GB2171880A - Local area network - Google Patents

Abstract

A local area network (LAN) consists of two closed ring sub-networks, interconnected by a bridge node. Thus the arrangement is extended in that the two rings interconnected by the bridge node use different bit rates and also different message protocols. Thus bit rate and format conversion is provided in the bridge node. Both rings use token ring techniques. <IMAGE>

Description

SPECIFICATION Locate area network The present invention relates to local area networks (LANs), a LAN being a telecommunication system serving a relatively small area. Such a system often uses a closed-ring arrangementfor interconnecting the nodes, each of which nodes acts as an interface between one or more terminals and the system.

The number of nodes on a LAN is often limited with the resultthatwhere a relatively large number of nodes have to be served, two or more interconnected rings have to be used. Each interconnection between two nodes uses a bridge node, and a system using bridge nodes is described in ourAppin. No. 2138651 (R. I. Swindle petal 7-3).

The use of bridge nodes is also described at page 1514 of a paper entitled "An introduction to Local Area Networks", by Clark, Pogran and Reed, Proc. IEEE, Vol.

66 No. Nov.1978, pp 1497-1517.

The above Patent application describes a system of the LAN type using closed rings each serving a number of nodes for subscribers' circuits and possibly for facilities circuits, communication being effected digitally. These rings are interconnected by bridge nodes each of which interconnects two such rings.

Each bridge node includes two stores, one per direction oftransmission, and a controller which monitors each message which its bridge node receives. If a message from one of the rings served by that bridge node is forthe other ring served thereby, it goes into one ofthe stores from which it is released when traffic on the destination ring permits. This has the effect that the rings a retreated "traffic-wise" as if they were one big ring instead of two or more interconnected rings.

An object ofthis invention is to extend the principles ofthe use of bridge nodes in plural-ring LAN's.

According to the present invention there is provided an automatictelecommunication system ofthe local area network (LAN) type, which includes first and second sub-networks each consisting of a closed ring serving a number of nodes, which nodes include nodes for subscribers served by the system, transmission over the rings being digital, and a bridge node which interconnects the sub-networks, in which the sub-networks operate at different digital bit rates, in which said bridge node includes a first interface between itself and the first sub-network, a second interface between itself and the second sub-network, buffer storage means for each direction oftransfer between the sub-networks, and a controller, in which the controller monitors each message which reaches the bridge node via one orthe other of the interfaces and determines from the message's destination information wherethat message should go, inwhich if said monitoring indicates that the message should pass from one sub-networkto the other it is placed in the buffer storage means from which it is sent to the other of the sub-networks when traffic conditions on said other sub-network permit, in which the controller also monitors each message to check that it is correct according to the sub-networkfrom which it came, and in which the controller also controls the conversion of a message to be sentfrom one sub-network to the other between the formats used on said sub-networks.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 showsschematicallya LAN with two rings operating at different bit rates interconnected by a bridge node.

Fig. 2 shows the bare essentials of the bridge node of Fig. 1, and Fig. 3 is a more detailed block diagram ofthe bridge node of Fig. 2.

The system described provides for interworking between ring-based systems operating at different bit rates. Fig. 1 shows atypical arrangement,with ring 1 operating at a different bit rate from ring 2. Each ring serves a number of subscriber attachment stations or nodes S, and is linked to the other ring by a bridge node B. Note that each subscriber node may serve more than one terminal, and that one or more nodes may be facilities nodes. Interworking takes place by the exchange of packets or frames of data between a source node on one ring and a destination node on the other, via the bridge node.The functions provided by the bridge node to enable interworking are confined to the MAC sublayer of level 2 and to the physical layer (level 1) of the International Standards Organisation (ISO) 7-layer Reference Model for Open Systems Interconnection (OSI).

The bridge node consists of a station with two interfaces, one to each ofthetwo rings, plus two sets of buffers, Fig. 2. The lower speed (LS) ring interface performs similarfunctionsto the ring interfaces ofthe nodes ofthe LS ring. Likewise, the higher speed (HS) ring interface performs similarfunctionsto the ring interfaces ofthe nodes ofthe HS ring. In normal operation, frames of data transmitted by a source node on the LS ring for a station on the HS ring, are copied by the LS buffer. After completion of any reformatting,theLS buffer retransmitstheframes at the required bitrate via the HS interface to the HS ring.

