United States Patent
`
`[191
`
`[11] Patent Number:
`
`4,697,281
`
`Sep. 29, 1987
`[45] Date of Patent:
`O’Sullivan
`
`[54] CELLULAR TELEPHONE DATA
`COMMUNICATION SYSTEM AND METHOD
`
`[75]
`
`Inventor:
`
`Harry M. O’Sullivan, Red Oak, Tex.
`
`[73] Assignee:
`
`Spectrum Cellular Communications
`Corporation, Iuc., Dallas, Tex.
`
`[21] Appl. No.: 839,564
`
`[22] Filed:
`
`Mar. 14, 1986
`
`Int. Cl.“ ............................................ .. H04M 7/04
`[51]
`
`. . . . . .. 379/59; 455/33
`[52] U.S. Cl. .. . .. . . . . . . . .
`[58] Field of Search ....................... 379/59, 60, 63, 58;
`455/33
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,178,476 12/1979 Frost ................................... .. 379/57
`
`4,562,872 12/1985 Goldman et al.
`.
`370/80
`.......................... 371/37
`4,637,022
`1/1987 Burke et al.
`
`Primary Examz'ner—Robert Lev
`Attorney, Agent, or Firm——Sixbey, Friedman & Leedom
`
`ABSTRACT
`[57]
`The cellular telephone data communication system and
`method involves the use of a mobile data processing
`interface and a cooperating static data processing inter-
`face to effectively transmit data over a cellular tele-
`
`phone system. Each data processing interface includes a
`processor which operates in the transmitting mode to
`add an error control correction data format
`to data
`received from an external data source. The data is di-
`vided into packets and provided to a modem which is
`uniquely operated to eliminate the action of the modem
`scramble system and to remain active in spite of a car-
`rier signal loss. The modem is deactivated or discon-
`nected by a disconnect signal from the processor, and
`when carrier signal loss occurs, this disconnect signal is
`provided only after the lapse of a delay period without
`the resumption of the carrier signal. The error control
`correction data format causes a receiver to evaluate the
`received data for error and to retransmit an acknowl-
`edgment signal for each acceptable packet of received
`data. In the absence of an acknowledgment signal, the
`processor will again provide a data packet
`to the
`modem for retransmission. Also,
`the processor will
`determine the frequency of error in the received data
`from the acknowledgment signals and subsequently
`adjust the data packet size in accordance with this error
`frequency.
`
`25 Claims, 4 Drawing Figures
`
`Microfiche Appendix Included
`(1 Microfiche, 75 Pages)
`
`
`
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`Sep. 29, 1987
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`Sheet 3 of4
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`Sep. 29, 1987
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`1
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`4,697,281
`
`CELLULAR TELEPHONE DATA
`COMMUNICATION SYSTEM AND METHOD
`
`The present invention incorporates a microfiche ap-
`pendix with one microfiche having 75 frames.
`-
`TECHNICAL FIELD
`
`The present invention relates to telephone data com-
`munications systems generally, and more particularly to
`a data communication system which is adapted to effec-
`tively transmit a data stream over a cellular telephone
`network.
`
`BACKGROUND ART
`
`Modern computer and telephone system technology
`have made the transmission of computer originated data
`over conventional
`telephone lines a commonplace
`event.
`In such systems, a computer
`is connected
`through a suitable interface, such an a RS 232 interface,
`to provide serial data signals to a conventional wire line
`modem. With modems of this type, when signal quality
`changes induce errors in the modem data stream, an
`ARQ (Automatic Repeat Request) or packet repeat
`scheme is conventionally employed for controlling
`these errors. This requires a complete repeat of numer-
`ous bytes of data until such time as all of the bytes of
`data in the packet are received correctly. However, the
`low frequency of signal quality change induced errors
`in a wire line environment makes this an efficient
`method of controlling error.
