United States Patent [191
`Dufresne et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,920,533
`Apr. 24, 1990
`
`[54] CATV SUBSCRIBER TERMINAL
`TRANSMISSION CONTROL
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`[75] Inventors: Michel Dufresne, Boucherville;
`Samir Sammoun, Brossard; Dave
`Gregory, Calgary; Jean-Paul
`Champagne, Brossard; Alain Tessier,
`Piedmont; Pierre Scott; Francois
`Methot, both of Montreal, all of
`Canada
`
`[73] Assignee: Videotron LTEE, Montreal, Canada
`
`[211 App]. No.: 262,802
`
`[22] Filed:
`
`Oct. 26, 1988
`
`Foreign Application Priority Data
`[30]
`Nov. 2, 1987 [CA] Canada ................................. .. 550764
`
`[51] Int. Cl.5 ............................................. .. H04N 7/08
`[52] U.S. Cl. ................................ .. 370/85.2; 370/94.1;
`455/5; 358/86
`[58] Field of Search ................... .. 370/85, 89, 94, 124,
`370/69.l, 123; 325/45, 46; 455/3, 4, 5, 6;
`340/8255; 358/84, 86; 380/10, 20
`
`4,533,948 8/ 1985 McNamara et al. .................. .. 455/ 5
`4,536,875 8/1985 Kume et al. ..... ..
`370/85
`4,553,161 11/1985 Citta ............. ..
`358/86
`4,623,920 11/1986 Dufresne et a1.
`380/10
`4,752,954 6/1988 Masuko ........ ..
`358/84
`4,773,065 9/1988 Kobayashi et a1. ................. .. 370/85
`Primary Examiner-Douglas W. Olms
`Assistant Examiner-Alpus H. Hsu
`Attorney, Agent, or Firm-Antonelli, Terry & Wands
`[s7]
`ABSTRACI‘
`A bidirectional cable communication system compris
`ing a head and, a bidirectional transmission network
`connected to the head end for transmitting signals
`downstream from the head end to a plurality of sub
`scriber stations and for transmitting data signals up
`stream to the head end from the subscriber stations,
`means at the head end for detecting collisions between
`signals received from the subscriber stations and for
`generating a collision signal, means at the head end for
`transmitting downstream on the transmission system
`status bits of data signal indicative of the collision,
`whereby upon detection thereof by a subscriber station
`it can be enabled to cease transmission.
`
`30 Claims, 6 Drawing Sheets
`
`SUBSCRIBER
`TERMINAL
`
`'rilunNATloN
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 1
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`

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`US. Patent Apr. 24, 1990
`
`Sheet 1 0f 6
`
`4,920,533
`
`DATA
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`TERMINAL
`
`FIG. 1
`
`TERMINATION
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 2
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`

`
`US. Patent .Apr.24,1990
`
`Sheet 2 on
`
`4,920,533
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`ARRIS GROUP, INC.
`IPR2015-00635 , p. 3
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`

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`US. Patent
`
`Apr. 24, 1990
`
`Sheet 3 of 6
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`4,920,533
`
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`FIG.5
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 4
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`ARRIS GROUP, INC.
`IPR2015-00635 , p. 4
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`

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`US. Patent
`
`Apr. 24, 199$
`
`
`
`Sheet 4 of6
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`4,920,533
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`
`
`
`
`37
`
`BUSYBITIDLE
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` 34
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 5
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 5
`
`

`
`U.S. Patent
`
`‘
`
`BUSYBIT
`
`IDLE
`
` Apr. 24, 1990
`TRON/OFF
`
`FIG7A
`
`Sheet 5 of6
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`4,920,533
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`ARRIS GROUP, INC.
`IPR2015-00635 , p. 6
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`ARRIS GROUP, INC.
`IPR2015-00635 , p. 6
`
`

`
`U.S. Patent
`
`Apr. 24, 1990
`
`Sheet 6 of 6
`
`4,920,533
`
`FlG.7B
`
`ARRI_S GROUP, mc.
