United States Patent [19]
`Baran
`
`[11]
`
`[45]
`
`4,438,511
`Mar. 20, 1984
`
`[54] PACKETIZED ENSEMBLE MODEM
`[75]
`Inventor: Paul Baran, Menlo Park, Calif.
`[73] Assignee: Telebit Corporation, Cupertino,
`Calif.
`[21] Appl. No.: 205,744
`[22] Filed:
`Nov. 10, 1980
`[51]
`Int. CI.3 .......................... H04J 11/00; H04J 4/00
`[52] U.S. CI ......................................... 370/19; 370/60;
`370/94; 370/16; 370/50
`[58] Field of Search ....................... 370/18, 70, 13, 16,
`370/17, 19,94, 60, 50
`References Cited
`U.S. PATENT DOCUMENTS
`3,706,929 12/1972 Robinson .....•.................... 179/1 SA
`3,875,394 4/1975 Shapely et aI .•..•..••.••.•.•...•.•.. 370/19
`4,206,320 6/1980 Keasler et a1. .............•.......... 370/19
`
`[56]
`
`OTHER PUBLICATIONS
`Product Brochure for Gandalf SM9600 Super Modem,
`Gandalf Data, Inc.
`"Digital Super Modem: Why and How It Was Devel-
`
`oped", Data Communications, Jun. 1980, pp. 87-95.
`Hick.
`Product Brochure for AT&T 2096A, 9600 bps. Data(cid:173)
`phone II.
`Primary Examiner-Gerald L. Brigance
`Attorney. Agent. or Firm-Townsend and Townsend
`ABSTRACT
`[57]
`A high speed digital data modem particularly suited for
`use on a dial up telephone line is disclosed. For the
`transmit ensemble. the telephone passband is divided
`into sixty-four sub-bands each with a carrier located
`approximately in the center of each sub-band. Each
`carrier is amplitude and phase modulated in order to
`encode five (5) bits. One carrier is used as a reference
`signal for phase and amplitude. The modulated carriers
`can be changed in data content every epoch. By use of
`packetization of data, individual amplitude correction,
`and individual phase correction for each carrier, the
`high speed modem may achieve up to 12000 bps over a
`dial up line with a simultaneous 300 bps reverse chan(cid:173)
`nel.
`
`10 Claims, 14 Drawing Figures
`
`~~~~~~!:========~l==!~~~~REFERENCE
`
`CORRECTORI
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`U.S. Patent Mar. 20, 1984
`
`Sheet 1 of 4
`
`4,438,511
`
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`
`U.S. Patent Mar. 20, 1984
`
`Sheet 2 of 4
`
`4,438,511
`
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`u.s. Patent Mar. 20, 1984
`• • • X11
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`

`
`u.s. Patent Mar. 20, 1984
`
`Sheet 4 of 4
`
`4,438,511
`
`SHIPPING
`DEPARTMENT
`
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`
`1
`
`4,438,511
`
`5
`
`PACKETIZED ENSEMBLE MODEM
`
`2
`pays a premium and is assured by the telephone com(cid:173)
`pany that the line is of lower (or customized) noise
`characteristics. Conditioning is tailored to the high
`BACKGROUND OF THE INVENTION
`speed modem manufacturer's specifications, and condi-
`tioning cannot obviously be obtained for a dial up line
`1. Field of the Invention
`(that would mean conditioning aI/lines). Second, 9600
`The invention relates to the field of data communica-
`bps full duplex operation can only be obtained with
`tions and finds particular application as a high speeq,
`modems of the prior art when utilizing two lines (i.e.
`full duplex, modem.
`.
`four wires), certainly not on a single channel and never
`2. Description of the Prior Art
`While at one time telephone lines only carried voice 10 on dial up lines. Third, modems of the prior art which
`communications, for many years now it has been com-
`do manage to achieve 9600 bps on conditioned lines do
`mon for digital data to be conveyed over the telephone
`not gracefully degrade their performance in the pres-
`network. However, this development has not" been
`ence of impairments. That is, a condititioned line does
`without significant technical limitations and obstacles.
