United States Patent [19J
`Yamano et al.
`
`[54]
`
`[75]
`
`METHOD AND APPARATUS FOR
`REDUCING SIGNAL PROCESSING
`REQUIREMENTS FOR TRANSMITTING
`PACKET-BASED DATA WITH A MODEM
`
`Inventors: Larry C. Yamano, Sunnyvale; John T.
`Holloway, Woodside; Edward H.
`Frank, Portola Valley; Tracy D.
`Mallory, Palo Alto; Alan G. Corry,
`Santa Clara; Craig S. Forrest; Kevin
`H. Peterson, both of San Francisco;
`Timothy B. Robinson, Boulder Creek;
`Dane Snow, Santa Clara, all of Calif.
`
`[73]
`
`Assignee: Broadcom HomeNetworking, Inc.,
`Sunnyvale, Calif.
`
`[21]
`
`Appl. No.: 08/853,683
`
`[22]
`
`Filed:
`
`May 9, 1997
`
`Int. Cl? ....................................................... H04B l/38
`[51]
`[52] U.S. Cl. ............................................. 375/222; 455/574
`[58] Field of Search ..................................... 375/222, 223;
`370/311, 318; 455/557, 574, 343
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,234,952 11/1980 Gable et a!. .
`4,680,773
`7/1987 Amundson .
`9/1987 Bergins eta!. .
`4,691,314
`4,756,007
`7/1988 Qureshi eta!. .
`4,856,030
`8/1989 Batzer et a!. .
`9/1989 Kaku et a!. .
`4,868,850
`5,463,661 10/1995 Moran, III et a!. ..................... 375/222
`2/1996 Cornelius eta!. ...................... 375/222
`5,491,721
`8/1996 Garcia-duarte et a!. ................ 364/707
`5,544,082
`5,625,651
`4/1997 Cioffi ...................................... 375/354
`6/1997 Perlman .................................. 370/281
`5,636,209
`5,745,860
`4/1998 Kallin ...................................... 455!574
`
`FOREIGN PATENT DOCUMENTS
`
`WO 86/03642
`91 07038
`
`6/1986 WIPO .
`5/1991 WIPO ............................ H04L 29/06
`
`OTHER PUBLICATIONS
`
`R. Aber: "XDSL Supersharges Copper. DSL schemes prom(cid:173)
`ise multimegabit rates over local phone lines-and carriers
`and vendors are nearly ready to deliver"Data Communica-
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006075814A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,075,814
`Jun.13,2000
`
`tions, vol. 26, No. 3, Mar. 1997, pp. 99-100, 102, 104/105
`XP000659545.
`
`Alvarez et al.: "Data-Pump Implementation for Automatic
`Interworking Between Auto mode Modems and other CCITT
`& Bell Modems" Signal Processing Theories and Applica(cid:173)
`tions,Brussels,Aug. 24-27, 1992, vol. 3, No. CONF. 6,Aug.
`24, 1992, pp. 1645-1648, XP000356561.
`
`F. Gao: "DSP Algorithms and Software for Modem, Fax, and
`Telephony" Electronic Design, vol. 44, No. 11, Nay 28, 1996,
`pp. 123/124, 126 XP000623737.
`
`"Digital Signal Processor Modem for Multiple telephone
`Lines" IBM Technical Disclosure Bulletin, vol. 39, No. 4,
`Apr. 1, 1996, pp. 263/264 XP000587492.
`
`Primary Examiner-Don N. Vo
`Attorney, Agent, or Firm-Christie, Parker & Hale, LLP
`
`[57]
`
`ABSTRACT
`
`A modem and method for operating same. A receiver circuit
`of the modem is coupled to receive a continuous analog
`signal from a communication channel. This analog signal
`includes both packet and idle information. The receiver
`circuit monitors the analog signal to detect the presence of
`idle information. Upon detecting idle information, the
`receiver circuit enters a standby mode in which the process(cid:173)
`ing requirements of the receiver circuit are reduced. A burst
`mode protocol is also provided, in which packets of digital
`information are modulated by a transmitter circuit of the
`modem, thereby converting the packets of digital informa(cid:173)
`tion into analog signal bursts of discrete duration. These
`analog signal bursts are transmitted from the transmitter
`circuit to a telephone line. However, the transmitter circuit
`does not generate any signals between the analog signal
`bursts. A receiver circuit monitors the telephone line to
`detect the analog signal bursts. Upon detecting the presence
`of the analog signal bursts on the telephone line, the receiver
`circuit demodulates the analog signal bursts using full
`processing capabilities of the receiver circuit. However,
`upon detecting the absence of the analog signal bursts on the
`telephone line, the demodulating function of the receiver
`circuit is disabled. The burst mode protocol enables multi(cid:173)
`drop and multi-cast operation, as well as reducing required
`DSP resources.
