United States Patent [19]
`Bartholet et al.
`
`[54] DISTRIBUTED TELEPHONE SYSTEM
`[75] Inventors: Chauncey I. Bartholet, Scarsdale;
`Joseph Hardy, Bedford, both of N.Y.
`[73] Assignee: Bard Technologies, Bedford, N.Y.
`[21] Appl. No.: 628,744
`[22] Filed:
`Jul. 9, 1984
`[51] Int. Cl* ................................................ H04J 1/10
`
`[52] U.S. Cl. .......‘g a tº a s : * * * * * * s a s a 2 & a sea e s tº sº e < * * * 370/124; 370/76
`
`... 370/124, 76, 110.1,
`[58] Field of Search .....
`370/85, 89, 95; 179/2 DP, 18 AD; 340/825.5
`References Cited
`U.S. PATENT DOCUMENTS
`3,573,379 4/1971 Schmitz et al. ..................... 370/124
`4,020,289 4/1977 Anderson .............................. 370/76
`4,107,471 8/1978 Reed ..................................... 370/76
`4,262,171 4/1981 Schneider et al.
`370/124
`4,339,816 7/1982 Reed ..................................... 370/76
`4,373,183 2/1983 Means et al. .....
`... 370/85
`4,513,413 4/1985 Gorman et al. ......
`6/1985 Stapleford et al. ................. 370/124
`4,521,881
`OTHER PUBLICATIONS
`“Putting Broadband into Perspective”, Dec. 1984, pp.
`67–68.
`“Performance Issues in Local-area Networks” by W.
`Bux, vol. 23, No. 4, 1984, pp. 351–374.
`Primary Examiner—Douglas W. Olms
`
`[56]
`
`- - - - - - 370/76
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,646,296
`Feb. 24, 1987
`
`Attorney, Agent, or Firm—Nixon & Vanderhye
`[57]
`ABSTRACT
`A communication system is provided for the intercon
`nection of telephones and other instruments capable of
`communicating via the voice spectrum, the system pro
`viding a single transmission line such as a coaxial cable
`by which full duplex communication is provided be
`tween any pair of telephones. Each instrument is cou
`pled to the transmission line by a control unit which
`includes computer operated transceiver equipment for
`paging other ones of the control units to initiate a tele
`phonic communication. Each control unit is provided
`with an identification number and circuitry for recog
`nizing the identification number contained within a
`paging signal so as to initiate a response. In a separate
`spectral portion of the transmission line, there are pro
`vided sets of analog communication channels each of
`which has upper and lower spectral portions to provide
`two-way communication in opposite directions be
`tween a pair of telephones engaged in communication.
`A transmitter and a receiver in each of the control units
`is provided for the communication of analog signals of
`both voice and data via the voice channel, each control
`unit including circuitry for tuning the transmitter and
`the receiver to the selected channel in accordance with
`digital data transmitted via the transmission line and
`specifying a selected one of the voice channels.
`
`20 Claims, 7 Drawing Figures
`
`
`
`
`
`
`
`VOICE / DATA Hº?