`IPR2015-00635 , p. 7
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 7
`
`

`
`1
`
`CATV SUBSCRIBER TERMINAL TRANSMISSION
`'
`CONTROL
`
`10
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`This invention relates to a bidirectional transmission
`system, and particular to one in which many subscriber
`stations can transmit data signals upstream to a head
`end.
`Cable transmission systems typically carry various
`TV signals downstream in a tree-type network contain
`ing a main trunk and branches from the trunk, with
`subscriber terminals connected to drops via splitters to
`the branches. Typical networks are fabricated of coaxial
`cable, with repeater ampli?ers spaced along the net
`work to maintain the signals to be distributed above a
`predetermined minimum level.
`.
`It has been an objective to allow signals to be origi
`nated at the subscriber terminals for transmission up
`stream to the head end as requests for service, for selec
`tion of programs, responses to polling, etc. However
`various problems as will be described below have pro
`hibited successful provision of services whereby signals
`could be originated at the subscriber terminals, passed
`upstream through the network, to the head end. Conse
`quently such systems have utilized a separate transmis
`sion network, e.g. telephone system or only pseudo
`interactive systems in which all programs or informa
`tion signals are presented to the subscriber terminals
`and the programs or information are selected at the
`terminals. In both cases the signals originating at the
`subscriber terminals are not passed upstream in the
`system.
`Two very signi?cant problems prohibited successful
`two-way communication via the same CATV network.
`With typically thousands of subscriber terminals having
`the possibility of transmitting upstream within the same
`frequency band, there is a signi?cant likelihood that
`several might be transmitting at the same time. The
`result at the head end is pollution of the signals received
`from one subscriber terminal by signals received from
`other simultaneously transmitting subscriber terminals.
`The problem is made worse by the existence of various
`time delays within,.the system presented to the signals
`transmitted from various subscriber terminals due their
`locations being at various different line lengths (trans
`mission distances) from the head end.
`The second problem involves noise gathering. It will
`be appreciated that signi?cant upstream noise is gath
`ered from the various branches feeding into a single
`trunk, which noise often can be signi?cantly higher
`than the signal level transmitted by a single subscriber
`terminal The problem is increased with the use of two
`way ampli?ers in the system, those amplifying in the
`upstream direction attempting to amplify whatever
`signals are applied to their inputs, e. g. noise. Such am
`pli?ers typically use automatic gain controls, which
`operate with maximum gain at the time of minimum
`signal, thus outputting maximum noise collected from
`all branches feeding into that ampli?er.
`The present invention is a cable transmission system
`which substantially reduces or eliminates the problem
`of collisions between signals from various subscriber
`terminals arriving at the head end at the same time. Also
`disclosed is means for substantially reducing or elimi
`nating the problem of noise gathering.
`The head end in the cable system of the present in
`vention provides television signals as in a normal
`CATV system, but also transmits data signals in a sepa
`
`55
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`65
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`4,920,533
`2
`rate downstream data channel. The signals in the data
`channel are transmitted in packets divided into time
`slots, the time slots being separated by a special byte of
`data referred to herein as a status byte. The status byte
`is formed of bits which are referred to herein as “busy
`bits”.
`The downstream data signal is frequency
`multiplexed with the TV signals, using a separate
`channel, and is transmitted downstream through the
`network to the subscriber terminals. The subscriber
`terminals each detect the busy bits and after decoding,
`determine whether the network is idle, busy, or
`whether a collision has occurred and if desired, a prior
`ity level. Each subscriber terminal if it has data to trans
`mit, transmits back to the head end, with a certain prob
`ability factor. The probability factor is modi?ed de
`pending on whether the busy bits which the terminal
`continuously receives indicates a collision (which indi
`cates that another subscriber terminal has transmitted,
`polluting its data signal). If a collision is detected, the
`probability factor is changed, and the subscriber termi
`nal retransmits the data from the time slot which has‘
`been polluted at a time dependent on the probability
`factor.
`The transmitting subscriber terminal also counts col
`lisions, i.e. the number of collisions indicated by the
`busy bits. If the number of collisions is in excess of a
`predetermined threshold, it recycles the probability
`factor and retransmits the entire data sequence at a time
`which is controlled by the new probability factor.
