United States Patent
`Ciofii
`Apr. 29, 1997
`[45] Date of Patent:
`
`[11] Patent Number:
`
`5,625,651
`
`[19]
`
`US005625651A
`
`[54] DISCRETE MULTI-TONE DATA
`TRANSMISSION SYSTEM USING AN
`OVERHEAD BUS FOR SYNCHRONIZING
`MULTIPLE REMOTE UNITS
`
`[75]
`
`Inventor:
`
`John M. Ciofli. Cupertino. Calif.
`
`[73] Assignee: Amati Communications, Inc..
`Mountain View. Calif.
`
`[21] Appl. No.2 252,829
`
`[22] Filed:
`
`Jun. 2, 1994
`
`Int. Cl.6 ........................................................ H04L 7/00
`[51]
`[52] US. Cl. .......................... 375/354; 375/222; 375/260;
`375/340; 375/356; 375/362; 370/506; 455/542;
`455/561; 340/825.2
`
`[58] Field of Search .................. 370/71. 124. 185.1.
`370/100.1. 103. 105. 105.1. 105.2. 108.
`118; 375/354. 356—358. 362. 371. 373.
`219—222. 260. 340; 455/51.1. 51.2. 53.1.
`54.1. 56.1. 69. 70. 54.2; 340/825.14. 825.2.
`825.21
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,438511
`4,528,656
`5,043,982
`5,177,739
`5,285,474
`5,440,561
`
`3/1984 Baran ........................................ 370/19
`..... 370/30
`7/1985 Morais .
`
`8/1991 Werner .........
`370/100.1
`
`1/1993 Basnuevo et al.
`.. 370/85.8
`
` 2/1994 Chow et a]. .......... 375/13
`8/1995 Wenonen ............................. 370/105.l
`
`Primary Examiner—Stephen Chin
`Assistant Examiner—~Amanda T. Le
`Attorney, Agent, or Firm—Hickman Beyer & Weaver
`
`[57]
`
`ABSTRACT
`
`A bi-directional data transmission system that facilitates
`communications between a central unit and a plurality of
`remote units using a frame based discrete multi—tone (DMT)
`transmission scheme is disclosed. The discrete multi-tone
`
`data transmission system has a multiplicity of discrete
`subchannels including an overhead bus. In a method aspect
`of the invention, frames transmitted from the plurality of
`remote units are synchronized at the central unit. When a
`selected remote desires to initiate communications. it loop
`times it own clock to the clock of the central unit and
`
`transmits a remote initiated synchronization signal to the
`central unit over a dedicated overhead subchannel in the
`
`overhead bus. The central unit responds with a centrally
`initiated synchronization signal that contains information
`indicative of a frame boundary phase shift required to
`' synchronize the selected first remote unit with other remote
`units that are currently communicating with the central unit.
`The remote responds by shifting the phase of the frames it
`outputs by an amount indicated by the centrally initiated
`synchronization signal. This synchronizes the frame bound—
`aries of the frames outputted by the selected remote unit with
`frame boundaries of frames output by the other remote units
`that are currently communicating with the central unit. The
`synchronization is arranged to occur such that the frame
`boundaries from the various remotes substantially coincide
`when they are received at the central unit. Specific central
`and remote modern designs suitable for implementing such
`a system are also described.
`
`15 Claims, 7 Drawing Sheets
`
`OBSERVE DOWNSTREAM
`TRANSMISSIONS THAT
`
`
`INI-IEREN'ILY CONTAIN
`CENTRAL MODEM CLOCK
`INFORMATION
`
`
`
`PHASE LOCK
`
`REMOTE UNIT—CLOCK
`WITH CENTRAL MODEM CLOCK
`
`
`
`
`310
`
`SEND SYNCHRONIZATION
`
`SIGNAL FROM REMOTE UNIT
`
`T0 CENTRAL UNIT
`
`320
`
`330
`
`340
`
`
`
`DETERMJNE FRAME BOUNDARY
`PHASE SHIFT RE UIRED
`
`
`T0 BETTER SYN RONIZE
`REMOTE UNIT WITH OTHER
`REMOTE UNITS
`
`
`SEND RETURN SYNCHRONIZATION SIGNAL
`FROM CENTRAL UNIT TO REMOTE
`
`
`UNIT INDICATING SYNCHRONIZATION OR
`REQUESTING A PHASE SHIFT
`370
`
`
`
`ADJUST
`YES
`
`
`PHASE OF
`SYNCHRONIZED
`
`FRAME
`
`
`
`
`350
`
`Z7
`
`360
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 1
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 1
`
`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 1 of 7
`
`5,625,651
`
`'2!
