GB2177878A - Data transmission system of the local area network type - Google Patents

Abstract

In a ring type local area network which handles both circuit switched (speech) traffic and packet switched (data) traffic, the nodes include nodes which can act as monitors as well as traffic handling nodes. A monitor node when active acts as the master control for the ring, and in the course of doing this it emits a framing bit sequence once per frame, which sequence's bits provide the various control and synchronisation functions. One such function is to allot to each node which can act as a monitor a bidding number. When the ring is initialised, or after it has lost sync., each potential monitor node sends its own bidding number and the node with the highest bidding number is selected. This selection defines which node is the active monitor, or system's master. The potential monitor nodes each include a shift register via which the ring bit stream passes and which sets the ring delay at the time between two speech samples. As usual each node may serve a number of terminals, and the terminals serving a node may together form a mini-exchange.

Description

SPECIFICATION Data Transmission System of the Local Area Network Type The present invention relates to a data transmission system ofthe local area network (LAN) type, one example of which is described in ourAppln. No. 2139852A (M.T. Shortland 1). In such a system each of the nodes on the ring, which may each serve a number of terminals, has access to the ring for the processing of both circuit-switched and packet-switched connections.

The protocol used allows a number of nodes to share a ring and to communicate using both circuit and packet switching. For circuit switching it is necessary first to reserve a circuit slot, which is maintained for the duration of the connection, offering a fixed bandwidth, and released at the end of the connection.

Any ring capacity not used for circuit switching is available for packet switching. In one version of the above system, four different priority levels are available and these allow the available non circuit-switched bandwidth to be allocated efficiently among the various services on the ring.

This invention seeks to provide further facilities for use in a system such as that of our above-mentioned application.

According to the invention there is provided a data transmission system of the local area network type which handles both circuit-switched and packet-switched traffic, in which a number of nodes are linked serially by a transmission medium to form a ring, in which the nodes include nodes which can each act as a station so as to handle traffic on the ring and nodes each of which can act as a monitor to provide functions necessary to maintain operation of the ring, in which a said node acting as a monitor generates as part of said functions a regular framing structure of bits for the medium, in which part of the bit sequence forming the framing structure is used to allow nodes capable of acting as monitors each to bid to become an active monitor, in which said bidding is effected by using a bidding number conveyed in the framing structure, the bidding numbers of such nodes capable of acting as monitors being sent to the line each from its own node, and in which one of said bidding numbers is automatically selected on the basis of preset criteria, the node whose bidding number is thus selected becoming the active monitor.

According to the invention there is also provided a data transmission system of the local area network type which handles both circuit-switched and packet-switched traffic, in which a number of nodes are linked serially by a transmission medium to form a ring, in which the nodes include nodes which can each act as a station so as to handle traffic on the ring and nodes each of which can act as a monitor to provide functions necessary to maintain operation of the ring, in which a said node acting as a-monitor generates as part of said functions a regular framing structure of bits for the medium, in which a said node capable of acting as a monitor includes delay means via which data packets in the ring pass, which delay means is so proportioned that the ring delay time exactly equals the time interval between two TDM speech samples, in which packet switching connections are controlled by the use of a token passing along the ring and originated by a said active monitor, in which part of the bit sequence forming the framing structure is used to allow nodes capable of acting as monitors each to bid to become an active monitor, in which said bidding is effected by using a bidding number conveyed in the framing structure, the bidding numbers of such nodes capable of acting as monitors being sent to the line each from its own node, and in which one of said bidding numbers is automatically selected on the basis of preset criteria, the node whose bidding number is thus selected becoming the active monitor.

The framing structure includes, as will be seen later, an n-frame, multijrame, sequence, and the bidding numbers each conveyed as one bit of the frame slot, a plural-bit bidding numberhus extending over a number of successive frames.

Introduction In the system to be described, nodes are linked serially by a transmission medium to form a ring. Each node ring performs functions which can be associated with one of two distinct entities, a Station and a Monitor. A Station provides services to the node by its Link Level Protocols, while a monitor provides functions necessary to maintain the operation of the ring. Note that in some cases the same node functions both as a Station as just defined and as a Monitor as just defined.

When operating normally, the Monitor in one node is in the Active Monitor State and the rest are in the Standby Monitor State. The Active Monitor generates a regular framing structure on to the transmission medium that is repeated by all other nodes. As a prime objective is to support 64 kbitls circuit switched calls, the frame structure uses frames of 125 microsecond duration. To achieve this, the Active Monitor uses a shift register (called a Latency Buffer) to "build out" the delay around the ring to a fixed 125 microseconds.