At the same time, the copied frames may be repeated onto the ring by the LS ring interface to be removed from the ring back atthe source node. This serves to notify the "calling" nodethatthe message has at least reached the bridge node. A similar set of operations is performed by the HS buffer, but in the other direction, i.e. from higher speed to lower speed rings.

The specific functions associated with a bridge node to enable interworking between rings operating at different bit rates are described below. The functions described are based on an example consisting ofthe IEEE 802.5 Token Ring operating art a bit rate of 4 Mbits/sec., interworking with the ANSI-FDDI Token Ring operating at 100 Mbitslsec. However,thefunctional description ofthe bridge node, ora subset thereof, holds for other interworking combinations of ring bit rates and ring access protocols.

Functional Description ofthe Bridge Node Fig. 3 is a functional block diagram of a bridge node for interworking between the token rings. Only the majorfunctions relevant to interworking are shown.

Otherfunctions, e.g. those for re-establishment ofthe ringsfollowing seriouserrorconditions,are not described since they can follow known practices and are not relevantto the present invention.

A brief description is given below of each ofthe blocks shown in Fig. 3. The operations needed for interworking between the rings are then described.

The abbreviation LS means lower speed, i.e. concerned with the lower bitrate ring, whilstthe abbreviation HS means higher speed, i.e. concerned with the higherbitrate ring.

LS Ring Interface The LS ring interface 1 performs similarfunctionsto the ring interfaces ofthe subscriber nodes on the LS ring. These include: (a) reception ofthe serial bitstream from the LS ring input, (b) decoding ofthe Differential Manchesterencoded serial bitstream from the ring into its equivalent4 Mbit/sec. symbol stream, (c) error checking ofthe input data frames, based on the Frame Check Sequence (FCS) polynomial in each data frame, (d) clock extraction from the input bitstream and clock distribution to the appropriate bridge functions, (e) provision of an FCS for inclusion in rames originated onto the LS ring by the bridge node, (f) encoding ofthe output symbol stream into a Differential Manchesterencoded bitstream fortransmission.

(g) synchronisation ofthe output bitstream to the correct clock frequency and transmission ofthe serial bitstream to the LS ring output.

LS Frame Delimiter Detector (2) This detector checks the incoming serial 4 Mbit/sec.

symbol stream received from the LS ring for both start offrameand end offrame delimiterfields. Thesefields are present in all correctlyformed and acceptable inputframes.

LS Frame Checker (3) This checks all frames received from the LS ring foracceptability in all respects otherthan the FCS check.

LSAddress Checker (4) This checks the address fields ofall frames received from the LS ring. The Destination Address (DA) fields are checked forframes from the LS ring but destined for the HS ring. The Source Address (SA) fields are checked for returning frames initiated by the bridge node onto the LS ring from the HS ring.

LS Token Detector{5) This decodes and examines the Access Control (AC) fields in all frames received from the LS ring for a token, and for determination ofthe currentframe priority and reservation levels.

LS Transmit Oueue (61 This stores frames in priority order pending their transmission onto the LS ring. Its storage capacity is at least enough forthe longest permitted data frame from the HS ring. It also provides the source offill symbols during the appropriate parts of the protocol, and generates a new Token following frametransmission.

LS PriorityRegisters and Stacks (7) These store records ofthe priority and reservation levels ofthe most recently receiver' and transmitted frames. They enable decisions to betaken concerning token reservations and frame and token transmissions.

LS Message Buffer(8) This is a temporary store, and is afirst-infirst-out (FIFO) register It holds the contents ofthe incoming frame from the LS ring pending decisions on its further processing or erasure.

LS Translator and Reformatter (9) This translates the contents ofthe component fields ofthe data frames from the LS ring and reformats them into the equivalentfields as needed fortransmission onto the HS ring. If the information field of an incoming frame from the LS ring is too long for transmission in a single frame on the HS ring, thatfield is split amongst an appropriate number of framers for transmission onto the HS ring.