`
`12 Claims, 9 Drawing Sheets
`
`CSCO-1006
`Cisco v. TQ Delta
`Page 1 of 24
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`CSCO-1006
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`CSCO-1006
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`

`
`U.S. Patent
`
`Jun.13,2000
`
`Sheet 5 of 9
`
`6,075,814
`
`CSCO-1006
`Page 6 of 24
`
`

`
`U.S. Patent
`
`Jun.13,2000
`
`Sheet 6 of 9
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`Jun.13,2000
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`Sheet 7 of 9
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`
`Jun. 13,2000
`
`Sheet 8 of9
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`Page 10 of 24
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`

`
`6,075,814
`
`1
`METHOD AND APPARATUS FOR
`REDUCING SIGNAL PROCESSING
`REQUIREMENTS FOR TRANSMITTING
`PACKET-BASED DATA WITH A MODEM
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to the reduction of the
`required amount of signal processing in a modulator/
`demodulator (modem) which is transferring packet-based
`data or other information which is intermittent in nature on
`a communication channel.
`2. Related Art
`Modern data networks commonly use complex digital
`signal processing (DSP) devices called modems to transport
`data over communication channels. Data is typically trans(cid:173)
`ported via an analog transmission signal which is represen(cid:173)
`tative of a synchronous, constant rate bit stream. This form
`of communication channel is suitable for the transmission of
`real-time information such as voice or video. However, it is
`increasingly common to use modems for the transmission of
`packet-based information. For example, packet-based infor(cid:173)
`mation is used to access the Internet and the World Wide
`Web. However, packet-based information is typically bursty
`in nature, with an average data rate which is often much less
`than the available peak data transfer rate of the communi(cid:173)
`cation channel.
`FIG. 1 is a block diagram of a transmitter circuit 100 of
`a conventional modem. Transmitter circuit 100 includes
`packet queue 101, framer 102, channel coding circuit 103,
`output shaper 104, modulator 105 and digital-to-analog
`(D/A) converter 106. In accordance with conventional
`modem protocols, transmitter circuit 100 transforms source
`data received by packet queue 101 into a continuous time 35
`analog transmit signal, which is provided at the output
`terminal of D/A converter 106.
`More specifically, within transmitter circuit 100, the
`source data is grouped into packets and stored in packet
`queue 101. These packets are not synchronous with respect
`to the modem bit clock, but arrive at packet queue 101 at
`random times. Framer 102 receives the packets from packet
`queue 101, and in response, composes a continuous bit
`stream which is synchronous with respect to the modem bit
`clock. To create such a synchronous bit stream in response
`to the asynchronous packets, framer 102 generates idle
`information (i.e., nulls or a marking tone) when no packets
`are available, and generates packet data when packets are
`available. The packet data and idle information are delin- 50
`eated in such a way that a receiver circuit of a modem (see,
`e.g., FIG. 2) can determine where the packet boundaries lie.
`The synchronous bit stream generated by framer 102 is
`then coded by channel coding circuit 103. Channel coding
`circuit 103 is used to compensate for noise and distortion in
`the communication channel. Channel coding circuit 103
`provides redundant information (e.g., convolutional
`encoding) to allow for error correction. Channel coding
`circuit 103 further performs a scrambling function, as well
`as mapping the coded bit stream onto symbol values. The 60
`stream of symbol values generated by channel coding circuit
`103 is provided to output shaper 104.
`Output shaper 104 digitally filters the stream of symbol
`values received from channel coding circuit 103. Output
`shaper circuit 104 limits the frequency bandwidth of these
`symbol values within a predetermined range and may also
`be adjusted to help compensate for channel distortion. The
`
`2
`filtered sample stream provided by output shaper 104 is
`provided to modulator 105, which modulates a carrier signal
`by the filtered sample stream. The output of modulator 105
`is provided to D/Aconverter 106, which generates an analog
`5 TRANSMIT signal for transmission on the communication
`channel (i.e., telephone line).