`
`gt:a I
`
`REMEVICE
`
`REMOTEDEVICE
`
`22
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`18
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`18
`
`E
`25
`
`MODEM
`
`17
`
`CENTRALMODEM
`
`10
`
` 8%?
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 2
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`Petitioner Cisco Systems - Exhibit 1008 - Page 2
`
`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 2 of 7
`
`5,625,651
`
`
`
`
`
`
`
`0
`
`.
`
`1.0
`
`33
`
`FREQUENCY (MHz)
`
`figure 2
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 3
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`Petitioner Cisco Systems - Exhibit 1008 - Page 3
`
`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 3 of 7
`
`5,625,651
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`Petitioner Cisco Systems - Exhibit 1008 - Page 4
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`Petitioner Cisco Systems - Exhibit 1008 - Page 4
`
`
`
`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 4 of 7
`
`5,625,651
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`Petitioner Cisco Systems - Exhibit 1008 - Page 5
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`Petitioner Cisco Systems - Exhibit 1008 - Page 5
`
`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 5 0f 7
`
`5,625,651
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`8
`
`140
`
`147figure5
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 6
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`Petitioner Cisco Systems - Exhibit 1008 - Page 6
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`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 6 of 7
`
`5,625,651
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`phase offset
`
`
`
`figure 6
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 7
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`Petitioner Cisco Systems - Exhibit 1008 - Page 7
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`

`

`US. Patent
`
`Apr. 29, 1997
`
`Sheet 7 of 7
`
`5,625,651
`
`
`
`
`
`
`
`
`
`300
`
`
`
`OBSERVE DOWNSTREAM
`TRANSMISSIONS THAT
`INHERENTLY CONTAIN
`CENTRAL MODEM CLOCK
`INFORMATION
`
`
`
`PHASE LOCK
`REMOTE UNI'IZCLOCK
`WITH CENTRAL MODEM CLOCK
`
`310
`
`
`
`SEND SYNCHRONIZATION
`SIGNAL FROM REMOTE UNIT
`TO CENTRAL UNIT
`
`320
`
`DETERMINE FRAME BOUNDARY
`PHASE SHIFT RE UIRED
`TO BETTER SYN
`ONIZE
`REMOTE UNIT WITH OTHER
`REMOTE UNITS
`
`330
`
`
`
`
`
`
`
`
`
`
`340
`
`SEND RETURN SYNCHRONIZA'HON SIGNAL
`FROM CENTRAL UNIT TO REMOTE
`UNIT INDICATING SYNCHRONIZATION OR
`REQUESTlNG A PHASE SHIFT
`
`370
`
`
`
`
`ADJUST
`YES
`
`?
`SYNCHRONIZED
`
`
`
`
`
`PHASE OF
`
`FRAME
`
`
`
`360
`
`350
`
`FIG. 7
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 8
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 8
`
`

`

`1
`DISCRETE MULTI-TONE DATA
`TRANSMISSION SYSTEM USING AN
`OVERHEAD BUS FOR SYNCHRONIZING
`MULTIPLE REMOTE UNITS
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates generally to systems for the
`transmission and reception of high speed data signals
`between a central station and a plurality of remote units
`using a discrete multi-tone (DMT) multi—carrier approach.
`More particularly, the use of a dedicated overhead bus for
`synchronizing frames transmitted from the various remote
`units is described.
`
`At the time of this writing. the Alliance For Telecommu-
`nications Information Solutions (ATIS). which is a group
`accredited by the ANSI (American National Standard
`Institute) Standard Group. is nearing finalization of a stan-
`dard for the transmission of digital data over Asymmetric
`Digital Subscriber Lines (ADSL). The standard is intended
`primarily for transmitting video data over ordinary tele-
`phone lines. although it may be used in a variety of other
`applications as well. The standard is based on a discrete
`multi—tone transmission system. The pending North Ameri-
`can Standard is referred to as the T1E1.4 ATIS Standard, and
`is presently set forth in Stande Contribution No. 94-007.
`rev. 2. dated April of 1994. which is incorporated herein in
`its entirety. Transmission rates are intended to facilitate the
`transmission of information at rates of at least 6 million hits
`per second (i.e., 6+Mbps) over ordinary phones lines.
