IN the case of a CMTS, the error can be eliminated by
setting the timestamp received from the CMTS under test into the
local timestamp counter in the timebase of the sniffer. If there is
no difference, processing returns to step 70. In some embodiments
of this process, instead of using the difference to correct the
actual count, the difference is used to generate an error signal to
change the frequency of the clock that drives the local timestamp
counter.
[0076] The master clock in the CMTS 32 is generated from, or, in
some embodiments is, a 20.48 MHz clock on line 46 generated by a
clock generator 48. Any clock generator structure which can
generate a 20.48 MHz reference clock on line 46 which is phase
coherent with the recovered downstream symbol clock in DOCSIS 2.0
SCDMA burst capture mode will suffice to practice the invention. In
the preferred embodiment, the reference clock on line 44 is
supplied as one input to a multiplexer 50, and a 10.24 MHz
reference clock from a CMTS under test is supplied to the other
input of multiplexer 50. PC 37 is programmed to allow a user to
select the desired reference clock input as between the signals on
lines 52 and 44, and generates a clock select signal on line 54
either directly or indirectly through a command sent to CPU 34 over
the data path 38. The clock signal on line 52 is the 10.24 MHz
master clock of the CMTS 10 under test. The clock signal on line 64
is the 20.48 MHz clock available from CMTS receivers manufactured
by Terayon Communication Systems, Inc. of Santa Clara, Calif. The
clock signal selected by multiplexer 62 is controlled by a signal
on line 35 from computer 34, and is output on line 56 to serve as a
reference clock input to a PLL 58. The PLL 58 is controlled by the
PC 37 though commands sent on data path 38 to CPU 34 to multiply
the reference clock signal by a factor of 2 or a factor of 4, and
output a 20.48 MHz reference clock signal on line 60. The PLL 58
also acts as a sort of filter on the reference clock signal from
the CM 40 to clean up its waveform. The PLL 58 should have a
bandwidth which is narrow enough to make sure the phase noise is
small. In TDMA mode, the clock signal output by the CM 40 can be
jittery because it is controlled by the received timestamps. In
SCDMA mode, the upstream reference clock on line 44 is much cleaner
because the CM 40 uses an internal PLL to generate it from the
recovered downstream symbol clock.
[0077] The reference clock signal on line 60 is coupled to one
input of a multiplexer 62. The other two inputs to the multiplexer
are a 20.48 MHz reference clock output on line 64 from a CMTS under
test or an internal 20.48 MHz reference clock on line 66 generated
by linecard 26. The other clock inputs on lines 52, 64 and 66 are
for testing and greater utility purposes, and may be eliminated in
some embodiments.
[0078] Any phase differences between the master upstream symbol
clock of CMTS 10 and the upstream symbol clock of the CMs under
test 18 is corrected out in a timing recovery loop in the CMTS 10.
There are two processes that go on in the CMTS 10 to receive
upstream bursts and in CMTS 32 to capture upstream bursts. The
first is recovery of the upstream symbol clock in 1.x and 2.0
advanced TDMA bursts. The second process involves recovery of the
QAM symbols themselves where phase, amplitude, timing and frequency
offsets are measured during the processing of a ranging burst
transmitted upstream by a particular CM and the measured offsets
are sent back to this CM to make corrections in its
transmitter.
[0079] This rotational amplifier uses known preamble data prepended
to each burst to develop phase and amplitude correction factors
which are used to remove phase and amplitude errors in the data
portions of the burst.
[0080] The CMTS receiver 32 in the sniffer 20 also has a timing
recovery loop like CMTS 10. CMTS 32 generates phase error
correction factors from the preambles and data symbols of training
bursts its captures from any of the cable modems 18 which are used
to correct phase errors in the data symbols of data bursts. In the
preferred embodiment, the CMTS receiver 32, like CMTS 10, has a
preamble processor which processes the known preamble symbols of
each burst to generate initial phase and frequency offset
correction factors. These offset correction factors are then
provided to a timing recovery loop which uses them as a starting
point to again process the symbols of the preamble to develop fined
tuned phase and frequency correction factors and then uses these
correction factors to correct phase and frequency offsets in the
data symbols of the burst. However, any DOCSIS 2.0 compatible
receiver structure will suffice for circuit 32.
[0081] The personal computer 37 has any user interface suitable to
control the sniffer, and is programmed to receiver user commands
indicating which channels on the upstream and downstream to monitor
and capture bursts and message traffic. In particular, the user
picks the upstream channel ID. When the captured bursts and/or
message traffic is transferred to the CPU 34 from the CMTS receiver
32 or the cable modem 40 (which receives downstream bursts and
messages from the CMTS 10), the PC 37 can then display the data of
the captured bursts or messages for analysis on a display (not
shown). The personal computer is programmed with any program that
allows the user to specify which upstream and downstream logical
channels to monitor, which bursts and/or messages to capture and
which data and/or messages received from the sniffer to process and
display.
Downstream Sniffing
[0082] In the preferred embodiment, shown in FIG. 1, the sniffer 20
uses two cable modems 40 and 42 to sniff the downstream. CM 40
captures only downstream DOCSIS messages such as ranging
invitations, MAP messages, UCD messages, ranging response messages
and all the other DOCSIS 2.0 downstream messages. CM 42 captures
only the downstream data bursts. A local area network or other data
path 80 couples this CM to the CPU 34 so that recovered downstream
data and other things may be sent to the CPU 34 and transferred to
the PC 37, and so that any control data needed by the CM can be
supplied by computer 34 such as control data that defines which
downstream RF carrier to tune and which downstream bursts are to be
captured.
[0083] The division of labor between CMs 40 and 42 improves the
performance of the system since there may be hundreds of cable
modems such as CM 18 coupled to the same CMTS 10. In alternative
embodiments such as is shown in FIG. 2, only a single cable modem
40 is used to capture all downstream data bursts and message
traffic for all cable modems such as 18. The cable modems 40 and 42
can be any DOCSIS cable modem structure which is capable of
receiving DOCSIS 2.0 bursts and message traffic which are modified
in accordance with the teachings herein. In the preferred
embodiment, both cable modems 40 and 42 can also receive DOCSIS 1.1
and DOCSIS 1.0 bursts and message traffic.
[0084] The cable modem 40 and the CMTS receiver 32 must each be
capable of receiving at least DOCSIS 2.0 bursts and messages
regardless of the type of multiplexing, symbol rate, channel
frequency, modulation type and all the other programmable
parameters of DOCSIS 2.0 systems.
