U.S. patent application number 11/215898 was filed with the patent office on 2006-01-12 for transmit and receive system for a cable data service.
Invention is credited to Bhavesh N. Desai, Nemmara K. Shankaranarayanan, David Hilton Shur, Aleksandra Smiljanic, Todd J. Totland, Jacobus E. van der Merwe, Sheryl Leigh Woodward.
Application Number | 20060007929 11/215898 |
Document ID | / |
Family ID | 26957528 |
Filed Date | 2006-01-12 |
United States Patent
Application |
20060007929 |
Kind Code |
A1 |
Desai; Bhavesh N. ; et
al. |
January 12, 2006 |
Transmit and receive system for a cable data service
Abstract
A transmit and receive system for transmitting data between a
transmit site and a receive site. The system includes a tunnel
source, router and modulator for dividing a transmit data stream
having a first bit rate into multiple data streams with each of the
multiple data streams having a bit rate which is lower than the
first bit rate, transmitting each of the multiple data streams over
a plurality of RF channels. The system further includes a
demodulator and destination source for recombining the multiple
data streams at the receive site to provide a receive data stream
having a bit rate equal to the first bit rate.
Inventors: |
Desai; Bhavesh N.;
(Livingston, NJ) ; Shankaranarayanan; Nemmara K.;
(Bridgewater, NJ) ; Shur; David Hilton; (Holmdel,
NJ) ; Smiljanic; Aleksandra; (Atlantic Highlands,
NJ) ; Totland; Todd J.; (Lincroft, NJ) ; van
der Merwe; Jacobus E.; (Providence, NJ) ; Woodward;
Sheryl Leigh; (Holmdel, NJ) |
Correspondence
Address: |
Mr. S.H. Dworetsky;AT&T Corp.
Room 2A-207
One AT&T Way
Bedminster
NJ
07921
US
|
Family ID: |
26957528 |
Appl. No.: |
11/215898 |
Filed: |
August 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09924617 |
Aug 8, 2001 |
|
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11215898 |
Aug 31, 2005 |
|
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60275665 |
Mar 14, 2001 |
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Current U.S.
Class: |
370/389 |
Current CPC
Class: |
H04L 25/14 20130101;
H04L 12/2898 20130101; H04L 12/2856 20130101; H04L 12/2861
20130101; H04L 69/14 20130101; H04L 12/2801 20130101; H04L 29/06
20130101 |
Class at
Publication: |
370/389 |
International
Class: |
H04L 12/28 20060101
H04L012/28 |
Claims
1. A data receiving system having at least one input, the data
receiving system comprising: a demodulator system for receiving
packets in parallel over multiple channels; and a tunneling
destination, coupled to said demodulator system, said tunneling
destination for receiving the packets from the demodulator system
and for serializing the packets.
2. The data receiving system of claim 1, wherein each of the
multiple channels are RF channels and each of the multiple channels
are received at a single input of the means for receiving.
3. The data receiving system of claim 2, wherein each RF channel
carries packets that are compliant with the DOCSIS standard.
4. The data receiving system of claim 1, further comprising an
analog to digital converter having an input adapted to receive RF
input signals and having an output; a plurality of digital filters,
each of said filters having an input coupled to the output of said
analog to digital converter and having an output; and a plurality
of demodulators each of said plurality of demodulators having an
input coupled to the output of a respective one of said filters and
having an output and the output of each demodulator being coupled
to said tunneling destination.
5. The system of claim 4 further comprising a down-converter
circuit which receives a first RF input signal at the input of the
demodulators and provides a down-converted signal to said analog to
digital converter.
6. The system of claim 5 wherein the digital signal processors
simulate filters having a band pass filter characteristic.
7. The system of claim 4 wherein said demodulators are provided as
QAM demodulators.
8. The system of claim 4 further comprising a data transmission
system.
9. The system of claim 8 wherein said data transmission system
comprises: a tunneling source having an input and a plurality of
output channels, said tunneling source for receiving one or more
packets at the input and for distributing the packets a plurality
of output channels coupled to an output of said tunneling source; a
cable modem termination system (CMTS) coupled to receive packets
from each of the plurality of tunneling source output channels and
to transmit signals on a plurality of parallel output channels.
10. The data transmitting system of claim 9, wherein the plurality
of CMTS output channels are RF channels.
11. The data transmitting system of claim 10, wherein each RF
channel carries packets that are compliant with the DOCSIS
standard.
12. The data transmitting system of claim 9, wherein said CMTS
further comprises: a CMTS router, having an input coupled to
signals from said tunnel source and having a plurality of output
ports; a plurality of channel modulators, each of said plurality of
channel modulators coupled to receive signals from a corresponding
one of the CMTS router output ports.
13. The data transmitting system of claim 12 further comprising: a
hybrid fiber coaxial (HFC) network coupled to the output of port of
each of said plurality of channel modulators. a plurality of
demodulator circuits, each of the plurality of demodulator circuits
having an input coupled to said HFC network and having an output; a
serializer having a plurality of input ports, each of the plurality
of input ports coupled to a respective one of the output ports of
said plurality of demodulator circuits and having a single output
port.