`Transmitter circuit 100 exhibits three distinct disadvan(cid:173)
`tages. First, because transmitter circuit 100 transmits con(cid:173)
`stantly (either packet data or idle information), a modem can
`10 be functionally connected to only one telephone line at any
`given time. Moreover, only a small percentage of the total
`information carrying capacity of the communication channel
`is used to transmit data, while a large percentage of this
`capacity is used to transmit idle information. Additionally,
`15 transmitter circuit 100 is unsuited to multi-drop operation on
`a single communication channel. The first disadvantage
`mentioned above is particularly deleterious where a number
`of xDSL modems are collected together in a central office to
`provide data communications to a number of remote loca-
`20 tions. In this case, each remote location requires a dedicated
`xDSL modem in the central office.
`The analog TRANSMIT signal is transmitted over the
`telephone line to the telephone company central office.
`Within the central office, an analog to digital converter
`25 converts the analog TRANSMIT signal into a digital signal.
`This digital signal is multiplexed onto a digital backbone
`circuit and routed to a second central office location. The
`digital signal is demultiplexed within the second central
`office location and routed over a digital trunk to a digital
`30 server which performs additional processing on the digital
`signal.
`FIG. 2 is a block diagram of a receiver circuit 200 of a
`conventional modem. Receiver circuit 200 includes analog(cid:173)
`to-digital (AID) converter 201, resampler 202, equalizer
`203, carrier recovery circuit 204, symbol decision circuit
`205, channel decoding circuit 206, framer 207, packet queue
`208, echo canceler 209, timing update circuit 210, equalizer
`update circuit 211 and carrier update circuit 212. Carrier
`40 recovery circuit 204 and symbol decision circuit 205 are
`sometimes referred to as a demodulator circuit. ND con(cid:173)
`verter 201 is coupled to the telephone line to receive the
`analog signal from the telephone company central office.
`AID converter 201 samples this analog signal, thereby
`45 converting the analog signal into a digital signal.
`The modem which includes receiver circuit 200 also
`includes a transmitter circuit (i.e., a near end transmitter
`circuit, not shown) which is similar to transmitter circuit
`100. During full duplex operation, this near end transmitter
`circuit may be generating a TRANSMIT signal at the same
`time that receiver circuit 200 is attempting to receive the
`analog signal from the remote (or far end) transmitter circuit
`100. Under these conditions, receiver circuit 200 may
`receive an echo of the TRANSMIT signal. Echo canceler
`55 209 generates a signal which is a replica of this echo. The
`signal generated by echo canceler 209 is then subtracted
`from the output signal provided by AID converter 201.
`Resampler 202 adjusts the raw input samples received
`from ND converter 201 to match the symbol rate of the
`transmitter circuit 100. Timing update circuit 211 extracts
`timing information which is used to control resampler 202.
`Equalizer 203 compensates for linear distortions introduced
`by the communication channel (e.g., the telephone line).
`Carrier recovery circuit 204 extracts the carrier signal from
`65 the received signal and provides rough symbols (or a soft
`symbol decision) to symbol decision circuit 205. Symbol
`decision circuit 205 quantizes the rough symbols and makes
`
`CSCO-1006
`Page 11 of 24
`
`

`
`6,075,814
`
`30
`
`3
`hard decisions as to the identity of the received symbols.
`Equalizer update circuit 211 and carrier update circuit 212
`receive the symbols provided by symbol decision circuit
`205. In response, equalizer update circuit 211 and carrier
`update circuit 212 determine quantizer error. In response to 5
`this quantizer error, equalizer update circuit 211 and carrier
`update circuit 212 adjust the coefficients used by equalizer
`203 and carrier recovery circuit 204, respectively, thereby
`improving the accuracy of subsequent hard symbol deci-
`SlOnS.
`Channel decoding circuit 206 uses redundant information
`present in the received analog signal to correct for quantizer
`errors. Channel decoding circuit 206 typically implements a
`maximum likelihood sequence estimator (MLSE) circuit
`(such as a Viterbi decoder or other form of error correction.
`Channel decoding circuit 206 provides a decoded bit stream
`to framer 207. Finally, framer 207 decodes the bit stream
`into packet data, discarding the idle information, and loading
`the packets of data into packet queue 208.
`The operation of receiver circuit 200 is significantly more
`complex than the operation of transmitter circuit 100. Sub(cid:173)
`stantial signal processing is performed by receiver circuit
`200, typically many hundreds or thousands of operations per
`symbol processed. Much of the signal processing is concen(cid:173)
`trated in equalizer 203, echo canceler 209, and channel 25
`decoding circuit 206. A significant percentage of this signal
`processing is dedicated to the processing of the idle infor(cid:173)
`mation generated by transmitter circuit 100.