[0085] The cable modems 40 and 42 are conventional except that a
few software changes are necessary for the sniffer function. These
modifications are:
[0086] (1) collecting downstream bursts addressed to other cable
modems, and packetizing them into a LAN packet addressed to CPU 34
or otherwise transferring them to CPU 34 by any other data
path;
[0087] (2) sending the captured downstream bursts and the TLVs
(data elements) of the UCD and MAP messages to the linecard 26 by
packetizing them into a LAN packet addressed to CPU 34 or otherwise
transferring them to CPU 34 by any other data path;
[0088] (3) capturing and sending the ranging corrections from
ranging response messages addressed to cable modems under test to
the linecard 26 by packetizing them into a LAN packet addressed to
CPU 34 or otherwise transferring them to CPU 34 by any other data
path;
[0089] (4) receiving and implementing control messages from the
linecard CPU 34 such as messages specifying SIDs of particular CMs
under test for which the cable modems of the sniffer are to capture
the downstream data bursts and messages directed thereto by the
CMTS under test; and
[0090] (5) enabling a debug mode and a mode to capture upstream
SCDMA bursts wherein a 2.56 MHz reference clock generated from the
recovered downstream clock in SCMDA mode at least is output at
upstream RF output 44 for use by the linecard 26 for generating an
upstream symbol clock which is phase coherent with the recovered
downstream clock. Generation of this reference clock from the
sniffer cable modem which is phase coherent with the recovered
downstream clock is only necessary where upstream DOCSIS 2.0 SCDMA
bursts are to be captured or in debug mode, and, in alternative
embodiments, the reference clock generated is 10.24 MHz or some
harmonic thereof, but in all cases where SCDMA bursts are to be
captured, the reference clock must be phase coherent with the
captured downstream clock. Generation in the sniffer of an upstream
symbol clock is not necessary to the invention if TDMA or DOCSIS
2.0 advanced TDMA bursts are the only bursts to be captured.
[0091] Any DOCSIS 2.0 compatible cable modem which has been
modified to have the functionality of at least elements 1, 2, 3, 4
and 5 above will suffice to practice the invention to capture
DOCSIS 2.0 SCDMA bursts, and will be referred to in the claims as a
"sniffer cable modem". However, the term "sniffer cable modem"
should also be interpreted to not require generation in the sniffer
of an upstream symbol clock which is phase coherent with the
recovered downstream clock where TDMA or DOCSIS 2.0 advanced TDMA
bursts are the only types of bursts to be captured.
Self Ranging
[0092] The CMTS receiver 32 receives both timestamps and timestamp
snapshot messages from the CM 40 in the sniffer which receives
downstream messages in 2.0 SCDMA mode. The CM 40 can send these
messages to CPU 34 via a local area network link 82 or other data
path which couples the CM 40 to the CPU 34. The sniffer linecard 26
receives the upstream training bursts from the CM 18 under test
just like the CMTS 10 under test receives them. The training burst
is processed in CMTS 32 just like it is processed in CMTS under
test 10 to make time, frequency, phase and gain offset measurements
and to develop upstream equalization coefficients from the known
preamble data in the training burst. However, the CMTS 32 is often
not located at the same position in the system as the CMTS 10, so
these offset measurements and equalization coefficients developed
in CMTS 32 will not be the same as are developed in CMTS 10.
[0093] Because the CMTS 32 in the sniffer is at a different
location than the CMTS under test (sometimes), the time, frequency,
phase and amplitude offset corrections and the equalization
coefficients developed by CMTS 32 are not sent to the CM under test
18. To do so would interfere with the actual time, frequency, phase
and amplitude offset corrections and the equalization coefficients
sent to the CM 18 by the CMTS 10 under test and developed from CM
18's upstream ranging burst by CMTS 10. Instead, the time,
frequency, phase and amplitude offset corrections and the
equalization coefficients developed by CMTS 32 from a captured
training burst from a CM under test 18, after transfer to the CPU
34, are modified and then used to adjust certain circuits in the
sniffer. This is done to achieve proper synchronization with CM 18
whose upstream bursts are to be captured. The same process happens
in CMTS 32 for every other CM in the system. More specifically, the
adjustment of the sniffer's circuitry to achieve self-ranging is
done using the ranging corrections developed by CMTS 32 from a
particular CM's upstream training burst after subtracting the
ranging corrections sent downstream to that CM by the CMTS 10. The
modified time, phase, frequency, amplitude offsets and upstream
equalization coefficients are used to set the coefficients of the
programmable digital upstream equalization filter in FPGA 30, the
delay of a programmable delay circuit in FPGA 30, the coefficients
of a rotational amplifier in the CMTS 32 and the gain of an
amplifier, and the frequency offset in the RF demodulator section
28. CPU 34 makes the subtraction or other processing necessary to
get the right frequency, phase, gain and timing offsets for use in
the sniffer. The results are loaded into the appropriate circuits
on the linecard 26 to get it into a state where it can receive
upstream bursts from a particular CM under test.
[0094] Thus, for example, the results of a subtraction of the
upstream equalization coefficients sent by CMTS 10 from the
upstream equalization coefficients developed by CMTS 32 are loaded
as the filter coefficients of the programmable equalization filter
in FPGA 30. The result of a subtraction of the gain adjustments
developed by the CMTS 10 and the CMTS 32 is used to adjust the gain
of a programmable gain amplifier in RF section 28 via control
signals on bus 36. The frequency offset developed by CMTS 10 from
the training burst sent by CM 18 is subtracted from the frequency
offset developed by CMTS 32, and the result is used to adjust a
local oscillator signal feeding a mixer in RF demodulator 28 to
achieve the proper IF frequency. A subtraction of the time offset
developed by the CMTS 10 from the time offset developed by CMTS 32
from the same training burst from a particular CM is used to set
the amount of a programmable delay imposed by a programmable delay
circuit in FPGA 30. This allows the sniffer to achieve proper
timing, phase, frequency and amplitude alignment and proper
upstream equalization to receive upstream bursts from a particular
CM transmitting on a particular upstream.
[0095] In some embodiments, the ranging corrections developed by
CMTS 32 will be transferred to the CPU 34 for transfer to the
computer 37 and the captured downstream ranging corrections from
the CMTS 10 will be sent from CM 40 to CPU 34 and from there to
computer 37 for comparison with the ranging corrections determined
by CMTS 32 to see if they are the same. This comparison is only
meaningful as a test on the ranging correction functionality of
CMTS 10 if the sniffer 20 is located at the same location on the
HFC upstream medium 14 as the CMTS 10.
[0096] Note that if the self-ranging corrections are done while a
burst is being received by the CM 40 or 42, the burst may be
corrupted. Therefore, the corrections should be done only when they
are big enough. Also, in the preferred embodiment, the corrections
are scheduled when no bursts are scheduled to be received, a fact
which is known from the downstream MAP messages captured by the CM
40. The trigger signal generated by CMTS receiver 32 can be used to
schedule the corrections.
[0097] The frequency correction offset is made in the RF
demodulator circuit 28 using the difference by subtracting from the
frequency offset correction developed by the CMTS receiver 32 from
the captured training burst from CM 18 the frequency offset
correction developed by the CMTS under test 10 from the same
training burst from CM 18. The difference is used to adjust the
local oscillator frequency in the RF demodulator circuit 28 to a
frequency necessary for the upstream channel being tuned to develop
the correct intermediate frequency.