Data received from the ring is thus queued in this Buffer before being clocked out on to the ring.

Frame Format In the system to be described below, the ring transmission medium supports a 125 microsecond frame structure comprising 128 1 O-bit slots. The first two slots are known as frame slots, and are used for specific, non-user generated information carrying purposes. The remaining 126 slots are numbered 0 to 125 and are used to carry circuit switched and packet switched data.

The format of a frame is shown below:

1111111111 110QFABNLR PC01234567 I PC01234567 PC01234567 Framing Slots Slot O | Slot 1 Slot 125 The initial sequence of twelve ones followed by a zero provides a unique pattern that cannot be reproduced elsewhere in the frame, except by transmission errors. This sequence is thus used to define the frame structure. The remaining seven bits of the Framing Slots are used to allow the Monitors to bid to become the Active Monitor, to allow nodes to determine their physical location on the ring, and to supervise circuit slots.

These remaining seven bits are: Active Monitor Present Indication F frame synchronisation pattern A Monitor Bidding Nurnber B Monitor Beacon Number N Node Sequence Number L 128-frame synchronisation pattern R Slot supervision bit.

The functions of these bits, and of the further bits PCO1 234567 are described below.

Short Multiframe Structure A short multiframe consists of eight contiguous 125 microsecond frames. The F bit in the framing slots has a continuous 8-bit sequence used to define the 1-millisecond short multiframe. In the eight frames the F bit successively takes the values: 11101000. This multiframe starts with 3 ones and ends with 3 zeros, a sequence with the property that any three similar bits received in succession define the position in the short multiframe.

This structure controls the transmission of the A bit, the B bit and the N bit. Each of these fields contains an octet sent over the short multiframe. The A and B bits define the A and B Numbers, see below. Each of these numbers is one octet long and is sent Most Significant Bit first.

The N bit defines the Node Sequence Number, an 8-bit binary number, incremented on the fly as it arrives at each node. To do this, it is useful to send N Least Significant Bit first.

Below is an example of a 1-millisecond multiframe showing only the F, A, B and N bits:

Frame 1 Frame 2 Frame 3 Frame 4 1001 | 1001 | 1000 | 0000 FABN FABN FABN FABN 1 1 millisecond Frame 5 Frame 6 Frame 7 Frame 8 1000 0000 0010 0100 FABN FABN FABN FABN 1 1 millisecond F=11101000 Repeats Fixed Pattern A=00000001 transmitted MSB first 1 B=00000010 transmitted MSBfirst=2 N=11000000 transmitted LSB first=3 Long Multiframe Structure A long multiframe structure consists of 128 contiguous 125 microsecond frames. The L bit in the framing slots provides a continuous 128-bit long sequence, generated by the Active Monitor. The sequence repeats every 16 milliseconds and is used to define the long multiframe.

The accompanying drawing shows a circuit usable to generate the L-bit sequence. It comprises a 7-bit shift register SR1-SR7 with a six input NAND gate and a three input EXCLUSIVE OR gate providing feedback to the input. The circuit is clocked once every 125 microseconds.

The L bit in the current received frame, plus the L bits in the previous six frames, define a 7-bit binary number formed by taking the most recently received bit as the most significant bit of the number. During a long multiframe this number takes for one frame all values between 0 and 127 inclusive, and defines the slot to be supervised in that frame. Slot supervision, which uses the R bit in conjunction with the L-bit sequence, is described below.

Slot Format The 126 10-bit slots, used for carrying circuit switched and packet switched data, each comprises eight data bits and two control bits. The two control bits distinguish between idle slots, circuit slots, packet data slots and delimiters which define the beginnings and ends of packets. These two control bits are referred to as the P bit and C bit (for Packet and Circuit).

Circuit slots and Packet slots carry information in their eight data bits. Delimiters carry only seven bits of information. The first data bit in a delimiter is always zero, which prevents a sequence of more than eleven consecutive ones occurring outside the frame slots.

The 126 10-bit slots are encoded according to the following table:

P C 01234567 Slot Usage 0 0 00000000 Idle Slot 0 1 XXXXXXXX Circuit Slot (8 data bits) 1 O XXXXXXXX Packet Slot (8 data bits) 1 1 OXXXXXXX Delimiter (7 data bits) X=User Data.

