U.S. patent application number 10/032127 was filed with the patent office on 2002-05-09 for dynamically-assigned voice and data channels in a digital-subscriber line (dsl).
Invention is credited to Aalaei, Faraj, O'Toole, Anthony J.P..
Application Number | 20020054597 10/032127 |
Document ID | / |
Family ID | 22414257 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020054597 |
Kind Code |
A1 |
O'Toole, Anthony J.P. ; et
al. |
May 9, 2002 |
Dynamically-assigned voice and data channels in a
digital-subscriber line (DSL)
Abstract
A Digital-Subscriber Line (DSL) modem dynamically allocates
bandwidth among one or more voice calls and unchannelized data, as
needed. The channelized voice and unchannelized data traffic are
transmitted over a digital subscriber line according to the
bandwidth allocations therefor.
Inventors: |
O'Toole, Anthony J.P.; (Los
Gatos, CA) ; Aalaei, Faraj; (Atherton, CA) |
Correspondence
Address: |
FENWICK & WEST LLP
TWO PALO ALTO SQUARE
PALO ALTO
CA
94306
US
|
Family ID: |
22414257 |
Appl. No.: |
10/032127 |
Filed: |
December 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10032127 |
Dec 20, 2001 |
|
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09124333 |
Jul 29, 1998 |
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Current U.S.
Class: |
370/395.41 ;
370/468 |
Current CPC
Class: |
H04L 2012/6456 20130101;
H04L 2012/6478 20130101; H04L 5/1446 20130101; H04M 11/062
20130101; H04L 5/1423 20130101 |
Class at
Publication: |
370/395.41 ;
370/468 |
International
Class: |
H04L 012/56 |
Claims
We claim:
1. A DSL modem comprising: a bandwidth allocator adapted to
dynamically adjust a bandwidth allocation based on voice channel
demand, the bandwidth allocation defining a bandwidth for each of
one or more voice channels and unchannelized data; and a formatter
coupled to the bandwidth allocator, the formatter adapted to
combine the voice channels and unchannelized data onto a digital
subscriber line according to the bandwidth allocation, thereby
creating a transmission signal.
2. The DSL modem of claim 1, further comprising: an off-hook
detector coupled to the bandwidth allocator, the off-hook detector
adapted to couple to one or more local customer premises voice
lines for measuring voice channel demand thereon.
3. The DSL modem of claim 2, further comprising: a next-format
storage coupled to the off-hook detector for storing a next
bandwidth allocation, the next bandwidth allocation based on a
detected change in voice channel demand.
4. The DSL modem of claim 1, wherein the transmission signal
includes next bandwidth allocation data, the next bandwidth
allocation data defining an anticipated bandwidth for the voice
channels and data.
5. The DSL modem of claim 1, wherein the bandwidth for each voice
channel is associated with a timeslot in the transmission signal,
the remaining transmission signal bandwidth available for data.
6. The DSL modem of claim 5, wherein the bandwidth allocator is
adapted to adjust the bandwidth allocation at integer multiples of
the periodicity of the timeslots.
7. The DSL modem of claim 1, wherein the formatter is adapted to
format the transmission signal into a series of superframes, each
superframe including a plurality of network frames, each network
frame including a plurality of low-level frames, each low-level
frame including a plurality of timeslots, the timeslots containing
a voice call or data.
8. The DSL modem of claim 7, wherein the bandwidth allocator is
adapted to adjust the bandwidth allocation at the frequency of the
superframe.
9. The DSL modem of claim 7, wherein the network frames are
synchronized to a telephone-network timing reference.
10. The DSL modem of claim 1, wherein at least one voice channel
includes voice data selected from the group consisting of: voice
data, facsimile data, analog modem data, and digital service
data.
11. The DSL modem of claim 1, wherein the DSL modem is a central
office modem.
12. A DSL modem comprising: a DSL connection for transmitting
information over a digital subscriber line; a module coupled to the
DSL connection for transmitting channelized data and unchannelized
data over the digital subscriber line, the module adapted to
dynamically allocate bandwidth for transmitting the channelized
data based on availability of channelized data, and to dynamically
reallocate unused channelized data bandwidth for transmitting the
unchannelized data.
13. A method of dynamically allocating bandwidth in a digital
subscriber line among channelized data from one or more local phone
lines and unchannelized data, the method comprising: establishing a
connection to a digital subscriber line; allocating a portion of
the bandwidth for each of the local phone lines in use, the
remaining bandwidth available for unchannelized data; transmitting
the channelized and unchannelized data over the digital subscriber
line in their respective allocated bandwidths; detecting a change
in phone line usage; and reallocating the bandwidths among the
local phone lines and unchannelized data based on the detected
change.
14. The method of claim 13, further comprising: transmitting a
bandwidth allocation over the digital subscriber line, the
bandwidth allocation defining bandwidths corresponding to the
channelized and unchannelized data.
15. The method of claim 13, wherein the bandwidths allocated for
each of the local phone lines in use are substantially equal and
are capable of carrying a voice call.
16. A method of transmitting voice calls and digital data over a
digital subscriber line, the method comprising: transmitting
digital data over the digital subscriber line in a bandwidth;
detecting a new voice call; responsive to the new voice call,
dynamically reallocating a first portion of the bandwidth to the
voice call and a second portion of the bandwidth to the digital
data; and combining the voice call in the first portion of the
bandwidth and the digital data in the second portion of the
bandwidth for transmitting over the digital subscriber line.