[0098] Power corrections based upon the power offset measurement
made by CMTS receiver 32 less the gain corrections made by CMTS 10
from the same training burst are made in the RF demodulator circuit
28 based upon the calibration of the RF gain. The RF gain is
adjusted according to the gain difference between the gain offset
calculated by the sniffer CMTS 32 and the gain offset calculated by
the CMTS 10 under test. Gain correction resolution is 1/4 dB. The
gain corrections at the linecard RF demodulator circuit 28 use a
calibration table which provides a translation between the gain
control number sent by the sniffer CPU to control the gain of a
programmable gain amplifier in the RF demodulator circuit 28 and
the resulting attenuation in the RF signal that results.
[0099] Small time offset corrections in response to time offset
measurements made by the CMTS receiver 32 are made in the sniffer
by adjusting time delay in a programmable delay circuit in FPGA 30.
If larger time offset corrections are needed, the timestamp counter
in the cable modems 40 and 42 and the CMTS receiver 32 need to be
adjusted. The first step in making corrections in the sniffer based
upon time offset corrections is to set the timestamp counter in the
CM 40 and 42 (for just 40 or just 42, depending upon the
embodiment) to the value of captured timestamp. The next step is to
correct the sniffer CMTS 32 timestamp counter to a new value based
upon the time offset correction developed by CMTS 32 in the sniffer
from the captured upstream training burst from CM 18 minus the time
offset correction developed by CMTS under test 10 from the same
training burst.
[0100] The programmable equalizer filter in FPGA 30 obtains new
filter taps by convolving the current equalizer filter taps with
the new correction tap coefficients after self ranging. The new
correction tap coefficients are the equalizer tap weights developed
during a tap weight adaptation process carried out by the CMTS
receiver 32 on the known preamble symbols of a training burst from
a CM under test. The results of the convolution have subtracted
therefrom the tap weights of the equalizer tap corrections
developed by the CMTS 10 under test during processing of the
preamble of the training burst received from a CM and sent
downstream to the CM under test in a ranging response message.
Equalizer tap weights for the equalizer filter in FPGA 30 are
developed in this way for every CM under test. To simplify the
implementation, the equalizer filter in FPGA 30 can be updated only
when the equalizer corrections are big enough.
[0101] Since the signal received by the FPGA 30 is centered at an
IF frequency of 5.12 MHz, the signal should be first down converted
to baseband, and after the equalizer filter, the signal should be
up converted back to the IF frequency of 5.12 MHz.
[0102] The local oscillator whose signal is used to mix the
received signal down to baseband is: LO=cos(2*.pi.*5.12
MHz*t)-j*sin(2*.pi.*5.12 MHz*t) (1) Since the time t is samples at
20.48 MHz=4*5.12 MHz, the down conversion local oscillator is a
simple signal comprised by +1, -1 and zeros: LO=(1,0,-1,0,1,0,-1,0
. . . )-j(0,1,0,-1,0,1,0,-1 . . . ) (2) The up conversion local
oscillator that is used after the equalizer filter to upconvert the
signal back to IF is similar to the down conversion local
oscillator except a positive sign exists before the imaginary
component beginning with j in the complex number. Sniffer Timebase
Synchronization Processes
[0103] The way that the cable modems under test get their timebases
in synchronization is as follows.
DOCSIS 1.x and 2.0 Advanced TDMA and DOCSIS 2.0 Timebase Synch in
CM's Under Test
[0104] The timebase of the cable modems under test are synchronized
according to the normal operations of cable modems in the
system.
DOCSIS 2.0 Sniffer Timebase Synchronization Process
[0105] The process the sniffer 20 goes through to get into
synchronization with the upstream and downstream is illustrated in
the flowchart of FIGS. 10A through 10D. The process of FIGS. 10A
through 10D essentially boils down into four basic steps: (1)
recovering a downstream symbol clock so as to enable capture of
downstream messages by the sniffer, and generating a local upstream
reference clock in the sniffer from the recovered downstream clock
which is phase coherent with and locked to the recovered downstream
clock; (2) using the local upstream reference clock to synchronize
a timebase in the sniffer with the timebase including a local
upstream minislot counter; (3) performing a self ranging step to
make adjustments in circuits in the sniffer to synchronize it with
an upstream upon which bursts to be captured are to be transmitted;
and (4) using the upstream minislot counter and data in captured
downstream messages to capture upstream bursts including SCDMA
bursts transmitted from a cable modem under test.
[0106] In step 178, the sniffer of FIG. 1 or FIG. 2 receives a
command from the computer 37 indicating which DOCSIS 2.0 downstream
to which the sniffer is to monitor. This is the preferred
embodiment where there may be multiple DOCSIS downstreams sharing
one upstream and/or multiple DOCSIS upstreams sharing a DOCSIS
downstream, and there may be multiple logical channels on each
DOCSIS upstream and there may be flexible mapping between the
groupings of downstreams and corresponding upstreams so that more
upstream or downstream capacity can be added as warranted by
traffic conditions.
[0107] In step 180, the cable modem in the sniffer recovers the
downstream clock of the designated DOCSIS 2.0 downstream from step
178, and uses that recovered downstream clock to recover downstream
messages and bursts. In some embodiments, all downstream messages
and bursts directed to all CMs are captured, and, in other
embodiments, only messages and bursts directed to particular CMs
designated by computer 37 or pertinent to particular upstreams
designated by computer 37 and linked to the downstream designated
in step 178 are captured. The CM, (CM 40 in the case of the
embodiment of FIG. 1) then generates a reference clock from the
recovered downstream clock which is locked to or phase coherent
with the recovered downstream clock. In the preferred embodiment,
the reference clock is a 2.56 MHz clock, but in other embodiments,
the reference clock generated from the recovered downstream symbol
clock can be a 10.24 MHz clock or a 10.24/N MHz clock where N is
any integer. This step is performed for DOCSIS 2.0 SCDMA upstream
burst recovery only and the reference clock must be phase coherent
with the recovered downstream symbol clock. Phase coherent means
that for every X cycles of the downstream clock, where X is an
integer, there will be Y cycles of the downstream clock where Y is
an integer an the zero crossings at the end of X and Y cycles of
the two clocks, respectively, will be simultaneous.
[0108] In step 182, the sniffer receives commands from computer 37
indicating which upstream or logical channel(s) linked to the
downstream designated in step 178 to which the sniffer linecard
should synchronize, and which bursts on the designated upstream or
logical channel(s) (designated by SID, IP address, minislot number,
etc.) which are to be captured.
[0109] In step 184, the CM in the sniffer captures the UCD and MAP
messages for all the upstreams or logical channels, or at least
captures the UCD and MAP messages for the upstream or logical
channel designated in step 182. The captured UCD and MAP messages,
or at least the TLV data items therefrom are sent to the
linecard.
[0110] In step 186, the CM in the sniffer captures the downstream
sync messages of the designated downstream which corresponds to the
designated upstream, and captures all the timestamp snapshot
messages in the designated downstream for all the upstreams or
logical channels that share the designated downstream, or at least
for the upstream or logical channel designated in step 182.