`It would therefore be desirable to have a modem system
`which is capable of utilizing a greater percentage of the
`information carrying capacity of the telephone line to trans-
`fer packet based data. It would also be desirable to have a
`modem system which minimizes the signal processing
`which must be dedicated to the processing of idle symbols.
`It would further be desirable to have a modem system which
`enables a common modem to be functionally connected to a
`plurality of telephone lines at the same time. It would further
`be desirable to have a modem system which enables a
`common telephone line to be used with a plurality of
`modems in a multi-drop configuration.
`
`4
`receiver circuit to determine when packet data ceases to be
`transmitted on the communication channel, and the trans(cid:173)
`mission of idle information commences. At some point after
`the receiver circuit detects the start of the idle information,
`the receiver circuit enters the standby mode. At this time,
`various elements within the receiver circuit are disabled
`and/or operated with reduced precision. In addition, an idle
`bit pattern, which is synchronous with the idle bit pattern
`generated by the associated transmitter circuit, is converted
`10 to a plurality of expected idle symbols. The expected idle
`symbols are then compared with a plurality of soft symbols
`which are generated by the receiver circuit in response to the
`analog signal using reduced processing within the receiver
`circuit. The receiver circuit remains in the standby mode as
`15 long as the expected idle symbols match the soft symbols.
`The receiver circuit can further store a most recent history
`of the analog signal in a buffer. After the standby mode is
`exited, this buffer can be accessed, thereby enabling the
`receiver circuit to reprocess the most recent history of the
`20 analog signal. This helps ensure that no packet information
`is lost due to the inherent delay in detecting the presence of
`packet information.
`In accordance with another aspect of the present
`invention, the receiver circuit can monitor the quality of the
`analog signal on the communication channel and reduce the
`amount of processing performed by the receiver circuit if the
`channel quality exceeds a predetermined level. This further
`reduces the processing requirements of the receiver circuit.
`In accordance with another embodiment of the invention,
`a burst mode protocol is provided for operating a modem on
`a telephone line. The burst mode protocol involves modu(cid:173)
`lating packets of digital information by a transmitter circuit
`of the modem, wherein the packets of digital information are
`converted into analog signal bursts of discrete duration.
`These analog signal bursts are transmitted from the trans(cid:173)
`mitter circuit to the telephone line. However, no signal is
`provided from the transmitter circuit to the telephone line
`between the analog signal bursts. In one embodiment, a
`non-idle state signal is appended to the beginning of the
`analog signal bursts by the transmitter circuit, thereby sig(cid:173)
`nalling the presence of the analog signal bursts.
`A receiver circuit of the modem monitors the telephone
`line to detect the presence and absence of the analog signal
`45 bursts. This monitoring step is performed by a non-idle
`detector within the receiver circuit. When the non-idle
`detector detects the presence of the analog signal bursts on
`the telephone line, the non-idle detector causes the receiver
`circuit to demodulate the analog signal bursts using full
`processing capabilities of the receiver circuit. However,
`when the non-idle detector detects the absence of the analog
`signal bursts on the telephone line, the non-idle detector
`disables the demodulating function of the receiver circuit.
`This greatly reduces the processing requirements of the
`55 receiver circuit when there are no analog signal bursts
`present on the telephone line.
`In one embodiment, the non-idle detector determines the
`presence and absence of the analog signal bursts on the
`telephone line by monitoring the telephone line for the
`presence and absence of carrier energy. Alternatively, the
`non-idle detector can monitor the telephone line for the
`presence and absence of a non-idle state signal provided by
`the transmitter circuit.
`In accordance with the burst mode protocol, there are
`certain periods during which the transmitter circuit is not
`transmitting any signals. During these periods, the echo
`canceler of the associated local receiver circuit can be
`
`35
`
`40
`
`SUMMARY
`
`Accordingly, the present invention provides a method for
`operating a modem on a communication channel which
`includes the following steps. A receiver circuit of the modem
`is coupled to receive a continuous analog signal which is
`transmitted on the communication channel. This continuous
`analog signal includes both packet information and idle
`information. The receiver circuit monitors the analog signal 50
`to detect the presence of the idle information. Upon detect(cid:173)
`ing the presence of the idle information, the receiver enters
`a standby mode. In the standby mode, the amount of
`processing performed by the receiver circuit is reduced.