`including twisted-pair phone lines. The standardized dis-
`crete multi-tone (DMT) system uses 256 “tones” that are
`each 4.3125 kHz wide in the forward (downstream) direc—
`tion. That is. in the context of a phone system. from the
`central office (typically owned by the telephone company) to
`a remote location that may be an end-user (i.e.. a residence
`or business user).
`The Asymmetric Digital Subscriber Lines standard also
`contemplates the use of a duplexed reverse signal at a data
`rate of 16—800 Kbps. That is, transmission in an upstream
`direction. as for example. from the remote location to the
`central oflice. Thus. the term Asymmetric Digital Subscriber
`Line comes from the fact that the data transmission rate is
`substantially higher in the forward direction than in the
`reverse direction. This is particularly useful in systems that
`are intended to transmit video programming or video con-
`ferencing information to a remote location over the tele-
`phone lines. By way of example. one potential use for the
`systems allows residential customers to obtain videos infor-
`mation such as movies over the telephone lines rather than
`having to rent video cassettes. Another potential use is in
`video conferencing.
`The discrete multi—tone (DMT) transmission scheme has
`the potential for use in applications well beyond data trans-
`missions over telephone lines. Indeed. DMT can be used in
`a variety of other digital subscriber access systems as well.
`For example. it may be used in cable based subscriber
`systems (which typically use coaxial cable) and wireless
`subscriber systems such as digital cellular TV. In cable
`systems. a single central unit (central modem) is typically
`used to distribute digital signals to more than one customer
`which means more than one remote unit (remote modem).
`While all of the remote modems can reliably receive the
`same digital signals. the upstream transmissions must be
`coordinated to prevent confusion at the central modem as to
`the source of the upstream signals. Presently.
`in cable
`systems (which do not use discrete multi-tone transmission
`
`10
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`5,625,651
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`2
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`schemes). each remote unit is given a dedicated frequency
`band over which it is to communicate with the central
`station. However. such an approach is inherently an ineffi—
`cient use of transmission bandwidth and typically requires
`the use of analog filters to separate transmissions from the
`various remote units. Stationary digital cellular transmission
`systems face similar obstacles.
`ADSL applications have the potential for a similar
`problem. although it is typically more limited in nature.
`Specifically. a single line may service a plurality of drop
`points at a particular billing address (which may typically be
`a home or an oflice). That is. there may be several telephone
`“jacks” through which the user may wish to receive signals.
`To facilitate service to multiple locations (jacks) over a
`single line. the use of a master modem has been proposed to
`facilitate synchronization. However.
`this is perceived as
`being a relatively expensive and undesirable solution.
`Accordingly. it would be desirable to provide a mechanism
`in discrete multi—tone data transmission systems which
`facilitates the synchronization of signals from a plurality of
`remotes so that a central unit can coordinate and reliably
`interpret signals sent from the remotes.
`SUMMARY OF THE INVENTION
`
`To achieve the foregoing and other objects and in accor—
`dance With the purpose of the present
`invention. a
`bi—directional data transmission system that facilitates com—
`munications between a central unit and a plurality of remote
`units using a frame based discrete multi—tone (DMT) trans-
`mission scheme is disclosed. The discrete multi—tone data
`transmission system has a multiplicity of discrete subchan-
`nels including an overhead bus.
`In one aspect of the
`invention. frames transmitted from the plurality of remote
`units are synchronized at the central unit. When a selected
`remote desires to initiate communications. it loop times its
`own clock with the clock of the central unit and then
`transmits a remote initiated synchronization signal to the
`central unit over a dedicated overhead subchannel in the
`
`overhead bus. The central unit responds with a centrally
`initiated synchronization signal that contains information
`indicative of a frame boundary phase shift required to better
`synchronize the selected first remote unit with other remote
`units that are currently communicating with the central unit.
`The remote responds by shifting the phase of the frames it
`outputs as indicated by the centrally initiated synchroniza-
`tion signal. The synchronization may be done in either an
`iterative manner or as a single step. This synchronizes the
`frame boundaries of the frames outputted by the selected
`remote unit with frame boundaries of frames output by the
`other remote units that are currently communicating with the
`central unit. The synchronization is arranged to occur such
`that the frame boundaries from the various remotes substan-
`tially coincide when they are received at the central unit.