17. The method of claim 16, wherein the first portion of the
bandwidth is outside POTS band frequencies.
18. The method of claim 16, wherein the voice call includes data
selected from the group consisting of: voice data, facsimile data,
analog modem data, and digital service data.
19. The method of claim 16, further comprising: responsive to the
voice call's ending, reallocating the first portion of the
bandwidth to the digital data.
20. A method of dynamically allocating bandwidth among voice and
data traffic, the bandwidth comprising a plurality of timeslots,
the method comprising: allocating timeslots among the voice and
data traffic; composing a first superframe, the first superframe
containing a plurality of network frames, each network frame
containing a plurality of low-level frames, each low-level frame
containing the voice and data traffic in their allocated timeslots;
sending the first superframe over a digital subscriber line; in
response to detecting a change in the voice traffic demand,
reallocating the timeslots among the voice and data traffic;
composing a second superframe, the second superframe containing a
plurality of network frames, each network frame containing a
plurality of low-level frames, each low-level frame containing the
voice and data traffic in their reallocated timeslots; and sending
the second superframe over the digital subscriber line.
21. The method of claim 20, wherein composing the first superframe
includes synchronizing the network frames to a telephone-network
timing reference.
22. The method of claim 20, further comprising: sending a next
allocation of the timeslots over the digital subscriber line to the
remote modem, the next allocation being encoded within the current
superframe.
Description
RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn. 120
from U.S. patent application Ser. No. 09/124,333, filed Jul. 29,
1998, which is herein incorporated in its entirety by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to telephone systems, and more
particularly to Digital-Subscriber Lines (DSL) carrying both voice
and data traffic.
BACKGROUND OF THE INVENTION
[0003] The telephone system was originally constructed for carrying
voice calls. With the widespread acceptance of personal computers
and wide-area data networks such as the Internet, telephone
networks are carrying more and more data traffic. Digital telephone
lines such as Integrated Services Digital Network (ISDN) and T1
lines carry both data and voice traffic. Some higher-speed
Digital-Subscriber Lines (DSL) also reserve some bandwidth for
voice calls.
[0004] ISDN Bearer Channel Allocation--FIG. 1
[0005] FIG. 1 shows an ISDN line used for both voice and data
traffic. The customer premises equipment (CPE) includes ISDN
terminal adapter or modem 12 that receives a data stream from a
computer. ISDN modem 12 may include one or two
plain-old-telephone-service service (POTS) voice ports that can be
connected to standard telephone or fax equipment. ISDN modem 12
transmits data or digitized voice over telephone line 20, which is
an ISDN line using digital rather than analog signaling.
[0006] At the Phone Company's central office (CO), ISDN line card
14 terminates ISDN telephone line 20. The data or digitized voice
is sent over the public switched-telephone network (PSTN) that
includes circuit switched network 22. When ISDN modem 12 transmits
a voice call, the call is sent over circuit switched network 22 to
other voice telephones over voice lines 26, 27. However, when data
is sent by ISDN modem 12, the remote number called is connected to
another computer (not shown) at Internet Service Provider (ISP)
21.
[0007] ISDN telephone line 20 carries three independent channels: a
low-bandwidth data channel D is primarily used for control signals,
while bearer channels B1, B2 are each 64 Kbps channels, each
capable of carrying one voice call. When no voice calls are being
made, ISDN modem 12 routes the data stream over both bearer
channels B1, B2, providing a combined bandwidth of 128 Kbps. These
two bearer channels remain as two separate calls within the PSTN,
since ISDN line card 14 makes two connections 24, 25 to circuit
switched network 22. These two calls are sent over two lines 28, 29
to ISP 21. At ISP 21, upper-layer software 18 combines data from
the two lines 28, 29 into a single data stream. Thus the single
data stream from the user is split into two separate calls to ISP
21.
[0008] When the user makes or receives a voice call, one of the
bearer channels is dropped and no longer carries data. The data
bandwidth then falls to 64 Kbps since only one bearer channels B1
in ISDN telephone line 20 is used for data, and only one connection
24 and only one of two lines 28, 29 are used. The other bearer
channel B2 is used for the voice call, connecting the user's
telephone or fax with a remote telephone or fax on voice line 26.
ISDN line card 14 uses the other connection 25 to connect with
circuit switched network 22 and voice line 26. When two voice calls
are simultaneously made, then both bearer channels are used and no
data can be transmitted. Thus ISDN modem 12 is able to drop one of
the bearer channels for a voice call. Once the voice call ends,
ISDN modem 12 reconnects the second bearer channel to ISP 21.
[0009] While ISDN bearer channels can be dropped and reconnected as
voice and data loads change, the low bandwidth of ISDN limits its
future usage. Having to separate data connections through the PSTN
that must be recombined by software at the ISP is also
undesirable.
[0010] Fixed Allocation of Voice and Data Channels of T1
Lines--FIG. 2
[0011] FIG. 2 highlights a high-speed T1 phone line that has fixed
allocations of its bandwidth to voice and data traffic. Larger
corporations often connect to the telephone network over 1.5-Mps T1
or 45-Mbps T3 lines.