HS Ring Interface (10) This performs similarfunctionstothe ring inter- faces ofthe nodes on the HS ring. These include: (a) reception of the serial bitstream from the HS ring input, (b) decoding ofthe NRZI (Non Retrun to Zero, with 1 bits inverted) encoded serial bitstream from the ring into its equivalent 25 Mbit'sec. symbol stream, (c) error checking ofthe input data frames, based on the FCS in each data frame, (d) clock extraction from the input bitstream and clockdistribution to the appropriate bridgefunctions, (e) provision of an FCSforinclusion inframes originated onto the HS ring bythe bridge node, (f) encoding of the output symbol stream into a NRZI encoded bitstream fortransmission.

(g) synchronisation ofthe output bitstream to the correct clock frequency and transmission ofthe serial bitstream to the HS ring output.

HS Frame Delimiter Detector (71) This checks the incoming 25 Mbitlsec.symbol stream received from the HS ring for both start of frame and end of frame delimiterfields, which are present in all correctly formed and acceptable input frames.

HS Frame Checker (12) This checks all frames received from the HS ring for acceptability in all respects otherthan the FCS check.

Address Checker (13) This checks the address fields of all frames received from the HS ring, in conjunction with the L-bitinthe FC field which indicates the length ofthe addressfields The DAfields are checked forframesfrom the HS ring butdestinedforthe LS ring. The SAfields are checked for returning frames initiated by the bridge node onto the HS ring from the LS ring.

HS Token Detector (14) This decodes and examines the FCfields in all frames received from the KS ring for a token.

HS Transmit Oueue (15) This storesframes in priority order pending their transmission onto the HS ring. The storage capacity is at least sufficientforthe longest permitted data frame from the LS ring. It also provides the source of idle symbols during the appropriate parts of the protocol, and generates a new Token following frame transmission.

HS Message Buffer {16) This is a temporary store, and is a first-in first-out (FIFO) register. It holds the contents ofthe incoming frame from the HS ring pending decisions on further processing or erasure.

HS Translator and Reformatter (17) This translates the contents ofthe component fields ofthe data frames from the HS ring and reformats them into the equivalent fields as needed fortransmission onto the LS ring. If the information field of an incomingframefromthe HS ring istoo long for transmission in a singleframe on the LS ring, itis split amongst an appropriate numberofframesfortrans mission onto the LS ring.

Operational Description The operation of that portion ofthe system con cernedwith reception and transmission of data frames on the LS ring is described first, followed by a description ofthe portion of the system concerned with the reception and transmission of data frames on the HS ring.

Reception and Transmission on the LS Ring Information frames circulating on the LS ring are presented in 4 Mbit/sec. bit-serial form to the detector 2 by the interface 1. With no outgoing transmissions from the bridge node onto the LS ring, the LS ring input is always repeated at the LS ring output after a few bits-worth of processing delay, often with modifications to certain of the frame control bits. This occurs whether or not the currentframe is destined for a node on the HS ring.

The detector 2, on detection of a start of frame delimiter, passes the start offrame field, followed by the otherframefields as it receives them, in bitparallel form on its output bus. A start delimiter (SD) indication is madeavailabletothefourblocks connected to the output bus, informing them of the availability ofthe start offramefield.The state ofthe intermediate bit (I-bit) in the end offrame delimiter is also made available as the Final Frame (FF) indication to the LS Transmit Queue 6 to permit decisions concerning the transmission of Fill characters following token transmission.

On receipt of the SD indication, the LS Frame Checker 3 begins two check the incoming frame for acceptability. If at anytime during incoming frame reception the frame is found unacceptable, or an FCS input is received from the Interface 1 indicating an FCS failure, an error detected (ERR) output is produced which resets the LS Message Buffer 8, clears the currentframe from the HS Transmit Queue 15 and disables the LS Translator and Reformatter 9. The Checker 3 also controls the error detected bit (E-bit) in the end offrame delimiterfield ofthe incoming frame as it is repeated onto the LS ring by the interface 1.

