Note: Descriptions are shown in the official language in which they were submitted.
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CABLE DATA SERVICE METHOD
FIELD OF THE INVENTION
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
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.
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.
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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."
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
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.
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
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 l OQBaseT connection.
On the upstream channel, data from the user's local area network (LAN) is
transmitted to
3 0 the headend using an RF channel in the 5-42 MHz band of the upstream
channel. Typically,
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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.
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.
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.
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
3 0 modem technology (i.e. so called legacy users who use the DOCSIS standard)
while at the same
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time supporting users of a new cable service which provides higher connection
speeds to the
users.
SUMMARY OF THE INVENTION
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.
Certain exemplary embodiments can provide a method of sending data from a
transmit
site to a receive site, the method comprising: 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 cable network
comprising a plurality of RF DOCSIS channels, wherein at least one of the RF
channels serves a
plurality of users; and 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.
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Certain exemplary embodiments can provide a method of sending data comprising:
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 cable network comprising a plurality of RF
DOCSIS
5 channels, wherein at least one of the RF channels serves a plurality of
users.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of the invention, as well as the invention itself may
be more fully
understood from the following detailed description of the drawings, in which:
Fig. 1 is a block diagram of a downstream path of a transmission system;
Fig. IA is a block diagram of a downstream path of a transmission system that
includes a
Transmission Control Protocol (TCP) gateway;
Fig. 2 is a block diagram of an upstream path of a transmission system;
Fig. 3 is a block diagram of a demodulator portion of a FastChannel modem;
Fig. 4 is a block diagram of a modulator portion of a FastChannel modem;
Fig. 5 is a block diagram which illustrates bundling Data Over Cable Standard
Interface
Specification (DOCSIS) channels via internet protocol (IP) tunneling; and
Fig. 6 is a block diagram of a demodulator portion of a FastChannel modem.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
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
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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.
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.
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 sigmal path 26
without passing
through the tunneling node 16. If, however, a packet having a destination
address in the F
2 o 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.
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.
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
3 0 generally denoted 24. In this particular embodiment, where it is desirable
to be compatible with
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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.
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 Conununications Magazine, Volume: 37
Issue: 10, Oct.
1999 page(s) 56 -62.
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.
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.
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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
To 2.
In another embodiment, the virtual links are provided via an Intern.et
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
(ITDP), 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.
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.
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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
5 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.
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.
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
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digital techniques.
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 riot be compatible
with supporting
simultaneously single-channel transmission and multiple-channel transmission
it may not be
appropriate for this application.
The MAC frame level technique involves distributing the MAC transmission
frames
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 distributionof 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.
The packet level method allows the tunneling end-points to be separate from
the CMTS
and DOCSIS cable modems. Furthermore, a tunneling end-poirit that is separate
from the CMTS
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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.
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.
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.
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.
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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.
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.
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.
Referring now to Fig. lA, 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.
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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 lA
includes subscriber
systems 36 which transmit signals through 1P 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.
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.
In this manner signals can be transmitted in the upstream direction within the
transmission system.
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.
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.
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.
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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.
The downconverter module output signal is provided to a filter 68 having a
band pass
5 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.
The ADC 70 is followed by processors 72a-72d generally denoted 72 each of
which
10 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.
Each band-pass filter 72a-72d is followed by processors 74a-74d, generally
denoted 74,
15 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.
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
2 5 packets to provide a serial signal at an output port 76a.
Referring now to Fig. 4, a modulator portion 80 of a modem includes a packet
inverse
multiplexor (mux) 82 adapted to receive signals from a user. In this
particular example the
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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 Fl-
F4 in Fig. 4.
The modulators 84 provide signals to a digital signal processor DSP 86 which
combines
the signals at frequencies Fl-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.
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 channel system and protocol with a conventional DOCSIS system and
protoeol.
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 T 1. The packet
110 originated at a
source with address S (identified with reference designator 1 l0a in Fig. 5)
and is destined to the
PC 106 having an address D (identified with reference designator 110b in Fig.
5). It is further
CA 02372415 2002-02-19
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assumed in this example that address D is an element of address space F (i.e.
a FastChannel
address space).
The tunnel source 96 having the address T I 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.
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.
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.
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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.
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.
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.
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,
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multiple-channel demodulator since it has more components, including multiple
RF tuners and
bandpass filters.
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.