`not guarantee zero impairments; it only statistically
`For example, the bandwidth of a typical udial up" 15 reduces the probability of impairments. Yet, when 9600
`telephone line is only approximately 3 kilohertz (kHz)
`bps modems of the prior art encounter noise, as they
`which serves as an upper limit to the data transfer rate
`inevitably will, they typically reduce the transmitted
`upon the line. Further, impairments or performance
`data rate to 7200 bps, 4800 bps, 2400 bps, 1200 bps, and
`limitations of the dial up telephone network have se-
`so on until reliable communication is re-established.
`verely limited the .<tbility of the network to reliably 20 Often, though, impairments of the telephone line are
`transfer digital data at high speeds. For example:
`limited to particular frequency bandwidths, and thus,
`Telephone lines suffer from frequency distortion or
`reducing the net data .transmission rate usually by a
`attenuation of the high and low frequencies across the
`factor of two or four is a needless and uneconomic
`available 3 kHz bandwidth.
`waste of the available telephone bandwidth.
`There is commonly phase distortion or a difference in 25 One high speed digital modem of the prior art is the
`SM9600 Super Modem manufactured by Gandalf Data,
`the time delay for each frequency component across the
`available 3 kHz bandwidth.
`Inc. The SM9600 is nominally a full duplex, 9600 bps
`Frequently a hetrodyne on!set may be encountered
`modem which can operate over a dial up line. However,
`11:
`which shifts the received component frequencies with
`the SM9600 data sheets recommend conditioning of the
`respect to the transmitted component frequencies; this 30
`results from frequency variations between the tele-
`line. In the presence of impairments, the SM9600 will
`phone company's hetrodyne oscillators.
`"gearshift" or drop back its transmitted data rate to
`Amplitude distortion is common and often caused by
`4800 bps or 2400 bps. Further, the SM9600 cannot oper-
`non-linear amplification of the telephone company's
`ate full duplex at 9600 bps over a single channel but
`AID converters.
`35 must allocate a portion of the available spectrum for a
`Impulse noise is common and often results from line
`reverse channel if a single channel is all that is avialable
`"hits" (i.e. lightning) or dial "clicks" from other tele-
`or utilize a second line (i.e., a total of four wires) for full
`phone circuits.
`duplex operation. Further, still, the SM9600 cannot
`Cross-talk is not a rare occurrence; cross-talk is the
`cope with single or multiple extraneous tones within the
`"leaking" of voices or tones from one line to another. 40 passband.
`Phase jumps (i.e. the instantaneous changes of time
`The most common class of 9600 bps digital data
`delay or phase) of the carrier are common.
`modems of the prior art is available from AT&T (Model
`Amplitude jumps (i.e. instantaneous changes in ampli-
`2096A). This class of modem is extremely sensitive to
`impulse noise (i.e., dial clicks and lightning strikes). It
`tude generally as a result of alternate microwave link
`switch overs) are all too common.
`45 will tolerate but 5 Hz frequency heterodyne offset, and
`Gaussian noise is an ever present impairment that
`it requires use of a conditioned line for 9600 bps. Fur-
`plagues all electrical systems.
`ther, this class of modem is extremely sensitive to cross-
`Echo suppressors are installed by the telephone com-
`talk, and the unit does not have any capability for error
`pany to permit very long distance voice communica-
`suppression.
`tion, but they must be disabled for long distance data 50 U.S. Pat. No. 3,706,929 to Robinson et al. discloses a
`two way communication.
`combined modem and vocorder pipeline processor. In
`Typically, high speed modems have suffered from
`Robinson, the modem function is implemented using 16
`one hybrid loss or the undesired return of a portion of
`frequency division multiplexed channels, data being
`the transmitted signal into the local receiver channel.