Other combinations are not valid and should be converted into valid slots before being retransmitted.

Packet Operation Any of the 126 10-bit slots not in use for circuit data can be used for packet data.

In the token passing protocol used, a single token is passed around the ring, from one station to the next, carrying with it the right to transmit packets. Each station can then transmit one or more packets while it holds the token and on completion passes the token on to the next station in the ring.

There are four priority levels, which are implemented by assigning a priority to the token and allowing nodes to reserve a high priority token if they have a high priority packet waiting while low priority traffic is using the ring. The token is a special kind of delimiter, and delimiters are also used to start and end packets.

Packets can be split by embedded idles, circuit slots and framing slots. Packets and tokens are passed through the latency buffer just like circuit slots and so take 125 microseconds to go once round the ring.

The packet functions at each node examine the received data, when the active monitor is present, and may modify the data according to the packet protocol. It is this modified data that is used by the circuit handling functions described below.

When there is no active monitor present on the ring, the packet functions are disabled.

Circuit-Switched Operation Each station maintains a slot counter, synchronised to the received frame structure, such that it knows which slot is being received at any time. When a pair of stations are using a slot as a circuit switched channel, the P bit of that slot is set to a zero and the C bit to a one for the duration of the connection. That slot can then be used to transfer 64 kbit's in each direction between the two stations with each station replacing the received data byte with its own outgoing data.

Broadcast slots are also supported, in which one station writes data to a circuit slot and other stations can only read from that slot.

Receiving a Circuit Slot The slots which can be used for circuit switched calls are always less than the complete set of 126 slots, and a value of N of 100 is recommended. However, this might be reduced for a very low priority request or increased for an emergency call.

For a station, i.e. a node with "station mode", to reserve a circuit slot, it first decides which range of slot numbers is available to it for the class of call being initiated. The station then examines all slots whose slot number is within that range, and if it receives a slot idle indication (afterthe received data has been processed by the packet functions) it converts that slot into a circuit slot by changing the C bit into a one and leaving the P bit a zero. This slot is now "owned" by the station and the station may start to write data into it.

When a slot reservation is required, the ring may be completely filled with packet data. To prevent a slot reservation being blocked by packets, each station maintains a flag called Repeat Only that is set at the start of each frame and is cleared as soon as an idle slot has been transmitted. This flag prevents the token passing protocol at that node from initiating and transmitting packet data slots or delimiters, although it allows these slots from upstream nodes to be repeated (possibly with bits flipped). The effect of the Repeat Only flag is to leave the first non-reserved slot in a frame free for new slot reservations.

Since several stations may request slots at the same time, this one free slot may be reserved, although a new idle slot will be created in the next frame. Hence the slot request is held for 1 millisecond (eight frames), before deciding that the request has failed and that the ring is full.

If a station reserves a slot that it already owns, because the slot has been turned into an idle by a transmission error in the current frame, the station must reject the slot and attempt a new slot reservation.

Owning a Circuit Slot As stations can fail and as transmission errors may occur on the links between stations, it is necessary to ensure that slots do not appear to be booked when no-one owns them, or become booked by two stations at once. These are described below under the heading of Slot Supervision.

The station that owns a slot, or a station writing circuit data to a slot owned by another station, sets the control bits to 01 in every frame to keep the slot reserved.

Using the framing slots, the monitor polls the stations to find out whether each slot is owned and the station must now look for a crossed call indication on the owned slot. During the frame in which supervision is being performed on its reserved slots, the station examines the R bit in the framing slots. If the R bit is already set, this indicates that an upstream station is claiming ownership of the slot. If this clash is confirmed during the next slot supervision frame, the station relinquishes the slot.

Using a Circuit Slot A station can use a reserved slot owned by someone else. Thus in a duplex voice call the originating (slot owning) station places data into its reserved slot and the data is read by the terminating station, which replaces the data by its own data for sending to the originating station, and so on. Alternatively, a station may just read data from the slot owning station, as in a broadcast call.

If a slot is being written to by a station, that station should set the two control bits to 01 on each cycle.

However, if a station is just reading data from a slot, it takes no action on the control bits.

The higher layers of the system's protocol ensure that once a slot has been reserved by a station, that ownership remains with that station. Under normal working conditions, all stations reading from or writing to the slot should stop using it before the owning station releases it. If a fault causes the owning station to lose the slot, or the owning station is removed from the ring, then all other stations should stop using that slot until it has been reserved again.

