COMMUNICATION SYSTEM AND METHOD

Abstract: A communication system comprises a plurality of communication stations (1-3, 5, 6) connected in a ring around which a first number of blocks of first information and a second number of block of second information are alternately transmitted together with a leading synchronisation block. For example, the first set of information may comprise voice information while the second comprises data. Each communication station (1-3) to which both sets of information are fed is adapted to separate the incoming information into the first and second sets, and is further adapted to insert information of both sets into the ring as a number of interleaved blocks for transmitting to a downstream station.

Description: COMMUNICATION SYSTEM AND METHOD The invention relates to a communication system and a method of transmitting information around a ring of communication stations. A common form of communication system involves a number of communication stations arranged in a ring with informatio being transmitted in the same direction between each station in the ring. In order to control the way in which information is transmitted, the information must be transmitted in a controlled manner and in one example use is made of variable length "frames", each frame containing the addresses of the sending and receiving stations, checking information, and various other matters together with the information concerned. This information is transmitted by the sending station and each succeeding station checks to see whether it is the designated receiving station. If it is not, the information is simply repeated around the ring whereas if it is to receive the information, the information is copied prior to being transmitted on to the downstream station. Eventually, the information arrives back at the sending station which then removes the information. This is one example of a token ring system and examples of such arrangements are the networks standardised by the IEEE Project 802 and by the ANSI FDDI Committee. For efficiency, these frames should be capable of being several thousand bytes long, thus taking several ns per frame on a 10-20 M bit/s network. One common requirement for such communication systems is to be able to transmit two different sets of data around the same ring. For example, it may be required to transmit synchronous (voice) and asynchronous (data) information around the same ring. This will occur, for example, where telephones and computers both make use of the same ring. Voice data is conveniently transmitted as 64 Kbit/s synchronous data streams, that is to say a stream of data whereby one 8 bit byte is exchanged every 125 μs. These 64 Kbit/s data streams are standard of communication into digital telephone networks. Although it is acceptable to have some delay between the receipt of each byte, it is inconvenient if the overall "round trip" delay exceeds 1-2 ms. On the other hand, for asynchronous data it is convenient to be able to transmit as much data as possible in one group where for example the data corresponds to a digitised file of information which is to be copied from one computer to another. It would thus appear that to design a network or system capable of carrying both synchronous and asynchronous information there are two options: a) make it inefficient for data (by having very small frames) or b) make it unsuitable for voice (by having long round trip delays) . In accordance with one aspect of the present invention, a method of transmitting first and second sets of information around a ring of communication stations comprises transmitting alternately a first number of blocks of the first set of information and a second number of blocks of the second set of information together with a leading synchronisation block. The invention provides relatively short time intervals between successive blocks of information of the same set which, particularly in the case of voice where delays of the order of the time for transmitting information around the ring are unacceptable, is important. The two sets of information may differ in a number of ways. As mentioned above, the invention is particularly applicable to the transmission of synchronous and asynchronous information. More generally, however, the invention could be used for transmitting data of the same type (ie. synchronous or asynchronous) but using different protocols where for example each communication station only communicates with a certain number of other stations operating with the same protocol. The invention also enables substantially simultaneous communication between stations to be achieved with information from each station being transmitted as interleaved blocks. The way in which the blocks of each set of information are interleaved in the ring is particularly simple. In general, a group of V blocks of the first set will be followed by a group of D blocks of the second set. The exact level of granularly of the symbols used will vary between types of network, but for simplicity the symbols can be considered as octets. Conveniently, V = 1 and D = 1. However, more general arrangements are also possible. In general, one of the sets of information will be transmitted in accordance with a data protocol such as that promulgated by the IEEE 802 standards committee and in this case it is convenient for the synchronisation block to be provided in association with a data block, and it is preferably provided in the start data block. The synchronisation block is preferably an octet which is specifically recognisable at a low level. For example, where the synchronisation character is contained in a data block and the IEEE 802.5 protocol is used, the synchronisation character can contain the Code Violation symbols J, K in the pattern