`RANSCEIVER
`
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`

`

`4,646,296
`º
`
`U.S. Patent Feb. 24, 1987
`
`Sheet 1 of 4
`
`#
`
`
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`

`

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`U.S. Patent No. 4,646,296
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`

`

`U.S. Patent Feb. 24, 1987
`
`sheets of4
`
`4,646.296
`
`25KHz
`
`
`
`XTAL
`OSC
`66’
`
`
`
`|MH:
`FREQ
`DIVIDE
`68
`
`
`
`
`
`FIG. 4
`
`76
`
`74
`º
`FILTER
`
`
`
`VARIABLE
`FREQUENCY
`DIVIDER+ N
`
`
`
`
`
`
`
`ENTER ATN
`KEY PAD
`
`DETECT CONTROL
`
`CHANNEL CARRIER º
`
`YES
`
`
`
`
`
`
`
`| NO
`TRANSMIT CALL REQUEST
`WITH KEY PAD NUMBER
`
`FIG.5
`
`
`
`
`
`DETECT
`RESPONSE
`
`YES
`
`SET CARRIERS TO CHANNEL
`DESIGNATED IN RESPONSE
`
`
`
`
`
`
`
`SIGNAL THAT
`CIRCUIT IS READY
`
`YMAX CORPORATION v. FOCAL IP, LLC
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`U.S. Patent Feb. 24, 1987
`
`Sheet 4 of 4
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`4,646,296
`
`CALL REQUEST
`DETECTED
`
`YES_^pHONE
`BUSY
`NO
`DETECT CONTROL
`
`-
`
`DETECT CONTROL
`
`|- CARRIER |- CARRIER
`
`YES
`SEND Y
`'BUSY'ANSWER
`
`NO
`SEND
`\'RING|NG' ANSWER
`
`RING TELEPHONE |
`
`HANDSET NS
`LIFTED
`
`YES
`READ MEMORY FOR
`AVAILABLE CHANNEL
`
`COMPARE SENDER & TARGET
`10 NUMBERS FOR CHANNEL
`USAGE PROTOCOL
`
`-
`
`SET WOICE
`CARRIERS TO
`AVAILABLE CHANNEL
`
`DETECT CONTROL
`
`CHANNEL CARRIER
`
`N0
`
`-
`SEND CODE WORD
`DESIGNATING
`WOICE CARRIERS
`
`|
`
`FIG 6
`
`LOWER PORTION OF CHANNEL
`RECEPTION BY HIGHER NUMBERED UNIT
`TRANSMISSION BY LOWER NUMBERED UNIT
`UPPER PORTION OF CHANNEL
`TRANSMISSION BY HIGHER NUMBERED ' UNIT
`RECEPTION BY LOWER NUMBERED UNIT
`Hmºtº BAND
`| " |
`|
`|
`|
`|
`|
`\–V—’ e--- \–2–’ FREQUENCY
`WOICE
`VOICE
`CONTROL DATA
`CHANNEL
`CHANNEL
`-
`CHANNEL
`
`F| G. 7
`-
`
`|
`|
`
`YMAX CORPORATION v. FOCAL IP, LLC
`FOCAL IP, LLC EX2056 - 5
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`
`

`

`1
`
`10
`
`DISTRIBUTED TELEPHONE SYSTEM
`BACKGROUND OF THE INVENTION
`This invention relates to telephone systems for trans
`mission of both voice and data and, more particularly,
`to a system wherein all telephone sites are intercon
`nected by a common coaxial transmission line with
`switching and routing functions being performed by
`on-site control units which couple the telephones to the
`transmission line.
`Presently, there are two basic types of telephone and
`data switching systems in use. The first type of system,
`which is the most widely used, incorporates central
`office switching. Such systems include the private 13
`branch exchange (PBX) which operates in the manner
`of a miniature central office. The second type of system
`is the local area network (LAN) which employs digi
`tized voice and data transmission over a common cable.
`The central office or PBX type of system is charac- 20
`terized by the use of individual wires, such as a pair of
`wires or group of such pairs, to connect each telephone
`instument to a central switch. The overall configuration
`of such a system has the form of a star with the central
`office switching at the middle and the individual tele- 25
`phone instruments being connected to the central office
`by their respective telephone lines. The central switch
`and its control equipment performs all of the necessary
`management functions for providing the voice and data
`communications, thereby to allow individual telephone 30
`instruments to communicate with any other one of the
`telephone instruments. In recent years, the telephone
`management functions, whether in a central office or in
`an on-site PBX, have been expanded to include a vari
`ety of sophisticated capabilities such as call forwarding, 35
`call camp-on, conference calling, call back, autodial,
`and least-cost routing as well as providing statistical
`information for efficient management of the system.
`The major disadvantages of the central switching
`approach arise from the dependence on the star wiring 40
`configuration, this resulting in an excessively large size
`and complexity to the central switch for the intercon
`nection of thousands of telephone instruments. The star
`wiring configuration is not economical for small tele
`phone systems. Furthermore, the star wiring configura- 45
`tion requires very large amounts of wire because the
`telephone line must extend from the location of each
`telephone instrument all the way to the central switch.
`Changes in the instrument may require new wiring as is
`the case wherein a call director instrument is substituted 50
`for a single line instrument necessitating the installation
`of multiple telephone lines. If the original single line
`instrument were re-installed, the surplus wire would be
`useless.
