(12) United States Patent
`Zhang et al.
`
`USOO6181711B1
`(10) Patent No.:
`US 6,181,711 B1
`(45) Date of Patent:
`Jan. 30, 2001
`
`(54) SYSTEM AND METHOD FOR
`TRANSPORTING A COMPRESSED WIDEO
`AND DATA BIT STREAM OVERA
`COMMUNICATION CHANNEL
`
`OTHER PUBLICATIONS
`f Moving Pi
`ion Technol
`Generic Codi
`Inf
`t
`ec Ešar enerIC UOding OI MOVIng. P1c
`SS and ASSociated Au 1O, ISO/IEC 1–13818-1, Nov. 13,
`
`(75) Inventors: Ji Zhang, San Jose; Wen H. Chen,
`Sunnyvale; Fang Wu, San Jose, all of
`CA (US)
`(73) Assignee: Cisco Systems, Inc., San Jose, CA (US)
`(*) Notice:
`Under 35 U.S.C. 154(b), the term of this
`patent shall be extended for 0 days.
`
`sk -
`
`"Network and Customer Installation Interfaces Asymmet:
`C Digital Subscriber Line (ADSL) Metallic Interface,
`American National Standards Institute, ANSI T1. 413-1995.
`The ATM Forum Technical Committee User-Network Inter
`face Specification, Version 3.1 (UNI 3.1), Aug. 18, 1995.
`cited by examiner
`Primary Examiner Joseph L. Felber
`(74) Attorney, Agent, or Firm-Beyer Weaver & Thomas,
`LLP
`ABSTRACT
`(57)
`Digitally compressed video/audio bit streams, when trans
`mitted over digital communication channels. Such as digital
`subscriber loop (DSL) access networks, ATM networks,
`Satellite, or wireleSS digital transmission facilities, can be
`corrupted due to lack of sufficient channel bandwidth. This
`invention describes Schemes to ensure lossleSS transmission
`of bit Streams containing pre-compressed Video signals
`within the communication channels. The Schemes herein
`comprises a rate conversion System that converts the bit rate
`of a pre-compressed Video bit stream from one bit rate to
`another, and that is integrated with a digital communication
`channel, and a means to convey the maximum channel
`transmission rate to the rate conversion System to allow
`Satisfactory transmission of the bit Stream from the input of
`the rate converter through the transmission facility.
`
`37 Claims, 16 Drawing Sheets
`
`(21) Appl. No.: 08/947,480
`(22) Filed:
`Oct. 10, 1997
`Related U.S. Application Data
`(60) Provisional application No. 60/051,109, filed on Jun. 26,
`1997.
`7
`(51) Int. Cl.' ........................................................ Ho: 3/16
`(52) U.S. Cl. ............................................. 370/468; 370/538
`(58) Field of "Soss, 532,537,535.
`s
`
`s
`
`s
`
`s
`
`s
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`s
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`s
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
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`2/1996 Augenbraum et al. ................ 360/64
`5,493.456
`55: : gE. A. - - - - - - - - - - - - ".
`."S.
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`Eights al
`... 3 S.405
`5,812,786 * 9/1998 SeaZholtz et al.
`... 709/233
`6,026,097
`2/2000 Voois et al. .......................... 370/468
`
`Commmands
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`US 6,181,711 B1
`
`1
`SYSTEMAND METHOD FOR
`TRANSPORTING ACOMPRESSED WIDEO
`AND DATA BIT STREAM OVERA
`COMMUNICATION CHANNEL
`
`2
`Signal without compression. International Standards have
`been created on Video compression Schemes. These include
`MPEG-1, MPEG-2, H.261, H.262, H.263, etc. These stan
`dardized compression Schemes mostly rely on Several key
`algorithm Schemes: motion compensated transform coding
`(for example, DCT transforms or wavelet/sub-band
`transforms), quantization of the transform coefficients, and
`variable length encoding (VLC). The motion compensated
`encoding removes the temporally redundant information
`inherent in Video Sequences. The transform coding enables
`orthogonal Spatial frequency representation of Spatial
`domain Video signals. Quantization of the transformed coef
`ficients reduces the number of levels required to represent a
`given digitized Video Sample and is the major factor in bit
`usage reduction in the compression process. The other factor
`contributing to the compression is the use of variable length
`coding (VLC) So that most frequently used symbols are
`represented by the Shortest code word. In general, the
`number of bits used to represent a given image determines
`the quality of the decoded picture. The more bits used to
`represent a given image, the better the image quality. The
`System that is used to compress digitized video Sequence
`using the above described Schemes is called an encoder or
`encoding System.