`carried on each channel by means of phase shift keyed
`Echo on telephone lines is the typical occurrence of 55 modulation of each carrier. Robinson, however, re-
`return of the transmitted signal back into the receiver
`quires a four wire circuit or a completely separate line
`channel usually on long distance communications.
`for the reverse channel in order to achieve full duplex
`Satellite delay is a further line impairment caused by
`operation, and conditioning of the line is a necessity to
`the delay encountered by the distances travelled by
`achieve high data rates.
`telephone signals when beamed to geostable earth satel- 60 U.S. Pat. No. 4,206,320 to Keasler et al. discloses a
`lites.
`high speed modem suitable for operating with a
`In addition to the foregoing, several other factors
`switched network. However, Keasler requires a com-
`must be kept in mind when considering digital data
`pletely separate reverse channel to obtain full duplex
`telecommunications at, say, 9600 bits per second (bps).
`operation. Further, while utilizing 32 carriers to convey
`First, achieving reliable 9600 bps over a dial up line is 65 the information in a frequency division multiplexed
`virtually unheard of. To obtain reliable 9600 bps C91Jl-
`. manner, Keasler employs the inefficient mechanism of
`munication over a telephone line, conditioning is oftena
`forming a "hole" or delay at the beginning and end of
`necessity; a conditioned line is one of which the user
`each modulation sub-period in order to minimize the
`
`

`
`3
`effects of intersymbol distortion. The shortened modu(cid:173)
`lation period can cause undesired crosstalk between
`adjacent channels.
`Several other aspects tend to render all high speed
`digital data modems of the prior art, including those
`referenced hereinabove, obsolete. No modems of the
`prior art are capable of functioning in both synchronous
`and asynchronous modes. Synchronous transmission is
`the old standard form of data communication over a
`telephone line.' However, asynchronous transmission in
`the form of packetization vastly improves the error
`performance of the modem. In packet transmission data
`is formed into blocks (say, from 0 to 256 characters
`each) which are sent as self contained packets. Each
`packet contains housekeeping information for framing, 15
`routing, error detection, etc. This housekeeping infor(cid:173)
`mation is placed into packet header and trailing fields.
`During the last few years there has been an unprece(cid:173)
`dented, rapid adoption of an international standard for
`packet switched communication-International Stan(cid:173)
`dards Organization X.25. X.25 is a multilevel protocol
`with only the lower levels which deal with the trans(cid:173)
`mission of the data itself now unambiguously defined.
`There is a major advantage in having a high speed
`modem that is capable of X.25 protocol compatibility
`for obvious internet communication. There is a further
`significant advantage in having a modem that can com(cid:173)
`municate over internets utilizing protocols other than
`X.25 (each computer company devised its own protocol
`prior to X.25). There are simply no high speed digital
`data modems of the prior art which are compatibile
`with packet switched networks (not to mention any that
`are compatible with multiple packet switching internet
`protocols) and which operate in both synchronous and
`asynchonous modes concurrently through signal multi(cid:173)
`plexing.
`In addition to the foregoing, modems of the- prior art
`suffer from the inability to isolate or otherwise pinpoint
`the source of poor error performance to either the
`modem or the communication medium. High speed
`modems of the prior art commonly calculate a compos(cid:173)
`ite data error rate which aggregates causes originating
`in the modem itself and the causes originating from the
`telephone circuit. Often the most vexing problem to the 45
`data communication manager is assigning blame cor(cid:173)
`rectly for poor communication performance so that the
`problem can be fixed. It is usual for the telephone com(cid:173)
`pany to immediately deny that any transient fault is due
`to its equipment. By their nature, transient faults tend to
`disappear in time and later gross measurements often in
`fact find nothing wrong with the telephone circuit.