`In one embodiment of the invention the overhead bus
`includes two dedicated overhead subchannels and the
`remote initiated synchronization signal and the centrally
`initiated synchronization signal are transmitted over diifer-
`ent overhead subchannels. In other embodiments a single or
`multiple dedicated overhead subchannels may be used. The
`invention has application in a wide variety of data transmis—
`sion schemes including Asymmetric Digital Subscriber Line
`systems that includes the transmission of signals over
`twisted pair. fiber and/or hybrid telephone lines. cable sys-
`tems that includes the transmission of signals over a coaxial
`cable. and digital cellular television systems that include the
`transmission of radio signals.
`In some embodiments. the number of subchannels avail-
`able to the selected remote unit for transmission of data to
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 9
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`Petitioner Cisco Systems - Exhibit 1008 - Page 9
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`

`

`5 ,625.651
`
`3
`the central unit are dynamically allocated. Specific central
`and remote modern designs suitable for implementing such
`a system are also described.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention. together with further objects and advan-
`tages thereof. may best be understood by reference to the
`following description taken in conjunction with the accom-
`panying drawings in which:
`FIG. 1 is a block diagram of a data transmission scheme
`that uses a single transmission media to facilitate commu-
`nications between a central station and a plurality of remote
`units.
`
`FIG. 2 is a graph illustrating a multi-tone transmission
`band that includes a pair of dedicated overhead subchannels.
`FIG. 3 is a block diagram of a central office modem
`architecture suitable for implementing the synchronization
`of the present invention.
`FIG. 4 is a block diagram of a remote unit modem
`architecture suitable for implementing the synchronization
`of the present invention.
`FIG. 5 is a block diagram illustrating a remote unit
`synchronization arrangement suitable for implementing syn-
`chronization and upstream symbol alignment.
`FIG. 6 is a graph illustrating phase error versus frequency.
`The slope is proportional
`to the timing error and the
`y—intercept is proportional to phase error of the carrier.
`FIG. 7 is a flow chart illustrating the synchronization of
`a remote unit.
`
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`DETAILED DESCRIPTION OF THE
`INVENTION
`
`4
`As described in the background section of this
`application. one limitation of the discrete multi-tone trans-
`mission system is that in order to support a plurality of drop
`points serviced by a single line (such as occurs with the
`phone lines in many homes and ofiices). a master unit has
`generally been considered necessary to synchronize all of
`the units at that sight. This synchronization problem has
`limited the attractiveness of Discrete Multi-tone data trans-
`
`mission schemes in certain other applications such as cable
`systems and wireless cellular television delivery applica-
`tions since these systems use a single line to service a
`relatively large number of independent remote units. which
`would typically be owned by different subscribers.
`Referring initially to FIG. 1. a schematic transmission
`scheme for a typical multi—user subscriber network will be
`described. A central unit 10 (which includes a central
`modem) communicates with a plurality of remote units over
`a common transmission line 17 which is split into a plurality
`of feeds 18. Each feed 18 services an associated remote unit
`which typically includes a remote modem 15 which receives
`the signals and a remote device 22 which uses the data. A
`service provider 19 would typically be arranged to provide
`the data to the central modem for transmission to the remote
`
`modems 15 and to handle the data received by the central
`modena from the remote modems. The service provider 19
`can take any suitable form. By way of example. the service
`provider can take the form of a network server. The network
`server can take the form of a dedicated computer or a
`distributed system. A variety of transmission mediums can
`be used as the transmission line. By way of example twisted
`pair phone lines. coaxial cables. fiber lines and hybrids that
`incorporate two or more diflerent mediums all work well.
`This approach also works well in wireless systems.
`As will be appreciated by those skilled in the art. one
`requirement of discrete multi-tone data transmission sys-
`tems such as those contemplated herein is that if two or more
`units (typically two remote units) are attempting to inde-
`pendently transmit information to a third unit (i.e. the central
`unit). the signals from the remote units must by synchro-
`nized or at least some of the signals will be incomprehen-
`sible to the central unit. The problem with using discrete
`multi-tone transmissions in such a system is that the length
`of the feeds 18 will typically vary from remote to remote.
`Therefore. even if the remotes synchronize with the clock of
`the central unit, their communications back to the central
`unit will be phase shifted by an amount that is dependent at
`least in part on the length of the associated feed. In practice.
`these types of phase shifts can make remotely initiated
`communications unintelligible to the central modem.