`Other disadvantages arise with the star wiring config- 55
`uration in that dependence on the central switch causes
`system fragility because a switch failure shuts down the
`entire system. If more advanced functions are to be
`accommodated with the central switch, the switching
`equipment must contain a complex and powerful com- 60
`puter and very sophisticated software to manage the
`thousands of instruments simultaneously, as well as to
`switch voice traffic among them. Such complexity fur
`ther increases the fragility of the central office switch.
`The LAN incorporates a number of interconnected 65
`on-site PBX's via a common cable capable of high speed
`data transmission. Each PBX is substantially smaller
`than the usual central office switch. The resulting sys
`
`4,646,296
`2
`tem has the form of clusters of star-wired PBX units
`connected to each other by a common cable network
`which may be configured in either the shape of a ring or
`a star configuration. While such a system may employ
`somewhat less wire than a central office system accom
`modating the same number of instruments, the system
`suffers, essentially, from the same disadvantages as the
`aforementioned central office system. Although each
`PBX unit is smaller and therefore less complex than the
`central office system, the PBX unit must be provided
`with the necessary functions to permit the unit to coor
`dinate and connect with other ones of the PBX units.
`Another approach to the design of a LAN has been
`the use of a common cable to physically connect indi
`vidual instruments or data terminals. Significantly,
`these systems have all relied on digital signal processing
`implementations to allow the multiplexing of both voice
`and data signals in high speed transmission bands of the
`common cable. Such digital signal processing has lim
`ited the practical capacity of the cable to a few hundred
`simultaneous conversations, in contrast with a desired
`capacity of carrying tens of thousands of conversations.
`The poor utilization of the digital approach results in
`such impracticality that the LAN’s have been used
`primarily to carry high speed data traffic instead of
`voice communication.
`Attempts have been made to enlarge the signal-carry
`ing capacity of the cable by frequency division wherein
`plural transmission bands are provided to carry digital
`signals with the bands being spaced apart along the
`frequency spectrum. Such frequency division adds
`complexity to the coordinating and the connecting of
`communications among the plural transmission bands,
`thereby rendering such a system to be impractical for
`voice transmission. Additionally, broadband telephone
`transmission devices have heretofore required manual
`adjustment of their transmitting power levels to insure
`that all signals received on the common cable are of
`substantially the same strength. While the deployment
`of large numbers of such broadband devices would be
`desirable in the use of a telephone system, the manual
`adjustment of power levels would render such deploy
`ment impractical.
`SUMMARY OF THE INVENTION
`The foregoing disadvantages are overcome and other
`benefits are provided by a telephone system which, in
`accordance with the invention, is constructed with a
`transmission line that connects with the site of each
`telephone in the system. Preferably, a coaxial cable is
`employed as the transmission line. Connection of each
`telephone to the coaxial cable is accomplished with the
`aid of an intelligent control unit which controls all
`switching and routing of communication between the
`telephone and other telephones of the system.
`Both conversation and control signals are communi
`cated via the coaxial cable. Voice and data signals in
`analog format are transmitted between telephones via
`any one of a set of relatively narrow bandwidth chan
`nels within the relatively broad frequency passband of
`the cable. A single channel of relatively wide band
`width is reserved for the transmission of control signals
`in digital format.
`.
`The system comprises many control units with one
`control unit being provided at each site. The control
`units are identical to each other. Each control unit can
`be employed with a telephone handset or other source
`
`YMAX CORPORATION v. FOCAL IP, LLC
`FOCAL IP, LLC EX2056 - 6
`U.S. Patent No. 4,646,296
`IPR2016-01258
`
`

`

`10
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`15
`
`35
`
`-
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`4,646,296
`3
`4
`of telephonic signal such as a facsimile transceiver for
`mission of voice/data signals and a transceiver 60 for
`the the transmission and reception of picture informa
`the transmission and reception of control signals.
`tion. Each control unit includes a microcomputer, two
`Signals of a handset 52 are converted to RF signals by
`the transceiver 58, the RF signals then propagating
`transmitters and two receivers. One of the transmitters
`along the transmission line 56 to all of the control units
`and one of the receivers are used for the transmission
`and reception of radio-frequency (RF) narrow band
`54. In one of the control units 54, the transceiver 58 is
`telephone signals for voice and data communications.
`tuned to the frequency of the RF signal and, in that
`The second transmitter and the second receiver are
`control unit 54, the transceiver 58 receives the RF sig
`used for wide band RF transmission and reception of
`nal and translates the signal back to audio frequency for
`control signals generated by the control units for the
`reception by the handset 52 connected to that control
`switching and the routing of the telephone signals on
`unit 54. The transmission line 56 has a bandwidth which
`is sufficiently wide to carry many channels, each of
`the narrow bandwidth channels.