`In the prior art compression Schemes, the quantization
`Scheme is lossy, or irreversible process. Specifically, it
`results in loSS of Video textural information that cannot be
`recovered by further processing at a later Stage. In addition,
`the quantization proceSS has direct effect on the resulting bit
`usage and decoded Video quality of the compressed bit
`Stream. The Schemes at which the quantization parameters
`are adjusted control the resulting bit rate of the compressed
`bit Stream. The resulting bit Stream can have either constant
`bit rate, CBR, or variable bit rate, VBR. CBR compressed bit
`Stream can be transmitted over channel delivers digital
`information at a constant bit rate.
`A compressed Video bit stream generally is intended for
`real-time decoded playback at a different time or location.
`The decoded real-time playback must be done at 30 frames
`per second for NTSC standard video and 25 frames per
`second for PAL standard video. This implies that all of the
`information required to represent a digital picture must be
`delivered to the destination in time for decoding and display
`in timely manner. Therefore, this requires that the channel
`must be capable of making Such delivery. From a different
`perspective, the transmission channel imposes bit rate con
`Straint on the compressed bit stream. In general, the quan
`tization in the encoding process is adjusted So that the
`resulting bit rate can be accepted by the transmission chan
`nel.
`Because both temporal and Spatial redundancies are
`removed by the compression Schemes and because of Vari
`able length encoding, the resulting bit stream is much more
`Sensitive to bit errors orbit losses in the transmission process
`than if the uncompressed video is transmitted. In other
`words, minor bit error or loSS of data in compressed bit
`Stream typically results in major loSS of Video quality or even
`complete shutdown of operation of the digital receiver/
`decoder.
`Further, a real-time multimedia bit Stream is highly Sen
`Sitive to delayS. A compressed Video bit Stream, when
`transmitted under excessive and jittery delays, will cause the
`real-time decoder buffer to under flow or overflow, causing
`the decoded Video Sequence to be jerky, or causing the audio
`Video signals out of Synchronization. Another consequence
`of the real-time nature of compressed Video decoding is that
`lost compressed data will not be re-transmitted.
`
`This application claims benefit of provisional application
`Ser. No. 06/051,109, filed Jun. 26, 1997.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates generally to communication
`channels and Systems for transmitting data. In particular, the
`present invention relates to a System and method for trans
`mitting compressed digital Video signals over a communi
`cation channel. Still more particularly, the present invention
`relates to a System and method for transmitting compressed
`digital video signals over digital Subscriber loop (DSL)
`access networks and asynchronous transfer mode (ATM)
`networks.
`2. Description of the Background Art
`There are presently a variety of different communication
`channels for transmitting or transporting video data. For
`example, communication channels. Such as digital Subscriber
`loop (DSL) access networks, ATM networks, satellite, or
`wireleSS digital transmission facilities are all well known. In
`fact, many Standards have been developed for transmitting
`data on the communication channels. The present invention
`relates to Such communication channels, and for the pur
`poses of the present application a channel is defined broadly
`as a connection facility to convey properly formatted digital
`information from one point to another. A channel includes
`Some or all of the following elements: 1) physical devices
`that generate and receive the Signals (modulator/
`demodulator); 2) physical medium that carries the actual
`Signals; 3) mathematical Schemes used to encode and decode
`the signals; 4) proper communication protocols used to
`establish, maintain and manage the connection created by
`the channel. The concept of a channel includes but is not
`limited to a physical channel, but also logical connections
`established on top of different network protocols, Such as
`xDSL, ATM, TCP/IP, wireless, HFC, coaxial cable, etc.
`The channel is used to transport a bit Stream, or a
`continuous Sequence of binary bits used to digitally repre
`Sent compressed Video, audio or data. The bit rate is the
`number of bits per second that the channel is able to
`transport. The bit error rate is the statistical ratio between the
`number of bits in error due to transmission and the total
`number of bits transmitted. The channel capacity is the
`maximum bit rate at which a given channel can convey
`digital information with a bit error rate no more than a given
`value. And finally, a multipleX is a Scheme used to combine
`bit Stream representations of different Signals, Such as audio,
`Video, or data, into a single bit stream representation.