`Current wire line modern technology provides a
`scrambled modulated signal to the telephone line which
`will not be interpreted by telephone equipment as a
`valid switch command. To accomplish this, conven-
`tional wire line modems are provided with a scrambler
`circuit which assures that the modulated signal is con-
`tinuously changing. This changing signal is used by the
`modem PLL (Phase Locked Loop) circuitry to provide
`synchronization, for without this scramble modulated
`signal, a static condition of the modem will cause the
`PLL to loose synchronization and the telephone equip-
`ment to interpret the static signal as a switch command.
`Conventional telephone modems have operated ef-
`fectively to interface computers with a telephone sys-
`tem for data transmission, but these modems do not
`operate effectively to provide data transmission over
`conventional cellular telephone equipment. In a cellular
`telephone system, data transmission must occur to and
`from a moving vehicle which may be passing between
`zones or cells in the system. For example, a city with
`cellular service is divided into a plurality of adjoining
`geographic cells, each of which has its own transmit/-
`receive antenna controlled by a mobile switching office.
`For conventional cellular telephone voice communica-
`tion, an automobile travelling through a city passes
`from cell to cell, and the signal is transferred from an-
`tenna to antenna. This transfer process interrupts com-
`munications for a brief period, normally a fraction of a
`second, and does not cause a problem for voice commu-
`nication. However,
`for data communications,
`this
`“hand-off” process results in significant problem if con-
`ventional wire line modems are used.
`A wire line modem for use with normal telephone
`equipment will disconnect upon experiencing a carrier
`signal loss. Thus, such a modem, when used with a
`cellular telephone system, will disconnect each time the
`vehicle in which the modem is mounted travels between
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`cells, for some carrier loss will always be experienced
`when the radio signal used for cellular telephone com-
`munication is switched between the low power trans-
`mission stations of adjacent cells.
`In the cellular telephone environment, numerous
`errors are induced into data transmission because of the
`problems associated with cellular telephone communi-
`cation. Echo and fading problems cause multiple bit
`errors in the data stream, and such problems occur
`frequently with a moving vehicle. For example, the
`transmitted signal may hit a building or other obstacle
`and bounce erratically or fade as the vehicle is shielded
`from the cell antenna. This high frequency of error in
`the data stream transmitted by cellular transmission
`renders the error correction protocol present in con-
`ventional wire line modems unsuitable for cellular use.
`Errors occur so frequently in a cellular environment
`that the number of repeat requests becomes large and
`data transmission efficiency is reduced below an accept-
`able amount. In some instances, errors may occur so
`often that a correct packet may never be received.
`Thus, the error correction protocol present in conven-
`tional telephone modems is unable to cope with the
`problems presented in a cellular environment.
`Finally, as previously indicated,
`the conventional
`wire line telephone modem incorporates scrambler cir-
`cuitry to ensure that the modulated signal is continu-
`ously changing to provide synchronization for
`the
`modem PLL circuitry. However, such scramblers em-
`ploy a polynomial which has the effect of increasing the
`number of bit errors received. If a single bit error occurs
`during the transmission of data, that single bit error will
`be presented when received, but in addition, that error
`will propagate through the scrambler polynomial and
`later cause two additional errors in the received data
`presented to the user. These errors, coupled with those
`normally inherent with a cellular telephone system, will
`completely overwhelm the error correction circuitry
`present in a conventional modem.
`In the past, systems have been developed for commu-
`nicating data between a plurality of geographical zones
`and a host computer by means of portable radios. Such
`systems are disclosed in US. Pat. Nos. 4,525,861 and
`4,545,071 to Thomas A. Freeburg. Although these pa-
`tented systems effectively provide data communications
`from a host computer throughout a geographical area
`divided into zones, they do not address the problems
`presented by hand-off or echoing and fading in a cellu-
`lar telephone system.
`DISCLOSURE OF THE INVENTION
`
`It is a primary object of the present invention to pro-
`vide a novel and improved method and apparatus for
`transmitting data signals over a cellular telephone sys-
`tem.