JK0JK011. Because the synchronisation characters are clearly recognisable it is possible at a very simple level to establish the synchronisation required in the ring. One important and significant advantage of the invention over the prior art is that other schemes which depend on embedding the token protocol in some TDMA scheme require the TDMA scheme to be established before the separation of data and voice can be achieved. However under fault conditions or during reconfiguration this can be a lengthy process. It is also cheaper to construct hardware that performs the separation proposed here. This is particularly important because this part of the hardware will have to work at very high speeds. Another advantage of the invention is that the burst data rate of the ring is reduced by a factor of D: (D + V) . Other schemes require very high burst data rates which necessitate extensive buffering in the data station. It should be understood that although the invention has been discussed in terms of two sets of information, it is possible to extend the principle behind the invention to more than two sets. The advantage of just two sets is that this is much easier to implement using digital processing techniques. in accordance with a second aspect of the present invention, a communication system comprises a plurality of communication stations connected in a ring around which a first number of blocks of first information and a second number of blocks of second information are alternately transmitted together with a leading synchronisation block, each communication station to which both sets of information are fed being adapted to separate the incoming information into the first and second sets, and being adapted to insert information of both sets into the ring as a number of interleaved blocks for transmitting to a downstream station. Where the first and second sets of information comprise voice and asynchronous data, in order that the data and voice blocks can implement different protocols, it is preferable if the length of the ring is adapted to be a multiple of the voice transmission rate. This can be achieved by padding the ring as necessary. A communication station for use in such a system may comprise a synchronisation block detector; a demultiplexer responsive to a predetermined control function to separate the incoming information stream from the ring into first and second sets; and a multiplexer responsive to the control function to insert information of both sets into the ring as a number of interleaved blocks for transmitting to a downstream station. An example-of a communication system and a method of transmitting voice and data information around a ring in accordance with the present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of the communication system; Figures 2A and 2B illustrate different ways of interleaving voice and data blocks; Figure 3 illustrates in more detail the way in which voice and data blocks are interleaved in accordance with the Figure 2B method; Figure 4 is a block diagram of one of the communication stations shown in Figure 1; Figure 5 is a block diagram of the separator block shown in Figure 1; and. Figure 6 is a block diagram of the merger block shown in Figure 1. The communications system illustrated in Figure 1 is a local area network and comprises three primary communication stations 1-3 capable of receiving and transmitting voice and data information; a separator block 4; a voice station 5; three data stations 6; and a merger block 7. The stations 1-3 and the blocks 4, 7 are connected in a ring so that all information transmitted by one station passes completely around the ring back to that station. The stations are connected together by links 8, 9, 10 which may be constituted physically in any conventional form such as by optical fibres or radio transmission links. The station 5 is connected to a telephone (not shown) while the stations 6 are connected to computers (not shown) . The stations 1-3 may be connected to telephones or computers. The stations illustrated in the ring have the conventional ability to reconfigure the ring on the occurrence of faults and although single links have been illustrated between the various stations, these could be dual links and additionally redundant links could be provided. Data (ie. asynchronous data) is transmitted around the ring in accordance with the token ring protocol set down in the IEEE Standard 802.5. In this protocol, a token circulates around the ring and when one of the data stations wishes to transmit data to another station in the ring, it seizes the token and outputs a frame having the form shown in Figure 3A. In this Figure, the symbols are defined as follows: SD = Starting of Frame Sequence (1 octet) AC = Access Control (1 octet) FC = Frame Control (1 octet) DA = Destination Address (2 or 6 octets) SA = Source Address (2 or 6 octets) FCS = Frame Check Sequence (4 octets) ED = Ending Delimiter (1 octet) FS = Frame Status (1 octet) One of the characteristics of asynchronous data is that the amount of data in terms of the number of bytes to be transmitted will vary considerably from transmission to transmission as will the time between transmissions. Voice data may be transmitted by the voice station 5 or one of the primary stations 1-3 in the form of voice blocks using a different protocol. For example, a TDMA protocol could be used, a slot type protocol, or a modified form of the asynchronous data protocol. In this example, a modified form of the protocol shown in Figure 3A is used as illustrated in Figure 3B. One of the characteristics of voice data is its synchronous form due to the set sample rate which is used. Up to now, the only way in which voice and data have been transmitted around the same ring has been by