The Checker 4, on receipt of the SD indication, counts the number of fields to the start of reception of the frame's DAand SAfields. It analyses the DAand if the frame is notfora node on the HS ring it produces a reset (RS) output to clear the currentframe from the LS Message Buffer8. However, if the DAindicatesthat the frame isforthe HS ring, an enable (EN) output is produced to enablethe operation ofthe LS Translator and Reformatter 9, whilstthe Buffer 8 continues to receive and store the contents oftheframe.

Likewise, the SA is analysed so that returning frames originated onto the LS ring by the bridging node can be recognised and stripped. An SA indication is provided forthis purpose to the LS Transmit Queue 6 whenever a frame is detected with SA identical with the bridge node's own address.

The Checker 4 also controls the Address Recognised bit (A-bit) in the Frame Status field of the frame as it is repeated onto the LS ring by the interface 1.

The SD indication from the Detector 2 causes the Buffer 8to begin storing the frame contents. It continuesto do this either untiltheframe ends or until it is reset by the RS input from the Checker 4, or by the ERR input from the Checker 3, or by the CL inputfrom the LS Token Detector 5. On request from the LS Translator and Reformatter 9, the frame contents are made available on the output bus from the buffer 8 as bytes of data in the same order as they were received (i.e. the buffer behaves as a FIFO register).

The LS Token Detector 5 is activated by the SD output from the Detector 2 to lookforthe start ofthe AC field. It analyses the AC field and if a token is indicated, an enable (ENP) output is produced, enabling the LS Transmit Queue 6 and indicating the lowest level of priority offramesthat may be transmitted. At the same time, a clear (CL) output is produced to clear the Buffer 8 of its current token frame contents. An indication is also provided to the LS Priority Registers and Stacks 7 of the priority and reservation levels in the AC fields of all frames and tokens incoming from the LS ring.

The LS Transmit Queue 6 stores frames of data received from the HS ring and destinedfortransmission onto the LS ring, following translation and reformatting by the HS Translator and Reformatter 17.

These frames are received, one byte at a time, on the bus from the HS Translatorand Reformatter 17. If all storage space isfull,the LS Transmit queue 6 refuses to accept further data from the HS ring on its input bus and makes available a Queue Full (QF) indication. This causes the Frame Copied symbol (symbol) in the Frame Status field of the rejected frame to remain reset as the frame is repeated onto the HS ring by the HS Ring Interface 10.

Frame transmission onto the LS ring is enabled on receipt of the ENP input from the LS Token Detector 5, which results from the detection of a token on the LS ring. The LS Transmit Queue 6 can then initiate frame transmission if it has a frame available of priority at least as high asthe priority level indicated bythe ENP input. If no frame of sufficiently high priority is awaiting transmission, the token is repeated onto the LS ring ratherthan being changedto a start of frame sequence by the LSTransmit Queue 6. Otherwise, the waiting frames are passed in bit-serial form to the LS Ring Interface 1 for transmission onto the LS ring.

Frame transmissions continue until eitherthere are no more frames at or above the present priority level or until the maximum time allowed for transmissions has elapsed.

Either after, or during, frame transmissions initiated by the LS Transmit Queue 6 (depending on the duration ofthetransmissions) the LS Address Checker 4recognisesthe SA in a returning frame as the bridge node's own address. It passes an SA indication to the LS Transit Queue 6, which then initiatesthetransmis- sion of a newtoken onto the ring, provided it has completed all currentframe transmissions. If SA recognition does not occur until after all frame transmissions are complete, the LSTransmitQueue6 enables the transmission of Fill until it can initiate the newtoken.

Having completed transmission of a newtoken, the LS Transmit Queue 6 causes the transmission of Fill until it receives an FF inputfrom the LS Frame DelimiterDetector2 indicating that its final frame transmission has returned on the LS ring (I-bit = O in the frame End Delimiter field). If the FF indication arrives before the end of transmission ofthe token, when token transmission ends the interface 1 returns to repeating the LS ring input on the LS ring output without transmission of intermediate Fill.

