Patent GB-1385074-A (GB)
Title: Telecommunications networks
IPC: H04Q3/00
Filed: 1972-09-28
Abstract: 1385074 Automatic exchange systems SIEMENS AG 28 Sept 1972 [28 Sept 1971] 44760/72 Heading H4K In a non-hierarchical system including a plurality of exchanges connected to one another by a network of connection paths, pieces of hunting information for use in establishing a connection between a pair of subscribers connected to different ones of said exchanges are each accompanied by a piece of load information which indicates the load on at least one of the connection paths via which the particular piece of hunting information is passed. Such an arrangement is used to ensure that where several routes exist between the exchanges to which a particular pair of subscribers are connected, that route is chosen on which the most congested connection path is less congested than the most congested connection path on any of the other routes.  The arrangement is described when applied to a system of nine exchanges interconnected as shown. The exchanges may be p.c.m., t.d.m., exchanges and the connection paths may be cable or radio links. Speech signals may be transmitted via the relevant connection paths in accordance with frequency multiplex or t.d.m. principles. In the example it is assumed that each connection path comprises 30 speech channels and that a certain number of channels in each connection path (shown in brackets next to each connection path) are free at the time a calling subscriber 123 at exchange A calls a subscriber 456 at exchange I. As is well known in such systems exchange A transmits a piece of hunting information to the exchanges B and D to which it is connected. Each piece of hunting information is however accompanied by information relating to the loading on the connection path via which it is transmitted. Exchange B therefore receives a piece of hunting information S10 which indicates a subscriber 456 is required and that the most congested connection path over which the information has come has ten free channels. As exchange B does not have the subscriber 456 the piece of hunting information is transmitted on to the other exchanges to which exchange B is connected. As should now be evident, a piece of hunting information S4 is sent to exchange E and a piece of hunting information S10 to exchange C. At exchanges such as E, F, and H which can receive two pieces of hunting information it is arranged that, providing the two pieces of information are received within a predetermined time, it is the piece of hunting information which has come via the least congested out of the pair of most congested connection paths, one for each route, that is considered for retransmission. Hence exchange H transmits on S8 in preference to S5, and exchange E transmits on S10 in preference to S4 but because the connection path E to H only has five free channels frequency S5 is transmitted to exchange H. If an exchange should receive pieces of hunting information carrying identical load information, e.g. exchange F, the marking arrangement is such that the exchange will consider the piece of hunting information that has come into the exchange by the least congested connection path connected to the exchange, e.g. exchange F considers the hunting information to have come from exchange E rather than exchange C. As a result of the arrangement the connection between exchange A and I is made as shown by the thick line i.e. A-D-E-F-I. Two different methods of conveying the load information between exchanges are described. In the first the information is merely tagged on to the piece of hunting information and is transmitted as part of that information. In a second a series of thirty different interexchange signalling frequencies are used, i.e. equal in number to the number of transmission channels. The highest frequency is used for transmission of the hunting information when all channels are free &c. down to the lowest frequency which is used when only one channel is free. There is no correlation between particular signalling frequencies and particular speech channels.  Marking.-Three different marking arrangements are described, all apparently known, for marking the route from exchange A to exchange I. In the first arrangement a channel in the connection path over which a piece of hunting information is received by an exchange is seized. Where an exchange receives more than one piece of hunting information, a channel is seized in a connection path over which that piece of hunting information chosen for onward transmission has been received. In the second arrangement each exchange stores the identity of the exchange from which it received the piece of hunting information chosen for onward transmission and in the final arrangement there is added to each piece of hunting information for onward transmission at each exchange a piece of information indicating the connection path over which it was received by the exchange. Hence when the hunting information reaches exchange I, this exchange has all the information required for establishing a connection back to exchange A. There is described with reference to Fig. 2 (not shown) a further embodiment associated with the arrangement in which 30 different signalling frequencies are used. Instead of transmitting the hunting information at a frequency indicating the least congested connection path via which the piece of hunting information has been passed, the hunting information is always transmitted between adjacent exchanges at the frequency relating to the number of free channels on the particular interexchange path, e.g. with reference to Fig. 1 the information would be transmitted from exchange A to D at Sf10, where Sf10 is the frequency corresponding to 10 free channels, from exchanges D to E at Sf16, from exchanges E to F at Sf20 and so on. By arranging that the higher the frequency on which a piece of hunting information is to be transmitted the sooner after its arrival in the exchange it is transmitted, the first piece of hunting information to reach exchange I will have travelled on average via the least congested paths.