transmitting an entire data frame as for example shown in Figure 3A followed by an entire voice frame as shown in Figure 3B. This leads to large gaps between information of the two sets which is particularly undesirable in the case of voice data. The invention improves on the prior techniques by interleaving blocks of the voice and data frames as they are transmitted between adjacent stations. Figure 2 illustrates two ways in which this may be achieved. In Figure 2A groups of four blocks of voice information are interleaved with groups of three blocks of data information. Figure 2B illustrates a preferred form in which single blocks of voice and data information alternate. In the example shown in Figure 3, each block of information comprises an octet although other block sizes are also possible. It should be noted in Figure 2 that the first data symbol is a Data Start (DS) octet which, where the IEEE 802.5 protocol is used for the data information, contains the Code Violation symbols J, K, in the pattern JK0JK011. The protocol for the voice information may be a TDMA protocol. For that purpose it is desirable that the ring is padded so that at least one voice block passes each station regularly every 125 microseconds. The extent to which the number of voice and the number of data blocks are interleaved can either be specified by agreement at power-up or by a unique synchronisation character followed by a data word. In an IEEE 802.5 system the unique synchronisation character could be a version of the start delimiter: J K 0 J K 0 1 1 This could be followed by a single octet giving the values of V and D either explicitly as two four bit numbers ie JK0JK011 00010010 would set D = 1 and V = 2 or implicitly by giving the sequence of the voice and data octets in the transmission. ie JK0JK011 11110010 would define the sequence: D D D D V V D V D D D D V V D V D D D D V V D V D D D D etc. (where V, D indicate single voice and data blocks respectively) . The internal construction of each of the primary stations 1-3 is shown in more detail in Figure 4. Incoming information in the form shown in Figure 2A or 2B is received by a demultiplexer 31 which is controlled by control logic 32 to switch incoming blocks of information either to a shift register 33 or a shift register 34. In the Figure 2A example, the demultiplexer 31 will cause four voice blocks to be fed to the shift register 33 followed by three data blocks to the shift register 34 and so on. The contents of each shift register 33, 34 are then monitored by the control logic 32 to determine the presence of a synchronisation block and whether for example that station's address has been detected indicating that the incoming information is for that station. The data is also monitored to check for faults in the ring and for a free token enabling the station to transmit information. In any event, after a two bit delay, information from the shift registers 33, 34 is shifted into a multiplexer 35 also controlled by the control logic 32. This remultiplexes the information into the Figure 2A or Figure 2B form for output to the next station in the ring. If the station is not interested in either receiving or transmitting information, the incoming information is simply passed through to the multiplexer 35. If, however, the information is to be monitored then the contents of the shift register 33, 34 will be copied to an internal memory of the control logic 32 for further processing. If the station is to transmit information, this will be fed to the appropriate shift register 33, 34 for interleaving with incoming blocks of the other set. Figure 3 illustrates in more detail the way in which the multiplexer 35 interleaves the data and voice blocks. At the start of a transmission, a data start block 36 will be output by the shift register 34 and the control logic 32 will then cause the multiplexer 35 to output the first block V 1 from the shift register 33. This will correspond to the first block 37 of the destination address of the voice frame. The next block D. will comprise the first block of the data frame from the shift register 34, the Starting Delimiter. As can be seen in Figure 3, successive blocks from each of the data and voice frames are interleaved by the multiplexer 35. The separator block 4 separates incoming information into voice information and data information. The voice information is transmitted along a link 9 to the voice station 5 and from there to the merger block 7. The data information is transmitted around the links 10 through the data stations 6 to the merger block 7. The reason for this separation is that the stations 5, 6 are only capable of detecting voice and data information respectively. The separator block 4 is illustrated in more detail in Figure 5. The combined voice and data bit stream arrives along the link 8 and is switched by a buffered switch 11 into block latency buffers 12 (for voice) or 13 (for data) . The buffered switch 11 stores an entire block before switching and is under the control of a flip-flop 14. The flip-flop 14 is controlled by a voice block counter 15 and a data block counter 16. Operation of the counters 15, 16 is initially controlled by a data start detector 17 and, where appropriate, an auxiliary data start detector 17'. In the case mentioned above where the synchronisation block comprises a data start octet followed by a second octet giving the values of V and D, this second octet will be detected by the detector 17' and be used to set the maximum counts in the counters 15, 16 respectively. The counters are set to V-l and D-l respectively. The flip-flop 14 is set to "voice" when the voice block counter 15 is reset and is set to "data" when the data block counter 16 is reset. The counters are reset when the flip-flop 14 changes state