Communication networks of this type are known which may form a local area network or a wide area network. Early systems were designed to operate at one to ten megabits per second and more recently higher transmission speeds have been achieved using established protocols based on token passing or slotted rings. An example of a slotted ring protocol is described in an article titled "The Cambridge digital communication ring" by M W Wilkes and D J Wheeler for "Proceedings of the local area communications network symposium" in 1979. These known systems may provide a transmission rate up to about one hundred forty megabits per second.
A problem with known communication systems of the aforementioned type is that a higher transmission rate is often required. Examples of applications requiring a high transmission rate are speech, video transmission and mixed traffic applications. In proceedings of the International Switching Symposium May 1984, a paper by Takeuchi et al suggests the use of synchronous composite packet switching where nodes have unlimited access to a number of rings. A problem with this proposal is that the synchronous connections must have packets regularly at very short time intervals (every 125 microseconds). This restriction only allows composite packet voice to be handled. Further since a slot is continually re-used by a synchronous connection, this slot is effectively reserved so that if a ring failure occurs then synchronous connections using that ring are lost.
It is an object of the present invention to provide a network capable of operating at high speed without restrictions of the type mentioned above.
According to the present invention there is provided a communication network suitable for the transmission of synchronous services such as voice, and also data packets, comprising:
a plurality of nodes (13) each connected to at lesat two rings (11', 11") to allow transmission to occur in one direction around each ring, characterised by:
control means (27) for controlling the maximum access delay experienced by packets belonging to the synchronous services and selector means (17, 19) for allowing any synchronous connection to utilise the first available ring for the transmission of each of its packets.
Preferably information is transferred in packets which are transmitted within predetermined time slots wherein each packet has a predetermined number of digits. To ensure that a node does not attempt to initiate simultaneous transfer of two packets the time slots for each of the physical links may be relatively out of phase. However it is advantageous that once a phase relationship between the links has been provided this does not shift, therefore, in a preferred embodiment all the data links are substantially the same length and introduce substantially equal delays and a common clock may provide timing signals for all of the data links.
Further according to the invention there is provided a method of operating a communication network comprising a number of stations connected to a plurality of message rings said method characterised by:
allocating a number of message slots to each station dependent on traffic requirements to ensure delay sensitive messages are transmitted without undue delay; routing the passage of message blocks at any station to an available ring for transmission to a downstream station and interrogating each available ring to determine whether a further allocation can be provided.
In the embodiments of the invention to be described the network operates according to a protocol described in the present applicants European Patent Application No. 85305017.7 (now published as EP No. 0168265 and incorporated herein by reference) and in an article by R M Falconer and J L Adams entitled "Carrying Integrated Services on the Orwell slotted ring" for "Proceedings of the International Seminar on Computer Networking and Performance Evaluation", Tokoy, 1985. See also `Orwell: a protocol for integrated services local network` by R M Falconer and J L Adams, British Telecom Technology Journal Vol 3 No 4 October 1985. Therefore in a preferred embodiment a transmit node is prevented from transmitting more than a predetermined number of packets during a predetermined interval.