`The microcomputer is programmed to perform the
`which is at a separate portion of the frequency spec
`various operations necessary to effect the various func
`trum, and each of which can carry a voice conversation
`tions to be provided by the system, this programming
`between a pair of the control units 54. In addition, the
`including central-office channel selection and switching
`bandwidth of the transmission line 56 is sufficiently
`functions presently employed both in central offices and
`wide to incorporate a broad band channel having suffi
`private branch exchanges. If desired, the system can
`cient bandwidth for the propagation of digital control
`utilize an existing two way coaxial cable system cur
`signals between the transceivers 60 and the respective
`rently in use for the transmission of cable television or
`control units 54. The transmission line 56 may be a
`20
`other communications. In particular, the microcom
`coaxial cable or waveguide for the case of RF signal or,
`puter incorporates memory for storing the use and
`may be fabricated as a light pipe for the propagation of
`availability of the narrow band channels, and for select
`optical signals in which case the transceivers 58 and 60
`ing one of the channels in accordance with a preestab
`would have suitable well-known devices for the con
`lished protocol in response to a request to initiate a
`version of electrical signals to optical signals. In the
`25
`telephonic communication. The microcomputer also
`preferred embodiment of the invention, the transmission
`generates a data control word including an identifica
`line 56 comprises a coaxial cable such as that utilized for
`tion number of the telephone site which is to be called.
`the two way transmission of cable television programs.
`... The control word is received by all control units of the
`In the use of the system 50, control signals are trans
`system, and the control unit at the desired site responds
`mitted along the line 56 to initiate a telephonic commu
`30
`by way of a further control word identifying the chan
`nication between a calling party, or sender of the mes
`nel to be utilized in the communication. In the event
`sage, to another telephone or target which is to receive
`that the telephone at the desired site is in use (line is
`the message. It is, noted that while handsets 52 are
`busy), or in the event that no person answers the phone,
`shown by way of example, is to be understood that
`other control words are transmitted by way of response
`communication can be maintained between other de
`to indicate the condition of the telephone. It is noted
`vices such as facsmile in which signals are generated
`that all control units receive all messages transmitted
`within the audio spectrum for transmission over the
`* along the control signal channel, including channel
`usual telephone channel. As will be described in further
`usage, such information being most useful in the perfor
`detail, hereinafter, the control signals transmitted be
`mance of more elaborate functions such as conference
`tween the transceivers 60 are used for tuning the trans
`40
`calling.
`ceivers 58 to a common channel so that persons located
`at the sender site and the target side can converse with
`BRIEF DESCRIPTION OF THE DRAWING
`each other. In order to initiate a conversation, each
`The aforementioned aspects and other features of the
`control unit 54 is provided with an identification num
`invention are explained in the following description
`ber. Thus, when a sender wishes to call a target, the
`45
`taken in connection with the accompanying drawing
`control signal transmitted by the sender transceiver 60
`includes the identification number of the control unit 54
`FIG. 1 shows the configuration of the system of the
`associated with the target.
`invention including a set of telephone instruments cou
`Referring also to FIG. 2, there is provided a more
`pled by a corresponding set of control units to a com
`detailed description of components of the control unit
`50
`54. Each control unit 54 comprises two frequency syn
`mon transmission line;
`FIG. 2 is a block diagram showing components of a
`thesizers 1 which are further identified by the legends A
`and B such that the synthesizer 1A generates a carrier
`control unit of FIG. 1;
`FIG. 3 shows a data control word format for signals
`frequency in the upper half of a voice/data communica
`communicated between control units;
`tion channel of the line 56 for the transmission of a
`FIG. 4 shows a block diagram of a frequency synthe
`message in one direction between the parties; the syn
`thesizer 1B generates a carrier frequency at the lower
`sizer employed within the control units of FIG. 2;
`-
`FIGS. 5 and 6 are flow charts showing the operation
`half of the selected communication channel for trans
`of a microcomputer in the control unit of FIG. 2; and
`mission of the message in the reverse direction between
`FIG. 7 is a graph showing spectrum utilization.