[0012] T1 lines can be partitioned into a number of 64 Kbit DS0
channels to carry several separate voice calls, with the remaining
bandwidth used for data traffic. For example, the T1 line can have
256 Kbps allocated for voice calls, with the remaining 1.25 Mbps
for data traffic. The 256 Kbps allocated for voice can carry four
separate voice calls at one time.
[0013] While it is useful to allocate some of the bandwidth of a T1
line to voice calls, the allocation does not vary over time. Once
the T1 line is configured for four voice calls, the allocation
cannot be changed when five or more voice calls are received. The
fifth caller hears a busy signal. Since calls often occur together
at peak times such as just after lunch, bandwidth must be reserved
for these peak times. Data bandwidth is restricted for all times of
the day and night by the peak voice bandwidth than occurs only
infrequently.
[0014] DSL With POTS Band--FIG. 3
[0015] FIG. 3 is a graph of DSL frequency bands with a lower POTS
band. FIG. 3 shows frequency bands for asymmetric DSL, or ADSL
(T1.413) service using frequency-division duplex and voice calls.
FIGS. 3 is not drawn with a linear scale. Plain-old-telephone
service (POTS) voice calls are transmitted over low-frequency POTS
band 2, as they are for standard telephone lines. POTS band 2
operates from near D.C. to 4 kHz. Since this is the same frequency
range as standard telephones, ordinary telephone equipment or
voice-band modems can be used over POTS band 2.
[0016] ADSL upstream channel 4 is for uploads from the customer, or
for sending commands and user input from the customer to the
central office side. Some embodiments may use a bi-directional
channel in place of upstream channel 4. Upstream channel 4 operates
at up to 138 kHz, with the data rate up to 1 Mbps.
[0017] Wide-band 5 carries the bulk of the ADSL-line bandwidth.
Wide-band 5 carries ADSL data downstream to the customer at up to 8
Mbps. Wide-band 5 is a frequency band typically from 140-200 kHz up
to about 1.1 MHz. The lowest frequencies are reserved for POTS.
Other kinds of DSL use different frequency bands, but all use
relatively high frequency bands.
[0018] While ADSL can be configured to reserve some bandwidth for
services such as ISDN basic rate, POTS band 2 is frequency-limited
and can carry just one voice call. Other proposed DSL services do
not have a POTS band at all, and provide transport without
distinguishing between voice and data services.
[0019] DSL Equipment Includes Frequency Splitter--FIG. 4
[0020] Special equipment is needed at both the customer premises
and at the phone company's central office where the customer's
copper phone line ends. Analog devices called frequency splitters
are typically used to separate the low-frequency POTS band from the
high-speed data bands. FIG. 4 is a diagram of a DSL phone line
highlighting the frequency splitters.
[0021] Copper telephone line 20 is a pair of copper wires running
from central office 8 to the customer. The phone customer has
installed customer premises equipment 6. Since DSL uses high
frequencies for data traffic and POTS uses low frequencies for
voice calls, the signal received over POTS telephone line 20 must
be split into high- and low frequency components. Splitter 46
contains a low-pass filter that outputs the low-frequency
components from copper telephone line 20. These low-frequency
components carry the voice calls that are sent to telephone set 10.
Telephone set 10 is a standard POTS analog telephone set.
Additional phone sets, fax machines, or voice-band modem equipment
can be connected to telephone set 10 as phone-line extensions as is
well-known.
[0022] Splitter 46 also contains a high-pass filter that outputs
the high-frequency components to DSL modem 48. DSL modem 48
receives the high-frequency analog signal from splitter 46 and
converts it to downstream digital data during the receiving window.
During the transmitting window, it converts the upstream data into
high-frequency analog signal. Splitter 46 mixes this high-frequency
analog signal from DSL modem 48 with the low-frequency voice from
telephone set 10 and transmits the combined signal over copper
telephone line 20 to central office 8.
[0023] Central office 8 receives copper telephone line 20 and
splits off the high-frequency components with splitter 16. The
high-frequency components from splitter 16 are sent to DSL modem
47, which converts the analog high-frequency signal to an upstream
digital data. DSL line card 50 includes DSL modem 47 and, in some
embodiments, splitter 16. The data stream can then be connected to
a high-speed data highway or backbone.
[0024] Splitter 16 sends low-frequency components to conventional
telephone switch 19, which includes a line card similar to
conventional line cards that terminate POTS lines. Conventional
telephone switch 19 uses a PCM highway or circuit switched network
that connects this voice call to remote voice-band equipment such
as a telephone set.
[0025] Incoming voice calls received by conventional telephone
switch 19 are combined by splitter 16 with high-frequency data
traffic from DSL modem 47. The combined signal is transmitted over
copper phone line 20 to customer premises equipment 6.
[0026] While such ADSL equipment is useful, only a small portion of
the bandwidth is reserved for voice calls. While data rates are
high, the ADSL line is not a true replacement for a T1 line, since
the ADSL line cannot be partially allocated for multiple voice
calls.
[0027] What is desired is a DSL system that can carry multiple
voice calls. It is desired to use a high-speed DSL line to carry
many voice calls. It is further desired to use idle voice bandwidth
for data traffic. It is desired to flexibly allocate the bandwidth
of a DSL line to voice and data traffic. It is desired to change
the allocation of voice bandwidth as voice calls are received and
terminated. Dynamic allocation of the bandwidth of a DSL line among
voice and data traffic is desirable, without corruption or
interruption of the data.