14. The data transmitting system of claim 13 further comprising a
TCP gateway having an input adapted to be coupled to a router and
having an output coupled to an input of said tunnel source, said
TCP gateway for terminating a TCP connection and for providing an
acknowledgement signal a sending node.
15. A data transmission system having at least one input, the data
transmission system comprising: a tunneling source having an input
and a plurality of output channels, said tunneling source for
receiving one or more packets at the input and for distributing the
packets a plurality of output channels coupled to an output of said
tunneling source; a cable modem termination system (CMTS) coupled
to each of the plurality of tunneling source output channels, said
CMTS for receiving signals on each of the plurality of tunneling
source output channels and for transmitting signals on a plurality
of parallel output channels.
16. The data transmitting system of claim 14, wherein the plurality
of CMTS output channels are RF channels.
17. The data transmitting system of claim 15, wherein each RF
channel carries packets that are compliant with the DOCSIS
standard.
18. The data transmitting system of claim 14, wherein said CMTS
further comprises: a CMTS router, having an input coupled to
signals from said tunnel source and having a plurality of output
ports; a plurality of channel modulators, each of said plurality of
channel modulators having an input port coupled to receive signals
from a corresponding one of the CMTS router output ports and having
an output port coupled to provided one of the CMTS output
channels.
19. The data transmitting system of claim 14, further comprising: a
plurality of channel modulators, each of said plurality of channel
modulators coupled to receive signals from the output of said
tunneling source; a digital signal processor, coupled to receive
signals from each of said plurality of channel modulators; and a
digital-to-analog converter having an input coupled to receive
signals from said digital signal processor.
20. The data transmitting system of claim 17 wherein each of said
plurality of channel demodulators comprises: an analog-to-digital
converter having an input coupled to receive signals from a
corresponding one of the CMTS router output ports and having an
output; a plurality of bandpass filter circuits parallel coupled to
the output of said analog-to-digital converter, each of said
bandpass filter circuits having a passband characteristic which is
offset in frequency from each of the other bandpass filter
circuits; a plurality of demodulator circuits, each of the
plurality of demodulator circuits having an input coupled to the
output of a respective one of said bandpass filter circuits and
having an output; a serializer having a plurality of input ports,
each of the plurality of input ports coupled to a respective one of
the output ports of said plurality of demodulator circuits and
having a single output port.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
09/924,617, which claims the benefit of U.S. Provisional
Application No. 60/275,665 filed Mar. 14, 2001 which applications
are hereby incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] This invention relates generally to a data service and
related apparatus and more particularly to a cable data service and
system.
BACKGROUND OF THE INVENTION
[0003] As is known in the art, in addition to the transmission of
television signals, it has been recognized that a cable network can
also be used to transmit other types of data between remote
locations. Thus, the cable network of the cable industry may be
used as an alternative to communicating data via conventional
telephone networks, such as the public switched telephone network
(PSTN) for example.
[0004] In this regard, cable networks are currently being used to
transmit data to and from subscribers located at remote locations.
Each subscriber location includes a cable modem (CM) capable of
communicating with a cable modem termination system (CMTS) located
at a central cable station (or headend). The headend provides
television signals to customers, as well as modulated data signals
to each subscriber modem. Cable connections between the CMTS at the
central cable station and the subscriber modems currently exist so
that data packets such as internet protocol (IP) datagrams can be
transmitted between the central cable station and each of the
subscriber modems.
[0005] In general, each connection between a subscriber modem and
the central cable station includes two channels, an upstream
channel on which signals having one frequency range propagate and a
downstream channel on which signals having a different frequency
range propagate. The downstream channel is used to transmit data
from the central cable station to the subscriber modems, and the
upstream channel is used to transmit data from the subscriber
modems to the CMTS at the central cable station. Thus, the CMs are
coupled in communication with the CMTS to receive information on a
so-called "downstream channel" and to communicate information to
the CMTS on a so-called "upstream channel."
[0006] Particular characteristics (e.g., frequency, power levels,
etc,) of the upstream channel are determined at the time the CM is
initialized. The CM at the user or subscriber site typically
connects to a personal computer (PC) through an Ethernet port while
the CMTS typically enables connection to a network through a high
speed Ethernet interface, although other types of network
connection are possible
[0007] As is also known, The Radio Frequency Interface
Specification, Data-over-cable Service Interface Specifications,
(DOCSIS) available from the Cable Television Laboratories, Inc.
(hereinafter, "DOCSIS") describes operating parameters for a cable
modem network. DOCSIS is the de-facto standard for cable modem
products in North America. To carry data downstream, from the
headend to the subscribers, a single 6 MHz-wide radio frequency
(RF) channel is used.
[0008] The 6 MHz channel is located in the 55 to 860 MHz frequency
band. The RF modulation format used over this channel is typically
64- or 256-QAM. A CMTS resides in the headend. The CMTS typically
contains multiple line cards, each capable of transmitting 30 to 40
Mbps downstream. In practice, FEC reduces this number slightly and
27 Mbps is typically achieved 25 over a 64-QAM channel. This
downstream channel will be shared by the subscribers within the
serving area of that line card. Cable modems receive the data, and
transmit the user's data to his computer or LAN via a 10 or
100BaseT connection.