`the two parties. Thereby, by the use of the two carriers
`within the same communication channel, full duplex
`DETAILED DESCRIPTION
`communication is attained. Typically, a voice/data
`With reference to FIG. 1, there is shown a telephone
`spectrum has a bandwidth in the range of 3000 hertz
`system 50 constructed in accordance with the inven
`(Hz) to 3500 Hz. Two of the foregoing voice spectra are
`tion. The system 50 includes a set of telephone handsets
`incorporated within a single communication channel to
`52, each of which is connected by a control unit 54 to a
`provide for the full duplex capability, the center fre
`quencies of the two spectra being spaced apart in a
`common transmission line 56. Each control unit 54
`includes a transceiver/58 for the reception and trans
`range of 7 kilohertz (kHz) to 15 kHz with a nominal
`
`wherein:
`
`55
`
`60
`
`65
`
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`5
`6
`value of 10 kHz being utiized to inhibit crosstalk be
`are modulated onto a carrier by the modulator 19 and
`tween the two spectra of the communication channel.
`transmitted by the switch 21 and the transmitter 22 to
`As will be shown with reference to FIG. 4, each of the
`the other control units 54 of the system 50. Control data
`synthesizers 1 incorporates digital circuitry which is
`signals emanating from other ones of the control units
`responsive to a multiple-bit digital signal provided by a
`54 of the system 50 are received at the receiver 23 and
`set of K1 frequency-data input lines 2.
`demodulated by the demodulator 20 to be applied to the
`microcomputer 15.
`Each control unit 54 further comprises a selector
`switch 5 connected by lines 3 and 4 to the synthesizers
`Signals coupled between the transmission line 56 and
`1A-1B respectively. The switch 5 is operated in re
`each of the transceivers 58 and 60 are connected via
`sponse to a signal on a control line 6. The switch 5
`variable attenuators 26 and 29, the attenuators 26 and 29
`provides the capability of selecting either the upper or
`being of a well-known form for attenuating RF signals
`in response to multiple-bit digital signals applied via
`lower carrier frequency to be utilized for sending or
`receiving a message in accordance with a protocol to be
`lines 25 and 28 from the microcomputer 15 to the atten
`uators 26 and 29, respectively. Connection between the
`described hereinafter.
`The voice/data transceiver 58 in each control unit 54
`attenuator 26 and the transmission line 56 is made by
`comprises a demodulator 8 and a modulator 9 which
`coaxial cable 27, and connection between the transmis
`connect with the earpiece and mouthpiece, respec
`sion line 56 and the attenuator 29 is made by coaxial
`tively, of handset 52.
`cable 30. The attenuator 26 is used for adjusting the
`intensity of signals transmitted by the transceivers 58
`Also included within the transceiver 58 are a switch
`and 60 to a suitable amplitude for reception by other
`11, a receiver 12 and a transmitter 13. RF signals cou
`20
`pled to the transceiver 58 by the transmission line 56 are
`ones of the control units 54. The attenuator 29 is utilized
`for adjusting the intensity of signals received from other
`received by the receiver 12, the RF signals then being
`ones of the control units 54 to a suitable signal intensity
`demodulated by the demodulator 8 and then outputed
`along line 7 to be heard by the handset 52. Signals spo
`for reception by the transceivers 58 and 60.
`ken into the handset 52 are inputed by a line 10 to the
`In view of the fact that all of the control units 54
`25
`modulator 9 which then modulates the telephone signal
`transmit over the same transmission channel, with re
`onto a carrier. The modulated carrier signal produced
`spect to the transmission of the control data signals,
`each microcomputer computer 15 monitors the trans
`by the modulater 9 is coupled by the switch 11 to the
`mission line 56 for the presence of control data signals
`transmitter 13 for transmission by the transmission line
`of other ones of the control units 54 prior to the genera
`56 to a distant control unit 54 for the switch contacts of
`30
`the switch 5 as portrayed in FIG. 2, the carrier fre
`tion and transmission of a control data signal. The moni
`quency produced by the synthesizer 1A is employed by
`toring of the transmission line 56 is accomplished by
`the demodulator 8 in the demodulation of incoming RF
`means of a detector 31 of the carrier of such other trans
`signals while the carrier frequency produced by the
`missions, the carrier detector 31 being coupled to the
`snythesizer 1B is employed by the modulator 9 and the
`receiver 23 for the detection of the carrier of such other
`generation of the outgoing RF signal. Typically, the
`control data signal transmissions. Also included within
`each control unit 54 is an oscillator 32 for providing the
`modulation utilized by the demodulator 8 and the mod
`ulator 9 is phase modulation, it being understood that
`carrier to the modulator 19 and the demodulator 20 of
`other well-known forms of modulation maybe utilized if
`the transceiver 60.