[0111] In step 188, a phase lock loop oscillator in the sniffer
multiplies the reference clock signal generated by the CM in the
sniffer, if needed, by a suitable factor so as to generate a
reference clock for the linecard which is phase coherent with the
downstream symbol clock. This upstream reference clock generated by
the PLL has to be fast enough to allow generation of a chip clock
which is synchronized with the chip clock of the CM that sent an
SCMDA burst so that the chip clock can be used in demultiplexing
upstream SCDMA bursts in the CMTS receiver 32. In the preferred
embodiment, the reference clock generated by CM 40 is 2.56 MHz and
it multiplied by a factor of four in the PLL to generate a 20.48
MHz reference clock for the linecard. In other embodiments, the
reference clock for the linecard can be 10.24 MHz. The frequency is
not important other than to comply with DOCSIS specifications. In
general, outside of DOCSIS, the only requirement is that the
reference clock for the linecard be phase coherent with the
downstream symbol clock and the upstream symbol clock used by the
CMs and with the upstream symbol clock used by the CMTS.
[0112] In step 190, if necessary, the linecard uses the 20.48 MHz
reference clock (which is phase coherent with the recovered
downstream symbol clock) to generate a 10.24 MHz local clock which
is locked to the recovered downstream symbol clock and which is
synchronized (except for a phase difference caused by propagation
delays) with the 10.24 MHz master clock in the CMTS 10 which is
driving the master timestamp counter.
[0113] In step 192, the linecard sets the initial value of its
local timestamp counter to the timesample sample number received in
the first downstream sync message for the designated upstream
captured by CM 40 after the sniffer powered up. Different upstreams
tied to the same downstream in DOCSIS 2.0 can have different
minislot sizes so they will have separate timestamp snapshot
messages and they also have different sync messages, one for each
upstream. But the master timestamp counter from which the
downstream timestamp samples are taken and master 10.24 MHz clock
are shared by all upstreams. Thereafter, the local timestamp
counter is driven by the local 10.24 clock which has been slaved to
the master 10.24 MHz clock shared by all upstreams. This is by
virtue of the downstream symbol clock being slaved to the 10.24 MHz
master clock in the CMTS and the slaving (locked into phase
coherency) of the reference clocks generated in the sniffer to the
recovered downstream symbol clock. The local timestamp counter
should then track the changes in the master timestamp counter in
the CMTS without drift with some offset because of the propagation
delay. In some embodiments, no drift is assumed. In the preferred
embodiment, the timestamp counts in subsequent sync messages are
compared to the local timestamp count. If the local timestamp
counter is off by more than some programmable values, such as one
count, from the timestamp sample in subsequent sync messages, the
local timestamp counter is set to the value in the sync message.
This offset is set by the self-ranging process in the sniffer to
achieve minislot and frame synchronization with the upstream or
logical channel upon which bursts to be captured are
transmitted.
[0114] In step 194, the linecard CPU searches the UCD messages
received from the CM pertaining to the designated upstream and
retrieves the M and N values from the UCD message that corresponds
to the upstream or logical channel which will carry bursts to be
captured. These M and N values and the recovered downstream symbol
clock are used to generate an upstream symbol clock for DOCSIS 2.0
bursts to be used in the sniffer CMTS to recover 2.0 upstream
bursts by multiplying the ratio M/N times the 10.24 MHz reference
clock in the sniffer in an M/N PLL. This synchronizes the upstream
symbol clock in the sniffer with the upstream symbol clock in the
CM under test for the designated upstream or logical channel.
[0115] In step 196, the data in the captured timestamp snapshot
message pertinent to the designated upstream channel is used to set
the correct values into the linecard's local upstream minislot and
frame counters. If the sniffer has one set of minislot and frame
counters for each possible upstream it monitors, the minislot and
frame counter corresponding to the designated upstream are set to
correct values using the pertinent timestamp snapshot message.
[0116] In step 198, the linecard in the sniffer captures an initial
station maintenance burst from the CM under test. This training
burst continues on the HFC system to the CMTS 10 under test after
capture by the sniffer. The CMTS 32 in the sniffer then makes time,
phase, frequency and power offset measurements on the preamble of
known symbols of the captured initial station maintenance burst.
The CMTS 32 also develops upstream equalization filter coefficients
using the preamble of the captured training burst. All these offset
measurements and the equalization coefficients are sent to the CPU
of the linecard.
[0117] In step 200, the CM in the sniffer captures a downstream
ranging response message from the CMTS 10 under test which is sent
in response to the initial training burst. That ranging response
message is sent to the CPU of the linecard.
[0118] In step 202, the linecard CPU takes the time offset
measurement of the ranging response message and subtracts if from
the time offset calculated by the CMTS 32 in the sniffer. The
result is sent as a command to a variable delay circuit in the FPGA
in the linecard and is used to advance the count of the local
timestamp counter by enough to establish upstream frame and
minislot synchronization for the designated upstream and/or logical
channel.
[0119] In step 204, the linecard CPU uses the rest of the
measurements (phase, frequency and power offsets and equalization
coefficients) made on the training burst from the CM under test and
the ranging corrections from the ranging response message sent by
the CMTS under test to send appropriate power, phase and frequency,
and upstream equalization filter coefficient adjustment commands to
the RF demodulator circuit 28 and equalization filter in the FPGA
30 of the linecard so as to complete the process of upstream
synchronization. Subsequent periodic station maintenance bursts are
captured and measurements made and the ranging response downstream
messages are captured. The CPU repeats the self ranging process
using these subsequent measurements to keep the sniffer in
sync.
[0120] In step 206, the linecard searches the MAP message data
pertinent to the designated upstream or logical channel and
determines the minislot numbers of the upstream bursts to be
captured. The UCD data pertinent to the upstream or logical channel
on which a burst is to be captured is also searched and the burst
parameter data of each burst to be captured is retrieved and used
to generate suitable control signals to the sniffer burst capture
circuitry to set it up to capture the burst.
[0121] The linecard then monitors the local timestamp counter in
step 208, and when the minislot number which is the first minislot
at the beginning of a burst to be captured occurs at the linecard
upstream tap, the linecard circuitry to capture the designated
burst is turned on.
[0122] The captured burst is then sent to the CPU 34 of the
linecard for transfer to the computer 37 for display and/or
analysis, as symbolized by step 210.
[0123] FIGS. 11A through 11C illustrate the process the DOCSIS 2.0
upstream and downstream sniffers of FIGS. 1 and 2 go through to
establish synchronization with a designated downstream and a
designated upstream and capture downstream bursts and upstream
DOCSIS 1.x bursts and upstream DOCSIS 2.0 advanced TDMA bursts. In
step 212, the sniffer of FIG. 1 or FIG. 2 receives a command from
computer 37 indicating which DOCSIS 1.x downstream to which to
lock. The cable modem in the sniffer then recovers the downstream
symbol clock of the designated downstream, and uses the recovered
clock to recover downstream bursts and messages in step 214. In
step 216, the sniffer receives commands from computer 37 that
indicate which upstream or upstream logical channel linked to the
downstream designated in step 212 to which to synchronize and which
bursts on that upstream or logical channel (by SID, IP address,
minislot number etc.) are to be captured.