Releasing a Circuit Slot To release a slot the station that first reserved it, and now owns it, ensures that no other stations are still using the slot. The station can then release the slot by setting the two control bits to 00 to turn the slot back to an idle.

Monitor Bidding Monitor bidding is the process whereby the nodes bid amongst themselves to decide which node will generate the master clock to which the whole ring is locked. The nodes bid with an 8-bit number, called the Bidding Number, using the short multiframe, and the node with the highest bidding number is selected.

Bidding becomes necessary whenever a node loses synchronisation.

If two nodes have the same non-zero bidding number, then the ring may fail to synchronise correctly.

Every ring must have at least one node with a non-zero bidding number as a node with a zero bidding number can never become ring Master.

O-bit Operation The 0 bit is sent in every frame and indicates when there is an active monitor present. It is sent as a zero by a monitor when in the Beacon or Bidding state, and as a one by a monitor in the Active State. Monitors in the Standby state repeat the Q bit unaltered. The Bidding states are described below.

Nodes examine the 0 bit when in sync and indicate that there is an Active Monitor Present (AMP) if the O bit is set and the node is also in sync for all 8 frames of a 1-millisecond multiframe. AMP is cancelled immediately if the node loses synchronisation and is also cancelled if all 80 bits of the multiframe are set to zero.

F-bit Operation The F bit is sent one bit per 125 microsecond frame by nodes in the Active, Bidding, and Beacon states with an 8 bit pattern of 11101000 as described above. There must be no breaks in the short multiframe structure when changing between these states. When in the Standby state the F bit is repeated as received.

An internal 8 frame counter locks to the received pattern when out of sync and free runs when in sync.

The received F bit should be checked against the predicted F bit and must be in phase to come into sync.

The node must go out of sync following two consecutive F bits received different to their predicted values. It may be noted that any offset in the short multiframe structure causes such pairs of F bits to be in error.

A-bit Operation The eight A bits in a 1 millisecond multiframe carry the Bidding Number of the active monitor when there is an active monitor present on the ring and the most recent bidding or beaconing monitor under other circumstances.

The bidding number is sent most significant bit first to aid comparisons.

The node's bidding number is sent by a monitor in the active, bidding or beacon state. Normally monitors in the standby state repeat the received number. If there is an active monitor present and a monitor in the standby state has a bidding number greater than that received, then it should transmit its Bidding Number rather than the received one. This ensures that if a node has its Bidding Number changed to a higher one than that of the current active monitor, that the role of active monitor will be handed over in a controlled manner, AMP being removed before the new node enters the bidding state.

B-bit Operation The B bits carry the 8-bit beacon number over a short multiframe, and this is used to localise a fault to the segment between a pair of nodes. A monitor in the Active Monitor state transmits a zero Beacon Number, otherwise, if it is out of sync, it transmits its own Bidding Number and, if in sync, it repeats the received Beacon Number.

N-bit Operation The eight N bits in a 1-millisecond multiframe define the N counter, a binary number between 0 and 255 with the least significant bit sent first. It should be noted that this is the opposite way round to the Bidding and Beacon Numbers which are sent most significant bit first.

The N counter is sent as a zero by a monitor in the active monitor or beaconning states, described below, and otherwise as one more than the received value. On a working ring the N bit allows nodes to find their position on the ring relative to the Active Monitor, and in a broken ring relative to the Node downstream of the fault.

Bidding States There are four bidding states: Beacon, Bidding, Active and Standby. A monitor in the Beacon state is one which has failed to lock to the received data and thus beacons to inform others of a potential problem. A monitor in the bidding state is sending its own Bidding Number to the ring and looking to see what numbers it receives back. If it receives back its own non-zero Bidding Number, then it assumes that the bidding has been won and that all the other nodes are in the standby state, so it goes into the active monitor state. If it receives a larger bidding number then it assumes that it has lost the bidding and goes into the standby state where it repeats the received data.

Active Monitor Present Active Monitor Present (AMP) is detected in the Active and Standby states if Q is set for all of one multiframe. The Standby state is one which Q is set for all of one multiframe. The AMP signal enables the Packet Protocol, and allows new circuit slots to be booked. Nodes that are in sync, i.e. they are not in the Beacon State, repeat delimiters and packet data without examining them when AMP is false. Owned circuit slots are always written to and used slots may also be written to, whilst other circuit slots are passed on unchanged. No data may be read from circuit slots without AMP and in addition no slot supervision errors are recognised.