`DMT transmission inherently partitions a transmission
`media into a number of subchannels that each carry data
`independently. The data on each subchannel can correspond
`to different signals or can be aggregated into higher data
`rates that represent a single or fewer wider-bandwidth trans-
`missions. These subchannels are implemented entirely with
`digital signal processing in DMT. which eliminates the need
`for analog separation filters and maximizes spectral effi-
`ciency. However. the inherent multiplexing nature of DMT
`was previously restricted to point-to-point
`transmission
`because the dilferent transmissions must be synchronized for
`the all-digital multiplexing to function properly. The present
`invention provides a novel arrangement and method for
`synchronizing a plurality of remote units. to facilitate multi—
`point-to-point transmission.
`A representative DMI‘ transmission band is illustrated in
`FIG. 2. As seen therein. the transmission band includes a
`multiplicity of subchannels 25 over which independent
`
`Discrete Multi—Tone (DMT) data transmission schemes
`have been shown to facilitate high performance data trans—
`mission. Among the benefits of DMT architectures is that
`they have high spectral efliciencies and can adaptively avoid
`various signal distortion and noise problems. Since they
`have very high data transmission capabilities. in most appli-
`cations selection of a DMI‘ data transmission scheme will
`provide plenty of room for the expansion of service as the
`demands on the data transmission system increase. Discrete
`Multi—tone technology has applications in a variety of data
`transmission environments. For example. the presently pro-
`posed ATIS Asymmetric Digital Subscriber Line North ~
`American standard contemplates use of a Discrete Multi-
`Tone data transmission scheme.
`
`35
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`A detailed description of the protocols for ATIS Asym—
`metric Digital Subscriber Line Noah American standard
`Discrete Multi-Tone transmission scheme is described in
`detail in the above referenced ATIS contribution. which is
`incorporated herein by reference in its entirety. The stan-
`dardized discrete multi-tone (DMT) system in North
`America uses 256 “tones” which are each 4.3125 kHz wide
`in the forward (downstream) direction. The frequency range
`of the tones is from zero to 1.104 MHz. The lower 32 tones
`may also be used for duplexed data transmission in the
`upstream direction. An improvement in this system which
`contemplates doubling of the transmission bandwidth is
`described in my co—pending US. application Ser. No.
`08/227,778. now US. Pat. No. 5.519.731. which is also
`incorporated herein by reference. In other systems. the
`number of subchannels used may be widely varied. How-
`ever when IFFI‘ modulation is done, typical values for the
`number of available subchannels are power of two. as for
`example. 128. 256. 512. 1024 or 2048 subchannels.
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`Petitioner Cisco Systems - Exhibit 1008 - Page 10
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`Petitioner Cisco Systems - Exhibit 1008 - Page 10
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`

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`5,625,651
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`5
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`carrier signals (referred to as subcarriers 27) may be trans—
`mitted. To address the synchronization problems pointed out
`above.
`the applicant has proposed the use of dedicated
`overhead subchannels 33 and 34 to facilitate synchroniza-
`tion. Upstream overhead subchannel 33 carries synchroni—
`zation signals from the various remotes to the central
`modem. Downstream overhead subchanne134 carries syn-
`chronization signals from the central modem to the various
`remotes. The overhead subchannels 33 and 34 may be
`located at any suitable frequency position within the trans-
`mission band. In many embodiments such as the asymmetric
`digital subscriber line system discussed above. it may be
`desirable to locate the overhead subchannels near either the
`upper or lower frequency edge of the downstream signal so
`as to minimize their interference with adjoining subchan-
`nels. When the system constraints permit. it may be further
`desirable to separate the overhead subchannels from other
`subchannels used for data transmission by at least one or two
`subchannels in order to minimize potential interference
`caused by the synchronization signals. This is desirable
`since the synchronization signals will often be unsynchro-
`nized with other transmissions. Therefore. they will cause
`more distortion than other signals due to being out of synch.
`Accordingly. a small buffer is helpful. Along the same lines.
`it may also be desirable to use relatively low powered
`signals as the overhead subcarriers to further minimize
`interference issues in some cases.
`
`Referring next to FIG. 3. a central office architecture
`suitable for implementing the synchronization of the present
`invention will be described. The central unit in the illustrated
`embodiment includes a central modem 30. a network server
`19. and a network interface 41. The central modem includes
`a transmitter 40. a receiver 70. and a controller 60. The
`controller 60 is used to synchronize the clocks of the remote
`modems with the clock in the central modem. as well as
`synchronize frames transmitted from the remote modems.