BRIEF SUMMARY OF THE INVENTION
[0028] A dynamically-allocating Digital-Subscriber Line (DSL) modem
dynamically allocates bandwidth among voice calls and unchannelized
user data. The modem has a plurality of local voice lines. Each of
the local voice lines is for carrying a voice call.
[0029] A data stream sends user data to a telephone network. A DSL
connection to a DSL telephone line connects to a central office
connected to the telephone network. A formatter is coupled to the
DSL connection and receives the user data from the data stream and
voice calls from the plurality of voice lines. It formats the user
data and the voice calls into timeslots for transmitting over the
DSL telephone line to the central office.
[0030] A current-format storage is coupled to control the
formatter. It stores a current allocation of the timeslots. The
current allocation indicates which timeslots are carrying voice
calls and which timeslots are carrying the user data.
[0031] A next-format generator is coupled to the current-format
storage. It generates a next allocation of the timeslots. The next
allocation of the timeslots has more timeslots allocated voice
calls when a new voice call is initiated, but fewer timeslots
allocated to voice calls when a voice call is terminated. Thus
allocation of the timeslots for voice calls and user data is
dynamically adjusted as voice calls are initiated and
terminated.
[0032] In further aspects of the invention the next-format
generator has an off-hook detector that is coupled to the plurality
of local voice lines. It determines when a local voice line is
off-hook. State means stores a current state of local voice lines
that are off hook. An allocater is coupled to the state means. It
changes an allocation of a timeslot from the user data to a voice
call when an additional voice line is off-hook, but changes
allocation of a timeslot from a voice call to the user data when a
voice line is no longer off-hook.
[0033] In still further aspects of the invention the next
allocation of the timeslots is transmitted to the central office
over the DSL telephone line before the next allocation becomes the
current allocation. Thus timeslot allocations are transmitted over
the DSL telephone line before taking effect. The next allocation is
latched into the current-format storage from the next-format
generator when a next superframe begins to be transmitted over the
DSL telephone line. Thus allocations are changed at superframe
boundaries.
[0034] In other aspects the timeslots each transmit 64-Kbits per
second over the DSL telephone line, and each timeslot is able to
carry exactly one voice call. The DSL telephone line is divided
into enough timeslots so that a maximum number of voice calls that
can be simultaneously carried is at least three simultaneous voice
calls.
[0035] In other aspects of the invention the superframe contains a
plurality of network frames. Each network frame is synchronized to
a telephone-network timing reference. Each network frame contains a
plurality of low-level frames. Each low-level frame contains the
timeslots that are each allocated to either voice calls or the user
data. The timeslots are repeated for each low-level frame in the
network frame, and the network frame is repeated for each network
frame in the superframe. Thus the timeslots are repeated at a
lowest level of a three-level framing structure, but allocations of
the timeslots are changed only at a highest level of a framing
structure.
[0036] In further aspects, each timeslot contains 8 bits, and the
low-level frame is repeated at a rate of 8 kHz. Thus each timeslot
carries 64 Kbits per second of data or voice-call information.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 shows an ISDN line used for both voice and data
traffic.
[0038] FIG. 2 highlights a high-speed T1 phone line that has fixed
allocations of its bandwidth to voice and data traffic.
[0039] FIG. 3 is a graph of DSL frequency bands with a lower POTS
band.
[0040] FIG. 4 is a diagram of a DSL phone line highlighting the
frequency splitters.
[0041] FIG. 5 shows time slots on a DSL line allocated for voice
and data traffic.
[0042] FIG. 6 illustrates dynamic allocation of voice and data
traffic on a superframe basis.
[0043] FIG. 7 highlights the pipelining of voice/data allocation of
time slots.
[0044] FIG. 8 highlights pipelining of low-level timeslot
allocations to voice and data.
[0045] FIG. 9 shows a DSL line with DSL modems that re-allocate
timeslots to voice and data traffic.
[0046] FIG. 10 is a DSL modem that allocates time slots when local
voice lines are offhook.
[0047] FIGS. 11A-11C detail the framing structure for the
switchable voice/data DSL channels.
[0048] FIGS. 12A-12C illustrate an example of DSL framing for a
384-Kbit data rate.
[0049] FIG. 13 shows that the framing bits in the superframe
include the next-frame allocation format.
[0050] FIG. 14 is an example where all 9 timeslots are allocated to
unchannelized data.
[0051] FIG. 15 shows an example when one channel has been allocated
to voice.
[0052] FIG. 16 illustrates alignment of Reed-Solomon Codewords to
every fourth superframe boundary.
[0053] FIG. 17 shows that timeslots at the end of each low-level
frame can be used as padding to keep synchronization.
DETAILED DESCRIPTION OF THE INVENTION
[0054] The present invention relates to an improvement in
Digital-Subscriber Lines (DSL). The following description is
presented to enable one of ordinary skill in the art to make and
use the invention as provided in the context of a particular
application and its requirements. Various modifications to the
preferred embodiment will be apparent to those with skill in the
art, and the general principles defined herein may be applied to
other embodiments. Therefore, the present invention is not intended
to be limited to the particular embodiments shown and described,
but is to be accorded the widest scope consistent with the
principles and novel features herein disclosed.