[0009] On the upstream channel, data from the user's local area
network (LAN) is transmitted to the headend using an RF channel in
the 5-42 MHz band of the upstream channel. Typically, quadrature
phase-shift keying (QPSK) transmission is used, although the DOCSIS
standard includes more bandwidth efficient formats. Such efficient
modulation formats typically can be used in CATV systems having a
relatively small amount of interfering signals and noise. The CMTS
line card coordinates the upstream data channels, so that only one
cable modem transmits at a time. Frequently, a single CMTS card
will coordinate multiple upstream channels.
[0010] As 100 Mbps fast-Ethernet becomes more popular, consumers
will develop a growing desire for cable-modem connections that are
faster than currently available cable-modem connections. There are
a variety of ways that a user's bit rate can be improved. One
approach to improve the performance of a cable-modem service is to
segment the serving area so that fewer users share a channel. While
this increases the user's average bit-rate, and provides a better
user-experience for streaming media applications, the peak rate
remains unchanged. For "bursty" applications, improving the peak
rate not only reduces the time it takes to download large files, it
has the additional advantage of allowing more users to share the
limited available bandwidth without compromising the users'
service. The larger the bandwidth being shared by a population of
users with the same traffic demand, the more efficiently the
bandwidth can be used.
[0011] As is also known, there exist a variety of techniques for
improving the peak rate. These techniques can be broken into
several basic categories. One category of techniques includes those
techniques that utilize a more spectrally-efficient modulation
format. One problem with this category of solutions, however, is
that this places strenuous demands on the system's signal-to-noise
ratio (SNR), which current systems might not be able to meet.
Another category of techniques includes those techniques that
utilize serial transmission over channels broader than those
specified in the current DOCSIS standards. This approach would
allow an increase of the symbol rate but would require that
agreements be reached concerning new allocations of spectrum, and
the design of new electronic systems capable of transmitting at
these higher rates.
[0012] Clearly, it is desirable to provide a technique for
increasing cable-modem connection speed. It is also desirable to
provide a system which supports both users of existing cable modem
technology (i.e. so called legacy users who use the DOCSIS
standard) while at the same time supporting users of a new cable
service which provides higher connection speeds to the users.
SUMMARY OF THE INVENTION
[0013] It has, in accordance with the present invention, been
recognized that a third category of techniques exists which, in
combination with protocols to be described hereinbelow, can be used
to improve the peak rate. This category includes those techniques
which utilize parallel transmission. Employing parallel
transmission over conventional Data Over Cable Standard Interface
Specification (DOCSIS) protocol channels has the advantage of
allowing users of the DOCSIS protocol as well as users of the
protocols described hereinbelow to simultaneously share the same
channel. It has further been recognized in accordance with the
present invention, that although a trade-off must be made between
modem technology required for serial versus parallel transmission,
comparisons between these two categories of technology reveal that
the hardware for these two-types of modems will be similar at a
future point in time. It has been further recognized that CATV
network evolution, channel performance, and modem complexity should
all play a role in choosing between the various approaches. From an
IP networking perspective, it is simpler to provide a single "data
link" below the IP layer. However, CATV evolution considerations
favor the approach of transmitting the data over parallel RF
channels.
[0014] Thus, in accordance with the present invention, a method of
sending data from a transmit site to a receive site includes the
steps of dividing a transmit data stream having a first bit rate
into multiple data streams with each of the multiple data streams
having a bit rate which is lower than the first bit rate, and
transmitting each of the multiple data streams over a plurality of
parallel RF channels wherein at least one of the RF channels serves
a plurality of users. The method further includes the step of
recombining the multiple data streams at the receive site to
provide a receive data stream having a bit rate equal to the first
bit rate. With this particular arrangement, a method that improves
the peak data transmission rate from a headend to a subscriber is
provided. The technique of the present invention improves the
performance and efficiency of the network for transferring large
files downstream by dividing the data and transmitting the data
over parallel RF channels. Because the peak transmission rate of
transmission control protocol/internet protocol (TCP/IP) in the
downstream direction is affected by the performance of the upstream
channel, and because additional upstream bandwidth may enable new
services, the same principle can be applied to the upstream
direction.
[0015] In accordance with a further aspect of the present
invention, a system for sending data from a transmit site to a
receive site includes a router, a tunnel source coupled to the
router, and means for providing packets over multiple channels to a
tunneling destination which receives the packets over the multiple
channels and serializes the packets. With this particular
arrangement, a system for enabling a data service which allows
users to utilize a connection having a speed that is higher than
the speed allowed by the current DOCSIS protocol is provided. The
system of the present invention improves the performance and
efficiency of the network for transferring large files downstream
by dividing the data and transmitting the data in parallel over
multiple channels. In a preferred embodiment, the channels are
provided as multiple RF channels. Because the peak transmission
rate of transmission control protocol/internet protocol (TCP/IP) in
the downstream direction is affected by the performance of the
upstream channel, and because additional upstream bandwidth may
enable new services, same principle can be applied to the upstream
direction.