The LS Registers and Stacks 7 are used to maintain a record ofthe latest received and transmitted frame and token priorityand reservation levels, so that frame transmissionsfrom the bridge node can be managed in accordance with the priority operation rules of IEEE 802.5. The incoming frame priority and reservation levels are notified by the LS Token Detector 5. The LS Priority Registers and Stacks 7 are closely involved with the operation ofthe LS Transmit Queue 6, and effect any updating ofthe priority and reservation levels in the AC fields of all frames and tokens transmitted onto the LS ring.

The LSTranslatorand Reformatter9 is enabled by theEN inputfromtheLSAddressChecker4on detection of a frame addressed to a station on the HS ring. Requests are madetothe Buffer8forthetransfer of bytes of data, as required forfurther processing. A translation and reformatting operation is performed on each frame field received from the LS Message Buffer, to produce an equivalent set offields for transmission onto the HS ring. This includes changing the SA the frame received from the LS ring to a new SA identical with the mode's own address.Also, if the information field from the LS ring is too long for transmission in a single frame on the KS ring,the LS Translatorand Reformatter9splitsuptheLSframe into an appropriate numberofsmaller, properly formatted HS frames.

The operation ofthe LS Translator and Reformatter 9can be aborted atanytime during LSframe processing by an ERR inputfrom the LS Frame Checker 3.

Reception and Transmission on the HS Ring The operation ofthe HS functional blocks concerned with reception and transmission offrames on the HS ring issimilarto that of the equivalent LS blocks,with some departures due to detail difference in operation of the two ring access methods (IEEE 802.5 vs ANSI FDDI). However, for completeness a full description of each HS block is included below.

Information frames circulating on the HS ring are presented in 25 Mbitl'sec. symbol-serial (i.e. 4-bit parallel)form lo the Detector 11 bythe Interface 10.

With no outgoing transmissions from the bridge node onto the HS ring, the HS ring input is always repeated atthe HS ring output after a delay determined by the 'elastic buffer' in the Interface 10, often with modifications to certain ofthe frame control bits. This occUrs whetherornotthe currentframe i5 fora station on the LS ring.

The detector 1, on detection of a start offrame delimiter, presents the startofframe field, followed by the otherframefields as it receives them, in bitparallel form on its output bus. A start delimiter (SD) indication is made available to the four blocks connected to the output bus, plusthe Transmit Queue 15, informing ofthe availability ofthe start of frame field. An indication ofthe satisfactory reception of an end offramefield (ED) is also made available to the HS Token Detector 14 and the HS Transmit Queue 15.

On receipt ofthe SD indication,the Checker 12 begins to checkthe incoming frame for acceptability, based on the criteria in the ANSI FDDI specifications. If any anytime during incoming frame reception the frame is found unacceptable, oran FCS input is receivedfrom the interface-l0 indicating a FCS failure, an error detected (ERR) output is produced which resets the Buffer 16, clears the currentframefrom the LS Transmit Queue 6 and disables the HS Transistor and Reformatter 17. The Checker 12 also controls the Error Detected symbol (E-symbol) in the Frame Status field ofthe incoming frame as itis repeated onto the HS ring output by the interface 10.In addition, a separate specific error condition called a frame format error (FE) is reported to the HS Transmit Queue 15, which caused Idle symbols to be transmitted onto the HS ring, effectively stripping the cu rrentframe.

The HS Address Checker 13, on receipt ofthe SD indication, countsthe number offieldsto the start of reception of the frame DA and SAfields. It analyses the DA and iftheframe is notfor a station on the LS ring it produces a reset(RS) outputto clearthe currentframe from the Buffer 16. However, if the DA indicates that the frame is forthe LS ring, an enable (EN) output is produced to enabletheoperation ofthe HS Translator and Reformatter 17 whilstthe HS Buffer 16 continues to receive and store the contents ofthe frame.

Likewise, the SA is analysed so that returning frames originated onto the HS ring by the bridging station can be recognised and stripped. An SA indication is providedforthis purpose to the HS Transmit Queue 15 whenever a frame is detected with SA identical with the bridge node's own address.

The HS Address Checker 13 also controlsthe Address Recognised symbol (symbol) in the Frame Status field ofthe frame as it is repeated onto the HS ring by the interface 10.