`Any subscriber terminal is inhibited from transmit
`ting if the busy bits which it receives indicate that the
`network is busy in the upstream direction. Thus the
`collision effect occurs following the time that the busy
`bits indicate that the upstream data channel is idle, data
`has begun to be transmitted by more than one subscriber
`terminal, and prior to the time that subscriber terminals
`receive busy bits from the head end which indicate that
`the upstream channel has been seized, i.e. is busy. Thus
`the collision effect occurs typically at the initiation of a
`new transmission by more than one subscriber terminal
`when the channel is idle, increasing with increasing
`traf?c.
`The probability of transmission factor is varied using
`a random number generator, which, coupled with the
`likelihood of different time delays for signals transmit
`ted from the various subscriber terminals within the
`system, decreases thelikelihood that more than one
`would transmit at the-same time, than if the probability
`factor was varied in the same way at each subscriber
`terminal Further, since the number of collisions causes a
`decreased probability that a subscriber terminal is al
`lowed to transmit, the heavier the traf?c, the more the
`likelihood that a subscriber terminal is automatically
`caused to wait for a longer random period to transmit.
`The system thus automatically compensates and adjusts
`for increased traf?c.
`The problem of noise gathering is substantially solved
`by causing each subscriber terminal to transmits its data
`using frequency shift keying at two spaced frequencies,
`and by using a narrow band ?lter, having two passbands
`just suf?cient to pass the two frequencies, in the up
`stream direction in each branch of the network. The
`downstream data signals are unaffected The ?lters thus
`block virtually all noise except a small amount in the
`two narrow passbands from passing upstream to the
`head end. The ?lters are preferably remotely address
`able so that they can be controllably switched to pass
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 8
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`

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`status.
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`FIG. 5 is a block diagram of the extraction logic
`signals or upon receipt of an upstream signal. It has been
`found that this structure successfully inhibits noise gath-.
`circuit shown in FIG. 4,
`FIG. 6 is a block diagram of the media access circuit
`ering.
`The prior art problems of collision of upstream sig
`shown in FIG. 4, and
`FIG. 7A and 7B form a schematic of the control
`nals in a bidirectional cable communication system and
`transmitter at the subscriber terminal.
`noise gathering are thus substantially overcome
`Brie?y, the invention is a directional cable communi
`FIG. 1 illustrates a bidirectional cable communica
`tion network in accordance with the present invention.
`cation system comprising a head end, a bidirectional
`However it should be noted that the invention could be
`transmission network connected to the head end for
`carrying signals downstream from the head end to a
`used in a star type network, a local area network or
`plurality of subscriber stations and for carrying data
`other kinds of networks. In the present embodiment, a
`signals upstream to the head end from the subscriber
`system is comprised of a head end 1 which communi
`stations. A circuit at the head end detects collisions
`cates via a wideband communication medium such as a
`between signals received from the subscriber stations
`coaxial cable 2 with a plurality of subscriber terminals 3.
`The terminals and head end 1 communicate via a net
`and generates a collision signal, and a further circuit at
`the head end transmits downstream on the transmission
`work, such as a well known tree network, but could be
`another form of non-looping network. The tree net
`system a status byte of data signal indicative of the
`work is comprised of bidirectional ampli?ers 4 and
`collision, whereby upon detection thereof by a sub
`splitters 5 which are connected via drops (not illus
`scriber station it can be enabled to, cease transmission.
`Preferably the head end collision detector also in
`trat'ed) to the subscriber terminals. The network is ter
`cludes apparatus for detecting whether or not signals
`minated at a matching impedance 6 in a well known
`are being received from any subscriber station and for
`manner.
`generating a busy or idle status byte signal respectively,
`For ease of description only one subscriber terminal 3
`the byte transmitting circuit generating the byte of data
`has been shown However it should be clear that sub
`signal indicative of the collision, the idle, or the busy
`scriber terminals can be connected to splitters distrib
`uted along the network in a well known manner.
