Patent GB-1526230-A (GB)
Title: Communication network
IPC: H04J3/17
Filed: 1975-12-29
Abstract: 1526230 Automatic exchange systems INTERNATIONAL BUSINESS MACHINES CORP 29 Dec 1975 [30 Dec 1974 (5) 26 June 1975] 52962/75 Heading H4K A communications network includes a plurality of geographical areas each having an access unit 1-3 connected via digital t.d.m. trunks to a plurality of branch exchange units 1.1, 1.2, 1.3, etc. to which are connected duplex telephone and data links and the areas are linked via a communications facility providing demand assignable channels for linking the access units. The communications facility is preferably a geostationary satellite and voice activity compression VAC (TAS1) is used on the digital t.d.m. trunks and the satellite links. The branch exchanges and access units preferably use stored programme control, microprocessors being used for the branch exchanges. A complete description of the system is given in USA Specification 4,009,343 to which reference is directed for a detailed description. The branch exchanges 10, Figure 2, are digital t.d.m. extendable switching units which provide for local connections between individual duplex lines, for connections between duplex lines and digital trunks, and for inter trunk (tandem) connections. On the trunks a 6 m.s. frame of 48 24- byte channels are provided of which channels 2-47 are used to carry up to 96 simultaneous connections using VAC. Channel 0 is used for signalling and channel 1 is used to transmit a VAC mask comprising 96 bits together with checking bits. The mask has one bit position for each of the 96 possible connections and in any frame up to 46 of the bit positions contain a binary 1 to indicate that the respective connection is active in that frame. The network access units 12, Figure 2, transmit the frames received from a plurality of branch exchanges as a series of bursts to a satellite 4. Each access unit 12 transmits the bursts to the satellite on the same frequency f1 but at different non- overlapping intervals (time interleaved bursts). The satellite then retransmits all the received bursts to all the access units on a second frequency f2. Each access unit examines the received bursts which include the VAC masks as well as the information blocks and reorders the information according to its destination e.g. according to which of its branch exchanges 10 the information is bound. As more than 46 connections may extend to the same branch exchange, the access unit may have to provide VAC for each outgoing trunk using the received VAC mask and employing the same criteria as used in the branch exchanges. Satellite signalling:-The satellite frame is also 6 m.s. long but operates at a rate of 49.4 m.b.s. as compared with a trunk rate of 1À541 m.b.s. A burst period of each frame is sequentially allocated to the access units for order wire signalling over a superframe of 56 frames i.e. 56 access units may be served. There then follows up to 56 traffic bursts of variable lengths allocated according to demand assignment procedures (see USA Specification No. 4,009,343). Each traffic burst includes clock information for timing and a unique word to distinguish the burst. The traffic provided by the network may only be a fraction of the total satellite traffic. System Acquisition Procedure:-There is described in general a method of attaining system synchronization wherein one access unit is nominated as master and initiates synchronization with the satellite frame using its order wire signalling superframe period. All the other access units are then synchronized with the master access unit and the branch exchanges are synchronized by bits transmitted over one of the trunks from their associated access unit. Establishment and Release of Trunk Connections is fully described in USA Specification No. 4,009,343. The general sequence of events is as follows. The processor (34, Figure 4, not shown) of the branch exchange 10 connected to the calling duplex line receives the routing digits and checks its memory tables to determine the possible routes to the called line. If a route exists (there may be temporarily no possible route due to faulty equipment along the only available route, and each processor (34) maintains an up to date record of faults elsewhere in the system) the processor checks the availability of connection channels on the corresponding trunk e.g. to an access unit 12. Assuming a channel can be allocated the processor reserves a path through the exchange 10 and forwards the call details to the access unit 12 over the signalling channel of the respective trunk. The access unit 12 performs a similar operation as regards an outward path via the satellite to the access unit 12 in the called geographical area and forwards the call information thereto. This access unit then forwards the call details to the called branch exchange 10 which checks the status of the called duplex line. If it is free the processor of the called branch exchange then allocates a channel on the trunk to the called access unit 12 which then allocates a return path via the satellite to the calling access unit. The various reserved paths are then switched through to complete the connection. Failure to find a path at any of the stages causes a busy signal to be given at the calling branch exchange.