Patent GB-1402823-A (GB)
Title: Automatic telecommunication switching system
IPC: H04Q3/545
Filed: 1973-02-22
Abstract: 1402823 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 22 Feb 1973 [25 Feb 1972] 8636/73 Heading H4K A toll switching centre comprises a switching network which includes overflow links interconnecting the incoming and outgoing sides of the switching network, and the centre is controlled by a stored-program computer. General description.-The switching network is essentially of the type disclosed in Specification 1,256,042, which used PNPN triode crosspoints and the trunking (Fig. 2) is as shown in Fig. 7e of said Specification except that a number y of overflow links couple a number of the outlets of the switching network back to a like number of inlets. The path selection is controlled by a computer C, (Fig. 3), functioning similarly to that disclosed in Specification 1,181,182, and which controls the selection of a direct path, i.e. path A from terminal I 1  to terminal O 1 , between an incoming trunk IT 1  and an outing trunk OT 1  or the selection of an indirect path via an overflow link, i.e. path B, overflow link fl 1  and path A, when no direct path is available due to internal blocking. The computer includes a memory, an arithmetic unit, a control unit and an interface unit coupled to the switching network. Memory map.-The memory MEM includes a first part MOT in which the status of all outgoing trunks are registered in 16-bit data words, a second part MLI storing the status of all the a to e links (Fig. 2) of the network SN in 16-bit data words and a third part MFL storing the state of the y overflow links fl 1  to fl y  also in 16-bit data words. The remaining units of the computer are indicated as block ACIOU. Route selection.-After detecting a calling trunk, and having received the called subscriber's number or part thereof, the computer C analyses the prefix to determine the one or more outgoing routes having access to the wanted destination and then selects one such route which may include a number of outgoing trunks, e.g. trunks OT 1  to OT i . A 16-bit data word MW11 giving the busy/free conditions of some of the outgoing trunks of the selected route is read from memory part MOT and is stored in the computer working area, a free outgoing trunk then being sought by searching for a 1-bit in the real word MW11, e.g. by using the "find first one" instruction well known in the computer art, such that when a bit 1 is found it is set to 0 and the search stopped. If no free trunk is found in the particular data word, further words are read relating to further trunks on the same route and the search repeated. If no free trunks are found a search of the data words relating to alternative routes is made. When no free trunk is available, busy is returned to the calling trunk. Path selection.-On finding a free trunk, e.g. OT 1  a search for a direct path through the network SN is instigated as given under the paragraph path selection in B.S.T.J. September 1964, No. 5, part 1, p. 1869 (No. 1 ESS). Thus the computer reads a word in MW21 in memory part ML1 giving the conditions of the a, b, c, d and e links forming part of the 16 possible direct paths between inlet and outlet. If one of these paths is free the corresponding links are busied in memory ML1 and the path selected for use. If none of these are free a search for a further suitable free trunk is initiated, as described in "Route selection" above. The path selection procedure is then repeated for each free trunk found until a free path is found. Once all possible trunks and paths thereto have been searched an "Overflow" procedure is initiated. Overflow procedure.-The overflow links y are searched using memory unit MFL in a similar manner as the search for a free trunk. Assuming, a free overflow link fl 1  is found, computer C searches for a direct path, B, between the incoming trunk and the overflow link appearance on the network outlet side. If no free path is found a search for a further free overflow link is instigated and a free path search performed for any free overflow links found. This is carried out until a free "B" path is found for a predetermined number of the overflow links which may be the total number. Once a "B" path has been found, a fresh search for an outgoing trunk is performed and a path search is carried out between any free trunks found and the network input appearance of the overflow link chosen. Once all the possibilities have been tested without result, a search through the overflow links is continued until a further free link is found, A path "B" is again searched for and if the search is successful a free trunk is again searched for. A path between the outgoing trunk and overflow link appearance is again searched for. This proceeds until a connection is finally established or the predetermined number of overflow links has been searched through. Instead of directly connecting the overflow links of between the inlets and outlets of the network, they may be connected via an additional switching network such that all the overflow outlets O n+1  to O n+y  can be connected to any of the overflow inlets I n+1  to I n+y . This network may be a non-blocking network. In this case the overflow procedure is modified in that when searching for a free path between an outgoing trunk and an overflow inlet, all the overflow inlets can be tried without affecting the choice of overflow outlet, i.e. there are a greater number of possibilities of connection between a selected B path and a outgoing trunk via an overflow path.