[0016] In accordance with a still further aspect of the present
invention, a system for transmitting signal packets from a source
to two or more destinations includes a router having at least two
address groups and a tunnel source having an input coupled to the
router and having an output. A packet destination address for each
signal packet is mapped to one of the at least two address groups
in the router. Packets having an original destination address which
belongs to the first address group are provided to the tunnel
source. In response to the tunnel source receiving a packet having
an original destination address which belongs to the first address
group, the tunnel source assigns each packet it receives to one of
a plurality of addresses, each address being associated with a
tunnel destination address and each one of the tunnel destination
addresses being mapped to one of a plurality of output channels.
Packets having an original destination address which belongs to the
second address group, on the other hand, are mapped onto a single
output channel based upon their original address. With this
particular arrangement, a system which automatically assigns
messages to one of two address groups and which provides improved
faster access is provided for destinations associated with the
first group. The packets having the original destination address
which belongs to the first address group are transmitted
substantially simultaneously to the destination address via the
plurality of output channels. At the destination address, the
packets on each of the plurality of channels are combined.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The foregoing features of the invention, as well as the
invention itself may be more filly understood from the following
detailed description of the drawings, in which:
[0018] FIG. 1 is a block diagram of a downstream path of a
transmission system;
[0019] FIG. 1A is a block diagram of a downstream path of a
transmission system that includes a Transmission Control Protocol
(TCP) gateway;
[0020] FIG. 2 is a block diagram of an upstream path of a
transmission system;
[0021] FIG. 3 is a block diagram of a demodulator portion of a
FastChannel modem;
[0022] FIG. 4 is a block diagram of a modulator portion of a
FastChannel modem;
[0023] FIG. 5 is a block diagram which illustrates bundling Data
Over Cable Standard Interface Specification (DOCSIS) channels via
internet protocol (IP) tunneling; and
[0024] FIG. 6 is a block diagram of a demodulator portion of a
FastChannel modem.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Referring now to FIG. 1, a downstream path of a transmission
system 10 includes a first router 12 coupled to a tunnel source
(also referred to as a sending tunnel end-point) 16 through a first
signal path 14 (referred to hereinbelow as a FastChannel path).
Tunnel source 16 is coupled to a cable modem termination system
(CMTS) 20 through a second signal path 18 here shown as signal
paths 18a-18d. It should be appreciated that the tunnel source 16
can functionally reside in a separate box upstream of the CMTS 20
as shown in FIG. 1. Alternatively, however, the tunnel source 16
can functionally reside within the CMTS 20 or the router 12.
[0026] The CMTS 20 includes a CMTS router 22 and a plurality of
quadrature amplitude modulators (QAMs) 24a-24d generally denoted
24. Router 12 is also coupled to the CMTS 20, and in particular to
the CMTS router 22, via a signal path 26. The purpose of the signal
paths 14 and 26 will next be described in general overview.
[0027] In the system of the present invention, a packet
encapsulation and tunneling procedure can be used which includes
two different IP address spaces associated with IP over cable
offerings. A first address space (referred to as an L address
space) is for existing single-channel users operating in accordance
with the Data Over Cable Standard Interface Specification (DOCSIS).
A second address space (referred to as an F address space) is for
FastChannel users (i.e. users of the protocol described herein).
The router 12 is adjacent to and upstream of the CMTS 20, such
that, if a packet having a destination address in the L address
space is received, the router 16 directly routes the packet to the
CMTS 20 via signal path 26 without passing through the tunneling
node 16. If, however, a packet having a destination address in the
F address space is received, the router 16 forwards the packet to
the tunnel source 16. Tunnel source 16 receives data provided
thereto from the router 12 and divides the serialized data stream
into a plurality of parallel channels which are fed via the signal
path 18a to the CMTS 20.
[0028] It should be noted that in FIG. 1, signals paths 18b-18d are
shown in phantom to indicate that the parallel signals are
logically separate but can be transmitted on a single physical
signal path (e.g. a single wire) such as the signal path 18a.
[0029] Whether fed to the CMTS router 24 via the FastChannel path
14 or via the legacy path 26, the CMTS router 22 provides each of
the signals to one of a plurality of modulators 24a-24d generally
denoted 24. In this particular embodiment, where it is desirable to
be compatible with the DOCSIS protocol, the modulators 24 are
preferably provided as quadrature amplitude modulators (QAMs). It
should be appreciated, however, that in other embodiments it may be
desirable or even necessary to utilize other types of modulators
including but not limited to quadrature phase-shift keyed (QPSK),
spread spectrum, orthogonal-frequency-division multiplexed (OFDM)
and code-division multiple-access (CDMA) modulators.
[0030] A plurality of parallel channels 28a-28d are formed via the
CMTS router 22 and the QAM modulators 24a-24d. Each of the
modulators 24 modulates the signal fed thereto and provides the
so-modulated signal to a corresponding one of a plurality of radio
frequency (RF) channels in a hybrid fiber coaxial (HFC) network 30.