[0124] In step 218, the CM in the sniffer captures downstream UCD
and MAP messages for all the upstreams linked to the downstream
designated in step 212 or at least for the upstream or logical
channels upon which the upstream bursts designated in step 216 will
be transmitted. In step 220, the CM in the sniffer captures sync
messages of the designated downstream for all linked upstreams or
at least for the upstream or logical channel(s) designated in step
216. These captured sync messages, or at least the data therefrom,
are passed on to the linecard CPU 34 in step 220.
[0125] In step 222, the linecard generates a free running 10.24 MHz
clock signal using a PLL. In step 224, the linecard sets the
initial value of a timestamp counter in the sniffer to the
timestamp sample value in the first sync message captured by the CM
in the sniffer in the downstream after powerup or after receiving
the message indicating what downstream to lock to and locking to
that downstream. In step 226, the linecard receives subsequent sync
messages from the CM in the sniffer. The timestamp samples in these
subsequent sync messages are compared to the timestamp count of the
local timestamp counter. If there is a difference, the new
timestamp value is loaded into the timestamp counter in the CMTS 32
or wherever the timestamp counter in the timebase of the sniffer is
located. In the CMs of the sniffer and the CMs under test, the new
timestamp value difference is used to generate an error signal that
is used to adjust the frequency of the local 10.24 MHz clock in a
direction to reduce the difference in the timestamp counts between
the local timestamp counter and the master timestamp counter in the
CMTS.
[0126] In step 228, an initial station maintenance burst from the
CM under test is received by the linecard, and the CMTS in the
sniffer makes time, phase, frequency and power offset measurements
using the known symbols of the preamble. The CMTS in the sniffer
also uses the preamble of the training burst to adaptively develop
upstream equalization filter coefficients. The CMTS in the sniffer
then sends all these offset measurements and equalization filter
coefficients to the CPU 34 on the linecard.
[0127] Next, in step 230, the CM in the sniffer captures the
ranging response message sent by the CMTS 10 under test to the CM
under test that sent the training burst. This ranging response
message contains the time, phase, frequency and power offset
measurements made by the CMTS under test on the same training burst
that was processed by the sniffer CMTS in step 228. The captured
ranging response message is sent to the CPU on the linecard.
[0128] In step 232, the linecard CPU takes the time offset
measurement from the CMTS in the linecard and subtracts the time
offset measurement made by the CMTS under test, and sends a command
to the variable delay circuit in the FPGA on the linecard to
establish a proper delay to implement the difference. The
difference is also used to advance the timestamp counter count by
an amount sufficient to achieve upstream minislot boundary
synchronization between the sniffer and the CMs under test.
[0129] In step 234, the linecard CPU uses the rest of the ranging
measurements developed by the CMTS in the sniffer and the ranging
measurements developed by the CMTS under test and generates
suitable commands based upon the differences to control circuitry
in the RF demodulator to make appropriate phase, power and
frequency adjustments and to set the filter coefficients of the
equalization filter in the FPGA to achieve proper upstream
equalization for the position of the sniffer on the HFC upstream
medium relative to the CMTS under test. This completes
synchronization to the upstream or logical channel designated in
step 216.
[0130] In step 236, the linecard searches the MAP data pertinent to
the designated upstream and determines the minislot numbers of the
designated upstream bursts to be captured. The UCD data pertinent
to the upstream or logical channel on which a burst is to be
captured is also searched and the burst parameter data of each
burst to be captured is retrieved and used to generate suitable
control signals to the sniffer burst capture circuitry to set it up
to capture the burst.
[0131] In step 238, the linecard monitors the count of the linecard
minislot counter, and when the beginning minislot number of a burst
to be captured occurs on the linecard minislot counter (or just
before it occurs), suitable commands are generated by CPU 34 to
turn on the burst capture circuitry of the sniffer and capture the
burst.
[0132] In step 240, the CMTS receiver in the sniffer recovers the
upstream symbol clock used by the CM under test that transmitted
the burst that was captured. The symbol clock is recovered from the
preamble of the captured burst, and is used to synchronize a local
upstream symbol clock which is used to recover the data in the data
portion of the burst.
[0133] Finally, in step 242, the captured burst data is sent to the
CPU 34 for transfer to the computer 37 for display and/or
analysis.
Sniffer Interfaces
[0134] The sniffer interfaces for the embodiments of FIGS. 1 and 3
are the upstream RF interface to the upstream medium of the HFC
system, the downstream RF interface to the downstream medium of the
HFC system, a 20.48 MHz reference clock input to couple to a CMTS
under test, a 10.24 MHz clock interface to the CMTS under test, and
a 10/100 BaseT Ethernet interface to the personal computer 37.
Alternative Sniffer Embodiments Using Only a Cable Modem
[0135] FIGS. 4, 5 and 6 are block diagrams of sniffers which use
only a cable modem to capture upstream bursts from any of the other
cable modems in the system. Since the cable modem in these
alternative embodiments sniffers receive the downstream MAP
messages, it knows the assigned minislots for all the other cable
modems in the system. Since it is locked onto the same downstream
as all the other cable modems in the system, it also has its
upstream timestamp counter synchronized using the same timestamp
samples from the sync messages and timestamp snapshots in DOCSIS
2.0 downstreams as all the other cable modems in the system. Since
every cable modem in the system uses the same free running upstream
clock frequency and the same sync messages to keep their local
upstream timestamp counters in synchronization with the master
upstream timestamp counter in the CMTS, each cable modem in the
system will be using the same timestamp counts to determine the
boundaries of the upstream minislots at the location of that cable
modem. Thus, the cable modem in the sniffer can be used
advantageously to control a separate tuner and buffer in the
sniffer to capture every upstream burst in the system.
[0136] Referring to FIG. 4, the sniffer 84 has a cable modem 96
which is coupled to both the downstream medium and the upstream
medium of HFC system 86 (separate DS and US mediums not shown). To
be able to capture the upstream bursts from all other cable modems
on the system using the sniffer of FIG. 4, it must be connected to
the HFC system 86 at a point closer to the CMTS 88 than all the
other cable modems on the system. The other cable modems on the
system are shown at 90, 92 and 94. If it is not so coupled, it will
only be able to intercept and capture the upstream bursts of the
other cable modems on the HFC system farther away from the CMTS
than the sniffer.
[0137] The cable modem 96 is coupled to the downstream from the
CMTS 88 via a diplexer 100, a programmable tuner 102, and a
programmable gain amplifier 104. All these circuits are
conventional and are controlled by the modem 96 via control signals
on bus 106 to tune the correct downstream channel after initially
searching for and locking onto any legitimate DOCSIS downstream
channel at power up and registering as a cable modem in the system.
The CMTS will then send a message to the cable modem telling it
what downstream channel to which it should tune. The cable modem 96
is also coupled to the upstream by a conventional DQU (digital
quadrature up converter) 108 which receives constellation points of
upstream bursts in digital format and modulates them onto
quadrature upstream carriers and transmits them on the upstream
medium through diplexer 100. Diplexer 100 controls the direction of
transmission of the upstream and downstream signals so that
downstream signals enter tuner 102 and signals received from
programmable gain amplifier 110 enter the upstream medium of HFC
system 86.