`Further, even if the circuit impairment persists, mea(cid:173)
`surements made by the telephone company are often
`incomplete with respect to the range of possible impair(cid:173)
`ments. There are simply no modems of the prior art
`which can separate modem performance errors from
`telephone circuit impairments. Moreover, there are
`simply no modems of the prior art which permit the
`modem user to completely characterize the telephone
`circuit so that the user can adequately direct repairs to
`the telephone circuit by the telephone company.
`
`SUMMARY OF THE INVENTION
`It is thus an object of the present invention to provide
`a digital high speed data modem that can operate in full
`duplex mode on a dial up telephone line at data rates in
`excess of 9600 bps.
`
`4,438,511
`
`4
`It is another object of the present invention to pro(cid:173)
`vide a digital high speed data modem that can operate in
`the synchronous, aynschronous, and packet modes.
`It is yet another object of the present invention to
`5 provide a digital high speed data modem that can oper(cid:173)
`ate on telephone lines exhibiting higher phase error and
`frequency attenuation
`roll-off than permitted by
`modems of the prior art.
`It is another object of the present invention to pro-
`10 vide a digital high speed data modem that can operate in
`full duplex mode in excess of 9600 bps on a dial up
`telephone line and which can be manufactured at a
`lower cost to performance ratio compared with that of
`prior art modems.
`It is yet another object of the present invention to
`provide a digital high speed data modem that can oper(cid:173)
`ate with mUltiple higher level protocols as commonly
`used with packet switched networks.
`It is another object of the present invention to pro-
`20 vide a digital high speed data modem which reduces its
`transmitted data rate in small increments in response to
`telephone line impairments.
`It is yet another object of the present invention t6
`provide a digital high speed data modem which exhibits
`25 significantly lower error rates than modems of the prior
`art while operating at 9600 bps.
`It is yet another object of the present invention to
`allow flexibility of data input format and the intermix(cid:173)
`ing of a multiplicity of sources of asynchronous, syn-
`30 chronous, and packet format data in an essentially error(cid:173)
`free manner.
`It is yet ~nother object of the present invention to
`permit local or remote diagnostics to be performed in an
`unambiguous manner in order to pinpoint errors by
`35 characterizing the channel's noise, frequency attentua(cid:173)
`tion, phase shift, and frequency offset solely in terms of
`the corruption by the transmission medium and not as
`an artifact of the modem's data characteristics.
`It is yet another object of the present invention to
`40 determine whether the telephone line or the modem is
`at fault when errors occur and to unambiguously inform
`a remote site as to the specifics of the problems in mea(cid:173)
`sured quantitative terms with both data and voice
`backup.
`In a preferred e.mbodiment of the present invention,
`an ensemble of sixty-four (64) orthogonally modulated
`carriers is digitally generated. Each such carrier or tone
`is individually modulated to contain five (5) bits. One
`(1) carrier within the ensemble of sixty-four (64) serves
`50 . as a pilot tone for precise coordination between the
`transmitter and receiver sections of the modem of the
`present invention. This tone maintains timing and fre(cid:173)
`quency calibration or "alignment" irrespective of the
`telephone network's carrier heterodyne errors or
`55 changes in transmission path length. The transmitter
`and receiver portions of the modem of the present in(cid:173)
`vention operate in conjunction with one another in a
`coordinated manner to deduce real time information on
`the performance of the telephone channel being used.
`60 All significant signal parameters are measured and cor(cid:173)
`rective signals are returned to the originating modem on
`a simultaneous reverse channel. Carrier frequencies,
`generally located at the end of the usable spectrum (but
`not always), which are impaired from any number of
`65 causes are removed from the ensemble. In the modem of
`the present invention, the spectrum spacing is mini(cid:173)
`mized so as to permit the simultaneous transmission of
`both 9600 bps or greater in one direction (as high as
`
`

`
`4,438,511
`
`5
`1200 bps in a preferred embodiment) together with 300
`bps in the reverse direction on a single, two wire, dial
`up telephone line.