`
`Another object of the present invention is to provide
`a novel and improved method for transmitting data
`over a cellular telephone system by means of a modem
`connected to the cellular telephone system. The modem
`is maintained in the activated state for a predetermined
`time period after a loss of the system carrier signal be-
`fore it is permitted to deactivate and is caused to remain
`in the activated state after the loss of the carrier signal
`if the carrier signal resumes within the predetermined
`time period.
`A further object of the present invention is to provide
`a novel and improved method for transmitting data
`over a cellular telephone system by means of a modem
`
`6
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`3
`connected to the cellular telephone system which in-
`cludes adding error control correction data to the data
`signal before providing the data signal to the modem.
`Yet another object of the present invention is to pro-
`vide a novel and improved method for transmitting data
`over a cellular telephone system by means of a modem
`connected to the cellular telephone system which in-
`cludes repetitively providing a unique data byte to said
`modem during a break in the data signal to the modem.
`A further object of the present invention is to provide
`a novel and improved method for transmitting data
`over a cellular telephone system by means of a modem
`connected to the cellular telephone system which in-
`cludes adding error control correction data to a data
`signal before it is provided to the modem and the re-
`moving said error control correction signal from the
`data signal at the receiver before the data signal is pro-
`vided to a receiver use device.
`Yet another object of the present invention is to pro-
`vide a novel and improved method for transmitting data
`over a cellular telephone system by means of a modem
`connected to the cellular telephone system which in-
`cludes providing no scrambler polynomial
`in the
`modem, but instead adding error control correction
`data to the data signal before providing said data signal
`to the modem. This error control correction signal
`consists of a sliding packet ARQ wherein the packet
`size changes or slides based on the transmission quality
`of the transmission. The packet size is increased for a
`_,_.good transmission signal and decreased for a bad trans-
`.._:mission signal. This is combined with a forward error
`....correction signal.
`Another object of the present invention is to provide
`a novel and improved cellular telephone data communi-
`cation system for transmitting data from a computer
`over a cellular telephone unit. This system includes a
`-microprocessor which is connected between the com-
`..puter and a special cellular telephone modem to control
`the operation of the modem. The microprocessor pre-
`,.;vents modem disconnect upon the loss of a carrier sig-
`_..nal for periods less than a predetermined disconnect
`..period.
`A further object of the present invention is to provide
`a novel and improved cellular telephone data communi-
`cation system for providing communication over a cel-
`lular telephone network between a portable computer
`and a computer connected to conventional telephone
`lines by use of unique modems. The portable computer
`is connected to a modem maintained in a unique state
`for cellular transmission by means of a microprocessor
`which controls the operation of the modern. The micro-
`processor adds error control correction data to a data
`signal from the computer before the data signal is pro-
`vided to the modem. The data signal with the modu-
`lated error control correction data is provided by the
`modem to a cellular telephone transceiver which trans-
`mits the data to a receiving system capable of retrans-
`mitting the data over conventional telephone lines. At
`the central computer, a unique modem and micro-
`processor combination receives the data and removes
`the error correction control signals therefrom before
`providing the data signal to a use device.
`Yet another object of the present invention is to pro-
`vide a novel and improved cellular telephone data com-
`munication system for transmitting data from a com-
`puter over a cellular telephone unit. A microprocessor
`connected between the computer and a unique cellular
`modern senses a static condition of the modem and
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`provides a repetitive synchronization byte to the cellu-
`lar modem during a break in the data stream thereto.
`A further object of the present invention is to provide
`a novel and improved cellular telephone data communi-
`cation system for transmitting data from a computer
`over a cellular telephone network which incorporates a
`mobile data programming interface adapted to operate
`with a static data programming interface. Both such
`interfaces operate to either transmit or receive data and
`cooperate with an associated external computer used to
`provide a stream of data to be transmitted. Each inter-
`face includes a microporcessor which is programmed to
`adapt the transmitted data to the high error frequency
`prevalent in the cellular telephone environment and to
`control a modem operating in a unique mode for cellu-
`lar transmission.