`One problem with existing communication channels is
`their ability to handle the transportation of video data. Video
`data is much larger than many other types of data, and
`therefore, requires much more bandwidth from the commu
`nication channels. Since transmission of Video data with
`existing communication channels would require excessive
`amounts of time, compression is an approach that has been
`used to make digital Video images more transportable.
`Digital video compression Schemes allow digitized video
`frames to be represented digitally in much more efficient
`manner. Compression of digital Video makes it practical to
`transmit the compressed signal by digital channels at a
`fraction of the bandwidth required to transmit the original
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`One particular communication channel that is becoming
`increasingly common is ASynchronous Transfer Mode
`(ATM) networks. ATM networks are based on the ATM
`transport protocol which can be used to transport data or
`multimedia bit stream with pre-Specified quality of Service.
`Unlike the xDSL standard, ATM protocols specifies how
`data is first packetized into fixed sized data units, called
`cells. It also specifies how Such a cell Stream can be
`multiplexed, de-multiplexed, Switched and routed between
`different locations to Support end-to-end connections at
`given bit rate and/or quality of service (QOS). In ATM
`networks, data bit stream to be transported are first con
`verted into fixed sized ATM cells, each cell has a 5 byte
`header and up to 48 bytes of payload. Of particular interests
`to our invention is the capability of ATM networks to carry
`MPEG transport streams.
`In ATM networks, connections can be established with
`pre-determined grade of QOS. Conditions of network
`utilization, along with call admission control Sometimes
`may prevent a new connection from being established at the
`given bit rate or given quality of Service. In Such cases, the
`requested connection may either have to be rejected or a new
`Set of admissible connection parameters have to be negoti
`ated between the network Service provider and the user
`requesting the connection.
`ATM networks can be used to carry either constant bit rate
`(CBR) or variable bit rate (VBR) bit stream. The bit stream
`may be compressed bit stream or data. In either case, an
`agreement must be made between the user requesting the
`connection and the network Service provider. The connec
`tion agreement includes the bit rate profile of the bit Stream
`and quality of Service. If Such an agreement cannot be
`reached, either the connection must be rejected, or the bit
`rate profile must be modified, or the mismatched bit rates
`may cause periodic loss of data within the ATM networks,
`which results in corrupted bit stream in the receiver/decoder.
`Specific decoded Video quality depends on the decoder
`design.
`The process of modifying the bit rate profile of the input
`bit stream is called traffic shaping. The objective of the
`traffic shaping is to smooth out the burstiness of the ATM
`40
`cell Stream So that the resulting bit rate profile is more
`constant than that without traffic shaping. Existing methods
`to do traffic shaping include using a large buffer to Smooth
`out the bit rate fluctuation, or to selectively drop ATM cells
`during high bit rate period. These methods were originally
`designed for connections carrying data Streams or non-real
`time multimedia bit stream. In the case of using large
`buffers, data bit stream is not Sensitive to large delay
`variations. In the case of Selective cell dropping, higher layer
`communication protocols will retransmit the lost informa
`tion at a later time. However, in the case of transporting
`real-time compressed Video, none of the method is accept
`able because of the tight constraint on the end-to-end delay
`variation. For example, when ATM networks are used to
`transport MPEG-2 transport stream, the end-to-end jitter
`typically shall not be more than 1 millisecond. The use of
`rate converter as traffic shaper will Solve exactly this prob
`lem.
`Within ATM networks data loss may also occur when
`there is not enough channel bandwidth to send all of the
`ATM cells. In this case, ATM cells may be dropped from the
`overflowing buffers. If the bit stream carried in the connec
`tion complies with the connection agreement, Such event
`will occur very infrequently and falls within the bounds of
`the quality of Service constraints.
`65
`Also, it is important to point out that in general com
`pressed Video bit Streams are generated by either real-time
`
`4
`encoderS or pre-compressed Video Server Storage Systems,
`which are likely to be in a remote site, away from the
`network facility itself. This implies that in general it is
`difficult to encode the Video Signal with a resulting bit rate
`profile tailored to the connection bandwidth available from
`the ATM network.
`ATM network protocols are under intense international
`Standardization effort and Several industry wide inter
`operable Specifications have been produced, including the
`one specific on means to carry MPEG-2 transport Streams
`over ATM networks using ATM Adaptation Layer 5 (AAL
`5).