`Several levels of close interaction and coordination
`<'Ire important between the receiver and the transmitter 5
`sections of the modem of the present invention in order
`to permit maximum error-free data transfer. Data pack:'
`ets are exchanged between the transmitting andreceiv:'
`ing sections and use interleaved cyclic redundancy
`checking (CRCs). Repeat transmission is used upon the 10
`detection of errors. This permits the error-free corivey(cid:173)
`ance of data, housekeeping information (e.g. ordering of
`packets), and diagnostic signals. This arrangement per(cid:173)
`mits adaptation of the modulation of the carriers in
`order to achieve data transmission at the maximum 15
`possible effective error-free rate for the channel. in the
`event of impairment, the actual data throughput capac(cid:173)
`ity gradually decreases by dropping out defective indi(cid:173)
`vidual carriers until a new eqUilibrium point of error(cid:173)
`free transfer is reached. It is thus clear that unlike 20
`modems of the prior art which "gearshift" dowri the
`transmitted data rate by a factor of two or four in the
`presence of impairments to the telephone line, the
`modem of the present invention reduces its throughput
`data rate by only approximately l/64th of its capacity 25
`per step. Typically, single step reductions in data
`throughput will be all that is necessary to surmount
`most impairments.
`The present invention also includes the use of a
`pulsed pilot tone to effect correction of telephone line 30
`amplitude variations, amplitude hits, frequency offsets,
`phase drift, and phase hits. Frequency distortion is es(cid:173)
`sentially removed in the mod~m of the present inven(cid:173)
`tion by modulation of the amplitude of the local oscilla(cid:173)
`tor equivalent circuit for each of the sixty-four (64) 35
`tones in the ensemble, Phase distortion and· channel
`cross"talk . are removed by processing calculations
`which are based upon initial transmission of a known
`test pattern.
`An important advance over the prior art of the 40
`modem of the present invention is packetization of all
`data and' housekeeping information. The modem of the
`present invention handles all data on a packetized basis
`and permits intermixing of a multiplicity of data
`streams. Each data stream may operate at almost any 45
`speed or protocol combination by virtue of packetiza(cid:173)
`tion permitting a degree of flexibility never before
`achieved. In a preferred embodiment the duration of
`each frequency ensemble is 2/75th of a second. In this
`period are transmitted up to 320 bits as an ensemble 50
`packet which contains both data and error detection
`means. This permits closed loop feedback between the
`transmitting and receiving modems to be based upon
`new error-free intelligence.
`The packetizing and. extensive test signal arrange- 55
`ments in the modem of the present invention also permit
`ready use of packets containing solely test data. These
`packets are useful in ,order to provide remote diagnos(cid:173)
`tics and to· pinpoint transmission faults. The modem of
`the present invention.measures amplitude, noise, phase 60
`delay, and frequency offset for each of a large ensemble
`of frequencies; these measured parameters are charac(cid:173)
`teristics solely of the communication circuit and not of
`the modem. Such measurements are stored over a series
`of periods to provide operating statistics to a remote 65
`modem or diagnostic center in a form that unambigu(cid:173)
`ously defines transmission line problems. To aid in the
`process of rapid line fault diagtiosis" a separate tele-
`
`6
`phone line. can be made, available that automatically
`connects to a remote data cc::nter. This separate circuit
`forms the transmission path for the readout of statistics,
`and it also forms a voice intercom permitting the oper(cid:173)
`ating personnel at the mode!p site to communicate with
`the connected modem or to a remote diagnostic center.
`In a preferred embodiment of the present invention,
`sine and cosine vectors for each of sixty-four (64) sepa(cid:173)
`rate frequencies are generated for each carrier. Each is
`derived.digitally. The symbol transmission period is
`2/75th of a second, and each symbol conveys five (5)
`bits using thirty-two (32) combinations of phase and
`amplitude modulation. The definition of the phase and
`amplitude combinations is called a "constellation" and
`in the present invention is selected as a function of the
`characteristics of the real time impairments encountered
`on the telephone line.