The data output from the node is generated according to the following table:

Beacon Bidding Standby Slot Type No AMP No AMP No AMP Frame Slots New New Repeat Used Circuit Slots Write Write Write Unused Circuit Slots Strip Strip Repeat Packet Data Strip Strip Repeat

Standby Active Active Slot Type AMP No AMP No AMP Frame Slots Repeat New New Used Circuit Slots | Write | Write | Write Unused Circuit Slots Repeat Repeat Repeat Packet Data | Modify I Repeat | Modify Bidding Finite State Machine The following table shows the transitions between states for the Bidding Finite State Machine:

Current State AMP In Syne A bit New State Beacon X Yes 1 X Bidding Bidding X Yes 2 A > My2 Standby Bidding X Yes 2 A=My2 Active Bidding X No X Beacon Standby No 1 Yes 2 A < My2 Bidding Standby | X No X Beacon Active | X | Yes 2 A > My2 Bidding Active No 1 Yes 1 X Bidding Active X No X Beacon Key: AMP Active Monitor Present X Don't care No False at anytime No 1 False for 1 short multiframe Yes 1 True for 1 short multiframe Yes 2 True for 2 short multiframes A > My2 A Number > My Bidding Number for two consecutive short multiframes A=My2 A Number=My Bidding Numberfortwo consecutive short multiframes A < My2 A Number < My Bidding Number for two consecutive short multiframes Slot Supervision Slot Supervision is the function of accounting for all the reserved slots, ensuring that all reserved slots are owned by exactly one station.

One slot is checked in each frame, allowing all slots to be checked in 128 frames or 16 milliseconds. This period is referred to as a long multiframe. All 128 slots are polled during this period, although as no one can reserve the two frame slots, no errors are indicated for them.

L-bit Operation The L bit in the frame slots defines the long multiframe. There is no specified relation between the 16-millisecond long multiframe and the 1-millisecond short multiframe.

The slot being supervised during a particular framing slot is defined by the L bit in that slot and in the previous six frames. These seven L bits define a binary number by taking the current L bit as the most significant bit and with the previous six bits used in order.

Lbit 0 0 0 0 0 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 0 0 11111 1111 Slot being 6 9 1 2 2 2 2 6 9 1 1 2 2 6 3...

supervised 0462046735193521 The table above shows which slots are to be supervised as the L bits are received. It marks with *'s the two points in the long multiframe when framing slots are specified and so no supervision is performed. The number of the slot being supervised must be read vertically. That is they are 0, 64, 96,... The first six bits do not show the number of the slot as they are derived from one or more earlier unshown L bits.

R-bit Operation The R bit is sent as a zero by the active monitor. Each station sets the R bit if it owns the specified slot. If AMP is set and a station receives an R bit set while it owns the slot being supervised then this indicates a crossed call. Before reporting this fault the node verifies it by waiting for 16 milliseconds until that slot is again supervised, and checking that the fault recurs.

If the active monitor receives a frame slot without the R-bit set but with the slot, as it was last sent by the active monitor, a circuit slot, then the active monitor has detected an unowned slot. Again before reporting the error the monitor should wait 16 milliseconds to check that the fault recurs.

The Latency Buffer The latency buffer is used to store the received data by any node in the Bidding or Active State. Slots 0 to 125 are stored for up to 125 microseconds to align the received data with the transmitter. For the frame slots only bits B, and N must be stored, but these are delayed by up to 1 millisecond to bring them into multiframe alignment.

The latency buffer is initialised on entering the active monitor state from which point it can accommodate up to +/- 3 bits of jitter. As the jitter may have been at its worse when the latency buffer was initialised, the maximum jitter allowed round the ring is only half this or +/- 1.5 bits. As clock jitter will affect the CODEC's performance a lower figure may be required on this account.

General Notes on the System In many local area networks of the ring type, each node in fact serves several terminals, which in the applications visualised for the present system may include both speech and data terminals. In fact, one such system may include as each node what is in effect a mini-telephone exchange with a number of lines. In such case calls can be set up wholly within the node and also between the different nodes. Each such node capable of acting as a monitor also includes the shift register, known as a latency buffer, which makes up the delay in the ring to the interval between two consecutive speech samples. In contemporary practice, in which speech is sampled at 8 KHz this means that the ring delay is made up to 125 microseconds.

