WO1983002206A1

WO1983002206A1 - A distributed control communications system

Info

Publication number: WO1983002206A1
Application number: PCT/AU1982/000210
Authority: WO
Inventors: Pty Limited Xitel
Original assignee: Davis, Barrie, William; Norman, Michael, Leslie
Priority date: 1981-12-17
Filing date: 1982-12-17
Publication date: 1983-06-23
Also published as: EP0097180A4; JPS58502129A; EP0097180A1

Abstract

A local area network communications system for a plurality of subscribers has distributed network control and provides random access capability. Each subscriber (130, 230, 330, 430) is connected to a transmission medium (100) via a respective access unit (120, 220, 320, 420). Each access unit contains a control unit (121), and network control is progressively transferred from one control unit to the next in a cyclical manner without the need for centralized control. When a particular control unit has network control, it provides the logical connection between its subscriber and the transmission medium if communication with another subscriber is requested.

Description

"A DISTRIBUTED ^'CONTROL- COMMUNICATIONS SYSTEM" The present invention relates to a local area network communications system with distributed control.

BACKGROUND ART A Local Area Network (LAN) Communications System is a communications system that is able to interconnect a number of communication devices, such as information storage, retrieva and processing devices, over a limited geographic area. Current LANs have a typical linear topology from 500 metres to approximately 10 kilometres, compared to telecommunication data networks which can be world wide. However, whereas the maximum data rates achievable with telecommunication data networks are limited by the public telephone network to a few thousand bits/seconds, LANs can achieve data rates of several megabits/second.

LANs are able to provide these high data rates by using a transmission medium such as low attenuation, wide bandwidth coaxial cable which can support the data modulation techniques used for high speed data transmission. As the area serviced by a LAN is quite small, the additional cost of the wide bandwidth cable, compared to normal telecommunications cable, is insignificant in relation to the amount of information that can be transferred over the net work^'.

Baseband transmission LANs utilize the wide bandwidth capability of coaxial cable to support the transmission of high speed (several megabits/second) digital signals along the cable. Because pulse transmission techniques are used, only one data path can be accommodated on the cable, as a wide frequency bandwidth is required to maintain the symmetry and timing of the data pulses.

Baseband LANs employ a shared channel topology and Time Division Multiplexing is used to allow each device attached to the network the opportunity to access the cable to transmit information to another device. To increase the network reliability. Distributed Network Control is used; however, central network synchronization is commonly

- \JRE_A required .

Broadband transmission LANs use the wide frequency > bandwidth available with coaxial cable to support a number of radio frequency carriers, which are separately modulated by the data to be transmitted over the network. The

Broadband LAN subdivides the frequency spectrum of the coaxial cable into a number of discrete radio frequency carriers. Each carrier frequency in a Broadband LAN can provide a separate data channel. The radio frequency carriers in a Broadband LAN are sinusoidal signals and only require a narrow portion of the available frequency spectrum. The narrow bandwidth required by each carrier frequency allows many separate data channels to coexist on the one coaxial cable. Data is transferred across the network by modulating the" carrier at the data rate using standard radio frequency modulation techniques (Amplitude, Frequency, Phase etc.).

Unlike the Baseband LAN where a single data channel is used and each device Time Division Multiplexes its data onto the cable, the devices connected to a Broadband LAN have many data channels on which they can transmit data. To maintain data integrity the network access control of a Broadband LAN requires the non-conflicting assignment of an appropriate data channel to each device requiring network access.

A disadvantage of such local area network communications system having central network control is that failure of the central network control will prevent the operation of the whole communications system. U.S. Patent No. 3,573,379 discloses a communications system for a plurality of subscribers having random access capability without the requirement for the usual central exchange. However, the system requires a master timing clock to which the subscribers are connected. Thus, centralised control is still present and failure of the clock will prevent proper operation of the system.

It is an object of the present invention to overcome, or substantially ameliorate, the above escrib ^ disadvantage by providing a local area network communications system having distributed network control.

In the present invention, the failure of a control unit will only cause the associated device to become inoperative, and the rest of the system continues to operate. Furthermore, due to distributed network control, a cut or break in the transmission medium need not render the whole system inoperative. Devices which are still connected to each other can communicate with each other regardless of a break in the line outside the link between these devices.