`A still further object of the present invention is to
`provide a novel and improved cellular telephone data
`communication system for transmitting data from a
`computer over a cellular telephone network which
`incorporates a mobile data programming interface
`adapted to operate with a static data programming in-
`terface to overcome the effects of signal error causing
`factors in the cellular telephone system environment.
`The transmitting interface adds an error detection and
`correction format to the data signal and the receiving
`interface removes this format from a received data sig-
`nal which is sent to a use device. The receiving interface
`responds to the error detection and correction portion
`of the received signal to check the data for error and to
`either acknowledge receipt of acceptable data or to
`provide an error indication to the transmitting interface
`by withholding the acknowledgment. The transmitting
`interface evaluates the errors in the received data
`stream and varies a data packet repeat size for subse-
`quently transmitted data. Also the transmitting interface
`retransmits previously transmitted data which was
`found to be erroneous at the receiving interface.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram of the cellular telephone
`data communication system of the present invention;
`FIG. 2 is a diagram illustrating a manner in which the
`data signal to be transmitted by the cellular telephone
`data communication system of the present invention is
`modified to provide an error detection and correction
`capability;
`FIG. 3 is a flow chart showing the control functions
`of the microprocessor for a transmitting interface of the
`cellular telephone data communication system of the
`present invention; and
`FIG. 4 is a flow chart showing the data processing
`function of the microprocessor for a transmitting inter-
`face of the cellular telephone data communication sys-
`tem of the present invention.
`BEST MODE FOR CARRYING OUT THE
`INVENTION
`
`The system for transmitting data over a cellular tele-
`phone network of the present invention is indicated
`generally at 10 in FIG. 1. A vehicle mounted mobile
`cellular telephone system conventionally includes a
`transceiver 12 which transmits or receives voice signals
`in the radio frequency range by means of an antenna 14.
`Voice signals transmitted by the antenna 14 are re-
`ceived by an antenna 16 connected to a transceiver 18
`located in a specific cell area of the cellular telephone
`network. The transceiver 18 is connected to cellular
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`land line equipment 20 which is operative to transmit
`the received signal over conventional telephone lines
`22. Voice signals from the telephone lines 22 may also
`be transmitted by the transceiver 18 and the antenna 16
`back to the antenna 14 to be provided by the transceiver
`12 to a conventional mobile cellular telephone unit. The
`transceiver 12 is controlled by a cellular telephone sys-
`tem control unit 24 which is connected to the trans-
`ceiver by means of a cellular telephone bus 26. The
`cellular bus 26, for purposes of description, may be one
`which meets the AMPS (Advanced Mobile Phone Ser-
`vice) specifications determined by the Federal Commu-
`nications Commission during test of the cellular system
`in Chicago. These specifications indicate that control
`signals between the transceiver 12 and the control unit
`24 should be on an eight bit parallel party line bus and
`that the analog signals should be a differential signal
`with a nominal —-20 dbV level using a 24 wire intercon-
`necting cable. A very complete description of the
`AMPS system is provided in “The Bell System Techni-
`cal Journal”, 1979, Vol. 58, No. 1, pp 1-269.
`To this point, the cellular telephone system with
`which the present invention is combined is a conven-
`tional cellular telephone system, and the mobile portion .