`In a conventional network, ampli?ers 4 will be unidi
`rectional in the downstream direction, whereby they
`carry a plurality of television channels. The present
`invention, however, is directed to the class of networks
`which also carry data signals. Such signals have been
`provided in networks which carry data signals which
`are to be reconstructed into computer generated pic
`tures, computer programs, control signals for descram
`blers, etc. Such data signals are sometimes carried in the
`vertical interval of a television channel, but in the pres
`ent invention they are carried by a separate high speed
`data channel. Consequently the ampli?ers 4 should
`have the capability of transmitting the data channel in
`the downstream direction.
`In order to order speci?c services, the subscriber
`terminal 3 typically has a keyboard which generates
`signals, ultimately resulting in command or other data
`signals to be transmitted upstream to the head end.
`Several problems have characterized this upstream
`transmission, with the result that few, if any such practi
`cal systems exist. The present invention provides means
`for reliably transmitting data upstream from a large
`number of ' subscriber terminals. It should be noted,
`however, that bidirectional ampli?er 4 should also have
`the capability of transmitting the data signals received
`from the subscriber terminals in the upstream direction.
`Such ampli?ers exist and the present invention is not
`speci?cally directed to them.
`Problems associated with upstream transmission in
`clude noise gathering and data collisions. Noise gather
`ing involves the generation of noise from the many
`branches of e.g. the tree network, which all feed into
`the main trunk or port of the system. The noise is ampli
`?ed by each of the ampli?ers, resulting in a noise level
`at the head end which is excessive.
`Data collisions exist when more than one subscriber
`terminal transmits data signals in the upstream direction
`which arrive overlapping in time at the head end.
`Clearly this causes corruption of all data signals which
`overlap.
`The solution to the problem of avoidance of noise
`gathering is not the subject of the present invention. In
`
`It is also preferred that the status byte of data should
`be transmitted separating time slots of a downstream
`data signal, in which a maximum round trip propagation
`trip delay, being the sum of the delays of all components
`in the transmission loop to the longest delayed end of
`the network in both downstream and upstream direc
`tions, de?nes the time period between successive status
`bytes which should be at least as long as said propaga
`tion delay.
`The invention is also a subscriber terminal for use in
`a bidirectional cable communication system comprising
`apparatus for repetitively receiving a data signal from a
`bidirectional transmission network comprised of a sta
`tus byte indicative of the status of the network including
`signal collision, busy or idle, and preferably priority,
`apparatus for formulating and storing a data signal to be
`transmitted via the’ network to the head end, apparatus
`for transmitting the data signal to the head end via the
`network, apparatus for controlling the apparatus for
`transmitting, the apparatus for controlling comprising
`apparatus for receiving the status byte and for enabling
`transmission of the data signal with a predetermined
`probability P, apparatus for counting the number of
`signal collision status signals received and for adjusting
`the probability P in accordance therewith.
`It is preferred that the apparatus for controlling inhib
`its transmission for a period of time if the number of
`signal collision status signals received exceed a prede
`termined maximum, thereupon reinitiating retransmis
`sion of the complete data signal.
`A better understanding of the invention will be ob
`tained by reference to the detailed description below of
`a preferred embodiment, with reference to the follow
`ing drawings, in which:
`FIG. 1 is a block diagram of a bidirectional cable
`communication network in accordance with the inven
`tion,
`FIG. 2 illustrates the downstream data stream with
`time,
`FIG. 3 illustrates a data packet with time,
`FIG. 4 is a block schematic of a subscriber terminal in
`' accordance with the invention,
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`IPR2015-00635 , p. 9
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`the present invention the problem of collisions is solved,
`by the use of so-called “busy bits” which are transmit
`ted from the head end downstream to the subscriber
`terminals. A byte of two busy bits de?nes the busy or
`idle quality of the channel. The two bits can of course
`de?ne four different states, i.e. idle, busy, collision and
`a fourth state which can be used as a priority indicator
`or other information. However other number of busy
`bits can be used if desired.