[0055] The inventors have realized that voice-call bandwidth should
be dynamically allocated as needed by incoming and outgoing calls.
During peak calling times, additional bandwidth is taken away from
data transfers and used for voice calls. During low-calling
periods, most of the bandwidth is available for data transfers.
[0056] Traditional DSL using frequency-splitters provide only one
voice channel. While this is useful for home users, larger
organizations such as corporations need many voice lines. The
inventors have realized that the framing of data for DSL can be
extended to carry voice traffic. Timeslots can be reserved for
voice channels as new voice calls are made or received. The
inventors have further realized that low-level frames can be
allocated for voice or data traffic on an as-needed basis.
[0057] FIG. 5 shows time slots on a DSL line allocated for voice
and data traffic. As voice calls are received or made, additional
time slots are allocated to voice calls. As voice calls end, time
slots are de-allocated from voice and reallocated for data,
increasing the available data bandwidth.
[0058] The time slots allocated are low-level time slots that are
repeated over many frames. The relatively slow rate that voice
calls are made and terminated allows allocation to be made
infrequently. While making allocations each frame might be the
straightforward, logical solution, the inventors prefer to make
allocations less frequently.
[0059] The inventors group frames together into a superframe. Each
superframe contains six 1-KHz frames, or 48 8-KHz frames.
Allocations are made for each superframe and remain in effect for
the entire superframe. Thus voice/data allocations are made every 6
milliseconds.
[0060] FIG. 6 illustrates dynamic allocation of voice and data
traffic on a superframe basis. Each superframe lasts for 6
milliseconds. Allocations are made just once for the entire
superframe. While the allocations are for timeslots of low-level
frames, the allocation of timeslots in these low-level frames is
repeated for each of the low-level frames in a superframe.
[0061] During superframe 1, voice calls are allocated about half of
the available bandwidth, leaving data traffic with the other 50% of
the DSL-line's bandwidth. Additional voice calls are made or
received during superframe 1, and additional bandwidth is allocated
to voice calls for superframe 2. In superframe 2, voice calls
occupy about 80% of the bandwidth, reducing data traffic to
20%.
[0062] One or more of the voice calls ends, so that in superframe
3, voice-call bandwidth drops from 80% to about 70% of the
bandwidth, increasing data traffic to 30%. Many of the voice calls
end during superframe 3, so that only 20% of the bandwidth is
needed for voice in superframe 4. Data bandwidth increases to 80%.
The last voice calls end, so that in superframe 5, all the
bandwidth is allocated to data traffic.
[0063] Some of the bandwidth is reserved for overhead signaling,
such as sync patterns or error correction. In particular,
forward-error-correctio- n (FEC) bytes can be sent with each
superframe. FEC bytes use Reed-Solomon encoding for error
correction.
[0064] Allocation Pipelining--FIG. 7
[0065] FIG. 7 highlights the pipelining of voice/data allocation of
time slots. Superframe N has a current format that allocates 12
time slots of each low-level frame to data, while allocating 4 time
slots to voice calls. No new calls are received during superframe
N, so the next format generated during superframe N for the next
superframe N+1 is identical.
[0066] During superframe N+1 a new call is made or received.
Another timeslot needs to be allocated to voice. The next format is
set to 5 voice timeslots and data timeslots are reduced to 11. This
becomes the current format in the next superframe N+2.
[0067] In superframe N+2 another new call comes in, and another
timeslot is allocated by changing the next format to 6 voice and 10
data timeslots. This next format becomes the current allocation
format for superframe N+3. Finally, in superframe N+3, no change in
voice calls occurs, so the next format remains as the current
format.
[0068] Pipelining of Time-Slot Allocations--FIG. 8
[0069] FIG. 8 highlights pipelining of low-level timeslot
allocations to voice and data. In this example each superframe
contains many low-level frames. Each low-level frame is composed of
six timeslots that can be allocated to either data or voice
traffic. These allocations are repeated for all the low-level
frames in a superframe. The allocations are determined in the
preceding superframe, but are not performed until the next
superframe begins.
[0070] A set of time-slot bits is used to indicate which time slots
are allocated to voice or to data. A zero bit indicates that the
time slot is to be allocated to data for all low-level frames in
the next superframe; a one bit indicates that the time slot is to
be allocated as a voice channel.
[0071] During superframe 1, all time-slot bits are zero. This
allocates all time slots to data in the next superframe, N+1.
During superframe N+1 a voice call is received. The third time slot
is allocated to voice by setting the third of six time-slot bits.
When this allocation is implemented at the beginning of superframe
N+2, the third of every six timeslots in each of the many low-level
frames is allocated to voice. This one timeslot can carry one voice
call, known as a DS0 channel.
[0072] The timeslot bits only control timeslot allocation in one
direction. Allocations are made by both modems for their outgoing
data, and each modem receives an allocation format for its incoming
data stream. The apparatus shown for each modem is duplicated on
the other modem for allocation control of the opposite
direction.
[0073] Voice/Data Allocating DSL Modems--FIG. 9
[0074] FIG. 9 shows a DSL line with DSL modems that re-allocate
timeslots to voice and data traffic. When a channel needs to be
added or removed for voice calls, the new format of time-slot bits
are generated and loaded into next-format register 32 in CPE modem
30. This next format is transmitted through formatter 36, over DSL
telephone line 20 to central-office (CO) modem 40. The next format
is contained in frame-overhead bytes, such as the frameformat field
shown later in FIG. 13. Formatter 45 extracts the next-format field
from the incoming line data and writes it to next-format register
42 in CO modem 40.