DISCLOSURE OF THE INVENTION According to the present invention, there is provided a distributed control communications system for providing communications between a plurality of subscriber devices over a limited geographical area, said system comprising a transmission medium having a plurality of communication channels; a plurality of access means each connected to said transmission medium and providing a terminal for connection to a respective subscriber device, whereby each said access means is adapted to provide a logical connection between its respective subscriber device and said transmission medium, each said access means having a control unit, said control units communicating with one another via a common one of said communication channels, characterised in that network control is provided by each control unit progressively in turn on a cyclical basis. The present invention provides a self contained communications system suitable for use with a large number of information storage and retrieval devices which need to be interconnected at random intervals for varying periods of time for the purpose of transferring information.

In the preferred embodiment, the frequency spectrum of a coaxial cable transmission medium is used to support the simultaneous transmission of a large number of radio frequency carriers. A data channel consists of one or more carrier frequencies. Data is transmitted over the data channel by modulating a channel carrier frequency at the data rate by varying the Phase, Frequency and/or Amplitude of the carrier radio signal energy in synchronism with the data to be transmitted.

Typically, each access means is a Cable Access Unit which provides the functional interface between the passive coaxial cable transmission medium and a respective communications device. The Cable Access Unit logically resides between the device and the coaxial cable although it may be physically incoporated in the device, or external to the device, depending on the particular implementation. The Cable Access Unit typically comprises a microprocessor for its logic operation.

The System of the preferred embodiment has a distributed control network with each Cable Access Unit having a control unit managing all of the network functions for its host device. The host device can only access the network by requesting the Cable Access Unit to provide a logical connection .with another device in the network. The Cable Access Unit uses an automatically sequenced network access control function to communicate with the Cable Access Unit attached to the requested device, and to select the appropriate carrier frequencies for transmit and receive. When the data channel has been established the two host devices are able to commence data transfers with each other. Network access is performed by a Cable Access Unit in response to a request from the host device. Once a data channel has been established it is reserved for the exclusive use of the two communicating devices. When the data transfer is completed the Cable Access Units executes a logical disconnection of the devices from the network. The carrier frequencies associated with the data channel are then available for use by other devices attached to the network.

Normal data communications functions such as device to device protocol conversion, error checking and correction, receive acknowledgments, retransmission requests, code conversion and information type (voice or data) remain the responsibility of the attached subscriber devices. A device connected to- the system only requires a simple control sequence between itself and its Cable Access_' Unit to establish a logical connection with another device. If both devices have the same native protocol no data transformation is required and error checking is performed by the communication devices in the normal manner.

Each Cable Access Unit can independently establish a unique data channel between its host device and another device attached to the network. In addition, the host device can transfer control information, via the Cable Access Units, to the requested device enabling data transfer parameters such as transmission speed, protocol and data formatting to be established between the two devices prior to a data transfer. The common channel is typically a separate fixed carrier frequency data channel (System Control Channel) for exclusive use by the Cable Access Units attached to the network. Each Cable Access Unit preferably has two radio transceivers. One transceiver is locked to the System ^' Control Channel and is set to the same carrier frequency in all Cable Access Units. Any information transmitted on the Control Channel is available to all Cable Access Units. Host devices attached to the network via the Cable Access Units cannot access or transmit on the Control Channel. The other transceiver is frequency agile, and can be set by the Cable Access Unit to any of the predetermined carrier frequencies provided by the System. The frequency agile transceiver is the network data interface for the host device and data transferred through this transceiver is transparent to the Cable Access Unit.

Each Cable Access Unit is assigned a unique System address, which is used as its identifier when communicating with other Cable Access Units via the Control Channel.

As each Cable Access Unit is able to transmit on the one single Control Channel, random transmissions would cause data collisions and a collision detection and recovery mechanism is preferably provided.^' According to this mechanism, the time at which each Cable Access Unit can transmit information on the Control Channel is determined by a specific 'Transmission Authorisation' or TA word (packet) of a predetermined number of bits. Each Cable Access Unit monitors the TA packets on the Control Channel until it recognises its own address. At this time the addressed Cable Access Unit is the only device 'authorised' to transmit information on the Control Channel. A 'Network Access* or NA word (packet) comprising a number of bits is transmitted in response to the TA packet, which is received by all other Cable Access Units on the network.

When a Cable Access Unit receives a TA packet which has the destination address set to its own source address it becomes the highest priority Cable Access Unit on the network. The Cable Access Unit asserts its priority by transmitting its NA packet which notifies all other Cable Access Units that it has control of the network.