`Therefore, there is a need for a system and method for
`transmitting digital Video information over data channels,
`that is simple to implement and has Smaller delay, and can
`match the input bit rate to the channel bandwidth, resulting
`in a dramatically reduced bit error rate.
`SUMMARY OF THE INVENTION
`The present invention overcomes the deficiencies and
`limitations of the prior art with a system and method for
`converting a bit Stream of a given bit rate to a different bit
`rate for reliable transport over communication channels. In
`various embodiments, the present invention includes: trans
`mission of a compressed bit Stream over ASymmetric Digital
`Subscriber Lines (ADSL) with rate adaptive capabilities
`(RADSL); transmission of a compressed bit stream over
`Asynchronous Transfer Mode (ATM) networks; flexible
`Sharing of bandwidth of common communication channels
`among compressed bit stream and data traffic (including, but
`not limited to, data stream based on the ATM protocols or
`TCP/IP protocols, etc.); and statistical multiplexing of
`MPEG-12 transport streams.
`A preferred embodiment of the system of the present
`invention comprises: a bit rate converter, a transmitter, a
`physical medium, and a receiver. The bit rate converter is
`preferably coupled to receive an input Signal that is a Video
`bit stream. The bit rate converter adjusts the bit rate of the
`input Signal to match the communications channel and then
`outputs the bit stream to the transmitter. The transmitter is in
`turn coupled to the receiver by the physical medium. The
`transmitter sends the bit stream over the medium to the
`receiver, where the bit stream is received. Thus, the System
`of the present invention advantageously eliminates the need
`to decode and encode the bit Stream before transportation
`over the channel. The bit rate converter eliminates the need
`for decompression and compression and preferably com
`prises: a separation unit, a decoder and extractor, a plurality
`of modification units, an encoder and a combining unit. The
`bit rate converter essentially adjusts the bit rate by making
`modifications to the video data portion of the bit stream. The
`bit rate converter first Separates the Video data portion of the
`bit Stream and then decodes and extracts the video data. The
`data is then modified to change the bit rate, and then encoded
`and combined with the other Signals that make up the bit
`Stream.
`A preferred method for transporting data over a commu
`nication channel comprises the Steps of converting a first bit
`rate of an input bit stream to a Second bit rate, transmitting
`the bit Stream at the Second bit rate; and receiving the bit
`Stream at the Second bit rate. The converting a first bit rate
`of an input bit Stream to a Second bit rate preferably
`comprises the Sub-Steps of Separating the bit Stream into
`Video, audio and data portions, decoding the Video portion,
`extracting vectors and coefficients from the Video portion,
`modifying the extracted data, encoding the modified data,
`and combining the encoded Video data with the audio and
`data portions.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1A is a block diagram of a prior art system for
`transmitting Video data over a communication channel;
`FIG. 1B is a block diagram of a prior art system for
`transmitting Video data over a communication channel
`showing the encoding and decoding function in more detail;
`FIG. 2A is a high level block diagram of one embodiment
`of a System constructed according to the present invention
`for transporting video data;
`FIG. 2B is a block diagram of the preferred system for bit
`rate conversion according to the present invention, depicted
`using terms and reference numerals similar to the prior art
`system of FIG. 1B to show the advantages yielded by the
`present invention;
`FIG. 2C is a block diagram of a first and preferred
`embodiment of a System, constructed according to the
`present invention, for transporting video data including a
`communication channel;
`FIG. 3 is a block diagram of a second embodiment of a
`System for transporting video data integrated within the
`communication channel;
`FIG. 4 is a first embodiment of a bit rate conversion
`device according to the present invention;
`FIG. 5 is a second and preferred embodiment of a bit rate
`conversion device according to the present invention;
`FIG. 6 is a block diagram of a System including a plurality
`of bit rate converters for Sending a single Stream of Video
`data over a plurality of respective channels,
`FIG. 7 is a block diagram of a System including a plurality
`of bit rate converters for sharing the bandwidth of a single
`communication channel;
`FIG. 8 is block diagram of a System including a plurality
`of bit rate converters for performing a Statistical multiplex
`ing for use of a Single communication channel;
`FIG. 9 is a block diagram of a third embodiment of a
`System for transporting video data integrated within the
`communication channel, in particular, a rate adaptive asym
`metric digital Subscriber loop;
`FIG. 10 is a graphical representation of the reference
`model used for the ADSL transceiver unit-Central Office and
`the ADSL transceiver unit-Remote terminal;
`FIG. 11 is a block diagram of a System using the rate
`conversion device of the present invention as an input point
`for an asynchronous transfer mode (ATM) network;
`FIG. 12 is a block diagram of a System integrating the rate
`conversion device of the present invention into an asynchro
`nous transfer mode (ATM) switch; and
`FIG. 13 is a block diagram of a System integrating the rate
`conversion device of the present invention into an ATM/
`ADSL communication device.
