Patent GB-1347942-A (GB)
Title: Telecommunications systems
IPC: H04Q3/545
Filed: 1972-01-10
Abstract: 1347942 Automatic exchange systems SIEMENS AG 10 Jan 1972 [29 Jan 1971] 983/72 Heading H4K In a system including two different types of exchanges, the exchanges of each type are interconnected with other exchanges of the same type by junctions or trunks, and each exchange of one type is paired with a respective exchange of the other type and has two way communication therewith through tie lines. General description (Fig. 1).-Two sets of exchanges A1-L1 and A2-L2 are interconnected with exchanges of their own type via respective one way or both way trunks V1, V2, V1<SP>1</SP>, V2<SP>1</SP> &c. and pairs of different exchanges are interconnected via tie lines XY &c. Each exchange also has connected thereto subscriber's origin trunks, e.g. U1, U2 by which subscribers have access to the exchange in the incoming direction and subscriber destination trunks, e.g. Z1, Z2 by which the exchange has access to the subscribers. In order to prevent multiple transitions between the two exchange sub systems in setting up a single connection there exists a set of selection rules whereby the origin trunks of an exchange of one type are connectible, via the tie lines, in the paired exchange of the other type with the latter's outgoing trunks or junctions and with its destination trunks and whereby the incoming junction or trunks of an exchange of one type are connectible, via the tie lines, in the paired exchange of the other type exclusively with the latter's destination trunks. It will therefore be seen that a transition from one subsystem to the other can only occur at the pairs of exchanges to which the calling and called subscribers are connected. This arrangement permits subscribers on the different sub-systems to communicate and also permits two subscribers on the same sub-system to be connected via the trunk lines of the other sub-system. Several different tie line arrangements are disclosed. Detailed description of the embodiments (Fig. 2).-There is shown two exchanges A1, A2 of different types connected via tie lines X1/2, Y1/2, X2/1, Y2/1, i.e. a pair of tie lines for each direction of setting up a call. The exchange A1 is of the type employing selectors P11-P14 as the switching devices whilst exchange A2 is of the type centrally controlled by a computer S and may employ a switching network P2 as disclosed in Specification 1,058,893. In exchanges of the latter type, signalling between exchanges is effected by using central data channels which are separate from the lines over which speech is transmitted. In order to determine in exchange A2 whether it is an origin trunk, e.g. U1 or an incoming trunk, e.g. V1r of exchange A1 that has seized a tie line separate lines X1/2, Y1/2 are used for the two types of connection. Similarly tie lines Y2/1, X2/1 are used for calls set up in the reverse direction. By this arrangement the exchange A2 is enabled to make only the connections permitted by the selection rules. However in order to fully utilize the tie lines it may be arranged that the origin trunks of an exchange have access to both the tie lines, e.g. X1/2 and Y1/2. In a further embodiment a single set of tie lines is provided for each direction of setting up a call. In this case, when a tie line is seized at the first exchange a signal indicating whether the line has been seized by an origin trunk or an incoming trunk is transmitted to the second exchange in order that the correct selection rule might be applied. The set of tie lines for each direction of setting up a call may be replaced by a single set of two-way tie lines which can all be used in either direction of setting up a call.