At the completion of the NA packet transmission the Cable Access Unit then transmits a TA packet with the destination address set to the next sequential Cable Access Unit. The transmission of this TA packet advises the other Cable Access Units that it is relinquishing control of the network. The Cable Access Unit with the next sequential source address then asserts its priority by transmitting its- A packet. In this manner, a rotating transmission priority structure is established wherein each Cable Access Unit is allowed access to the control channel on a cyclical basis.

The NA packet transmitted by each Cable Access Unit, in response to a TA packet, contains the System network control information. The TA packets only schedule the orderly transmission of this information onto the Control Channel by each Cable Access Unit in the correct sequence required by the network control architecture. It is the sequencing of Cable Access Unit transmissions on the Control 'Channel which prevents multiple transmissions occuring and avoids the need for data collision detection and recovery.

When a device requires a logical connection with another device attached to the System it commands its Cable Access Unit to make the connection, provides the address of the destination device, the type of transmission required _* and the device dependent parameters, if required.

The source device Cable Access Unit reserves an appropriate data channel corresponding to the type of transmission required and transmits a 'connect request* to the destination device Cable Access Unit via a NA packet on the Control Channel. The destination device Cable Access Unit responds with its own NA packet and acknowledges the request or advises that it is busy. If the source device receives an acknowledge from the destination device it sets its frequency agile transceiver to the carrier frequencies that it previously reserved, and advises the host device that transmission can commence. The destination device Cable Access Unit also sets its frequency agile -transceiver to the same carrier frequencies, and advises its host device that a logical connection from another device on the System has been made. Any device dependent parameters that were received from the source device are passed transparently to the destination device.

The type of interface provided between the host device and its Cable Access Unit can vary depending on the capability of the host device, the complexity of the network, and the type and format of the data to be transferred over the network.

With regard to devices that cannot provide direct communication with the Cable Access Unit, such as simple CRT terminals, the operator provides the Cable Access Unit with the appropriate access request information via an attached keypad. When a connection is made the status is displayed to the operator and the serial interface control signals (Clear to Send, Data Set Ready, Carrier Detect etc.) are set to reflect the status of the logical connection. More complex devices, such as word processing terminals, can have the Cable Access Unit tightly coupled to their own internal control architecture. In ^"this case, the terminal control processor would interact directly with the Cable Access Unit, as if it were another peripheral device attached to its internal system structure. Although the access request would be initiated by the operator, the * logical connection would be established by a control sequence between the Cable Access Unit and the terminal control processor. In this type of implementation the ability to transfer device dependent parameters between the source and destination devices, via the NA packets, allows non-System functions, such as the data format, transmission speed and protocol to be established before the data transfer commences.

With the distributed control architecture of the System there is no central control node. The failure of any Cable Access Unit on the network has no effect on the ability of other Cable Access Units to use the network. Each Cable Access Unit in the network is- never idle. It is continuously receiving NA packets from the other active Cable Access Units on the network and transmitting its own NA packet in response to a TA packet. Each NA packet has a special field, known as the Reserved Channel field, which is used by a Cable Access Unit to reserve one or more carrier frequencies for its own use, to the exclusion of all other Cable Access Units.

When a host device requests a network access, in order to establish a data channel to another device on the network, the Cable Access Unit commences a Network Access Sequence. At the time of the access request from its host device, the Cable Access Unit is transmitting'idle' NA packets on the Control Channel, and is monitoring the incoming NA packets for a connect request from another Cable Access Unit.

Once the access request is received from the host device the Cable Access Unit goes 'busy* and waits for its next TA packet. The next NA packet transmitted is an 'idle* and 'busy' packet which informs all other Cable Access Units that is unavailable for a connection. After transmitting this NA packet the Cable Access Unit monitors the Reserved Channel field in each NA packet received from the other Cable Access Units on the network. After one complete Control Channel TA packet sequence rotation all active Cable Access Units have transmitted their NA packet, and all of the carrier frequencies that have been reserved ' by other Cable Access Units are known to the Cable Access Unit requiring network access. The Cable Access Unit requiring network access is the only one that is authorised to transmit at this time. A Channel Number corresponding to an unused channel is . arbitarily selected by the Units logic and a 'connect request' NA packet is transmitted on the Control Channel. This NA packet is received by all other Cable Access Units on the network and advises them that the carrier frequencies corresponding to the Channel Number in the Reserved Channel Field have been reserved and are unavailable for allocation. As no other Cable Access Unit can transmit at this time the NA packet from the^' access requesting Cable Access Unit reserves its carrier frequencies exclusively and without ambiguity.