[0075] At the end of the current superframe, the next format from
next-format registers 32, 42 are latched into current format
registers 34, 44. The same format of time-slot bits is contained in
registers 34, 44. In CPE modem 30, current-format register 34
controls formatter 36, time-multiplexing data and voice calls to
the allocated timeslots. In CO modem 40, current-format register 44
controls formatter 45, time-multiplexing data and voice calls to
the allocated timeslots.
[0076] Allocation to Voice Calls when Off-Hook Detected--FIG.
10
[0077] FIG. 10 is a DSL modem that allocates time slots when local
voice lines are off-hook. CPE modem 30 formats data and local voice
lines into frame timeslots based on a timeslot bits. These format
bits are stored in current-format register 34 and control formatter
36 that drives DSL telephone line 20. At the end of a superframe,
superframe clock SF_CLK clocks the next format from next-format
register 32 into current-format register 34, thus updating the
time-slot allocation.
[0078] When a local user initiates a new voice call by lifting a
handset of a local telephone, off-hook detector 52 detects the
off-hook condition by sensing loop current flow. Likewise, when a
local user hangs up the handset, off-hook detector 52 detects that
a local voice line is on-hook and no longer in use. The total
number of local voice lines that are off-hook are counted, and the
sum is the number of active voice calls.
[0079] The total number of voice calls indicates the voice
bandwidth that needs to be allocated for the next superframe. Each
timeslot can carry one voice call, so the number of timeslots
allocated is equal to the number of voice calls counted by
off-hook-detector 52. Bandwidth allocater 54 generates a string of
timeslot bits that have enough slots allocated to match the number
of voice calls. Bandwidth allocater 54 loads the next format into
next-format register 32.
[0080] New incoming calls from the CO modem are detected by digital
signaling bits or an analog ring voltage and are signaled over DSL
telephone line 20 using control signals in the framing overhead
fields. The number of incoming calls is extracted by formatter 36
and sent to bandwidth allocater 54. The total number of voice calls
is generated that includes the local voice calls counted by
off-hook detector 52 and the new incoming calls from the remote
modem. These incoming calls are those that are ringing but have not
yet been answered. Once answered, off-hook detector 52 counts the
established call.
[0081] Framing Structure--FIGS. 11
[0082] FIGS. 11A-11C detail the framing structure for the
switchable voice/data DSL channels. In FIG. 11A, the high-level
superframe is illustrated. Each superframe is exactly 6
milliseconds in duration, and contains 6 1-KHz frames (1
millisecond each frame) and a superframe pad of 0 to 5 bits. The
number of superframe pad bits depends on the exact modem bit-rate.
Line rates of 126 to 1536 Kbit are supported.
[0083] The framing fields in the superframe each contain 12 bits,
except for the first framing field, which contains 9 to 15 bits.
The number of framing bits in the first framing field is varied to
adjust the superframe's length to exactly match a multiple of the
8-KHz network reference clock at the central office. These framing
fields for each superframe are combined as shown later in FIG. 13
for DSL control signaling. In particular, the next-format of the
time slots, which contains the allocations of channels to voice or
data for the next superframe, is contained in these framing fields
for the superframe.
[0084] The six user-data fields each contain from 114 to 1076 bits,
depending on the DSL line rate. DSL lines can operate at different
rates, depending on the length of the phone line, noise, and other
factors.
[0085] FIG. 11B highlights one of the six user-data fields of the
superframe. Together with the 12 superframe-level framing bits,
this forms a 1 KHz frame. The 1 KHz frame includes eight 8-KHz
low-level frames. Each of the 8 KHz low-level frames contains 14 to
134 bits, depending on the DSL line rate. Pad bits for the 1 KHz
frame can include up to 7 bits.
[0086] FIG. 11C highlights a low-level 8-KHz frame that contains
the time slots that are allocated to voice or data traffic. The
8-KHz frame is divided into one or more time slots. Each time slot
contains 8 bits, and is repeated at the 8-KHz frame rate, for a
channel rate of 64 Kbit. This 64 Kbit rate is sufficient to carry
one voice call when allocated to voice. Up to seven pad bits can be
added to the 8-KHz frame. For the lowest DSL line rate, each 8-KHz
frame has just 14 bits, and thus has only one 8-bit timeslot and 6
pad bits. The maximum line rate has 134 bits in each 8-KHz frame,
which is divided into 16 timeslots, with 6 pad bits. Intermediate
line rates have between 2 and 15 timeslots.
[0087] The combination of the three levels of pad bits is used to
adjust the superframe's length to exactly 6 milliseconds. Having
pad bits for each level increases flexibility for different line
rates when matching fixed network clocks.
[0088] 384-Kbit Framing Example--FIGS. 12A-C
[0089] FIGS. 12A-C illustrates an example of DSL framing for a
384-Kbit data rate. The DSL modem line rate is 416.5 Kbit, but
framing overhead reduces the data rate to 384 Kbit. Since each
high-level framing field has 12 bits for each 1 millisecond frame,
the superframe-overhead is 12 Kbit for all line rates. In this
example, the pad bits occupy another 20.5 Kbit. The efficiency is
92%.
