(12) United States Patent
`Zhang et al.
`
`US006181711B1
`US 6,181,711 B1
`Jan. 30, 2001
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`(54)
`
`SYSTEM AND METHOD FOR
`TRANSPORTINGA COMPRESSED VIDEO
`AND DATA BIT STREAM OVER A
`
`OTHER PUBLICATIONS
`P.
`f M .
`T h 1
`G . C d.
`,
`I f
`n ormafiion ec. nodogy; ener1c 0 mg 0
`ovmg 1c
`an ASSOClate All 10,
`NOV. 13,
`
`(75)
`
`Inventors, Ji Zhang, San Jose; Wen H_ Chen,
`Sunnyvale; Fang Wu, San Jose, an of
`CA (Us)
`
`(73)
`(*)
`
`(21)
`(22)
`
`(60)
`
`(51)
`(52)
`(58)
`
`(56)
`
`Assignee; Cisco Systems, Inc” San Jose, CA (Us)
`
`Notice:
`
`Under 35 U.S.C. 154(b), the term of this
`Pawnt Shall be extended for 0 days-
`
`Appl. No.: 08/947,480
`Filed:
`Oct. 10, 1997
`
`Related US Application Data
`Provisional application NO_ 60/051,109’ ?led on Jun 26’
`1997,
`
`7
`Int. Cl. ...................................................... .. H'04J 3/ 16
`US. Cl. ........................................... .. 370/468, 370/538
`Fleld of Search ................................... .. 370/465, 468,
`370/545> 522> 537> 538> 203> 210> 484
`_
`References Clted
`Us PATENT DOCUMENTS
`
`574937456
`
`2/1996 Augenbraum ct a1~ ~~~~~~~~~~~~~~ ~~ 360/64
`
`' ' ' ' "3336;
`?oonb """"" ' ' ' ' ' ' ' '
`* 12/
`/
`ang erg et a '
`/
`’
`5’
`348/405
`,754,235 * 5/1998 Urano et al. .... ..
`709/233
`5,812,786 * 9/1998 SeaZholtZ et al.
`6,026,097 * 2/2000 Voois et al. ........................ .. 370/468
`
`“Netvvork and Customer Installation Interfaces'—Asymmet’-’
`r1c Digital Subscriber Lme (ADSL) Metallic Interface,
`American National Standards Institute, ANSI T1. 413—1995.
`The ATM Forum Technical Committee User—NetWork Inter
`face Speci?cation, Version 3.1 (UNI 3.1), Aug. 18, 1995.
`
`*
`
`.
`
`.
`
`cued by examlner
`Primary Examiner—Joseph L. Felber
`(74) Attorney, Agent, or Firm—Beyer Weaver & Thomas,
`LLP
`(57)
`
`ABSTRACT
`
`Digitally compressed video/‘audio bit streams, When trans
`IIIIIICd'OVCI‘ 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 su?icient Channel bandwidth‘ This
`invention describes Schemes to ensure 10551655 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
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`

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`US 6,181,711 B1
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`1
`SYSTEM AND METHOD FOR
`TRANSPORTING A COMPRESSED VIDEO
`AND DATA BIT STREAM OVER A
`COMMUNICATION CHANNEL
`
`This application claims bene?t of provisional application
`Ser. No. 06/051,109, ?led 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 de?ned 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 ?nally, 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 ef?cient
`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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`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
`?cients 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. Speci?cally, 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 or bit 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 How or over?oW, 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.
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`US 6,181,711 B1
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`3
`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-speci?ed quality of service.
`Unlike the xDSL standard, ATM protocols speci?es hoW
`data is ?rst packetiZed into ?xed siZed data units, called
`cells. It also speci?es 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 ?rst con
`verted into ?xed 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 pro?le 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 pro?le must be modi?ed, 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.
`Speci?c decoded video quality depends on the decoder
`design.
`The process of modifying the bit rate pro?le of the input
`bit stream is called traf?c shaping. The objective of the
`traf?c shaping is to smooth out the burstiness of the ATM
`cell stream so that the resulting bit rate pro?le is more
`constant than that Without traf?c shaping. Existing methods
`to do traf?c shaping include using a large buffer to smooth
`out the bit rate ?uctuation, 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 traf?c 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
`over?oWing 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.
`Also, it is important to point out that in general com
`pressed video bit streams are generated by either real-time
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`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
`dif?cult to encode the video signal With a resulting bit rate
`pro?le 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 speci?cations have been produced, including the
`one speci?c 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 de?ciencies 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; ?exible
`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 modi?cation units, an encoder and a combining unit. The
`bit rate converter essentially adjusts the bit rate by making
`modi?cations to the video data portion of the bit stream. The
`bit rate converter ?rst separates the video data portion of the
`bit stream and then decodes and extracts the video data. The
`data is then modi?ed 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 ?rst 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 ?rst 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 modi?ed data,
`and combining the encoded video data With the audio and
`data portions.
`
`Dish
`Exhibit 1053, Page 19
`
`

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`US 6,181,711 B1
`
`5
`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 ?rst 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 ?rst 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 Of?ce 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.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`1O
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`15
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`25
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`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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`65
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`6
`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 modi?cation 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
`
`Dish
`Exhibit 1053, Page 20
`
`

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`US 6,181,711 B1
`
`7
`specified by arrows A, B, C or D. Generally, motion com-
`pensation is n1ost 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 8()‘7n’9()% of the overall decode-encode compil-
`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 wfll 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 reali7e that
`the present invention is applicable to a variety of other types
`of communication channels such as any xDSL includes
`ADSL, HDSL, VDSL, SDSL.
`is a
`Asymmetric Digital Subscriber Loop, or ADSL,
`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 C0 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.
`
`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 C0 location and at the remote locations, in a
`conventional manner. The initialization a11d training process
`determines the proper coding configuration best matched to
`the current cha