Except for the Cable Access Unit whose Source Address is contained in the NA packet Destination Address Field, all other Cable Access Units only note the change in network status, and that the carrier frequencies corresponding to the Reserved Channel Field are reserved. The addressed Cable Access Unit recognizes a valid access to its host device by another device on the network and, with a NA packet, responds to the requesting Cable Access

Unit after it receives its TA. The requesting device Cable Access Unit is advised of the current activity status of the requested device. If the requested device is 'busy' the requesting device Cable Access Unit can either cancel the request, wait until the requested device is 'ready' or, if it has a higher network access priority, force a connect by exercising its priority status.

If the requested device is not 'busy' an acknowledge is transmitted to the requesting device Cable Access Unit in its next NA packet transmission. Both Cable Access Units then set their frequency agile transceiver to the carrier frequencies previously reserved and a logical connection is established between the two host devices.

Each Cable Access Unit advises its own host device that data channel is established and the data transfer can commence .

During the period of the logical connection the NA packets transmitted by the Source and Destination Cable Access Units maintain exclusive use of the reserved carrier frequencies by transmitting the Channel Number in the Reserved Channel Field. When the data transfer is completed a disconnect NA packet sequence is exchanged between the requesting and requested device Cable Access Units validating the disconnect. The next NA packet transmitted by both Cable Access Units has the Reserved Channel Field set to zero which relinquishes the reserved status of the carrier frequencies previously used for the data channel. As these carrier frequencies are no longer being used they are available for use by any other Cable Access Unit on the network.

BRIEF^" DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic block diagram of a limited area network communications system according to a preferred embodiment of the present invention.

Figure 2 is a schematic block diagram of a host device connection to the cable of the system of Figure 1. Figure 3 is a schematic block diagram of a Cable Access Unit of the system of Figure 1. Figure 4 is a schematic block diagram of a control unit of the system of Figure 1.

Figure 5 shows a transmission authorization packet format.

Figure 6 shows a network access packet format. Figure 7 illustrates the time sequence for TA and

NA packet transmissions.

Figure 8 illustrates the time sequence for TA and NA packet transmissions for a^'non-responding CAU.

Figure 9 illustrates the time sequence for TA and NA packet transmissions during recovery from a non-respondin CAU.

- &E Figure 10 illustrates the time sequence for TA and NA packet transmissions for two successive non- - responding CAUS.

DESCRIPTION OF THE PREFERRED EMBODIMENT A communication system in accordance with a preferred embodiment is illustrated schematically in Figure

1. A number of host devices 130, 230, 330, 430 are connected to a transmission medium such as a coaxial cable 100 via respective Cable Access Units 120, 220, 320, 420 and line couplers 110, 210, 310, 410..... Although the transmission medium is typically a coaxial cable, it can also be any other suitable medium such as a light fibre or even a wireless link.

The host unit can be any form of information retrieval, processing or storage device, such as video terminals, data base processors, personal computers, telemetry devices, or analog communication devices such as telephones. The host devices are connected to their

respective Cable Access Units- by a link appropriate to the type of signalling taking place, but typically by twisted >- pair wires.

Communication takes place over the coaxial cable 100 by means of radio frequency signalling. The bandwidth of the coaxial cable is several 100 MHz (e.g. 300 MHZ is accepted by the cable TV industry) , hence a high number of discrete radio channels can be supported by the cable, the number of channels being a function of the information bandwidth.

Communication is established between a pair of host devices when one host, say 130, instructs its respective Cable Access Unit 120 that a connection is required to another host, say 330. Cable Access Units 120 and 320 communicate over a common channel, being one of the plurality of channels provided on the cable, and known as a 'control channel* which, for a given network, is set to a radio frequency known to all Cable Access Units. When Cable Access Units 120 and 320 have established that communication is possible (e.g. that host 330 is not already communicating with some other host or is disconnected or inoperative) , Cable Access Units 120 and 320 select a pair of radio frequency data channels for the exclusive use of their hosts which then exchange information as required.