[0090] In FIG. 12A, each user-data field in each of the six 1-KHz
frames contains 404 data bits. The superframe pad is 3 bits. FIG.
12B shows that each of the six 1-KHz frames contains eight frames
of 50 bits each. The 1-KHz pad field has 4 bits, to give a total of
8*50+4=404 bits. The 50 bits in each 8-KHz low-level frame is
divided into six 64-Kbit timeslots, each with 8 bits. The remaining
2 bits are 8-Khz pad bits. Each of the six timeslots can be
allocated for a voice call (a DS0 channel) or for unchannelized
data.
[0091] The pad bits occupy a total of 20.5 Kbit of bandwidth. Each
intermediate 1-KHz frame has four pad bits. Since these frames
occur at a rate of 1 KHz, the intermediate pad bits occupy 4 KHz.
The three superframe pad bits are spread among the six 1-KHz
frames, and so add 0.5 pad bits per intermediate frame, or 0.5-Kbit
of bandwidth. The two low-level pad bits occur for each of the
eight 8-KHz frames, for a total of 2*8=16 pad bits per 1-KHz frame,
a bandwidth of 16 Kbit. The total bandwidth occupied by the pad
bits is thus 0.5+4+16=20.5 Kbit. Note that low-level pad bits
occupy more bandwidth than high-level pad bits, since the low-level
pad bits are repeated more frequently.
[0092] Rather than put all the pad bits at the highest (superframe)
level, the pad bits are placed in the low-level frames that are
spread throughout the superframe. This minimizes the offset or peak
delay on the DS0 voice channels. If the pad bits were simply
grouped in the superframe pad field, the transmission of this field
would take a long time at low bit rates, causing increased delay on
the DS0 data. Likewise, the superframe framing bits are spread
throughout the superframe in 12-bit fields for each 1-KHz frame.
This makes the timeslots occur at a more even rate.
[0093] Superframe Bits Contain Next-Frame Allocation--FIG. 13
[0094] FIG. 13 shows that the framing bits in the superframe
include the next-frame allocation format. The framing bits of each
superframe include the variable first field of 9-15 bits, and the
five 12-bit framing fields. All these bits are combined into a 69-
to 75-bit control word. The first framing field is variable to
match the modem line rate to the network timing reference, such an
8-KHz pulse-code-modulation (PCM) clock at a central office.
[0095] The control word begins with up to six stuff bits. These
bits are added so that the sync pattern that follows is aligned
with the 8-KHz network reference clock. The sync pattern is the
8-bit pattern 11011100. A valid CRC must also be found for the sync
pattern to be accepted as valid.
[0096] The Frame format field contains the next-format allocation
of the timeslots to voice or data for the next superframe. It
provides one bit for each of the 32 possible DS0 timeslots. It
allocates the switchable timeslots between the unchannelized data
and dedicated DS0 voice channels. The first bit transmitted
corresponds to the first 64 Kbit timeslot. A one indicates
channelized DS0 voice while a zero indicates that the timeslot
become part of the unchannelized data. This field refers the next
superframe. It is sent in advance so that both modems can change
timeslot allocations at the same time, at the beginning of the next
superframe. The new allocation format is only acted upon when the
CRC is good; line errors thus are prevented from causing the wrong
format to be set by a remote modem.
[0097] The timeslot bits only control timeslot allocation in one
direction. Allocations are made by both modems for their outgoing
data, and each modem receives an allocation format for its incoming
data stream.
[0098] Error correction is enabled by setting the Reed-Solomon bit.
When the Reed-Solomon Sync bit is set in a superframe, the first
data byte in the following superframe is the start of a new
Reed-Solomon code word. This bit transfers alignment information
between the modems, so codeword boundaries can be recovered. The
number of superframes between alignments is determined by a value
programmed into a register.
[0099] The 3-bit # of stuff bits field indicates how many leading
stuff bits will begin the next superframe. Up to six stuff bits
precede the next superframe's sync pattern. The error and control
channel (EOC) field has 14 bits used for managing the CPE modem.
Control information is sent from the CO modem to the CPE modem
using this field. The Alarm Status bits provide a direct signaling
path between the modems. These five bits are directly managed by
programming Alarm Control and Status registers in the modems. When
a bad CRC occurs, these bits do not cause an interrupt and are
instead ignored. The Alarm Status may be used to signal such
conditions as Loss of Sync or Loss of Signal.
[0100] The six-bit cyclical-redundancy-check (CRC) is computed over
all other overhead field in the control word, from the sync field
to the alarm and status bits. The CRC is computed using the CRC-6
polynomial (x.sup.6+x+1) and validates all the Sync, next-format,
EOC, Alarm and Status bits. Framing overhead received with a bad
CRC result is discarded and the previous valid version used.
[0101] 9-Timeslot Example--FIG. 14
[0102] FIG. 14 is an example where all 9 timeslots are allocated to
unchannelized data. The modem line rate in this example is
sufficient for nine 64-Kbit timeslots. The CPE modem can support up
to 32 timeslots for each frame clock period. The CPE PCM frame
clock operates at 8 KHz. The first nine CPE timeslots, 0-8, are
allocated for unchannelized data. The user's data is fed to all
nine timeslots.