[0138] A programmable gain amplifier 110 amplifies the upstream RF
signals generated by CM 96 in accordance with gain control signals
received from the cable modem on bus 112. The cable modem 96
controls the frequency of the upstream carriers via signals on bus
112 to the DQU 108 for transmissions to the CMTS 88. The cable
modem 96 controls the frequency and gain levels of its transmitted
signals in accordance with downstream instructions from the CMTS 88
just like any conventional DOCSIS 2.0 cable modem.
[0139] Because the sniffer is a cable modem which is tuned to the
CMTS downstream, it registers with CMTS 88 as a regular cable
modem, and it receives all the downstream messages including the
MAP, UCD, sync, ranging-request and ranging response messages.
Because it has the regular DOCSIS 2.0 cable modem functionality, it
will respond to ranging response messages by sending training
bursts and will receive ranging response messages which include
timing offset, power offset, phase offset and frequency offset
correction measurements made by the CMTS 88 on the training burst
sent by the sniffer 84. It will also receive upstream equalization
coefficients developed by the CMTS 88 from the training burst sent
upstream by the cable modem 96 in the sniffer. The cable modem 96
also recovers the downstream symbol clock and generates a DOCSIS
2.0 upstream clock for use in sending upstream 2.0 SCDMA bursts and
forwarding them to the CMTS under test in some embodiments. For
TDMA bursts, the sniffer's cable modem uses a free running symbol
clock counted by an upstream timestamp counter which is corrected
in accordance with the process of FIG. 3 using the downstream
timestamp messages whenever deviations from a synchronized state
occurs. When sniffer 96 makes all the requested corrections in the
ranging response message and has its local timestamp counter in
sync with the CMTS master upstream timestamp counter, it will be
completely synchronized to the CMTS 88 and will be synchronized
with all the other cable modems in the system and will be able to
determine when each upstream minislot is occurring at its tap point
98 on the HFC system.
[0140] Because sniffer 84 receives the MAP messages in the
downstream, it knows what the upstream assigned minislots are for
every other CM in the system. To capture the upstream bursts from
the other CMs in the system, the cable modem 96 sends control
signals on the bus 112 to a separate line of circuits called the
burst capture circuitry in the claims which functions to capture
the upstream bursts. This line of circuitry includes a tuner 114
which the cable modem controls by signals on bus 112 to turn on
during the duration of the burst and to set the frequency of a
local oscillator (not shown) in tuner 114 to tune to the upstream
carrier frequency of the burst to be captured. Typical structures
for the tuner are shown in the patent application which is
incorporated by reference herein and which is published in Europe
and covers the Jasper CMTS receiver (Docket Number TER-013, EPO
Publication number 1235402). Typical tuner structure for circuit
114 includes a mixer and local oscillator to mix the center
frequency of the desired channel with the burst to be captured down
to some intermediate frequency of a passband filter such as a SAW
filter. The SAW filter or other filter is then used to filter out
undesired RF signals on the high and low side of the band of
frequency components in the Fourier spectrum of the burst to be
captured. In some embodiments, an analog passband filter with
programmable filter coefficients which can be altered to adjust the
bandwidth of the passband of the filter is used to do this
filtering. In DOCSIS 1.0, 1.1 and 2.0 bursts, the symbol rate is
programmable so the bandwidth of the burst depends upon the symbol
rate. This means a filter with a programmable bandwidth can be used
to filter out unwanted RF signals outside the bandwidth of
interest. However, this is difficult to do in the analog domain and
the signals of the burst to be captured are still analog inside
tuner 114. Therefore, in the preferred embodiment, a SAW filter
with a fixed bandwidth passband which is set wide enough to
encompass the bandwidth of a DOCSIS burst having the highest
possible symbol rate is used to filter out most unwanted RF signals
inside tuner 114. Then, in an optional alternative embodiment (as
indicated by the dashed lines around filter 123), a digital
passband filter 123 is used. This programmable digital filter is
coupled to receive the digital samples output by an
analog-to-digital converter 118. The digital filter 123 has
programmable filter coefficients which are set by the cable modem
96 by signals on bus 112 to establish the passband of the filter
123 according to its symbol rate so as to filter out the unwanted
RF components outside the actual bandwidth of the burst to be
captured. The cable modem 96 knows the symbol rate of every burst
to be captured from the burst profile data in the UCD messages it
receives from the CMTS 88 in the DOCSIS downstream. There is one
UCD message for every channel. Data in these UCD messages define
the burst and channel characteristics for every burst the
corresponding channel.
[0141] A programmable gain amplifier 116 is controlled by signals
on bus 112 from cable modem 96 to turn on during the duration of
the burst to be captured and its gain is controlled to amplify the
received signal to the proper level to use the full dynamic range
of an A/D converter 118. The A/D converter is enabled by a signal
on bus 112 during the burst to be acquired. Finally, a buffer 120
captures the digital samples which define the captured burst. The
cable modem 96 controls the buffer via enable signals on bus 112 to
enable the buffer to store the samples of the burst when the burst
arrives.
[0142] The cable modem 96 uses the data in the downstream MAP
messages to know when each cable modem will transmit and uses its
local timestamp counter counts to know when those assigned upstream
minislots are occurring at the upstream sniffer tap 122. The
upstream sniffer tap 122 couples the upstream medium of the HFC
system 86 to the RF input of the sniffer tuner 114. The cable modem
96 uses information in the UCD message to know the frequency,
symbol rate and other burst parameters of the burst to be captured.
The cable modem 96 then suitably controls the tuner 114 and the
programmable gain amplifier 116 and the A/D converter 118 and the
programmable digital filter 123 (if used) and the buffer 120 to
capture the samples of each burst in buffer 120. The burst is not
actually removed from the upstream medium by the sniffer. It
continues to the CMTS so as to not cause the CMTS protocols to be
adversely affected.
[0143] Once the sample data in the burst is captured, it may be
processed in any way desired.
[0144] In some embodiments, the sniffer 84 captures every burst. In
other embodiments (as indicated by dashed lines in FIG. 4 to PC
124), the sniffer 84 is controlled by a user interface implemented
on a personal computer 124 so as to only capture selected bursts
such as all bursts from a particular CM or all bursts of a
specified type. The PC 124 is coupled to the cable modem typically
by a 100 BaseT LAN link 126. However, in alternative embodiments,
the connection to the PC 124 may be by any known interface such as
USB, SCSI, Firewire, Fibre Channel, 100 BaseT, token ring, etc. or
by a proprietary interface.
[0145] Referring to FIG. 7, there is shown a flowchart of the
process carried out by the sniffer of FIG. 4 to lock onto a DOCSIS
downstream, register as a cable modem, get into synchronization
with the CMTS and start capturing upstream bursts. Step 128
represents the process of the sniffer of FIG. 4 searching for any
legitimate DOCSIS downstream and locking onto it. This step is
performed conventionally in the manner any DOCSIS 2.0 cable modem
performs this process. Step 130 represents the process of the cable
modem in the sniffer of FIG. 4 registering with the CMTS as a
DOCSIS 2.0 cable modem. The CMTS may order the cable modem to
switch to another downstream after the cable modem in the sniffer
has registered with the CMTS, and, if this occurs, the cable modem
will tune to this requested downstream, lock onto it and may
re-register using the upstream associated with the new downstream.