After exchange of information is finished, either Cable Access Unit 120 or 320 is so advised and the duplex pair of radio frequencies channels allocated to hosts 120 and 320 are released for use by other host devices. Taking host 130 as an example, figure 2 illustrates the connection of the host device to the cable via a Cable Access Unit 120. The Cable Access Unit 120 may be physically part of the host device 130 (e.g. if the host is a telephone or personal computer) or may be a separate key pad 125 for devices such as CRT terminals.

The Cable Access Unit 120 is connected to a line coupler 110 by means of a 'drop' coaxial cable which is relatively short (less than 500 metres) and has the same electrical i pedence as the main cable 100. Line coupler 110 provides a junction in the cable to allow an external connection while providing the correct electrical . impendence to each coaxial cable terminal. Such line couplers are in common use throughout the cable television industry, and in various forms are known as 'line couplers', 'splitters', 'directional couplers' and 'splitter-combiners' .

If other transmission media (e.g. optic fibers) are used, appropriate couplers are provided to tap into the medium.

The Cable Access Unit 120 is shown in more detail in Figure 3, the other Cable Access Units being similar in architecture. The unit 120 has a control unit 121 which comprises a microcomputer and associated logic and memory. A transmitter 123 and receiver 124 are connected- between the cable 100 and the control unit 121, and form a fixed frequency transceiver for communicating over the common control channel. The transmitter 123 includes a modulator for digital signalling and can modulate the carrier using amplitude or angle techniques or a combination of these.

The receiver 124 has a demodulator capable of demodulating the transmitted control carrier and output digital data.

Transmitter 126 and receiver 127 form a frequency selectable transceiver which can be tuned over a set of discrete frequencies, the number of discrete frequencies being a function of the information bandwidth, and the dynamic range of a frequency synthesizer 122 which is used to tune the transceiver 126, 127.

The transmitter 126 also includes a modulator which may accept analog or digital information depending on the Cable Access Unit's function. The carrier signal can be either amplitude or angle modulated, or a combination of these. The receiver 127 demodulates the carrier signal and provides the requisite analog or digital demodulated signal as output to a user interface 129.

Any suitable known frequency synthesizer 122 can be used. The control unit 121 provides a digital number to the frequency synthesizer corresponding to a selected frequency. The output of the frequency synthesizer (a low level RF signal) is then used- to tune transmitter 126 and receiver 127. The input to transmitter 126 and output from receiver 127 are the signals passing between the frequency agile transceiver and the user interface 129. These signals may be analog or digital. User interface 129 provides the correct electrical level etc., between the user host device and the frequency agile transceiver.

User interface 128 provides the electrical interface between the host device 130 and the control unit 121. This interface exchanges digital data relating to the addresses of sending host device and receiving host device, and other information required to establish a connection. This interface may be driven by a simple key pad in the case of a basic CRT terminal or both interface 128 and Cable Access Unit itself can be embedded in the host device 120, as would be the case with a personal computer or data base processor.

The control unit 121 can be implemented in several ways including the use .of random logic and gate arrays, but the preferred embodiment uses a microcomputer and associated memory and logic. The control unit 121 is shown in more detail in the block diagram of Figure 4. The control unit comprises a microprocessor 500 which can be "single chip" device which includes on-chip Random Access Memory, Read Only Memory, etc., or it may be the central processing part only^"of the microcomputer. The microprocessor 500, Random Access Memory 510 and Read Only Memory 520 communicate via control bus 530. The frequency synthesizer 122, together with its RF outputs 550 and 560 which drive the frequency agile transmitter 126 and receiver 127, are also connected to the control bus 530. Digital latches and associated electronic circuits 570 and 580 provide the interfaces between the host and the Cable Access Unit, and staticizer-serializer 590 provides the serial interface to the fixed frequency control channel transmitter 123 and receiver 124.

Each Cable Access Unit is continuously monitoring the control channel for control information known as transmission authorisation (TA) packets and network access (NA) packets .

Each TA packet has three fields which command a particular Cable Access Unit to transmit its status and network control information on the control channel. The TA packet format is shown in Figure 5 and comprises a destination address 50 (address of the channel controller to which the TA packet is being sent) , a transmission control 51 (defines the particular transmission function required) and an error check 52 (calculated from fields 50 and 51 and used to determine the TA packet validity) .