It should be understood that the parallel channels may or may not
be adjacent one another in frequency. HFC 30 corresponds to a cable
network utilizing a combination of optical fibers and coaxial
cables of the types known to be used in the cable television
industry for transmission of television signals. Alternatively, HFC
30 could be replaced with a wireless system, wherein signals are
transmitted over the air, typically using the MMDS band, rather
than over HFC plant as described for example, in AT&T Labs
broadband fixed wireless field experiment, Byoung-Jo Kim;
Shankaranarayanan, N. K.; Henry, P. S.; Schlosser, K.; Fong, T. K.
IEEE Communications Magazine, Volume: 37 Issue: Oct. 10, 1999
page(s) 56-62.
[0031] Signals are provided via the parallel channels and the HFC
30 to a corresponding plurality of demodulators 32 here provided as
quadrature amplitude modulation (QAM) demodulators 32. The
demodulators 32 provide demodulated signals to a tunnel destination
34 (also referred to as a destination end-point) which receives the
demodulated tunnel source signals and re-serializes the data. Thus,
a plurality of channels are coupled between the tunnel source 16
and the tunnel destination 34.
[0032] The tunnel destination 34 is coupled to personal computers
(PCs) or other devices of a user or subscriber, typically via a
100baseT local area network (LAN) connection.
[0033] In one embodiment, each of the channels 28a-28d is provided
as an RF channel between the send and receive sites and virtual
links are established over each of the RF channels. Packets are
distributed over these virtual links in a controlled fashion. Thus,
virtual links can be established over each RF channel between send
and receive sites. As used herein the term "virtual link" means a
logical connection between a sender and a receiver, where both ends
are addressable via some type of address. Data is sent via packets
or link layer frames, which contain the sending and receiving
address (as well as other information) in a packet or frame header.
Many virtual links can share the same physical link. In one
embodiment, the virtual links are established via a MAC-layer
process. Those of ordinary skill in the art will appreciate that
the MAC layer is also known as an OSI layer 2.
[0034] In another embodiment, the virtual links are provided via an
Internet Protocol IP within IP encapsulation or tunneling process.
It should be appreciated, however, that other tunneling processes
including but not limited to IP within User Datagram Protocol
(UDP), IP within TCP can also be used. Technically it is possible
to encapsulate IP within the network layer packets of other
protocols such as X.25. It should be understood that as used
herein, the term "IP tunneling" includes IP over TCP and UDP or any
other mechanism by which IP is the inside layer, and IP, TCP or UDP
is the outside layer.
[0035] For the illustrative embodiment IP encapsulation within IP,
and IP tunneling are used. This technique allows an incoming IP
packet to be placed in the payload field of an encapsulation packet
having source and destination address headers which point to the
respective end-points of the tunnel. When received at the
destination tunnel, the encapsulation header is stripped off, and
the original packet is forwarded by the tunnel end-point toward the
original destination. The sending tunnel end-point can functionally
reside in a separate box upstream of the CMTS. The receiving tunnel
end-point will reside in a box, which terminates the N cable modem
MAC interfaces. Each cable modem interface is assigned an IP
address and multiple tunnels are created from the sending end-point
to the IP address endpoints of each cable modem.
[0036] The packets are distributed over the virtual links in a
controlled fashion. In one embodiment, control over the packet
distribution is provided by load balancing. As used herein, the
term "load balancing" includes but is not limited to adjustment of
system characteristics to adjust and fix congestion situations or
to avoid them. Load balancing can be achieved via monitoring or
scheduling techniques. When using a monitoring technique, system
characteristics are adjusted once a particular condition or state,
such as an overload condition, is detected. When using a scheduling
technique, on the other hand, system characteristics (e.g. quality
of service-QOS) are monitored and system adjustments are made prior
to an overload condition occurring.
[0037] Each virtual link (both upstream and downstream) may be
shared by multiple data flows, where data flows might have the same
or different sources and destinations. Scheduling policies provide
QoS to these flows, primarily bandwidth and delay. Flows carrying
interactive applications (including but not limited to voice calls
and video conferencing) have stringent delay requirements that
should be fulfilled. For the applied scheduling policy and existing
flows with their QoS requirements, resources will be assigned to a
new flow with the specified QoS requirements if they are available.
DOCSIS defines the admission control procedure: how the resource is
requested by the higher layer protocols, and how the information
about the resource availability is stored in CMTS. DOCSIS also
defines QoS parameters that applications may specify when
requesting the resource. In accordance with the present technique,
the resource will be assigned to users that utilize multiple
virtual links with the higher probability. The QoS capabilities of
IP that is likely to carry data in the access network in question
are currently under development.
[0038] In some embodiments, each of the plurality of RF channels
are adjacent in frequency while in other embodiments, each of the
plurality of RF channels are not adjacent in frequency. Allowing
the channels to be not adjacent in frequency permits greater
flexibility when interworking with an existing cable plant which
may already contain a high occupancy of video channels. Using
adjacent channels may simplify the modem design, as a single
down-converter and digital-to-analog converter may be used. The
adjacent channels can then be separated using digital
techniques.
[0039] There are various alternative methods for utilizing the
bandwidth of parallel channels, namely: (1) the bit-level, (2) the
media access control (MAC) frame level, and (3) the IP level. It
should be appreciated that since the bit-level method would not be
compatible with supporting simultaneously single-channel
transmission and multiple-channel transmission it may not be
appropriate for this application.