`The control data digital words transmitted between
`desired.
`The control unit 54 further comprises a keyboard or
`transceivers 60 of the various control units 54 has the
`keypad 14 and microcomputer 15, the keypad 14 being
`form presented in FIG. 3. Such control word includes
`used by the sender party to enter the identification num
`the line number identifying the sender, the line number
`ber into the computer 15 for the target party which is to
`identifying the target, a transaction code such as an
`be called. The computer 15 is coupled by the foregoing
`indication that the party wishes to initiate a communica
`lines 2 and 6 respectively to the synthesizers 1 and the
`tion, and when required a channel number identifying
`switch 5, and is also coupled by a control line 16 to the
`the voice/data channel which is to be employed in
`switch 11 for initiating and terminating the connection
`communicating between two of the control units 54.
`In FIG. 2, a ring signal generator 64 provides a ring
`of analog signals to the imput terminal of the transmitter
`signal under command of the microcomputer 15. In
`13. The transceiver 60 for the transmission and recep
`tion of control signals is coupled to the microcomputer
`addition, the microcomputer 15 can be employed for
`15 by lines 17 and 18, the line 17 coupling incoming
`the introduction of audible signals to the handset 52 via
`control data to the microcomputer 15, while control
`the line 33 , such as when the microcomputer 15 re
`data is outputed from the microcomputer via line 18 to
`ceives a data word via the transceiver 60 indicating that
`the telephone at the desired control unit 54 is in use.
`55
`the transceiver 60.
`.
`The transceiver 60 comprises a modulator 19, a de
`FIG. 4 shows details in the construction of the fre
`quency synthesizer 1A. The synthesizer 1B has the same
`modulator 20, a switch 21, a transmitter 22 and a re
`form as the synthesizer 1A and, accordingly, only the
`ceiver 23. The components of the transceiver 60 operate
`as do the corresponding components of the transceiver
`synthesizer 1A will be described to facilitate the de
`scription of the invention. The synthesizer 1A comr
`58, except that the components of the transceiver 60
`operate at a single carrier frequency. The modulator 19
`pises a crystal oscillator 66, a frequency divider 68, and
`a phase locked loop 70. The loop 70 comprises a phase
`and the demodulator 20 operate with FSK (frequency
`shift keying) or another well-known form of modula
`detector 72, a low pass filter 74, a voltage controlled
`tion for the transmission of digital data along the trans
`oscillator 76, and a variable frequency divider 78. By
`way of example in the construction of the synthesizer
`mission line 56. A control line 24 from the microcom
`65
`1A, the oscillator 66 provides an output signal at a fre
`puter 15 operates the switch 21 to initiate and terminate
`quency of one megahertz which is applied to the divider
`the transmission of digital signals by the transmitter 22.
`Control data signals initiated by the microcomputer 15
`68. Typically, the divider 64 comprises a counter, as is
`
`45
`
`50
`
`60
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`YMAX CORPORATION v. FOCAL IP, LLC
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`U.S. Patent No. 4,646,296
`IPR2016-01258
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`4,646,296
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`8
`well known, which counts cycles of the oscillator out
`place as follows. The control data receiver 23 in each of
`put signal. By way of example, the divider 68 has a
`the units 54 receives, over the control data channel, any
`division ratio of forty resulting in an output signal of 25
`words transmitted by other ones of the control units 54.
`kHz which is applied to the phase detector 72 of the
`When a user at unit X wishes to connect unit X to unit
`loop 70. The oscillator 76 applies an output signal to the
`Z, the user at X enters the number “430” into his mi
`crocomputer 15 via the keypad 14. The microcomputer
`divider 78 which operates in a manner analagous to that
`of the divider 68 to provide an output signal at a fre
`15 then checks the detector 31 for the presence of a
`quency of 25 kHz to the reference input terminal of the
`carrier on the control data channel.