Each NA packet has six specific fields which convey the intention and status of the transmitting Cable Access Unit to all other Cable Access Units of the network. The NA packet format is shown in Figure 6 and comprises a source address 61 (address of the CAU transmitting the NA packet) , an access control 62 (defining the access mode and the current status of the CAU transmitting the NA packet) , a destination address 63 (address of the CAU to which the NA packet is being sent; an "idle" packet has this field set to zero.), a reserve channel 64 (data channel number which has been reserved by the source CAU for the exclusive use of its host device and the destination address device; if a data channel is not reserved this field is set to zero) , a device control 65 (a free format field which contains device to device control information; it is received from the source address device and passed transparently to the destination address device) and a cyclic redundancy check 66 (calculated from the previous five fields and used by each CAU to check the received NA packet validity) .

A transmission priority structure is maintained by the CAUs on the network whereby only one CAU at any one time has the "authority to transmit" on the control channel. When a CAU receives a TA packet which has the destination address 50 set to its own source address, it becomes the highest priority CAU on the network. The CAU asserts its priority by transmitting its NA packet which notifies all other CAUs that it has taken control of the network. At the completion of the NA packet transmissi( the CAU then transmits a TA packet with the destination address 50 set to the next sequential CAU source address. ^y The transmission of this TA packet advises the other CAUs that it is relinquishing control of the network. The CAU with the next sequential source address then asserts is priority by transmitting its NA packet. A rotating transmission priority structure is established without the requirement of a central control. Packet collision is prevented since each CAU can only transmit after it receives a TA packet assigning the control channel to it to the exclusion of all other CAUs for the duration of the NA packet transmission.

In the time domain, the control channel can be considered to be a ring structure around which circulate the TA and NA packets generated by each CAU. Figure 7 illustrates, in the time domain, sequential TA and NA packet transmissions for a network having N CAUs. Each packet is a fixed length. The time to transmit a TA packet is Tt.a and the time to transmit a NA

Typically, for a control channel data rate of 128 kBits/sec, T is 375 microsecond for a 32 bit packet, and Tna is 437.5 microsecond for a 56 bit packet,

. -. Each packet transmit time is equal to the number of data bits multiplied by the control channel data transmission bit rate. In the described embodiment, all of these paremeters are constants and each CAU can receive and decode every packet on the control channel.

A second predetermined time T , the inter-packet wait time, is the minimum separation time between sequential packets. Typically T is of the order of 50 microsecond. During ³ time Tw the control channel is idle. When a CAU determines, from the last received TA packet destination address that it is the current highest priority CAU, it delays a time of T W from the end of the TA packet before commencing transmission of its own NA packet. At the completion of the NA packet transmission, the CAU waits time Tw and then transmits its TA packet.

CAUs which are not powered up, not physically present or faulty, are not able to transmit their NA *& packets when they are selected by TA packet. Figure 8 illustrates the TA and NA packet transmission in the time .' domain for a non-responding CAU. CAU (M-2) transmits its •

TA packet, with the destination address set to M-l, to CAU (M-l) . It then monitors the control channel for the transmission of the corresponding NA packet from the CAU

(M-l) . CAU (M-l) waits time T from the end of the TA packet and then asserts its control of the network by transmitting its NA packet. It waits other time T_w and transmits its TA packet to CAU (M) .

As CAU (M) is powered off it cannot respond to the

TA packet from CAU (M-l) and the control channel remains idle. CAU (M-l) waits time T , the minimum time from the completion of a TA packet that CAU (M) will commence transmission of its NA packet. CAU (M-l) then waits an additional time T ,, which corresponds to the maximum signal progation delay time for a cable segment.

Typically, T _d is 4.5 microsecond/kilometre of coaxial cable. At the end of time Tw + Tpd, from the transmission of its TA packet, CAU (M-l) assumes that CAU (M) is unable to transmit its NA packet. CAU (M-l) increments the destination address to M+l and transmits the next TA packet to CAU (M+l) . CAU (M+l) is active and asserts its control of the network by transmitting its NA packet. The recovery from the non-responsing CAU is shown in the time domain in Figure 9.