[0040] The MAC frame level technique involves distributing the MAC
transmission frames 10 across the multiple channels, and
recombining the frames into a single stream at the modem. The IP
packet level technique involves distributing the packets across the
multiple channels, and recombining the packets into a single stream
at the modem. The differences between these two alternatives are
that in the frame-level case, a channel number/frequency band must
be mapped to a different MAC destination address, while in the
packet level case a channel number/frequency band must be mapped to
a different IP address. The frame level method integrates the
recombining of packets with the cable modem. In contrast, the
packet level method allows the tunnel end-point to be placed
"outside" a DOCSIS cable modem. It should be appreciated that in
this approach, the FastChannel modem could be constructed from
multiple DOCSIS cable modems and a tunneling end-point. Similarly
the distribution of packets is most natural inside the CMTS with
the frame method, and may take place outside the CMTS with the
packet method. The frame level method will allow relatively tight
integration into the CMTS and modem components and therefore may be
most cost-effective in the long run. It should be noted that for
this option, in order to incorporate the relevant functionality
changes that one would need to make to the CMTS, the FastChannel
modems could not be created by simply combining together several
current DOCSIS cable modems. The packet level method, while
possibly not optimal in the long run, allows use of existing cable
modems and CMTS without requiring modification to the CMTS.
[0041] The packet level method allows the tunneling end-points to
be separate from the CMTS and DOCSIS cable modems. Furthermore, a
tunneling end-point that is separate from the CMTS can serve
multiple CMTS. This may make it easier to add capacity to a system,
as additional DOCSIS channels could be added, and served with the
FastChannel protocol, without needing to upgrade the previously
installed CMTS.
[0042] In one example, assume there are N parallel channels
assigned to FastChannel cable modems. In this case CMTS 20
maintains N separate output queues, one for each RF channel. In
FIG. 1, four queues 28a-28d are shown. Frames are thus placed into
one of the four output channels as they arrive.
[0043] It should be appreciated that it is important to minimize
the fraction of out-of-sequence packets. If packet sequence numbers
were employed (by means of a sequence number field in the
encapsulation header), out-of-sequence packets can be eliminated.
This is the approach taken with the known PPP Multi-link Protocol
(MLP). While the use of PPP MLP would lead to the desired result,
the PPP protocol is overkill for the job at hand. It is thus
suggested that it is possible to obtain a satisfactory
out-of-sequence packet minimization through a suitably chosen queue
management algorithm, and without the use of sequence numbers in an
encapsulation header. However some care is needed in the algorithm
selection. Placing frames into queues in a round-robin fashion
could lead to mis-ordering. For example, suppose one queue is
backed-up and another is empty, and the first frame is placed in
the backed-up queue, and the second frame placed in the empty
queue. It is possible that in this case the second frame may arrive
at the receiver before the first frame. To address this particular
problem, an alternative queuing discipline comprises insertion of
frames into the shortest queue, where the "shortest" metric should
represent frame service time. It is possible to estimate the frame
service time based on an appropriate combination of byte and packet
counts in the output buffer.
[0044] On the receiving side in the modem, a frame "serialization"
function is required, which simply plays out received frames
serially into the output in the MAC-level driver, in the order in
which they were received. Optimally, order should be measured as
the time at which the first byte of the frame is received rather
than the last byte, in order to further reduce the possibility of
frame mis-ordering.
[0045] In the case of packet level parallel transmission, IP
encapsulation within IP/IP tunneling can be used. This technique
allows an incoming IP packet to be placed in the payload field of
an encapsulation packet, having source and destination address
headers that point to the respective end-points of the tunnel. When
received at the destination tunnel, the encapsulation header is
stripped off, and the original packet is forwarded by the tunnel
end-point toward the original destination. The proposed use of this
technique is described in detail below in conjunction with FIG.
5.
[0046] The tunnel destination 34 can reside in a box, which
terminates the N cable modem MAC interfaces. Each cable modem
interface is assigned an IP address, and multiple tunnels are
created from the sending end-point (e.g. tunnel source 16), to the
IP address endpoints of each cable modem. A queue-scheduling
algorithm is employed at the end point of the sending tunnel 16,
which uniformly distributes the IP packets over each tunnel.
[0047] The choice of queue scheduling algorithm to minimize
mis-ordering is again relevant. One difference between the IP and
MAC approaches is that in the IP-based approach, the tunnel does
not have access to the output buffer state on the CMTS itself, only
on the tunnel machine. The tunnel buffer state may not be the same
as the CMTS buffer state. If it turns out that packet sequence
problems may arise because of this difference, it may be necessary
to add a sequence number field to the encapsulation header.
[0048] Referring now to FIG. 1A, in the case where the FastChannel
system is implemented in a downstream path but not an upstream
path, the downstream transmission rate of TCP is limited by the
speed at which an acknowledgement is received from the upstream
module. To increase this speed, a known transmission control
protocol (TCP) gateway 13 is interposed between the router 12 and
the tunnel source 16. In this optional embodiment, the TCP gateway
transparently terminates the TCP connection, provides
acknowledgements back to the sending node, prior to them being
received by the TCP receiver. The sender is therefore able to grow
its transmission window faster and send data faster than it would
otherwise be able to.