`"
`The preferred embodiment is described in terms of
`sinusoidal carrier frequencies. However, the invention
`is not so limited in theory; other orthogonal wave
`shapes could be utilized in lieu of conventional sinus(cid:173)
`oids. Specifically, pseudo-random but orthogonal noise
`streams could be used instead. Each' pseudo-random
`wavefornl would be stored. in a read only memory
`(ROM). ; The waveform would be one epoch in length
`and could ,be amplitude modulated. Separation of the
`waveforms occUrs by multiplication by an identical 'set
`of waveforms in the receiver by use of noise as a spread
`spectrum transmission concept. The nature of the detec(cid:173)
`tion process. in thiS modem of .the present invention in
`which each channel is antiCipating a particular" wave(cid:173)
`form over a relatively long epoch penod tends to lend
`itself to the transmission of a simultaneous ensemble of
`pseudo noise channels. This arrangement is particularly
`fitting to insure privacy.
`It is thus an advantage of the present invention to
`provide a digital data modem capable of reliably operat:(cid:173)
`ing in full duplex modem over a dial up telephone line at
`data rates in excess of 9600 bps by allowing closest
`spacing between adjacent carriers without. frequency(cid:173)
`to-frequency crosstalk permitting mdre clatachannels
`within the telephone passband. '.
`It is another advantage of the present invention that
`data throughput degradation in the presence of tele(cid:173)
`phone line impairment is gradual and -tailored to the
`particular form of line impairment.
`It is another advantage of the present invention to be
`able to utilize a dial up telephone line.with sigilificantly
`higher phase en:or and frequency aitenuationrolloff
`than previously possible.
`.
`.
`It is yet another advantage of the, present invention to
`utilize asynchronous data transmission and reception in
`the form of packetization of data and housekeeping
`functions.
`It is yet another advantage of the present invention to
`permit the concurrent multiplexing of a piurality of
`signals, both asynchronous and synchronous.'
`It is yet another advantage of the present invention to
`permit operation using"a wide range of data communi(cid:173)
`cations protocols. These aI\d other objects and advan(cid:173)
`tages of the present invention will become apparent 'by
`referring to the drawing figures in conjunction with the
`description of a preferred embodiment.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a high-level block diagram of the high speed
`modem of the present invention.
`
`

`
`4,438,511
`
`7
`FIG. 2is pictorial representation of the transmit and
`receive ensembles of the present invention.
`FIG.' 3 is a functional block diagram of the signal
`generatorlhybrid of the high speed modem ofthe pres(cid:173)
`ent invention.
`FIG. 4 is a functional block diagram of the signal
`extractor of the high speed modem of the present inven(cid:173)
`tion.
`FIG. 5 is a functional block diagram of the vector
`definer of the high speed modem of the present inven(cid:173)
`tion.
`FIG. 6 is a functional block diagram of the crosstalk
`reducing circuit of the high speed modem of the present
`invention.
`FIG. 7 is a functional block diagram of the reference
`correctorl generator of the high speed modem of the
`present invention.
`FIG. 8 is a functional block diagram of the constella(cid:173)
`tor of the high speed modem of the present invention.
`FIG. 9 is a functional block diagram of the loading
`docks and the shipping department of the present inven(cid:173)
`tion.
`FIG. lOA" is a pictorial representation of the bits
`transmitted for'each of the sixty-four (64) carners for
`two time epochs of the present invention.
`'
`FIG. lOB is a pictorial representation of the bits trans(cid:173)
`mitted in a: reverse channel for each of nine (9) frequen(cid:173)
`cies for'two time epochs of the present invention.
`FIG. llA is a pictorial representation illustrating
`binary signals as a function of time for two epochs to
`transmit a 640 bit packet according to a preferred em(cid:173)
`bodirnenfof the present invention.
`FIG. lIB is a pictorial representation illustrating the
`housekeeping and data assignments for a hypothetical
`packet according to a preferred embodiment of the
`present invention.