[0103] The unchannelized data from the nine timeslots is received
by the CO line card or modem, and assigned to CO timeslots 32-40.
The CO equipment can operate at higher speeds and thus can
accommodate as many as 128 64-Kbit timeslots for each period of the
central office's PCM frame clock. This frame clock also operates at
8 KHz.
[0104] One Channel Allocated to Voice--FIG. 15
[0105] FIG. 15 shows an example when one channel has been allocated
to voice. When a voice call is made, one of the 64-Kbit channels is
taken away from unchannelized data and allocated to a voice
channel. The fourth timeslot, TS-3, is assigned to voice. The CPE
modem's first 8 timeslots are allocated to unchannelized data, and
drive the DSL line's timeslots TS-0 to TS-2, and TS-4 to TS-8.
These data channels are received by the central office and assigned
to CO timeslots 32-39. Other DSL lines could be occupying timeslots
0-31 in this example.
[0106] The voice channel, timeslot TS-3, is generated from CPE
modem's PCM timeslot 19, and is received by the CO PCM timeslot
83.
[0107] Forward Error Correction--FIGS. 16, 17
[0108] Error correction can be employed with the invention. For
example, Reed-Solomon (RS) encoding can be used, such as 64/68
coding that attaches 4 bytes of error correction code (ECC) to
every 64 data bytes. The group of 68 bytes is called a Reed-Solomon
codeword. A syndrome can be generated from the 68-byte codeword
received and compared to an expected syndrome to detect errors. The
ECC bytes or syndrome can then be used to locate and correct any
errors within the codeword.
[0109] The additional 4 ECC bytes for every 64 data bytes are
transported in the timeslots, thus decreasing the effective data
bandwidth. The relatively long Reed-Solomon codewords may not align
exactly with each superframe, depending on the modem line rate.
Thus the Reed-Solomon sync bit shown in FIG. 13 is set to signal
that the next superframe begins a new RS codeword. The first
timeslot of the next superframe contains the first data byte of the
RS codeword, followed by the other 64 data bytes and then the 4 ECC
bytes that end the codeword. Other codewords follow.
[0110] FIG. 16 illustrates alignment of Reed-Solomon Codewords to
every fourth superframe boundary. In superframe 3, the RS sync bit
is set to signal that the next superframe (superframe 0) is the
start of a new series of RS codewords. The example shown is for a
256-Kbit data rate, which has four 64-Kbit timeslots. Nine RS
codewords span four superframes.
[0111] FIG. 17 shows that timeslots at the end of each low-level
frame can be used as padding to keep synchronization. Each 8-KHz
frame uses all available timeslots. Frames 4, 9, and 10 use one
fewer timeslot, only N-1 timeslots. This keeps the user payload
(total payload minus Reed Solomon ECC bytes) equal to an integer
multiple (N-1) of 64 Kbit. Codewords can begin and end within an
8-KHz frame, but always align with every fourth superframe
boundary.
[0112] When some of the timeslots are allocated to voice, the voice
data is considered part of the RS codeword. The voice timeslots can
be sent directly to the PCM highway or circuit switched network
from the receiving modem to reduce latency, rather than wait for RS
decoding.
[0113] ADVANTAGES OF THE INVENTION
[0114] A DSL system can carry multiple voice calls. Using the
invention, high-speed DSL lines are able to carry many voice calls.
Idle voice bandwidth can be allocated for data traffic. This
flexibility allows the bandwidth of a DSL line to be allocated to
voice and data traffic as needed. The allocation of voice bandwidth
is changed as voice calls are received and terminated. Dynamic
allocation of the bandwidth of a DSL line among voice and data
traffic more fully uses the available resources. An extension of
the services provided by DSL is thereby achieved.
[0115] Alternate Embodiments
[0116] Several other embodiments are contemplated by the inventors.
For example the RS codewords can use interleaving. The modems can
operate in many different modes and at different data and line
rates. Many circuit implementations are possible. The modems may
use different modulation techniques. A frequency-splitter may still
be used to provide a POTS service independent of the DSL modem.
[0117] The frequency range of the DSL band can be varied. The
frequency bandwidth can be reduced for lower data rates and/or for
shorter telephone lines. While the term "line card" has been used,
it is apparent that the functions described for the line card could
reside on a printed-circuit-board substrate, a metal or ceramic
substrate, or on other modular systems such as racks or boxes. The
functions of the line card can be arranged on multiple substrates
or integrated onto one or more silicon semiconductor chips.
[0118] The term "voice calls" and "voice channels" have been used
as a shorthand for channelized DS0 channels that most often carry
voice calls. However, DS0 channels can also carry analog modem
traffic, fax, or digital data services such as X.25, or may be
combined for video conferencing applications.
[0119] Processing of the DSL data streams can occur using
processors in a DSP, or with time-sharing of a fast DSP. A large
analog driver with enough drive for the phone line is also normally
added to the analog output of a D/A converter. Various signal
processing techniques, such as Trellis encoding/Viterbi decoding
and pre-coding/pre-emphasis can be used in an encoder/decoder, or
not used in cost-sensitive applications. CAP modulation, QAM
modulation, Discrete Multitone modulation, or other pass-band
modulation techniques can be used.
[0120] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of the above teaching. It is
intended that the scope of the invention be limited not by this
detailed description, but rather by the claims appended hereto.
* * * * *