under the control of the other counter. The data block counter 16 is also reset when the detector 17 detects a data start block. The counters 15, 16 are incremented by the switch 11. The merger block 7 is illustrated in more detail in Figure 6. The voice stream and data stream come into respective block buffers 18, 19 which are capable of storing at least a total of two more bits than the number of bits in the respective blocks. Where each voice and data block comprises an octet then the buffers 18, 19 are capable of storing at least 10 bits. The voice blocks are passed from the buffer 18 to a switch 20 whose output port is connected to a buffer 21 connected to the link 8 leading to the primary station 1. If data is sent in groups of more than one block (for example as shown in Figure 2A) then an alignment buffer 22 is required capable of buffering a number of blocks of data. Thus, data is fed from the buffer 19 to the buffer 22 and from there to the switch 20. The switch 20 is normally controlled by a flip-flop 23 which is driven by counters 24 (voice) and 25 (data) in the same way as the switch 11 of the separator described above. The counters 24, 25 are incremented by the switch 20, as appropriate. Control logic 26 is arranged to be alerted by a data start detector 27 of DS blocks on the data stream. If the states of the data counter 25 and the buffer 22 are such that this DS block will not be transmitted at the start of the data block then sufficient "fill" blocks (usually comprising zeros) are transmitted to align the DS blocks. This will normally have the effect of making the buffer 22 fuller. If the protocol used in the data blocks has a physically recognisable block as an Ending Delimiter (as in IEEE 802.5), so that after a fixed number of blocks after the ending delimiter the blocks transmitted on the medium are no longer significant until the start delimiter, then the merger block 7 can detect this with a detector 28 and this can cause the control logic 26 to signal the buffer 22 to ignore the input and transmit the data blocks it has stored. This can be simplified if each group of transmitted data blocks consists of a single block, which in practice is likely as mentioned above. It will be appreciated that simplified versions of the separator block and the merger block shown in Figures 5 and 6 will be present in each of the primary stations 1-3 as the demultiplexer and the multiplexer respectively together with suitable 2 bit latency buffers.

Claims: CLAIMS 1. A method of transmitting first and second sets of information around a ring of communication stations (1-3,5,6), the method comprising transmitting alternately a first number of blocks of the first set of information and a second number of blocks of the second set of information together with a leading synchronisation block (DS). 2. A method according to claim 1, wherein the first and second sets of information are transmitted in accordance with different protocols. 3. A method according to claim 1, wherein the first and second sets of information comprise voice and data information respectively. 4. A method according to claim 1, wherein the first and second numbers are unity. 5. A communication system comprising a plurality of communication stations (1-3,5,6) connected in a ring around which a first number of blocks of first information and a second number of blocks of second information are alternately transmitted together with a leading synchronisation block, each communication station to which both sets of information are fed being adapted to separate the incoming information into the first and second sets, and being adapted to insert information of both sets into the ring as a number of interleaved blocks for transmitting to a downstream station. 6. A communication station for use in a system according to claim 5, the station comprising a synchronisation block detector (32) ; a demultiplexer (31) responsive to a predetermined control function to separate the incoming information stream from the ring into first and second sets; and a multiplexer (35) responsive to the control function to insert information of both sets into the ring as a number of interleaved blocks for transmitting to a downstream station. 7. A station according to claim 6, wherein the demultiplexer (31) comprises a switch (11) , actuated by a flip-flop (14) to which information from the ring is fed; and a pair of counters (15,16) for setting and resetting the flip-flop respectively, the counters being adapted to generate suitable set and reset signals respectively when counts defined by the control function are reached by blocks of incoming information. 8. A station according to claim 7, wherein the synchronisation block detector is adapted to define the control function as a pair of block numbers corresponding to the number of blocks of the first and second sets of information which are alternately transmitted in response to detecting the synchronisation block. 9. A station according to claim 6, wherein the multiplexer (35) comprises a switch (20) to which the first and second sets of information are fed; a flip-flop (23) for actuating the switch; and a pair of counters (24,25) responsive to the control function and to the number of blocks of each set of information inserted into the ring to set and reset the flip-flop respectively when the respective counts defined by the control function are reached. 10. A local area network comprising a plurality of communication stations according to claim 6.

Inventors: BEALE NICHOLAS CLIVE LANSDOWNE, GLADSTONE PHILIP JOHN STEUART, REED ANDREW STEVEN, BEALE, NICHOLAS, CLIVE, LANSDOWNE, GLADSTONE, PHILIP, JOHN, STEUART, REED, ANDREW, STEVEN
Applicants: BEALE INT TECHNOLOGY
Application year: 1985
Main classification: H04L11
IPC codes: H04L12