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`networks or ATM networks. The signal sent over the physi
`cal medium 18 is received by the receiver 20 and input to the
`decoder 22. The decoder 22 restores the bit stream received
`by decompressing it into its original format.
`FIG. 1B shows a block diagram of a prior art system for
`transmitting Video data over a communication channel
`showing the encoding and decoding function in more detail.
`In particular, as shown, the encoding includes receiving raw
`Video data and processing the raw Video data with motion
`compensation 50, transform coding 52, quantization 54, and
`VLC encoding 56 to produce a compressed bit stream. The
`compressed bit Stream can then, because of its reduced size,
`be transmitted over any one of a variety of prior art trans
`portation Systems 58. The decoding proceSS is then applied
`to the compressed bit Stream received from the transporta
`tion system 58 to obtain the original raw video images. The
`decoding includes VLC decoding 60, Dequantization 62,
`inverse transform coding 64, and motion compensation 66,
`all in a conventional manner.
`FIG. 2A is a high level block diagram of one embodiment
`of a System 250 constructed according to the present inven
`tion for transporting video data. The System preferably
`comprises a compression unit 252, a bit rate converter 202,
`a transmission system 204 and a data formatting unit 258.
`The compression unit 252 receives video data and other data
`and produces a compressed bit stream. The compression unit
`may perform any one of a variety of types of compression
`including but not limited to MPEG compression, H.26X or
`H.32X compression for Video conferencing, compression
`using proprietary Video Stream formats, and compression of
`non-real-time data bit streams. Those skilled in the art will
`recognize that the data formatting unit 258 is optional and
`provided only if additional data beyond the video data is
`being transmitted. The data formatting unit 258 may add
`Such additional data by either providing it to the compres
`sion unit 252 as just described or by providing it directly to
`the transmission system 204 as also shown in FIG. 2A. The
`compression unit 252 provides a compressed bit stream to
`the input of the bit rate converter 202. The bit rate converter
`202 advantageously adjust the bit rate to match the band
`width of the transmission system 204. The bit rate converter
`202 in its various embodiments will be described below in
`more detail, however, the bit rate converter 202 can perform
`conversion by adjusting or modifying the encoded bit Stream
`Syntax Such as for the VLC decoding, the de-quantization,
`the inverse transform coding or the motion compensation.
`The output of the bit rate converter 202 is provided to the
`transmission system 204 which formats the data and trans
`mits it over a physical channel (not shown). The transmis
`sion system 204 may be any one of a number of conven
`tional transmission Systems, including but not limited to
`ASDL, ATM/ADSL, ATM, ISDN links, Ethernets, public
`data networks, T1, T3, DS-3, OC-3, wireless/terrestrial
`networks, digital Satellites, and digital cable networks, and
`particular ones are described below.
`FIG.2B is a block diagram of the preferred system for bit
`rate conversion according to the present invention, depicted
`using terms and reference numeral Similar to the prior art
`system of FIG. 1B for ease of understanding and to show the
`advantages yielded by the present invention. In particular,
`the FIG.2B is annotated with arrows to show the advantages
`of the present invention. As shown in FIG. 2B, the bit rate
`converter 204 includes a process of decoding, bit rate
`converting, and encoding in the compressed domain.