This process, although not specifically mentioned in steps 128 and
130, is also symbolized by steps 128 and 130.
[0146] Step 132 represents the process of receiving a downstream
DOCSIS ranging request message that defines a ranging contention
interval during which cable modems which have not performed their
initial training may do so. In step 134, the cable modem in the
sniffer performs an initial ranging offset determination and sets
its ranging offset, and then transmits an initial training burst
using this initial ranging offset in an attempt to hit the ranging
contention interval. If the training burst hits the ranging
contention interval and does not collide with another training
burst, the CMTS processes the training burst to determine a ranging
offset, frequency and amplitude offsets, a phase offset and
calculates upstream equalization coefficients from the known
preamble data. The data portion of the training burst tells the
CMTS which cable modem sent the training burst. The ranging offset
is a measurement of how far off the desired time of arrival at the
CMTS the training burst arrived (it is supposed to arrive at the
beginning of the ranging contention interval). The frequency offset
is a measurement of how far off the frequency of the training burst
is from the desired frequency of the upstream channel which this
cable modem transmitted upon. The amplitude offset is a measurement
of how much the CM must adjust its transmit power to cause its
burst to arrive at the CMTS at nominal or desired power. The phase
offset is a measurement of how far off the desired phase the
training burst was. The equalization coefficients are coefficients
developed by an adaptive equalization process which when set into
an upstream equalization filter in the CMTS equalizes the upstream
channel on which the training burst was sent to remove the
intersymbol interference effects of echoes and dispersion
(different propagation speeds of different frequencies) of the
Fourier frequency components of the transmitted signal. The CMTS
then sends these offset corrections and equalization coefficients
down to the cable modem in the sniffer, which receives them in step
136. The cable modem then makes the requested ranging offset
adjustments, adjusts it frequency and transmit power, adjusts its
phase and convolves the equalization coefficients with the
coefficients of the precode filter in the upstream transmitter of
the sniffer which were in use when the training burst was sent.
[0147] Step 138 represents the process of receiving downstream
synchronization messages and using the timestamps therein to make
any necessary adjustments in the localt timestamp counter in the
cable modem in the sniffer. This is done by the process of FIG. 2
typically, and results in the local timestamp counter being in
synchronization with a master upstream timestamp counter in the
CMTS used by the CMTS to determine the boundaries of upstream
minislots assigned to various CMs for upstream transmissions. The
state of synchronization means that the local timestamp counter
will be counting at a virtually identical rate and there will be a
fixed offset caused by propagation delays, and whenever the offset
gets out of sync, a correction will be made in the local timestamp
count using the timestamp sample from a sync message or timestamp
snapshot message.
[0148] Step 140 represents the process of receiving the downstream
UCD and MAP messages and storing them. The UCD messages define the
burst parameters such as frequency and symbol rate and modulation
type for every upstream burst and its logical channel. The MAP
messages are bandwidth grants which define the minislots during
which each cable modem may transmit upstream.
[0149] Step 142 represents the optional process of receiving input
commands indicating which bursts to capture or other data
indicating exactly which IP addresses or Service Identifiers (SIDs)
or upstream minislot numbers to sniff. In some embodiments, all
upstream bursts will be captured, but in others, data will be
received from an external control process such as a control process
running on a personal computer coupled to the sniffer by a Ethernet
connection controlling the sniffer to capture only selected bursts.
In some embodiments, the sniffer listens to registration traffic
and learns the SIDs of each cable modem in the system.
[0150] Step 144 represents the process of reading the UCD and MAP
messages to determine when the bursts to be captured are going to
be arriving at the sniffer (to determine the upstream minislots
assigned to the bursts to be captured), and to determine the burst
parameters such as the frequency of the logical channel on which
the burst will be transmitted and its symbol rate.
[0151] Finally, in step 146, the sniffer determines from its local
timestamp count when the burst to be captured will be arriving at
the sniffer and generates suitably timed control signals on bus 112
to control the burst capture circuitry to capture the burst.
Specifically, the tuner will be enabled and ordered to tune the
proper frequency, and the symbol rate will be used to set the
coefficients of the passband filter to set the proper passband
(assuming a programmable digital passband filter is used), and the
amplifier will be enabled and its gain set and the buffer will be
enabled and addresses to store the digital samples will be
generated. These addresses may be generated in any way and may not
be generated by the cable modem in all embodiments.
[0152] In alternative embodiments, bursts from all SIDs will be
captured. In other alternative embodiments, any process will
suffice to practice the invention which: performs upstream training
and registers with the CMTS and achieves upstream minislot boundary
synchronization with the CMTS through normal DOCSIS protocols;
receives data defining which bursts to capture or to capture
specified or all upstream transmissions from specified SIDs or IP
addresses; listens to registration traffic to learn the SID of each
CM in the system; receives UCD and MAP messages and has the
capability to search the MAP messages to find the corresponding
minislot numbers of authorized upstream bursts from specific SIDs;
and which can use the local timestamp counter to generate properly
timed control signals for the sniffer burst capture circuitry to
capture the designated bursts. One way to generate the proper
timing is for the sniffer to fake a MAP that authorizes a
transmission during the minislots that map to the burst to be
captured and then not actually transmit any upstream data but use
signals generated in the transmitter to generate the appropriate
timing to generate the control signals necessary to control the
burst capture circuitry. In some embodiments, the tuner and
programmable gain amplifier can be left on all the time and only
the A/D converter and address generation for the buffer is enabled
during the upstream minislots corresponding to a burst to be
captured.
[0153] The embodiments of FIGS. 5 and 6 are sniffer/repeaters. The
main difference between these embodiment and the embodiment of FIG.
4 is that in the embodiments of FIGS. 5 and 6, the original
upstream burst from the cable modem under test does not get to the
CMTS via the upstream medium 152 of the HFC system 86. Instead, all
upstream bursts get captured in buffer 120 and then repeated to the
CMTS 88 a fixed time delay later via path 150. In other words,
every upstream burst on upstream medium 152 is received by tuner
114, amplified by amplifier 116 to maximize the use of the dynamic
range of the A/D converter 118, filtered by the programmable
digital filter 123 (if used) and stored in buffer 120. A fixed time
delay later, the samples of each burst are converted back into an
RF signal of the same frequency they originally were and are
transmitted to the CMTS on upstream medium 150. In the meantime,
the samples of each burst in buffer 120 may be processed or moved
to other storage (not shown).
[0154] In the embodiment of FIG. 6, all upstream bursts are
diverted by diplexer 101 to tuner 114 and are captured in buffer
120. After a fixed delay, the samples of each burst are converted
back to RF and transmitted to the CMTS 88 on path 150.