[0049] Referring now to FIG. 2, an upstream path of a transmission
system such as the transmission system 10 described above in
conjunction with FIGS. 1 and 1A includes subscriber systems 36
which transmit signals through IP tunnel sources 38. The tunnel
sources 38 form a plurality of channels 40a-40d each of which are
coupled to one of a plurality of upstream modulators 42a-42d which
in turn are coupled to an HFC 44.
[0050] The upstream plurality of parallel channels are coupled to a
CMTS 46 and in particular, the parallel channels are coupled to
corresponding ones of a plurality of demodulators 48a-48d,
generally denoted 48. The upstream demodulators provide the signal
to a CMTS router 50 which in turn provides the signals to an IP
tunnel destination 52 and subsequently to a router 54.
[0051] In this manner signals can be transmitted in the upstream
direction within the transmission system.
[0052] The MAC frame level technique and the (IP) packet level
technique for utilizing the bandwidth of the parallel channels
discussed above in the downstream case can also be used in the
upstream case.
[0053] Referring now to FIG. 3, a demodulator 60 of the type which
may be used in a modem coupled to receive signals from a
FastChannel signal path includes a tuner 62 provided from a
downconverter module 64 having a local oscillator (LO) 66 coupled
thereto. The downconverter module 64 receives RF signals at a first
port thereof and an LO signal at a second port thereof and provides
an output signal having a frequency equal to the difference between
the frequencies of the RF signal and the LO signal.
[0054] It should be appreciated that the demodulator embodiment
shown in FIG. 3 requires that the parallel channels be adjacent to
one another. It should also be understood that other demodulator
embodiments may not require that the parallel channels be adjacent
one another.
[0055] The tuner, band pass filter and ADC can be provided having
performance characteristics that are similar or in some instances
even identical to those used in serial modems.
[0056] The downconverter module output signal is provided to a
filter 68 having a band pass filter characteristic. The so-filtered
signal is then fed to an input port of an analog to digital
converter (ADC) 70, which receives the analog signal at an input
thereof and provides at an output port thereof a stream of bits
which represents the analog signal fed thereto.
[0057] The ADC 70 is followed by processors 72a-72d generally
denoted 72 each of which simulates a filter having a band pass
filter characteristic. Thus, processors 72a-72d correspond to
digital filters. In one embodiment, the filters are provided having
a 5 megahertz (MHz) bandwidth.
[0058] Each band-pass filter 72a-72d is followed by processors
74a-74d, generally denoted 74, which perform a demodulation
process. In one embodiment, processors 74a-74d perform 5
Msymbols/sec QAM demodulation. It should be understood that
although multiple processors are shown, this does not mean that
multiple chips would be required. It should also be understood that
the processor requirements of this modem may be easier to meet than
those of a demodulator used in a serial modem, as a band-pass
filter is rather simple computation, and the symbol rate of each
QAM channel is lower. Thus, a single integrated circuit or "chip"
can contain multiple demodulators and digital filters.
[0059] The demodulators 74 provide the filtered, demodulated signal
to a serializer 76. Serializer 76 receives the signals in parallel
from the demodulators 74 and re-serializes the packets to provide a
serial signal at an output port 76a.
[0060] Referring now to FIG. 4, a modulator portion 80 of a modem
includes a packet inverse multiplexor (mux) 82 adopted to receive
signals from a user. In this particular example the inverse mux 82
is coupled to a home 100 base T LAN. The inverse mux 82 provides
signals to a plurality of upstream modulators 84a-84d, generally
denoted 84. Each of the modulators 84a-84d modulates the signals
fed thereto at a different frequency, designated F1-F4 in FIG.
4.
[0061] The modulators 84 provide signals to a digital signal
processor DSP 86 which combines the signals at frequencies F1-F4.
The DSP 86 provides a stream of bits to a digital to analog
converter (DAC) 88 which receives the bit stream and generates a
corresponding analog signal at an output port thereof The analog
signal is fed from the DAC 88 to a diplexor 90. Diplexor 90 is
adapted to provide signals to one of the coax signal port and a
downstream signal port. The diplexer 90 sends the upstream signals,
which are within a first frequency band (typically 5-42 MHz) to the
headend via the HFC infrastructure. It simultaneously sends the
downstream signals within a second frequency band (typically this
frequency band begins at 55 MHz and ends somewhere between 500 MHz
and 900 MHz) to the demodulator portion of the FastChannel
modem.
[0062] Referring now to FIG. 5, a system for processing data in a
series of parallel channels includes a router 92 coupled via a
signal path 94 to a tunnel source 96 and via a signal path 98 to a
CMTS 100. The CMTS is coupled via a plurality of cable channels
102a-102N to a like plurality of tunnel destinations 104a-104N
generally denoted 104 on a machine 105. The tunnel destinations are
coupled to a processor or computer 106 via a standard network
interface such as an Ethernet interface. Also depicted is a
Personal Computer (PC) 108 having an address E. PC 108 represents a
conventional DOCSIS user. This user simply uses one of the channels
102a-102N. In FIG. 5, the DOCSIS user is coupled to channel 102N.