`The reduction of the amount of processing performed by
`the receiver circuit can be achieved by disabling and/or
`reducing the processing precision of selected elements
`within the receiver circuit. For example, a symbol decision
`circuit, a channel decoder and a framer within the receiver
`circuit can be disabled during the standby mode in one 60
`embodiment of the invention. Moreover, the processing
`precision of other elements, such as an echo canceler, update
`circuits and an equalizer can be reduced when the receiver
`circuit is in the standby mode.
`To detect the presence of the idle information, the receiver 65
`circuit fully demodulates the analog signal to provide a
`digital bit stream. This digital bit stream is processed by the
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`processing (DSP) resource. This common DSP resource
`modulates digital data packets from different sources. The
`multi-line network access circuit then de-multiplexes the
`modulated digital data packets onto telephone lines corre-
`5 sponding to the destination addresses. In one variation, a
`common idle generator within the multi-line network access
`circuit is used to generate common idle information for each
`of the telephone lines. In another variation, a non-idle state
`signal generator within the multi-line network access circuit
`10 is used to generate non-idle state signalling for each of the
`telephone lines.
`Yet another embodiment of the present invention provides
`a method of implementing a multi-cast network access
`circuit. In accordance with this method, a digital data packet
`is transmitted from a source to the multi-cast network access
`circuit. In this embodiment, the digital data packet does not
`include idle information. The digital data packet identifies a
`plurality of destination addresses to which the digital data
`packet is to be transmitted. The digital data packet is routed
`to a digital processing resource and modulated. The modu(cid:173)
`lated digital data packet is demultiplexed to a plurality of
`telephone lines which correspond to the destination
`addresses, thereby completing the multi -cast operation.
`The present invention will be more fully understood in
`25 view of the following detailed description taken together
`with the drawings.
`
`5
`disabled, since there will be no echo signal to cancel during
`these periods. This further reduces the processing require(cid:173)
`ments of the receiver circuit.
`In accordance with another aspect of the present
`invention, the receiver circuit can monitor the quality of the
`analog signal bursts on the telephone line and reduce the
`amount of processing performed by the receiver circuit if the
`line quality exceeds a predetermined level. This further
`reduces the processing requirements of the receiver circuit.
`In accordance with another embodiment of the present
`invention, a plurality of remote transmitter circuits, which
`are coupled to separate telephone lines, generate analog
`signal bursts in accordance with the burst mode protocol.
`The separate telephone lines are connected together at a
`central location where the analog signal bursts are multi- 15
`plexed to a number of receiver circuits. A non-idle detector
`is coupled to receive the analog signal bursts from each of
`the transmitter circuits, and to detect the presence and
`absence of the analog signal bursts on the telephone lines.
`Typically, only a small number of the telephone lines will be 20
`transmitting analog signal bursts at any given time. The
`analog signal bursts are therefore multiplexed into a number
`of receiver circuits which is less than the number of tele(cid:173)
`phone lines. That is, each receiver circuit can process analog
`signal bursts from a plurality of telephone lines. As a result,
`the number of receiver circuits required to handle informa(cid:173)
`tion from a given number of telephone lines is advanta(cid:173)
`geously reduced. In a particular embodiment, different sets
`of update coefficients are enabled within the receiver
`circuits, depending upon which telephone line is currently 30
`coupled to the receiver circuit.
`The present invention also includes a method for operat(cid:173)
`ing a plurality of modems on a single telephone line (i.e.,
`multi-drop operation). This method includes the steps of (1) 35
`modulating packets of digital information by the modems,
`wherein the packets of digital information are converted into
`analog signal bursts of discrete duration, (2) transmitting the
`analog signal bursts from the modems to the telephone line,
`(3) providing no signal from the modems to the telephone 40
`line between the analog signal bursts, and ( 4) arbitrating the
`transmitting of the analog signal bursts from the modems to
`the telephone line such that only one modem is transmitting
`analog signal bursts to the telephone line at any given time.
`In one variation of the multi-drop method, each of the
`analog signal bursts includes a preamble and a correspond(cid:173)
`ing main body. Each preamble is transmitted in accordance
`with a predetermined first modem protocol. However, the
`main bodies can be transmitted in accordance with different
`modem protocols which are different than the first modem 50
`protocol. For example, the different modem protocols may
`implement different data rates, modulation formats and/or
`protocol versions. The modem protocol associated with each
`of the main bodies is identified by information included in
`the corresponding preamble. This variation enables devices 55
`having different operating capabilities (e.g., personal com(cid:173)
`puters and smart appliances) to be operably coupled to the
`same telephone line in a multi-drop configuration.
