Patent GB-1155682-A (GB)
Title: Improvements in or relating to Telecommunications Exchange Systems
IPC: H04Q3/00
Filed: 1967-05-26
Abstract: 1,155,682. Automatic exchange systems. SIEMENS A.G. 26 May, 1967 [27 May, 1966], No. 24560/67. Heading H4K. In a sectionalized switching network through which a free path is determined with the aid of a memory map that stores the free/busy state of every interstage link in the network, the latter is made highly symmetrical so that selection can be effected without the usual translation fields and also can be made simultaneously in respect of links extending from the first stage and links leading to the last stage, links extending from the second stage and links leading to the penultimate stage, and so on. Trunking.-The basic network section is depicted in Fig. 1. The A, B switches are formed into groups 1 ... m each of which has full availability. The outlets from one A stage switch are connected to a same level in each of the B stage switches, i.e. the k outlets of the first A switch in each group are connected to the first inlets of the k, B switches in the group. The A, B switch groups are linked to the groups of C switches in a similar manner. Thus the first outlet from each B switch in the first AB group is connected to the first inlet of each C switch in the first C switch group and the last outlet from the B switches in the first AB group are connected to the first inlets of the C switches in the last C switch group. By contrast, the first and last outlets from B switches in the last AB group are connected to the last inlets of C switches in the first and last C switch groups respectively. A number of each basic sections are linked together to form a full network as shown in Fig. 3. In Fig. 2 (not shown) there is depicted a folded network made up from such sections. More than one set of links may be used for connecting two sections together. Memory map and path selection (Fig. 4).-The free/busy condition of the AB, BC &c. links in the Fig. 3 network are stored in memories SpAB, SpBC &c. In setting-up a call, a control circuit inserts the relevant network inlet and outlet addresses into address registers ARG1 and ARG2. Each address in ARG1 comprises: KGA, the identity of the AB switch group viz. 1... m (Fig. 1); KVA, the identity of the A switch viz 1 ... l; LA ,the identity of the inlet; KT, the identity of the section viz KTel . . . KTeq (Fig. 3). The outlet address is entered in similar manner into ARG2. These addresses serve to interrogate the AB and EF link memories which produce an indication in their output registers LR1 and LR5 as to which of the links outgoing-from and incoming-to the relevant A and F switches are free. The conditions of the first outlet from the A switch and the first inlet to the F switch are compared by AND gate G1  and if both are free a suitable record is made in ZSp. As to why the first inlet and outlet are chosen, see below. Similar comparisons are made for the other outlets and inlets. One of these free pairs is selected by KVS in order to interrogate memories SpBC and SpDE and simultaneously therewith memory SpCD. It may be noted by reference to Fig. 3 that the above method of interrogating the memories ensures that only relevant links are considered. Thus for subscriber Tn2, initially, only links from his own particular A switch are checked, then only those BC links within his own AB group are checked and finally only those CD links from his own section are considered. The latter consideration is reduced still further because of the simultaneous search being conducted in section KTer to which the relevant outlet, e.g. relay set RS2, is attached. Returning now to Fig. 4, output registers LR2-LR4 are apprised of the free condition of the BCDE links and logic K1 &c. determines whether or not pairs of suitable links exist. If not, KVG chooses another pair of AB and DE links and the interrogation of SpBC-SpDE recommences. If and when suitable links are found, their addresses are passed to control circuit E3 which then proceeds to set-up the selected path and to clear the various registers. It should be noted that the reason for comparing like-numbered outlets-from and inlets-to the first stage and last stage switches respectively lies in the highly symmetrical nature of the network. It can be seen (of Fig. 3) that the first outlet of Tn2's A switch is the only outlet-from this switch which can be connected to the first inlet-to RS2's F switch (and similarly for the other corresponding outlets and inlets). Furthermore, due to the symmetry, selection of a BC link is effectively governed by the choice of the AB link. Thus if the first AB link from Tn2's A switch is chosen then only the links from the first B switch in Tn2's AB group can be considered. The restriction on the CD links is of course complete since there is only one CD link which can connect particular BC and DE links. In the present case it will be seen that whichever of the links from the first BC switch is chosen (these links all being first links according to the notation depicted in Fig. 1) the corresponding CD link will also be the first link into the selected DE switch group. PBX hunting.-If a group of outlets rather than a single outlet is under consideration, then outlet register ARG2 is initially supplied with the address of the first outlet in the group and a search is conducted to see if a free path exists to this outlet. If the search is unsuccessful, the address of the second outlet is inserted and the search is repeated and so on. Logic gates GC-GT (each of which is in fact a set of gates) supply central control with the identity of the outlet only after a successful path finding operation. If more than one set of links exists between two sections, discriminators Z1-Z4 are operated in order to ensure that during the search in memory SpCD each set is considered in succession-all the BC, CD and DE paths for one set being considered before any of the paths in the next set. Numerical separation of subscribers is allowed for by means of intra-stage links. Thus in Fig. 3, subscribers Tn3 and Tn4 connected to the same AB switch group are connectible by a direct link at the outlets of the B switch. Similar links on the A and C switches serve for connecting subscribers connected respectively to the same switch and the same section.