`The network server 19 provides digital data to the transmit-
`ter 40 through an asynchronous transfer modem switch 41
`(labeled network interface in the drawings). The network
`server 19 can provide data at any data rate up to the
`maximum data rate permitted in View of the transmitter’s
`capability. the transmission distance. the transmission line
`quality and the type of communications line used. The
`transmitter 40 incorporates several components including an
`encoder 43. a discrete multi—tone modulator 45 and a win-
`dowing filter 46. The encoder 43 serves to multiplex. syn-
`chronize and encode the data to be transferred (such as video
`data). More specifically. it translates incoming bit streams
`into in phase. in quadrature components for each of a
`multiplicity of subchannels. The encoding may be done
`using forward error correction and/or trellis coding. The
`encoder would typically be arranged to output a number of
`subsymbol sequences that are equal to the number of sub-
`channels available to the system. By way of example. in a
`system having 256 subchannels. the encoder would output
`256 subsymbol sequences. In the above-referenced ATIS
`standard. the subsymbol sequences would each represent 4
`Kbps. These inputs are complex inputs that are passed to a
`discrete multi—tone modulator 45. By way of example. a
`suitable encoder is described in detail in the referencedATIS
`standard.
`
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`The modulator 45 is an IFFI‘ modulator that computes the
`inverse Fourier transform by any suitable algorithm. Since
`the encoder outputs are complex numbers. the EFF modu-
`lator receives twice as many inputs as there are subchannels
`available. The bit distribution is determined adaptively in
`discrete multi-tone systems as described in both the refer-
`
`65
`
`6
`enced ATIS stande and Peter S. Chow et aL’s co—pending
`application serial number 08/057,301. now US. Pat. No.
`5.479.447. To facilitate this. the transmitter 40 also includes
`a line monitor that monitors the communication line to
`determine the line quality of each of the available subchan-
`nels. In one embodiment. the line monitor 64 (which may be
`part of the controller 60) determines the noise level. single
`gain and phase shift on each of the subchannels. The object
`is to estimate the signal—to-noise ratio for each of the
`subchannels. Therefore. other parameters could be moni-
`tored as well or in place of the parameters described The
`determination of which subchannels to transmit the encoded
`data over as well as how much data to transmit over each
`subchannel is dynamically determined on the basis of sev—
`eral factors. The factors include the detected line quality
`parameters. subchannel gain parameters. a permissible
`power mask. and the desired maximum subcarricr bit-error
`rates. It is noted that the various factors need not be constant
`between subchannels and indeed may even vary during use.
`Most notably. the line quality parameters are continually
`checked and adjustments in the modulation scheme are
`made in real time to dynamically adjust the modulatidn as
`the line quality over various subchannels changes during
`use. By way of example. a suitable discrete multi—tone
`modulator is described in the same ATIS standard document.
`
`After the encoded signal has been modulated to form a
`discrete multi-tone signal. a cyclic prefix is appended to the
`discrete multi—tone encoded signal. The cyclic prefix is used
`primarily to simplify the demodulation of the discrete multi—
`tone signals. In the ATIS standard. a 32-bit cyclic prefix is
`used. However. in systems that utilize larger bandwidths. it
`would be preferable to increase the length of the cyclic
`prefix as well. The modulated signal is then passed through
`a windowing filter 46 and/or other filters in order to mini-
`mize the out of band energy. This is desirable to help prevent
`the analog interfaces in the remote receivers from saturating.
`The windowng can be accomplished by a wide variety of
`conventional windowing protocols. The transmitter also
`includes an analog interface 48 which applies the discrete
`multi—tone signal to the transmission media. In hardwired
`systems such as twisted pair phone lines and coaxial cables,
`the analog interface may take the form of a line driver.
`The central modem 30 also includes a receiver 70 for
`receiving multi—tone signals from the remote units. The
`receiver 70 includes an analog interface 72. a windowing
`filter 74, a demodulator 76, and a decoder 78. Signals
`received by the central modem 30 are initially received
`through the analog interface 72. The windowing filter 74 is
`arranged to effectively perform windowing and/or filtering
`functions on the received signal. One suitable filter arrange-
`ment is a time domain equalizer 74. Again. the windowing
`can be accomplished by a wide variety of conventional
`windowing protocols. The demodulator 76 demodulates the
`equalized discrete multi-tone signal and strips the cyclic
`prefix. The decoder 78 decodes the demodulated signal. The
`demodulator 76 and the decoder 78 effectively perform
`inverse functions of the modulator 45 and encoder 43.