`25
`of this system is connected to the mobile data program-
`ming interface 28 of the present invention. This mobile
`data programming interface includes a cellular interface
`30 which renders the remainder of the mobile data pro-
`gramming interface compatible with the cellular bus 26
`of a particular cellular telephone system. For example,
`with AMPS compatible cellular telephones, the cellular
`interface 30 would consist of an eight bit parallel I/O,
`port party line drivers and receivers, operational ampli-
`fiers providing differential driving and receiving analog
`conversion between the -20 dbV signal on the cellular
`bus and the defined levels required by an analog switch
`and conditioning system 32 for the mobile data pro-
`gramming interface. Cellular interfaces of this type are
`known, commercially available items, as exemplified by
`cellular interfaces sold by Motorola Corporation of 40
`Schaumberg, Ill. or Oki of Japan.
`The analog switch and conditioning system 32 is
`implemented using currently known switching technol-
`ogy. The system may incorporate CMOS analog
`switches operative in response to microprocessor gen-
`erated control signals to switch the state of signal pro-
`cessing operational amplifiers. Basically,
`this analog
`switch and conditioning system operates to selectively
`connect various components of the mobile data process-
`ing interface to the cellular interface 30 in a manner to
`be described in greater detail.
`The heart of the mobile data programming interface
`is a microprocessor 34 which provides control functions
`for the cellular interface 30 and analog switch and con-
`ditioning system 32 as well as for other portions of the
`mobile data programming interface to be described.
`This microprocessor may be a conventional 8/16 bit
`microprocessor, such as the Intel 8088 manufactured by
`Intel Corporation. The microprocessor includes ran-
`dom access (RAM) and read only (ROM) memory
`storage systems which contain the control and data
`error programs necessary to adapt computer data for
`cellular telephone transmission. The use of a separate
`microprocessor in the mobile data processing interface
`frees the limited memory which is normally available in
`portable computers for other uses.
`A serial data stream is provided to the microproces-
`sor 34 from an external portable computer 36 by means
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`of a conventional RS 232 interface 38 included within
`the mobile data programming interface. Data received
`by the microprocessor from the portable computer 36 is
`provided with unique error‘ correction signal informa-
`tion in the microprocessor before being provided to a
`modem 40.
`The modem 40 may be one of a number of conven-
`tional modems used for telephone wire line transmission
`which has test mode capabilities for deactivating certain
`modem functions. A particular commercially available
`modem suitable for use as the modem 40 is the AMI
`3530 modem manufactured by Gould Advanced Semi-
`conductors of 3800 Hemstead Road, Santa Clara, Calif.
`Modems of this type, when employed for data transmis-
`sion over conventional telephone lines, will disconnect
`immediately in response to a carrier loss. When such
`modems sense a channel blanked status occasioned by a
`carrier loss, they provide a “break bit" output and dis-
`connect. Also, for normal use such modems include a
`scrambler system which assures that the data modulated
`signal is continuously changing, and this signal change
`is used by the modem PLL circuitry to provide syn-
`chronization. A non-scrambled modulated signal may
`be interpreted by the telephone operating equipment as
`a valid switch command, and this is particularly true
`when the modem is in the static condition. Normally the
`scrambler system in the modem prevents this static
`condition where loss of synchronization by the PLL or
`the interpretation of the static signal as a switch com-
`mand is most likely to occur. However, modems such as
`the AMI 3530 incorporate a test mode of operation
`wherein the modem is prevented from disconnecting in
`response to carrier loss and wherein the modem scram-
`bler can be deactivated or defeated. Normally, such
`modems would be incapable of effective operation in
`this test mode, but it is the availability of this test mode
`that renders modems of this type suitable for use as the
`modem 40.
`The data stream from the microprocessor 34, which
`has been provided with unique error correction data by
`the microprocessor,
`is transmitted by the modem 40
`through the analog switch 32 and cellular interface 30
`to the transceiver 12. This data is then transmitted as a
`radio frequency signal by the antenna 14 to the antenna
`16, where it is converted by the transceiver 18 and
`cellular land line equipment 20 to a signal suitable for
`transmission over conventional
`telephone lines 22.