If more than one sequential CAU is unable to respond to its NA packet, the CAU which has control of the network waits time Tw + Tpd, from the end of each successive TA packet transmission and then transmits a TA packet to the next sequential CAU. This sequence continues until an active CAU responds with its NA packet and takes control of the network. The TA and NA packet sequence for two sequential inactive CAUs is known in Figure 10. Therefore, the CAUs can be powered on or off without the requirement for synchronisation with the other CAUs. The presence or absence of a NA packet in response to the TA packet is sufficient to advise all other CAUs attached to the network of the status and availability of the respective CAU.

For the time delay figures quoted above, the total . period of a control transfer is typically 913 microsecond, and the poll rate is therefore approximately 1095 dual packets/second.

There are other packet formats which can be implemented to provide a similar network access mechanism to that described above. By increasing the complexity of the packet control structure, an increase in network access performance is achievable. The length of the NA packet can be varied, depending on the information being transferred on the control channel. An idle CAU need only notify other CAUs that it is active. This can be achieved by transmitting a truncated NA packet which contain the source address field, the access control field and the CRC field. The same truncation of the NA packet can be used if a device control field transfer to the requested device CAU is not required. In each case, the access control field would specify the type of NA packet being transmitted, which would allow receiving CAUs to determine the start of the CRC field for packet validation.

The establishment of a data channel between two devices can be significantly shortened if the CAU requesting a device access can reserve a number of sequential time slots on the control channel. The reserving of a number of time slots would enable the requesting and the requested device control units to exchange sequential NA packets and synchronise the logical connection without waiting for a complete control channel time rotation for each NA packet transmission. This would require the control channel transmission priority to be passed, via the exchange of TA packets, between the requesting and requested CAUs enabling NA packets to be exchanged before the control channel transmission priority is passed to the next sequential CAU.

Claims

CLAIMS 1. A distributed control communications system , for providing communications between a plurality of subscriber devices over a limited geographical area, said system comprising a transmission medium (100) having a plurality of communication channels; a plurality of access means (120, 220, 320, 420) each connected to said transmission medium (100) and providing a terminal for connection to a respective subscriber device (130, 230, 330, 430) , whereby each said access means is adapted to provide a logical connection between its respective subscriber device and said transmission medium, each said access means having a control unit (121) , said control units communicating with one another via a common one of said communication channels, characterised in that network control is provided by each control unit in turn on a cyclical basis.

2. A communications system as claimed in claim 1 wherein network control is transferred from one control unit to another in a progressive cyclical manner, only the control unit having network control being capable of transmitting control information signals along said common channel, characterised in that in transferring network control, a first control unit in control transmits a control information signal along said common channel to the other control units, said control information being indicative of a second control unit to which control is to be transferred, whereby on receipt of said control information signal, said second control unit, if operative, assumes network control.

3. A communications system as claimed in claim 2 wherein said plurality of channels includes at least one data channel and said subscriber devices communicate along said data channels via their respective access means and said transmission medium, characterised in that each control unit is responsive to a request by its associated subscriber device for communication with another subscriber device to transmit a control information signal along said common channel, said information signal being indicative of a further device with which communication is requested, said control unit also being responsive to a predetermined _' return control information signal from the further control unit to select an available data channel, whereby a logical connection is made by the respective access means to enable said devices to communicate via the selected data channel.

4. A communications system as claimed in claim 3 wherein each said access means also comprises a fixed frequency transceiver connected between said transmission medium and the respective control unit, said transceiver having a frequency corresponding to the common channel; a frequency agile transceiver connected between said control unit and the transmission medium; and frequency control means connected between said control unit and said frequency agile transceiver, said frequency control means being responsive to said control unit to set the frequency of the frequency agile transceiver to a selected one of said data channels.

5. A communication system as claimed in any preceding claim wherein said transmission medium is a coaxial cable.

6. A communication system as claimed in claim 5 wherein said communication channels are frequency division mutliplex --^" in the operating bandwidth of said coaxial cable.

7. A communication system as claimed in any prece ding claim, wherein each access means is connected to the transmission medium via a respective line coupler.

8. A communication system as claimed in any preceding claim wherein each said control unit comprises a microcomputer.

9. A method of network control distribution in a communications system as claimed in any one of claims 1 to 8, wherein network control is vested in one control unit at a time, said method comprising the steps of enabling the control unit in control to transmit a control information signal on said common channel indicating a further control unit to which network control is to be transferred, enabling the further control unit to transmit a return signal indicative of its status, network control being thereby vested in said further control unit, said steps being repeated for each control unit having network control whereby such control is transferred cyclically around said control units.