`In general, the head end transmits on a regular basis,
`busy bits indicatinq an idle channel, preferably using
`AM modulation of a courier. If it receives data signals,
`it changes the character of the busy bits to de?ne a busy
`condition. Upon analysis of the data, the head end de
`tects collisions between received data signals, and if it
`detects such collisions, it changes the busY bits to indi
`cate a collision condition.
`The subscriber terminal, on the other hand, transmits
`the upstream data, preferably in a frequency shift keyed
`form (e. g FSK) It constantly monitors the downstream
`busy bits, and transmits when the channel is indicated as
`being idle (assuming that there is some data to transmit).
`If, while transmitting, it detects collisions between busy
`bits, it stops transmitting, waits for a period of time as
`will be described below, and then retransmits the signal.
`In order to minimize noise on the trunk ?lters 7 are
`connected in series with various branches of the distri
`bution network, each ?lter having sharply de?ned nar
`row bandpasses for the upstream frequency shift data,
`each ?lter being connected in the transmission upstream
`direction. This reduces all upstream noise except that
`which is in the bandwidth of the ?lter. To improve the
`upstream data transmission performance preferably
`?lters are used which will remain shut off until they
`sense the presence of upstream carrier signal in the
`actual bandwidths of the ?lter, whereupon they will
`quickly open; e g. within microseconds, allowing the
`upstream data signal to pass. In addition, the ?lters
`could be addressable from the head end to open and
`close, for network maintenance purposes. The ?lters are
`transparent in the downstream direction; the television
`signals and downstream data signals pass through these
`?lters in the downnstream direction.
`Within the head end a source of television signals is
`connected to a modulator 8, which has its output con
`nected through channel filters 9 to the coaxial cable.
`Television signals are thus applied to the network as in
`the prior art.
`Receive bandpass ?lter 10 has its input connected to
`the coaxial cable 2, and its output to a data demodulator
`11. The frequency keyed data transmitted from sub
`scriber terminal 3 thus is output from demodulator 11.
`Also connected to the output of ?lter 10 is a collision
`detector 12. Collision detector 12 detects the presence
`of overlapping bits preferably by means of envelope
`detection. Since that type of collision detection must be
`very fast, it is preferably done in broadband.
`Logic within collision detector 12 also provides the
`pair of bits which is referred to herein as the busy bits,
`designating the four states referred to above, idle, busy,
`collision and priority or unde?ned. These bits, desig
`nated as IEC or channel state indicator are applied to a
`communication controller 13. Here the busy bits are
`interleaved (multiplexed) into a downstream data
`stream, which is applied to modulator 8. Modulator 8
`applies the combined data stream into a high speed data
`channel, from which the output is treated as described
`earlier with reference to the television signal source.
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`The busy bits preferably are sent through the down
`stream channel at 4 M bits per second periodically at the
`beginning of each time slot, as shown in FIG. 2 which
`illustrates the downstream data stream with time. The
`busy bits illustrated by arrows 18 therefore de?ne the
`time period of each time slot. The time slots can be
`programmed at the head end each containing e.g. 128,
`256, 512, 1024 bits, for example, the bit duration now
`preferably being 0.25 microseconds. The downstream
`data sent within the time slot is indicated as reference
`19.
`The communication controller also ensures that
`downstream data packets are not transmitted during a
`busy or collision state of the network. It also switches
`off the transmitter if it transmits for too long a period,
`e.g. if it transmits over a predetermined maximum
`packet size. The transmitter is thus locked and timed
`out.
`It is important that the period of time that separates
`the transmission event of consecutive pairs of busy bits
`should be equal to or greater than the maximum round
`trip propagation delay of the network. This delay is the
`sum of the delays of all of the components which consti
`tute the transmission loop with maximum cable length,
`going ?rst to the longest delayed end of the network, in
`both directions.