`FIG, llC is a pictorial representation illustrating the
`ntimber o(bits in each,field comprising an exemplary
`packet according to a preferred embodiment of the
`present invention.
`
`8
`leads to a constellator 20 which matches the received x
`and y ve.ctors to the closest fitting probably expected
`signal combination. The other path from the vector
`definer 18 leads to a reference corrector 22 where the
`5 frequency, time, and amplitude characteristics of the
`reference signal are extracted and compared with its
`expected value and corrections made accordingly.
`The output of the constellator 20 proceeds to a diag(cid:173)
`nostics analyzer 24 which extracts performance infor-
`10 mation necessary for the coordination of the sending
`and receiving modems and for diagnostics purposes.
`Diagnostics analyzer 24 performs such functions as
`determining whether any frequency channels are exhib(cid:173)
`iting excessive noise levels. Diagnostics analyzer 24 also
`15 generates statistical estimators of individual channel
`phase delay,· amplitude distortion, noise, etc. The infor(cid:173)
`mation from the diagnostics analyzer 24 is formed into
`packets and sent to a shipping department 26. The ship(cid:173)
`ping department 26 is responsible for the organization
`20 of information into packets for the purpose of transmis(cid:173)
`sion and for the disassembly of packets in the case of
`reception of information. Packet switching and the
`packetization of information itself is well known in the
`25' prior. art of data, communications. The techniques of
`packet switching will not be described herein other than
`to note that most information within the high speed
`modem 10 of the present invention is transferred in the
`form of standard size packets of information each con-
`30 taining a header together with housekeeping data which
`facilitates information transfer between a sending high
`speed modem 10 and a remote receiving high speed
`modem 10'.' Internal interlaced error correction sym(cid:173)
`bols permit the detection of error with a high degree of
`35 certainty. Errors are generally corrected by the repeat
`transmission of. defectively received packets. Packets
`between the shipping department 26 and the outside
`world (Le. a computer system to which high speed
`modem 10 is connected) are sent and received by one or
`40 more loading docks 28. Each loading dock 28 is tai(cid:173)
`lored, generally by software command, to match the
`precise interface requirement of the data user's equip(cid:173)
`ment. For example, many data terminals utilize a stan(cid:173)
`dard RS-232-C connector and voltage levels. A loading
`45 dock 28 would be used to connect to such an RS-232-C
`interface. The loading docks 28 also remove and add
`housekeeping data from the packets so that the output
`serial data stream appears to the user as input to the
`transmitter with the visibility of added information
`50 required by the packet switching process itself.
`
`Ensemble Description
`Referring to FIG. 2, a diagramatic representation is
`shown of the transmit and receive channels that form
`the transmit and receive ensembles for the high speed
`modem 10 of the present invention. High speed modem
`10 simultaneously transmits an ensemble of frequencies
`30 and receives an ensemble of frequencies 32. High
`speed modem 10 is designed in a preferred embodiment
`to operate wth either four-wire telephone circuits (i.e.
`two lines) or two-wire telephone circuits (i.e. a single
`line). With a four-wire circuit connection (not illus(cid:173)
`trated), the full spectrum from 300 Hz to 3 kHz is avail(cid:173)
`able in both directions simultaneously. In the two-wire
`case (that shown in FIG. 2), separation between the
`transmit ensemble 30 and the receive ensemble 32 is
`imperfect. Frequency division is thus used to separate
`the two directions. FIG. 2 shows the transmit ensemble
`
`DESCRIPTION OF A PREFERRED
`. EMBODIMENT
`Introduction
`Referring to FIG. 1, a preferred embodiment qf the
`high speed modem of the present invention is shown by
`the general reference numeral 10. High speed modem 10
`performs both transmitting and receiving functions over
`a telephone line 12 in conjunction with a similar high
`speed modem 10' '(not shown) located remotely at the
`opposite end of