`These telephone lines connect the signal to the second
`portion of the cellular transmission system of the pres-
`ent invention which is a static data programming inter-
`face 42 operative to pass data signals to and from the
`telephone line 22. The data signals which are passed to
`the telephone line originate at a host computer 44 which
`cooperates with the static data programming interface
`in a manner similar to the operation of the portable
`computer 36 with the mobile data programming inter-
`face 28.
`For transmission purposes, the computer 44 provides
`data to an RS 232 interface 46 which in turn provides
`the data to a microprocessor 48. This microprocessor is
`identical in construction and function to the micro-
`processor 34, and is programmed with the same control
`and error correction and other programming. The mi-
`croprocessor 48 adds error correction and control sig-
`nals to the data provided from the RS 232 interface, and
`then provides the modified data stream to a modem 50.
`The modem 50 is identical in construction and function
`to the modem 40, and operates to transmit the data
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`stream by means of an analog switch and conditioning
`system 52 to a conventional FCC interface 54. The FCC
`interface provides the data stream to the telephone lines
`22 where they are sent by means of the cellular land line
`equipment 20 to the transceiver 18. The data is then
`transmitted to the transceiver 12 which provides it by
`means of the cellular interface 30 and the analog switch
`32 to the microprocessor 34. The microprocessor then
`removes the error correction and control signals from
`the data stream and provides the data through the RS
`232 interface 38 for display and use by the portable
`computer 36.
`The static data programming interface 42 receives
`data transmitted over the cellular telephone line, and
`this incoming data from the FCC interface 54 is fed by
`means of the analog switch 52 to the microprocessor 48.
`Here the error and control signals are removed from the
`data and the data is then displayed and/or used by the
`computer 44. It will be noted that the static data pro-
`gramming interface 42 and the mobile data program-
`ming interface 28 are substantially identical in structure
`and operation with the exception that the static data
`programming interface is connected to telephone lines
`by an FCC interface while the mobile data program-
`ming interface is connected to a cellular telephone sys-
`tem by a cellular interface. Also, the mobile data pro-
`gramming interface may be powered from a vehicle
`battery 60 which operates through a conventional
`power converter 62 to provide power to a power bus
`‘ .64. This power bus 64 is connected to provide power to
`all of the operating units in the mobile data program-
`..ming interface 28 and may also be connected to provide
`power to the portable computer 36. The static data
`programming interface 42 includes a similar power bus,
`not shown, which is connected to any conventional
`power supply such as the power supply in a building
`containing the computer 44.
`Although the mobile data programming interface 28
`and the static data programming interface 42 must com-
`»._ municate with one another to effectively transmit data
`I over a cellular telephone system, both of these units can
`"also transmit and receive conventional cellular tele-
`phone audio transmissions. The mobile data program-
`ming interface 28 includes a telephone interface 66
`which may be connected to an external telephone-like
`handset 68 and which operates in conventional manner
`through the analog switch and conditioning system 32
`and cellular interface 30 to transmit and receive audio
`communications by means of the transceiver 12. In a
`similar manner, the static data programming interface
`42 includes a telephone interface 70 which may be con-
`nected to an external telephone to transmit and receive
`audio signals through the telephone interface, the ana-
`log switch 52 and the FCC interface 54. Thus, both the
`mobile data programming interface 28 and the static
`data programming interface 42 are adapted for normal
`audio communication. Like the mobile data program-
`ming interface, the static data programming interface
`includes a control and display section 72 which is con-
`nected to the microprocessor 48 and which operates to
`receive data from which the microprocessor has ex-
`tracted the control and error signals.
`Both the mobile data programming interface 28 and
`the static data programming interface 42 will operate
`with optional equipment, and an optional equipment
`block 74 is shown for the mobile data programming
`interface in FIG. 1. This optional equipment might
`include other modems, a microphone which may be
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`employed to provide audio communication in place of
`the cellular phone 68, and various memory and encrypt-
`ing devices known to the art to accomplish automatic
`dialing and similar functions.