`Each data packet sent from the head end should pref
`erably also be equal to or longer than one time slot
`period. A typical packet format is shown graphically in
`FIG. 3; it can be formed for example by a 2-7 byte
`preamble, followed by a one byte start frame delimiter
`followed by a 4 byte destination address, followed by a
`4 byte source address, followed by a two byte frame
`length indicator, followed by an indeterminate number
`of bytes containing the data to be transmitted, following
`by an indeterminate number of padding bytes (if neces
`sary), followed by a two byte frame check sequence
`Turning now to FIG. 4, a block diagram of the pre
`ferred form of subscriber terminal 3 shown in FIG. 1 is
`illustrated. Coaxial cable 2 from the network is con
`nected to both downstream bandpass ?lter 22 (which is
`similar to ?lter 9) and upstream bandpass ?lter 23, the
`latter of which can form two narrow passbands just
`sufficient to pass the two frequencies of the frequency
`shift keyed modulated output signal to be generated in
`the subscriber terminal
`The output of ?lter 22 is carried via splitter 23A to
`TV tuner/analog circuit 24. Circuit 24 may consist of a
`tunable down-converter, pay TV descrambler or the
`like. The output of the circuit 24 is applied 'to modulator
`25 for application to a TV set 26.
`-
`The output of ?lter 22 is also connected via splitter
`23A to the input of a digital demodulator 27, which
`demodulates the data stream in the digital channel.
`Digital demodulator 27 provides data and clock output
`signals on corresponding output lines, in a well known
`manner. These signals are applied via a CAD circuit 8
`where they are translated into parallel in a known man
`ner, and applied to data and clock inputs of a central
`processor/memory circuit 29. A data and address bus
`30 communicates with the central processing unit/
`memory circuit 29. Other peripheral digital circuits 31
`associated with the subscriber terminal are also con
`nected to the central processing unit/memory circuit
`29, e.g. to control a video display generator, to monitor
`alarm circuits, etc.
`The data and clock lines output at the digital demod
`ulator are also connected to corresponding inputs of
`
`ARRIS GROUP, INC.
`IPR2015-00635 , p. 10
`
`

`
`10
`
`25
`
`30
`
`35
`
`4,920,533
`8
`7
`packet transmission process is stopped. The packet is
`busy bits (IEC) extraction logic circuit 32, which is also
`returned to the buffer 34, where it waits a period of time
`connected to bus 30. A divide by N circuit 33 is con
`prior to transmission again as controlled by the MAC
`nected to the clock line, its output being a clock signal
`C1. The divide by N circuit thus generates un internal
`circuit.
`If the number of collisions already occurred is less
`clock signal C1 from the clock line, and is thus desired
`than the maximum, the packet transmission probability
`from the downstream data signal.
`A buffer 34 is connected to the bus 30, for receiving
`P is reduced by a factor K (where P=P/K) to a mini
`data signals from processor circuit 29 for transmission
`mum retransmission probability value Then it waits
`until the next pair of busy bits at whick time it repeats
`to the head end. The output of buffer 34 is connected to
`frequency shift key modulator 35, for modulating the
`the examination of the next set of busy bits.
`output signal from buffer 34 into a frequency shift keyed
`If the next pair of busy bits does not indicate a colli
`signal within the passband of the upstream data channel.
`sion, it will necessarily indicate a busy state, since the
`The output of modulator 35 is applied through a time
`subscriber station itself is transmitting. This means that
`the local subscriber terminal has uniquely captured the
`out circuit 36 which controls the transmission interval
`of the upstream signal. The output of time out circuit 36
`channel. Transmission is reinitiated and continued until
`is applied to upstream bandpass ?lter 23, from which it
`the end of the packet, after which the channel will be
`is applied to coaxial cable 2 for transmission to the head
`declared free. The collision counter is then initialized to
`end.
`zero, and its transmission probability P to l.
`The extraction logic circuit 32 and MAC circuit 37 of
`A media access management transmit logic circuit 37
`(MAC) (controlling means) is connected to bus 30, and
`FIG. 4 will now be described in detail. Turning to'FIG.