`phase detector 72. The detector 72 provides a loop
`When no carrier is detected, the microcomputer 15
`error signal proportional the difference in phase be
`activates the switch 21 to transmit a control data word,
`10
`tween the input and reference signals, the error signal
`having the format disclosed in FIG. 3, via the modula
`being filtered by the filter 74 in a well known manner to
`tor 19 and the transmitter 22; thereafter, the microcom
`provide a control voltage to the oscillator 76. Thereby,
`puter 15 turns off the switch 21 to terminate any further
`the frequency of the oscillator 76 is locked to the fre
`transmission. All of the units 54 receive the transmission
`quency at the output of the divider 68 by a fixed ratio,
`from the sender via the transmission line 56.
`N. The divider 78 comprises well known digital count
`In accordance with a feature of the invention, each of
`the units 54 maintains, in the computer memory RAM,
`ing circuitry wherein the ratio, N, is established in ac
`a table of the busy/free status of all voice communica
`cordance with digital words on the set of lines 2, these
`tion channels of the system 50. The microcomputer 15
`digital words being provided, as noted hereinabove, by
`the microcomputer 15. The output frequency on lines 3
`in each unit 54 is also programmed to select, from the
`20
`data received via the receiver 23, the information
`and 4 can be raised in increments of the 25 kHz fre
`quency appearing at the input to the phase detector 72.
`needed to maintain the status of other ones of the units
`Assuming, by way of example, that the butput fre
`54. When the microcomputer 15 of unit Z receives the
`quency on line 3 has a nominal value of 10 megahertz
`control data word transmitted by unit X, the microcom
`puter 15 of unit Z can than process the “call request”.
`(MHz), then the aforementioned nominal value of 10
`25
`The microcomputer 15 of unit Z then selects a currently
`kHz between the transmission and reception portions of
`a voice channel can be accomplished by setting the
`free channel from the channel status table and transmits
`synthesizer 1B to a frequency which differs from that of
`along the transmission line 56 a data control word con
`“the synthesizer 1A by 10 kilohertz (kHz). Thereby, the
`taining the sender's line numer (430 for unit Z), the
`carrier signals on lines 3 and 4 of FIG. 1 differ in fre
`target line number (210 for unit X), the transaction code
`quency by a nominal value of 10 kHz. As the carrier
`for “accepting call”, and the channel number to be used
`frequencies are scaled upwards and downwards by
`for the call.
`variations in the value of N, the frequency difference
`Upon receiving this control data word, the mi
`crocomputers 15 in all of the control units 54 designate
`between the carriers in lines 3 and 4 remains constant.
`This provides a desired frequency spacing between the
`the selected channel as being busy. Also, the microcorn
`35
`puter 15 in unit X sets the frequency-data input lines 2
`transmitted and received portions of a voice communi
`for the frequency synthesizers 1A-1B to the selected
`cation channel over a useful set of channels. The digital
`* 'words on lines 2 select the desired channel and also set
`channel.
`the carriers for the transmit and receive portions of the
`In accordance with a further feature of the invention,
`voice channels. Digitally controlled frequency synthe
`a protocol is established for each pair of units 54 com
`40
`sizers, such as the synthesizer 1, are commercial ly
`municating with each other wherein each unit 54 of the
`available, such as that provided by Radio Shack in the
`pair of control units 54 sets its selector switch 5 for
`citizen band radio model #TRC-451 or that provided
`transmission on the upper half of the voice/data channel
`if its “telephone line” number is higher than that of the
`by Heathkit in the “ham” radio model #HW-5400.
`In operation, the microcomputer 15 includes read
`other unit of the pair of control units 54, and to receive
`45
`only memory (ROM) and random access memory
`on the upper half of the voice/data channel if the line
`(RAM) with conventional input/output circuitry neces
`number of the control unit 54 is lower than that of the
`sary to exchange information with other components of
`line number of the other unit of the pair of control units
`54. In the foregoing example, unit Z has a higher num
`the control unit 54. The ROM contains the program
`ming logic which allows the control unit 54 to interact
`ber than unit X. Accordingly, unit Z will transmit on
`50
`with the other control units 54. The RAM provides
`the upper portion of the selected communication chan
`facility for storing information, such as identification of
`nel to unit X which is tuned to receive on the upper
`the sender, unique to the control unit 54.
`portion of the communication channel. Unit X will
`Upon installation of the system 50, each control unit
`transmit on the lower portion of the communication
`54 is provided with a unique “line” number