`Before considering in detail the operation of the mo-
`bile data programming interface 28 and the static data
`programming interface 42, it is necessary to understand
`the manner in which the microprocessor 34 and modem
`40 and the microprocessor 48 and modem 50 cooperate
`to adapt a data signal for cellular telephone transmis-
`sion. With reference to FIG. 2, there is diagramatically
`illustrated a data document 76 to be transmitted which
`has originated with the portable computer 36. This data
`document is divided, by the microprocessor 34, into a
`plurality of packets 78, and for purposes of illustration
`in FIG. 2, four packets of equal size are shown. In actu-
`ality, a document would be divided into many more
`packets which would not necessarily be of equal size.
`Further, each packet is divided by the microprocessor
`34 into a plurality of words, and for purposes of illustra-
`tion in FIG. 2, each packet 78 includes two words 80.
`Again, a packet would normally contain many more
`than two words, but two are shown for purposes of
`illustration. Each word in a packet includes three bytes
`a, b, and c, and the microprocessor 34 will determine
`whether the word is a control word or a data word.
`When the word is a data word, then bytes a and b will
`be data bytes, while byte c is a foward error correction
`(FEC) byte. Conversely, if the word is a control word,
`byte a will be the control word signifier, byte b will be
`the control word descriptor and again, byte c will be
`the FEC byte. Thus it will be noted that byte c is always
`the FEC byte for both data and control words.
`Data words are always synchronous with the packet
`stream, while control words may or may not be asyn-
`chronous to data. An example of an asynchronous con-
`trol word would be an acknowledgement word, while
`an example of a synchronous control word would be
`the packet “end” word which is the last word in every
`packet. Synchronous control words are added into the
`cyclic redundancy check (CRC) which is included in
`the packet “end” word, while asynchronous control
`words do not affect the CRC of a packet. An exemplary
`form for an asynchronous control word, for example,
`the acknowledgement word, would be a control word
`signifier for byte a, the packet number for byte b, and
`byte c, the FEC byte. On the other hand, an example of
`a synchronous control word, such as the packet “end
`word”, would be a control word signifier for byte a, a
`CRC byte for byte b and the FEC byte as byte c. A
`control word synchronous to the packet indicates that
`byte b is a data rather than a control byte.
`The FEC byte causes the receiving microprocessor
`to check the data bytes in a data word and determine
`whether or not an error exists in that word. If an error
`is detected, the microprocessor 48 will use the FEC
`byte to correct the word at reception. However, if a
`predetermined error level in any word within a packet
`is exceeded, an acknowledgment signal for the packet
`will not be transmitted back to the transmitting micro-
`processor, thereby causing this microprocessor to re-
`transmit the entire packet. For example, the predeter-
`mined error level could be a specified number of bits per
`word, for example 2 bits. Correction would occur for
`any error of 2 bits or less, but an error in excess of 2 bits
`would result in no acknowledgment being transmitted
`for the packet.
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`The microprocessor unit 48 operates in a manner
`identical to that of the microprocessor 34 to form data
`packets with control and data words to transmit data
`provided from the computer 44, and the microprocessor
`34 operates in the receiving mode to check the data byte
`and pass acceptable data onto the computer 36. If the
`transmitting microprocessor 48 or 34 does not receive
`acknowledgement signals back from the receiving mi-
`croprocessor, correction, or other procedures to be
`described, are initiated by the transmitting microproces- 10
`sor.
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`sion efficiency, or in extremely high error situations,
`prevent the reception of a correct packet.
`The microprocessors 34 and 48 maintain the modems
`40 and 50 on line in the event of a carrier signal loss
`during data transmission. As previously indicated, such
`a carrier transmission loss occurs when the vehicle
`bearing the mobile data programming interrace 28
`passes between cells in a cellular telephone system. The
`modems 40 and 50 are operated normally in the test
`mode or a similar mode which prevents the modem
`from automatically disconnecting in r