The preceding description is mainly based on the operating protocols used, since the logic circuitry used can in most respects follow established techniques.

Claims (15)

1. A data transmission system of the local area network type which handles both circuit-switched and packet-switched traffic, in which a number of nodes are linked serially by a transmission medium to form a ring, in which the nodes include nodes which can each act as a station so as to handle traffic on the ring and nodes each of which can act as a monitor to provide functions necessary to maintain operation of the ring, in which a said node acting as a monitor generates as part of said functions a regular framing structure of bits for the medium, in which part of the bit sequence forming the framing structure is used to allow nodes capable of acting as monitors each to bid to become an active monitor, in which said bidding is effected by using a bidding number conveyed in the framing structure, the bidding numbers of such nodes capable of acting as monitors being sent to the line each from its own node, and in which one of said bidding numbers is automatically selected on the basis of preset criteria, the node whose bidding number is thus selected becoming the active monitor.

2. A system as claimed in claim 1, and in which a said node capable of acting as a monitor includes delay means via which data packets in the ring pass, which delay means is so proportioned that the ring delay time exactly equals the time interval between two TDM speech samples.

3. A system as claimed in claim 1, and in which packet switching connections are controlled by the use of a token passing along the ring and originated by a said active monitor.

4. A data transmission system of the local area network type which handles both circuit-switched and packet-switched traffic, in which a number of nodes are linked serially by a transmission medium to form a ring, in which the nodes include nodes which can each act as a station so as to handle traffic on the ring and nodes each of which can act as a monitor to provide functions necessary to maintain operation of the ring, in which a said node acting as a monitor generates as part of said functions a regular framing structure of bits for the medium, in which a said node capable of acting as a monitor includes delay means via which data packets in the ring pass, which delay means is so proportioned that the ring delay time exactly equals the time interval between two TDM speech samples, in which packet switching connections are controlled by the use of a token passing along the ring and originated by a said active monitor, in which part of the bit sequence forming the framing structure is used to allow nodes capable of acting as monitors each to bid to become an active monitor, in which said bidding is effected by using a bidding number conveyed in the framing structure, the bidding numbers of such nodes capable of acting as monitors being sent to the line each from its own node, and in which one of said bidding numbers is automatically selected on the basis of preset criteria, the node whose bidding number is thus selected becoming the active monitor.

5. A system as claimed in claim 2, 3 or 4, and in which as a result of said predetermined criteria the node having the highest bidding number is selected as the active monitor.

6. A system as claimed in claim 1,2,3,4 or 5, and in which bidding is initiated on switch on of the system or in response to the loss of synchronisation by one of the nodes served by the system.

7. A system as claimed in any preceding claim, and in which the bidding number is conveyed in said framing structure using one bit per frame, said bidding numbers each having n bits.

8. A system as claimed in claim 7, in which the framing structure includes the bidding number of the currently active monitor sent as said bidding number, and in which a node capable of acting as a monitor receives a bidding number less than its own bidding number it causes its own number to be sent so that that node takes over as a monitor.

9. A system as claimed in claim 8, in which a further bit in the framing structure is used, over a set of m frames, to define an m bit number referred to as the N counter which forms a node sequence number which is conveyed least significant bit first, in which the N counter is sent from a node as zero if that node is in the active monitor state, and in which for a node not in the active monitor state the N counter number is incremented by unity compared to the received value.

10. A system as claimed in claim 9, and in which means is provided to check the N counter number so as to assess the position of a node relative to the node which currently acts as an active monitor.

11. A system as claimed in any one of the preceding claims, in which in each frame of the system one of the TDM slots of the system is checked to assess whether it is currently allocated to a node, and if so that it is currently allocated to only one node.

12. A system as claimed in any preceding claim, and in which the nodes include nodes each of which is capable of acting both as a station and as a monitor.

13. A system as claimed in any preceding claim, and in which each of the nodes is capable of handling traffic on the ring serves a number of terminals which may include both speech and data terminals.

14. A system as claimed in any one of claims 1 to 13, in which each of the nodes capable of handling traffic on the ring serves a number of terminals which may include both speech and data terminals so arranged that a said node and its terminals is in effect a mini-telephone exchange, and in which calls can be set up between terminals connected to the same node without using the ring while calls can also be set up over the ring between terminals connected to different nodes.

15. A data transmission system of the local area network type substantially as described with reference to the accompanying drawings.