`However, based on the modification units (see FIGS. 4 and
`5 below) used in the bit rate converter 204, the bit rate
`conversion process effectively follows one of the paths
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`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`Referring now to FIG. 1A, a prior art system 10 for
`transmitting Video data over a communication channel 19 is
`shown. The system 10 preferably forms a channel 19 using
`an encoder 14, a transmitter 16, a receiver 20, a physical
`medium 18 coupling the transmitter 16 to the receiver and a
`decoder 22. The encoder 14 receives an input bit stream 32
`and compresses the input bit Stream by encoding. The
`compress bit stream is then received by the transmitter 16
`and output over the physical medium 18. The transmitter 16
`can be any one of a variety of those known in the art for DSL
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`US 6,181,711 B1
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`specified by arrows A, B, C or D. Generally, motion com
`pensation is most computationally expensive, transform
`coding and inverse transform coding are also quite expen
`Sive. In general, without Special hardware to perform these
`functions, motion compensation and transform coding will
`take over 80%-90% of the overall decode-encode compu
`tation load. The key to a simplified rate conversion Scheme
`according to the present invention is therefore to bypass
`Some of these expensive steps. For example, in FIG. 2B, if
`we take the path B, motion compensation is avoided. If we
`take path C, both motion compensation and transform cod
`ing are eliminated. If we take path D, quantization Steps are
`also eliminated, in addition to motion compensation and
`transform coding. Of course, if we take path A, the entire
`decoding and encoding processes are performed, resulting in
`the most flexibility and quality potential, at the cost of being
`most expensive. Each of these paths can be specified by the
`use of different modification units, 404, 406, 408, 514, 516,
`518 as will be described below with reference to FIG. 4 and
`FIG. 5.
`While the present invention will now be described in the
`context of an Asymmetric Digital Subscriber Loop (ADSL)
`and ATM networks, those skilled in the art will realize that
`the present invention is applicable to a variety of other types
`of communication channels. Such as any XDSL includes
`ADSL, HDSL, VDSL, SDSL.
`Asymmetric Digital Subscriber Loop, or ADSL, is a
`physical line code modulation Scheme for transmitting digi
`tal bit Stream over a single pair of plain old telephone System
`(POTS) grade twisted copper wires, that are usually noisy
`and are Subject to outside interference. Several Schemes
`have been developed to achieve high channel capacity of the
`said twisted copper wires. xDSL Systems provide simple or
`full duplex raw bit pipes between the Central Office (CO)
`and the remote Site receivers. The material content and the
`format of the bit stream or the protocols used by the
`connections being established by the bit pipe is immaterial
`to the xDSL system itself. In ADSL, the downstream link,
`from CO to remote site, has higher bandwidth than the
`upstream direction. Downstream channel capacity is typi
`cally up to 8 Mbps, and upstream channel capacity is up to
`about 1 Mbps; the actual channel capacity depends on the
`noise level of the wires and the distance between the
`transmitter and the receiver.
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`Because broadcast Video quality can be achieved with
`compressed video, such as MPEG-2, at 3–6 Mbps, ADSL
`provides an ideal delivery Scheme for compressed digital
`Video and other high Speed data connecting COS with remote
`Sites, which can be either consumer Subscribers or busineSS
`establishments.
`However, because of the variation in physical line con
`ditions due to both the physical distance of the connection
`and the noise conditions of the lines, the ADSL transmission
`Schemes do not specify channel capacity in either direction,
`rather the channel capacity is determined at connection Set
`up time via channel initialization and training by the trans
`ceivers at the CO location and at the remote locations, in a
`conventional manner. The initialization and training proceSS
`determines the proper coding configuration best matched to
`the current channel condition in order to achieve the maxi
`mum channel capacity. During the connection, due to
`change in line condition or due to loSS of data, the trans
`ceivers may also re-initialize and retrain to Settle on the new
`coding configuration.
`In all the ADSL figures and discussions for the present
`invention below, it is important to point out that the ADSL
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`channel is bi-directional, even though the present invention
`is described and shown by focusing on a Single direction for
`ease of understanding. The present invention focuses on the
`transmission in a direction from a central office to a cus
`tomer as shown in FIGS. 2C and 9 from 210 to 214. In other
`words, the transmitter device (such as transmitter 906) is
`also a receiver device in the reverse direction, from the
`customer to the central office (from 208 to 206). For video
`over ADSL, the bit rate conversion over ADSL in the
`customer to central office direction appears pedantic but
`conceptually possible, though at a lower maximum bit rate.
`Still more particularly, if transmitter 906 in FIG. 9 is a
`transmitter as part of ATU-C (central office), then it must
`also be a receiver as part of ATU-C (remote). If box 906 is
`treated as part of an ATU-R then the end subscriber is
`Sen