This conventional user plays no part in the FastChannel
arrangement. It merely illustrates the co-existence of the
FastChannel system and protocol with a conventional DOCSIS system
and protocol.
[0063] Assume a packet 110 arrives via the router 92 at a tunnel
96. Tunnel 96 is connected to the CMTS via an interface having an
IP address designated as T1. The packet 110 originated at a source
with address S (identified with reference designator 110a in FIG.
5) and is destined to the PC 106 having an address D (identified
with reference designator 1110b in FIG. 5). It is further assumed
in this example that address D is an element of address space F
(i.e. a FastChannel address space).
[0064] The tunnel source 96 having the address T1 encapsulates the
packet by creating a new packet 112, placing the original packet
110 in the payload field of the new packet 112, and adding a new
packet header 114. In the new header 114, the source address is T1
(identified with reference designator 112a in FIG. 5) and
destination address is one of a, b, . . . , n, (identified with
reference designator 112b in FIG. 5) which are separate IP
interfaces on tunnel destination 104. It should be noted that
destination addresses a, b, . . . , n are part of L's address space
and that each address pair (T1, a), (T1, b), etc. defines a
separate tunnel. The CMTS 100 is configured such that the
subnetwork address of which address a is a member, is mapped onto
cable channel 102a; similarly b is mapped onto cable channel 102b,
and so on; finally n is mapped onto cable channel 102N. The
encapsulated packets 114 are then routed via the appropriate tunnel
to the tunnel destination 104. At the tunnel destination 104, the
encapsulation headers are removed to again provide packet 110, and
the packets are forwarded in their original order to the
destination, which in this case is the PC 106.
[0065] The net effect of this procedure makes available the sum of
the bandwidths of channels 102a through 102N to the path between
source tunnel 96 and the destination tunnel 104. It should also be
noted that the address allocation method of the present invention
allows legacy DOCSIS users to share channels with FastChannel
users. As depicted in FIG. 5, a PC 108 with address E (where E is
in the L address space) is able to receive data addressed to it,
while sharing channel 102N with the FastChannel-attached PC 106
with address D.
[0066] It should further be noted that it may be desirable to
maintain the same (or even greater) ratio of upstream to downstream
bandwidth for FastChannel as for legacy DOCSIS. One reason is the
well known limiting effect that bandwidth asymmetry has on TCP
performance. Hence the tunneling, encapsulation and channel
combining procedures described above can also be applied to group
together a corresponding set of upstream channels.
[0067] Referring now to FIG. 6, an alternate embodiment of a
demodulator portion 120 of a FastChannel modem includes a plurality
of tuners 122a-122d. Each of the tuners are provided from a
respective one of a plurality of down converter modules 124a-124d
having a respective one of a plurality of local oscillators (LO)
126a-126d coupled thereto. Taking tuner 122a as representative of
tuners 122b-122d, the down converter module 124a receives RF
signals at a first port thereof and an LO signal at a second port
thereof and provides an output signal having a frequency equal to
the difference between the frequencies of the RF signal and the LO
signal.
[0068] The output signals from the tuners 122a-122d are provided to
respective ones of filters 128a-128d with each of the filters
having a band pass filter characteristic. The filtered signals are
then fed to respective ones of a plurality of analog to digital
converters (ADC) 130a-130d. The ADCs 130a-130d receive the analog
signals at inputs thereof and provide at outputs thereof a stream
of bits which represents the analog signal fed to each ADC.
[0069] The ADCs 130a-130d are followed by processors 132a-132d each
of which perform a demodulation process, In one embodiment,
processors 132a-132d perform 5 Msymbols/sec QAM demodulation. It
should be understood that although multiple processors are shown,
this does not mean that multiple integrated circuits would be
required. The demodulators 132a-132d provide the filtered,
demodulated signal to a serializer 134. Serializer 134 receives the
signals in parallel from the demodulators 132a-132d and
re-serializes the packets to provide a serial signal at an output
port of the serializer 134.
[0070] The demodulator 120 illustrates one method for receiving
FastChannel data when parallel transmission is used. It should be
appreciated that in demodulator 120 multiple demodulators 132a-132d
are used, and the output is combined in the serializer 134. The
serializer would multiplex the received packets or frames. Such an
approach should not require extensive buffering, since the headend
controls the peak rate to each user. Such a demodulator can be
readily implemented using currently available commercial
components. An additional benefit of this approach is that any RF
channels can be used, they need not be adjacent to one another. One
drawback to this design is that it may be relatively expensive
compared with an integrated, multiple-channel demodulator since it
has more components, including multiple RF tuners and bandpass
filters.
[0071] All references cited herein are hereby incorporated herein
by reference in their entirety.
[0072] Having described preferred embodiments of the invention, it
will now become apparent to one of ordinary skill in the art that
other embodiments incorporating their concepts may be used. It is
felt therefore that these embodiments should not be limited to
disclosed embodiments, but rather should be limited only by the
spirit and scope of the appended claims.
* * * * *