The SD indication from the detector 11 causes the Buffer 16 to begin storing the frame contents, which it continues to do either until the frame ends or until it is resetbythe RS input from the Checker 13, or by the ERR inputfrom the Checker 12:or by the CL inputfrom the HS Token Detector 14. On request from the HS Translator and Reformatter 17, the frame contents are made available on the Buffer 1 6's output bus as a series of bytes of data in the same orderas they were received (i.e. the buffer behaves as a FIFO register).

The Detector 14 is activated by the SD inputfrom the Detector 11 to lookforthe start ofthe FCfield. It analyses the FC field and if a token is indicated, a clear (CL) output is produced to clear the Buffer 16 of its current token frame contents. In addition, a strip (ST) instruction is produced, which causes the KS Transmit Queue 15 to enable the transmission of Idle symbols onto the HS ring to strip the token. Later, an indication is received from the Detector 11 when the token ending delimiter (ED) has been recognised. Ifthe token is acceptable, an enable (EN) output resu Its from the Detector 14which causesthe HSTransmit Queue 15 to enable frame transmissions onto the HS ring.If the token is not acceptable, no EN output is produced and Idle symbols continue to be transmitted onto the HS ring until the start delimiter ofthe next frame incoming from the HS ring is detected.

The HS Transmit Queue 15 sto res fra mes.of data received from the LS ring and destinedfortransmission onto the HS ring, following translation and reformatting by the LS Translator and Reformatter 9.

The frames are received, one byte at a time, on the bus from the LS Translator and Reformatter 9. If all storage space is full, the KS Transmit Queue 15 refuses to acceptfurther data from the LS ring on its input bus and makes available a Queue Full (QF) indication. This causes the Frame Copied bit (C-bit) in the Frame Status field ofthe rejected frame to remain reset as the frame is repeated onto the LS ring by the interface 1.

Frame transmission onto the HS ring is enabled on receipt ofthe EN inputfrom the KS Token Detector 14, which results from the detection of an acceptable token on the HS ring. The HS Transmit Queue 15 can then initiate frame transmission if it has a frame available of priority at least as high asthe current HS ring priority level. This level is determined from the time taken by the token to circuiate around the HS ring.

This is carefully monitored by the HS Transmit Queue 15, which is thus aware ofthe prevailing priority level and ofthe amount of time availableforframe transmissions. If there is no frame of sufficiently high priority awaiting transmission, the ST and EN inputs from the Detector 14 are ignored and the token is repeated onto the HS ring ratherthen being stripped.

Otherwise, the waiting frames are passed in symbolserial form to the interface 10 fortransmission onto the HS ring. Frame transmissions continue until either there are no more frames at or above the present priority level or until the maximum time allowed for transmission has elapsed. The completion offrame transmissions is followed by the generation and transmission of a new token onto the HS ring from the KS Transmit Queue 15. This is followed by a return to the repeat mode of operation when the interface 10 repeats the HS ring input onto the HS ring output.

The HS Transmit Queue 15 is notified each time an SD and ED is detected by the Detector 11. The ED causes the HS Transmit Queue 15to enable the transmission of Idle symbols onto the HS ring while the SD causes Idle Symbol disablement. Idle symbols thus appear on the HS ring between repeated frames and tokens.

Either after, or during, frame transmissions initiated by the HS Transmit Queue 15 (depending on the duration ofthetransmission),theKSAddressChecker 13 recognisesthe SA in a returning frame as the bridge node's own address. It passes an SA indication to the HS Transmit Queue 15 each time it recognises a returning frame. If it has completed its frame transmissions and generated a new token, the HS Transmit Queue 15 initiatesthetransmission ofldlesymbols onto the HS ring on receipt of each SA indication, to strip its own returning frames.

The HSTranslator and Reformatter 17 is enabled by theEN inputfrom the Checker 13 on detection of a frame addressed to a station on the LS ring. Requests are made to the Buffer 16forthetransfer of bytes of data, as required forfurther processing. Atranslation and reformatting operation is performed on each of the frame fields received from the HS Message Buffer, to produce an equivalent set of fields for transmission onto the LS ring. This includes changing the SA in the frame received from the HS ring to a new SA identical with the node's own address.Also, if the information field from the HS ring is too long fortransmission in a singleframeonthe LS ring, the HSTranslatorand Reformatter 17 splits up the HS frame into an appropriate number ofsmaller, properly formatted LS frames.