`5, the busy bit extraction logic 32 circuit is shown in
`has other inputs connected to the extraction logic cir
`cuit 32 for receiving the busy bits. The MAC circuit is
`block diagram. The data and clock lines are connected
`to a descrambler circuit 40, for descrambling if neces
`also connected to a control input of buffer 34 and to a
`control input/ output of time out circuit 36. The purpose
`sary, if the downstream data signal has been received
`of the MAC circuit 37 is to detect the busy, idle or
`scrambled. Descrambler 40 is a mirror image circuit of
`collision status of the network by monitoring the busy
`the head end scrambler, which can be included as part
`of communication controller 13 (FIG. 1). The output
`bits, and to control the transmission and the timing of
`the transmission of signals stored in buffer 34 to the
`signals of descrambler 40 are clock and data signals
`head end.
`which are applied to the data and clock inputs of bit
`The digital channel signals are received from the
`counter 41. Counter outputs of bit counter 41 are con
`nected to flag determining circuit 42, which determines
`head via cable 2, are ?ltered in ?lter 22 and are applied
`to digital demodulator 27. The resulting data and clock
`the beginning of a packet from the contents of counter
`signals are applied to processor 29. A keypad or other
`41, and provides on its IND output a pulse or leading
`means operating through other digital circuits 31 com
`edge of a pulse which indicates the presence of the
`beginning of a received data packet.
`municates with processor 29 to cause it to formulate
`data signals for transmission to the head end. The mech
`The data and clock outputs of bit counter 41 are
`anism for formulating these signals in the CPU/ memory
`connected to zero extraction'circuit 43. This circuit is
`is not the subject of the present invention as techniques
`used in a well known manner to remove zeros which
`may have been inserted in the communication control
`known to persons skilled in the art can be used. Suffice
`ler 13 at the head end to distinguish the value of long
`to say that once the signals are formulated, they are
`passed via bus 30 to the input of buffer 34 where they
`bytes which all consist of ones. Thus zero extraction
`circuit 43 reconstitutes the original data. This corrected
`are stored.
`.
`The IEC extraction logic circuit 32 detects the busy
`data is applied to a status byte extraction circuit 44.
`bits on the data and clock lines at the output of demodu
`The corrected data signal is also applied to a counter
`lator 27. The busy bits are presented to the MAC circuit
`and interval control circuit 45, which is connected via
`37.
`address bus ADDR and data bus D0~D7 to bus 30.
`MAC circuit 37 operates using an internal algorithm
`Bus 30 is, as described earlier connected to processor
`to be described below If the MAC circuit receives busy
`29. The time slot interval is determined by the CPU and
`bits which indicate a busy channel or a collision, it waits
`stored in its memory, and this data is sent via the bus 30
`until the arrival of the next busy bits, and continues
`to circuit 45 which in turn presents a control signal on
`monitoring the busy bits state. If the busy bits indicate a
`the CTRL line to extraction circuit 44. The time slot
`free channel, the MAC circuit decides to transmit with
`can be found by changing the interval on the extraction
`a probability P, or decides not to transmit with a proba
`circuit until the received packet check sum matches.
`bility (l-P) and thus will wait until the arrival of the
`Extraction circuit 44, which can be a programmable
`next set of busy bits whereupon it will operate of a
`register, thus can receive the data within a time slot and
`probability of transmission adjusting algorithm.
`provide the bits associated with the time of the busy or
`The MAC circuit upon deciding to allow transmis
`status bits to its output terminals. Those output termi
`sion, applies a control signal to buffer 34, which causes
`nals are connected to a decode IEC circuit 46, which is
`it to begin outputting its stored packet At the same time
`a two bit decoder, for decoding the busy bits. The four
`the MAC circuit monitors the busy bits. If the busy bits
`output terminals of decode IEC circuit 46 thus reflect
`received, while the circuit is transmitting, indicate colli
`the four states of the two bits constituting the busy or
`status bits: idle, busy, collision state, and a priority or
`sion, which means that one or more other transmitters
`are transmitting during the same time period, it immedi
`other fourth state, which terminals are correspondingly
`ately controls buffer 34 to halt its transmission A colli
`labelled in FIG. 5. The lines connected to those termi
`sion counter is also incremented within the MAC cir
`nals are connected to the MAC circuit 37.
`cuit.
`Turning now to FIG. 6, media access circuit 37 is
`shown within the dashed lines in block diagram, along
`If the number of collisio