[0155] In these embodiments, an additional step is added to the end
of FIG. 7 to control the sniffer repeater circuitry (not separately
shown) to convert the samples of each burst back into an RF signal
of the same frequency of the original burst and re-transmit them on
upstream medium 150.
[0156] In these embodiments of FIGS. 5 and 6, if a conventional
CMTS 88 with burst acquisition circuitry is used, the buffer
circuit 120 includes circuitry such as a digital-to-analog
converter and digital quadrature up converter (DQU) to convert the
samples of the IF signal digitized by A/D converter 118 back into
IF signals and then up convert the analog IF signals back to the
original radio frequency signals received by tuner 114 which the
CMTS will reacquire. These optional circuits are shown in dashed
lines as circuits following or part of buffer 120 in FIG. 5 and are
present in embodiments like FIG. 6 also which use conventional CMTS
receivers 88 although they are not shown in the drawing. The CMTS
protocols do not get derailed by this process as long as there is a
fixed delay between the time the burst is captured and the time it
is repeated. This delay just causes the CMTS to think that the CM
that sent each burst is further away from the CMTS than it really
is.
[0157] In the embodiment of FIG. 5, the upstream medium 152 is
disconnected from the CMTS 88 and is connected only to the tuner
114 in the sniffer. In the embodiment of FIG. 6, a diplexer 101
separates the upstream signals on HFC 86 and supplies them on path
103 to tuner 114. Downstream signals from the CMTS enter the
diplexer 101 on path 105 and are coupled onto the downstream medium
of the HFC system 86. Diplexer 100 couples the upstream path 107
and the downstream path 109 of the cable modem 96 onto the
appropriate medium of the HFC system 86. If separate medium for
upstream and downstream are not used at the point of the tap 98,
then the diplexer uses the different frequencies of the upstream
and downstream transmission bands to separate the upstream and
downstream traffic and couple each for transmission in the proper
direction, and the same is true of diplexer 101.
[0158] All other software in the cable modem in the sniffer is
conventional.
[0159] One important class of embodiments for the sniffers of FIGS.
5 and 6 is for the buffer 120 to not include any circuitry that
converts the digital samples of the captured bursts back to analog
signals. In this class of embodiments, the CMTS 88 pushes the burst
acquisition function out to the optical nodes. In other words, in
these embodiments, the normal burst acquisition circuitry in the
CMTS 88 (which is a duplicate of the tuner 114, programmable gain
amplifier 116, A/D converter 118, programmable filter 123 and
buffer 120) is eliminated. This is because its functionality is
pushed out to the location of the optical node, so the CMTS only
needs the signal processing circuitry that processes the filtered
digital samples of each burst. This allows a more efficient use of
the CMTS receiver by placing a sniffer at the location of each
optical node (or at the CMTS in some embodiments) to capture all
upstream bursts as digital samples and send only the digital
samples of the captured bursts to the CMTS 88. The CMTS 88 in these
embodiments would contain only the signal processing circuitry of a
conventional CMTS which is normally located after the burst
acquisition circuitry which tunes, filters, digitizes, decimates,
and buffers each burst. This arrangement with a sniffer at each
optical node would replace the digital return structure of the
prior art shown in FIG. 8 with the sniffer based structure of FIG.
9.
[0160] The embodiment of FIG. 8 samples all minislots of the
upstream on line 154 output by the diplexer 156 at the optical node
using an A/D converter 158 which samples the amplified output from
amplifier 160 at a high sample rate. All minislots, including empty
ones, are sampled by A/D converter 158 and output to a framer 162.
The framer outputs bits to a digital laser diode 164 which converts
them to light pulses which are carried by optical fiber 166 to a
photodetector diode 168 at the head end. A buffer 170 stores the
samples, and a diplexer 172 separate the upstream from the
downstream and couples the signals from the buffer 170 to a CMTS
174 for the upstream and couples downstream signals from the CMTS
174 to a downstream 176.
[0161] FIG. 9 is a block diagram of a sniffer based digital return
structure from the diplexer at the optical node to the CMTS which
captures only the bursts and does not sample empty minislots like
the structure in FIG. 8. Only the actual bursts on the upstream
line 154 from the diplexer 156 are captured in sniffer 178 and
empty minislots are ignored. The bursts include the protocol
messages so no protocol upsets occur. The sniffer has the structure
of either FIG. 5 or FIG. 6. The digital samples of each burst are
output from the sniffer 178 to a framer 162 and converted to light
pulses by a laser diode 174. From there all the rest of the
circuitry is the same. The CMTS 174 does not need any burst
acquisition circuitry because that functionality is performed out
at the optical nodes.
[0162] The advantage of the system of FIG. 9 is that only the
bursts are captured and sampled and everything else is ignored.
This means that multiple optical nodes can be daisy chained
together for maximizing the efficiency of use of the optical fiber
166 and the CMTS 174. The CMTS 174 can be only signal processing
circuitry and needs no burst acquisition circuitry (the same is
true of FIG. 8 prior art). However, in FIG. 9, the CMTS is used
more efficiently than in FIG. 8 prior art because the pipelined
stages of CMTS 174 are kept filled with burst data at all times and
no pipeline stage is processing samples of empty minislots at any
time.
Timebase Search Mode
[0163] The linecard CMTS receiver 32 can adjust its timebase using
the initial IUC3 training burst of the CM under test if the CM
under test 18 has not yet performed initial training. However, if
the sniffer is coupled to the upstream medium 14 after the CM under
test 18 has performed its initial training, the linecard 26 may be
unable to receive the IUC4 periodic station maintenance bursts
because its timestamp counter may be too far off to know where the
IUC4 window beginning minislot is in time.
[0164] To resolve this problem, the operator may elect to reset one
of the CMs under test to force the CM to transmit an IUC3 initial
station maintenance training burst which will enable the linecard
26 in the sniffer to bet in synchronization.
[0165] If the CM under test is not reset, and the linecard is
unable to detect an IUC4 periodic station maintenance burst, then
the linecard will go into an automatic timebase search mode. In
this mode, the linecard changes its timestamp counter setting in a
trial and error manner and tries after every iteration to receive
and IUC4 periodic station maintenance burst from the CM under test.
If an IUC4 burst is successfully received, the timestamp counter
can be adjusted properly using the clock offset corrections
generated by the CMTS 10 under test from the IUC4 burst. If an IUC4
burst is not received, the operator is notified and must force a
reset of the CM 18 under test.
[0166] The only modifications that need to be made to the sniffer
cable modem and CMTS software in this second genus of sniffers is
to use the MAP and UCD data to generate suitable control signals to
control the burst capture circuitry to capture upstream bursts at
the proper times. In some embodiment, modifications must also be
made to receive user input defining which bursts to capture. In
other embodiments, no such medication to receive user input need be
made since the sniffer captures all upstream bursts.
[0167] Although the invention has been disclosed in terms of the
preferred and alternative embodiments disclosed herein, those
skilled in the art will appreciate possible alternative embodiments
and other modifications to the teachings disclosed herein which do
not depart from the spirit and scope of the invention. All such
alternative embodiments and other modifications are intended to be
included within the scope of the claims appended hereto.
* * * * *