The operation ofthe KS Translator and Reformatter 17 can be aborted atanytime during HSframe processing by an ERR input from the HS Frame Checker 12.

Conclusions A system has been described which enables interworking between ring communication systems operating at different bit rates. The description is based on interworking between the Token Ring specified in the IEEE 802.5 specification and the Token Ring specified in the NASI FDDI specification. However, the arrangement ofthe major blocks shown in Fig. 3, or an appropriate subset of the major blocks, can satisfy the interworking requirements of a range of different ring systems operating at a range of different bit rates. A virtue of the functions described above is that interworking is confined to the lower two levels of the ISO 7-Layer model for OSI.

Claims (6)

1. An automatictelecommunication system ofthe local area network (LAN) type, which includes first and second sub-networks each consisting of a closed ring serving a number of nodes, which nodes include nodes for subscribers served bythe system, transmission over the rings being digital, and a bridge node which interconnects the sub-networks, in which the sub-networks operate at different digital bit rates, in which said bridge node includes a first interface between itself and the first sub-network, a second interface between itself and the second sub-network, buffer sto rage means for each direction of transfer between the sub-networks, and a controller, in which the controller monitors each message which reaches the bridge node via one orthe other ofthe interfaces and determinesfromthe message's destination information where that message should go, in which if said monitoring indicates that the message should pass form one sub-network to the other it is placed in the buffer storage means from which it is sent to the other ofthe sub-networks when traffic conditions on said other sub-network permit, in which the controller also monitors each message to check that it is correct according to the su b-network from which it came, and in which the controller also controls the conversion of a messageto besentfrom one sub-networkto the other between the formats used on said sub-networks.

2. Asystem as claimed in claim 1,in which each message to be handled includes a headerwhich includes at least its source address (SA) and its destination address (DA), in which when a message is to be transferred from one ring to the otherthat message is also sent from the bridge node on to the first ring so that its return to the SA node corresponding to its SA indicates thatthe message has at least reached the bridge node.

3. Asystem as claimed in claim 1 or 2, and in which each said ring uses token ring techniques.

4. Asystem as claimed in claim 1 or2or3 or4, and in which the functions performed by the bridge node are confined to the lowertwo layers (the Physical layer and the Linklayer) ofthe International Standards Organisation 7-layer Reference Model for Open Systems Interconnection.

5. An automatictelecommunications system of the local area network type, substantially as described with reference to the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: New ortextually amended claims have been filed as follows:-

6. An automatictelecommunication system of the local area network (LAN) type, which includes first and second sub-networks each consisting of a closed ring serving a number of nodes, which nodes include nodes to which subscribers served by the system are connected, transmission overthe rings being digital, and a bridge node which interconnectsthesubnetworks, in which the sub-networks operate at different digital bit rates and message conveyance on the two sub-networks uses different message formats, in which said bridge node includes a first interface between itself and the first sub-network, a second interface between itself and the second sub-network, buffer storage means for each direction oftransfer between the sub-networks, and a controller, in which the controller monitors each message which reaches the bridge node via oneorthe otherofthe interfaces and determines from the message's destination information where that message should go, in which if said monitoring indicates thatthe message is for a node on the same sub-network it is transmitted from the bridge node overthe same sub-network, in which if said monitoring indicates thatthe message should pass from onesub-networkto the other it is placed in the buffer storage means appropriateto the desired direction of said passage from which it is sentto the other ofthe sub-networks when traffic conditions on said othersub-network permit, in which the controller also monitors each message to checkthat it is correct in respect of its format and contents according to the sub-networkfrom which it came, and in which the controller includestranslation and reformatting means for conversion from the lower speed subnetworkto the :lig her speed sub-network and viceversa, so that each message which has to pass from one sub-networkto the other is translated and reformatted as appropriate to the direction of transfer ofthat message via the bridge node, the controller therefore controlling the conversion of a message to be sent from one sub-network to th9 other betweeen both the bit rates and theformats used on said sub-networks.