`The present invention further includes a method for
`implementing a multi-line network access circuit. In this 60
`embodiment, digital data packets are transmitted from a
`plurality of sources (e.g., ISPs) to a multi-line network
`circuit. The digital data packets do not include idle infor(cid:173)
`mation. The multi-line network access circuit identifies the
`telephone lines associated with the digital data packets using 65
`a destination address monitor. Digital data packets from
`different sources are multiplexed to a common digital signal
`
`BRIEF DESCRIPTION OF DRAWINGS
`
`FIG. 1 is a block diagram of a transmitter circuit of a
`conventional modem;
`FIG. 2 is a block diagram of a receiver circuit of a
`conventional modem;
`FIG. 3 is a block diagram of a receiver circuit of a modem
`in accordance with one embodiment of the invention;
`FIG. 4 is a block diagram of a receiver circuit of a modem
`in accordance with a burst-mode protocol of the present
`invention;
`FIG. 5 is a block diagram of a multi-line network access
`circuit which can be located in a central office in accordance
`with one embodiment of the invention;
`FIG. 6 is a schematic diagram of packet data received on
`the multiple lines of the multi-line network access circuit of
`45 FIG. 5 in accordance with one embodiment of the invention;
`FIG. 7 is a schematic diagram of a multi-drop configu(cid:173)
`ration which includes modems in a subscriber's residence
`and a modem in the telephone company central office;
`FIG. 8 is a schematic representation of packet information
`which is transmitted by transmitter circuits in accordance
`with the burst-mode protocol of the present embodiment;
`FIG. 9 is a block diagram of a multi-line network access
`circuit in accordance with another embodiment of the
`present invention; and
`FIG. 10 is a schematic diagram of packet information
`received by and transmitted from the multi-line network
`access circuit of FIG. 9.
`
`DETAILED DESCRIPTION
`
`FIG. 3 is a block diagram of a receiver circuit 300 of a
`modem in accordance with one embodiment of the present
`invention. Receiver circuit 300 includes ND converter 301,
`resampler 302, equalizer 303, carrier recovery circuit 304,
`symbol decision circuit 305, channel decoder 306, framer/
`idle detector 307, sample buffer 308, echo canceler 309,
`timing update circuit 310, equalizer update circuit 311,
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`carrier update circuit 312, idle generator 314, idle symbol
`predictor 316, comparator circuit 317, packet queue 318 and
`summing node 319. In combination, carrier recovery circuit
`304 and symbol decision circuit 305 form a demodulator. In
`the described embodiment, ND converter 301 is imple(cid:173)
`mented by a coder/decoder (codec) chip, while the remain(cid:173)
`ing elements of receiver circuit 300 are implemented by a
`digital signal processor (DSP). In other embodiments, the
`elements of receiver circuit 300 can be implemented by
`other means, such as a general purpose processor. Receiver
`circuit 300 is coupled to receive an analog RECEIVE signal
`from communication channel 321, which in the described
`embodiment, is a telephone line. It is understood that other
`communication channels, such as twisted pair other than a
`telephone line, wireless, coaxial cable, infrared or optical,
`can be used in other embodiments.
`In the described embodiment, the RECEIVE signal
`received on communication channel 321 is an analog signal
`in accordance with a conventional modem protocol, such as
`xDSL or a voice band modem protocol. For example, this
`analog RECEIVE signal could originate from transmitter
`circuit 100 (FIG. 1) in the manner previously described.
`Thus, the analog RECEIVE signal received on communi(cid:173)
`cation channel 321 includes modulated packet data as well
`as idle information which is interleaved with the packet data.
`ND converter 301 samples the analog RECEIVE signal,
`thereby converting the analog RECEIVE signal into a digital
`signal. This digital signal is provided to a positive input
`terminal of summing node 319. Echo canceler 309 uses the
`local transmit signal to adaptively predict the echo signal on 30
`communication channel 321. As previously described, an
`echo of the local transmit signal may be present if the
`modem which includes receiver circuit 300 is operating in
`full duplex mode. Echo canceler 309 applies the predicted
`echo signal to the negative input terminal of summing node
`319, thereby canceling the echo signal from the digital
`signal.
`The digital signal output by summing node 319 is pro(cid:173)
`vided to a conventional resampler 302. Resampler 302 40
`interpolates this digital signal to generate samples which
`match the symbol rate of the transmitt