`respectively. The decoded signal is then passed from the
`decoder 78 to the networks server 19 or other appropriate
`user of the information through the interface 41. The func-
`tions of the time domain equalizer 74, the demodulator 76
`and the decoder 78. as well as algorithms suitable for
`accomplishing the desired functions are all described in
`more detail in Chow et al.’s US. Pat. No. 5.285.474 which
`are incorporated herein by reference.
`Referring next to FIG. 4. a remote unit architecture
`suitable for implementing the synchronization of the present
`
`Petitioner Cisco Systems - Exhibit 1008 - Page 11
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`Petitioner Cisco Systems - Exhibit 1008 - Page 11
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`

`

`5,625,651
`
`7
`invention will be described. In many respects the remote
`modena will be sirnilar‘to the central modem although its
`respective upstream and downstream communications
`capacities may be somewhat different. A signal transmitted
`by the central modem 30 is received by a remote unit 50
`through an analog interface 172. The remote unit 50 includes
`the analog interface 172. a time domain equalizer (TEQ)
`174. a demodulator 176 that demodulates the equalized
`discrete multi—tone signal and strips the cyclic prefix. and a
`decoder 178 that decodes the demodulated signal. The time
`domain equalizer 174 effectively performs a filtering func-
`tions on the received signal. A windowing filter may also be
`employed. The demodulator 176 and the decoder 178 per-
`form inverse functions of the modulator 45 and encoder 43.
`respectively. The decoded signal is then passed from the
`decoder 178 to a remote device 22 such as a television. a
`computer. or other suitable receiving apparatus. The func—
`tion of the time domain equalizer 174. the demodulator 176
`and the decoder 178, are similar to the functions of the
`corresponding components in the central modem. A notch
`filter 185 may optionally be provided at a location upstream
`of the receiver’s analog interface 172 in order to block
`energy in frequency bands outside of the subchannels that
`are of interest to the remote unit. This can help prevent the
`analog filter from saturating. By providing a notch analog or
`other suitable filtering mechanism for filtering out of band
`energy. lower cost receiver components can be used since it
`is not necessary for the receiver itself to handle as much
`energy.
`The upstream encoding and modulation may be done in
`exactly the same manner as the downstream data transmis—
`sion described above in the discussion of the central modem
`unit. Thus.
`the remote modem 50 will also include an
`encoder 143. a multi—tone modulator 145, a window or filter
`146. and an analog interface 148. It also requires a frame
`synchronizer 147 to time delay the multi—tone signals an
`amount suitable to synchronize the remote modem 50 with
`other remotes that are currently in communication with the
`central modem as will be described in more detail below. In
`subscriber type applications, a smaller number of subchan-
`nels are typically made available to facilitate upstream
`communications. However. it should be appreciated that any
`number of subchannels could be made available for such
`upstream communications.
`The remote modem 50 also includes a remote synchro-
`nization controller 80 that cooperates with the central con-
`troller 60 in the central modem unit. As briefly discussed
`above. in the described embodiment. two auxiliary overhead
`subchannels are provided to facilitate communications
`between the controllers. When the remote modem 50 is
`initialized and desires to come on stream. its remote con-
`troller 80 observes downstream signal transmissions that
`inherently contain the central modern clock information.
`This is sometimes done by employing pilot signals although
`other schemes can be employed as well. The remote modem
`is then “loop-timed”. That is, it phase locks its own clock
`with the clock of the central modem. The remote controller
`then sends a synchronization signal to the central unit 30 via
`overhead subchannel 33. The synchronization signal passes
`through the transmission media into the receiver portion of
`central modem unit 30. When the central modem 30 receives
`a remotely initiated (upstream) synchronization signal while
`it is currently in communication with other remote units. it
`compares the frame boundaries of the remotely initiated
`synchronization signal with the frame boundaries of signals
`being received from other remote units. Typically. there
`would be a phase shift between the frame boundaries that is
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`detected by the controller 60. The controller 60 then gener-
`ates a downstream synchronization signal that is transmitted
`back to the remote units via overhead subchannel 34.
`In the embodiment described and shown, the controller 80
`is responsible for generating the upstream synchronization
`signal when the remote modem desires to initiate commu—
`nications with the central modem. The upstream syn