Patent GB-904232-A (GB)
Title: Interconnecting network for a telecommunication system
IPC: H03K17/60
Filed: 1960-10-14
Abstract: 904,232. Automatic - exchange systems. STANDARD TELEPHONES & CABLES Ltd. Oct. 14, 1960 [Oct. 20, 1959], No. 35317/60. Class 40 (4). In a time division multiplex exchange in which connections are made by the series association of three multiplex highways subject to the same multiplex cycle, the terminal highways are arranged in sets such that a connection between one set and another set is made over an intermediate highway exclusively devoted to connections between these particular sets. The arrangement is suited to a large exchange and for a given traffic and blocking factor is designed to use the least number of gates with the least cross-talk between channels. The gates may be symmetrical transistors. As shown in Fig. 3, a total of G terminal highways is partitioned into S equal sets. Connections between sets two-by-two are made over sets of intermediate highways with Q<SP>1</SP> highways in each set. Connections between highways in each terminal set are made by way of Q<SP>11</SP> intermediate highways particular to each set. Connections between terminals served by the same highway are made with the use of storage capacitors, as described in Specification 822,297, where a different time channel is associated with each terminal. Connections set up over three multiplex highways in series use the same time channel in each highway and may be established by seizing a channel simultaneously free in the terminal highways and matching it with a corresponding channel free in an associated intermediate link, or by seizing a channel which is free simultaneously in all three highways. Cross-talk is measured by the maximum number of multiplex gates for which the physical arrangement prohibits wholly effective screening and which lie in the path of a connection. Cross-talk is, therefore, expressible in terms of the total number of terminal highways and the number of highways in sets of terminal and intermediate highways and this expression is found by inspection of the physical connection scheme employed. Two such connection schemes are shown in Figs. 5 and 6 where the five horizontal lines represent planes in each of which the multiplex gates particular to a set of terminal highways are disposed. Terminals have access to the left of the planes and the first four dots shown on each plane represent the physical order from left to right in which the multiplex gates to the intermediate highways are made. At the extreme right the unconnected dots represent the multiplex gates for the intermediate highways particular to a set of terminal highways and, should it be desired, these may be placed at the extreme left physically adjacent the terminal connections. The intermediate highways coupling the terminal highway sets twoby-two comprise coaxial cables and it is therefore, considered that cross-talk is confined within each plane of multiplex gates. With the arrangement of Fig. 3, the minimum number of gates needed is computed for a given traffic and blocking factor on the assumption that connections are set up by seizing a channel free in the relevant highway of each terminal highway set and then finding this channel free in an intermediate highway. For a given solution the connecting scheme of Fig. 5 or Fig. 6 is used depending on which gives the lowest measure of cross-talk. In the examples treated numerically the minimum number of multiplex gates does not coincide with the minimum of cross-talk and a compromise is effected in choosing the preferred system. A particularly desirable solution arises when the intermediate highways are limited to one for each two-by-two connection. This is equivalent to Fig. 3 with Q<SP>1</SP> and Q<SP>11</SP> taken as unity and is illustrated separately in Fig. 4 (not shown). Since an intermediate link is identified as soon as the terminals to be interconnected are identified a connection is set up by finding a channel that is free simultaneously in all three highways involved. Analysis of the arrangement shows that the number of multiplex gates required and the resulting cross-talk can be reduced by half for any given traffic and blocking factor as compared with values of Q<SP>1</SP> and Q<SP>11</SP> greater than one, albeit that a greater number of highways are employed. The random selection of free channels is not preferred although it is an assumption of the analyses given, an ordered selection of channels producing a smaller blocking probability without vitiating the results. The highway distribution of a 10,000 line exchange is shown in Fig. 8 in which groups of 100 subscribers have access to a highway of 25 channels, groups of ten such highways forming sets S1 to S10 and adjacent pairs of such groups forming inflated sets S<SP>1</SP>1 to S15. Two-by-two and internal connections of the inflated sets S<SP>1</SP>1 and S15 are made by way of the ten intermediate highways L<SP>1</SP> and the five highways L<SP>11</SP>. Eighty highways each serving fifteen junctions, half the highways serving incoming and the other half serving outgoing junctions, are arranged in sets of ten numbered S11 to S18 interconnected over the eighty intermediate links JL two-by-two with the terminal highway sets S1 to S10. A set S19 of two highways XH give access to the central exchange operators and connect over intermediate highways XL with all terminal and junction highways. Connections between terminals of the same highway are made with the use of a set of 12 storage capacitors SSD which are connected in common to all the terminal highways and store an amplitude modulated pulse between one channel and another in a multiplex cycle. Busy tone, ringing tone, and ringing supervision, are connected to all terminal highways and incoming junction highways from the sources BT, RT, and RS. Spare irtermediate highways OL are provided which can be taken into use to deal with any overflow should one of the intermediate highways JL, L<SP>1</SP>, or L<SP>11</SP>, be congested. If the overflow highways are associated with storage capacitors they may also take the overflow of connections between terminals to the same highway. Tandem junction connections can be made by connecting the incoming and outgoing junction sets two-by-two over extra intermediate highways. Maintenance is said to be facilitated by having two separate partitions of terminal highways into sets which overlap, each set of one partition including highways from two sets of the other partition and vice versa. Specifications 753,645, 765,681, 824,221, 904,231 and 904,233 also are referred to.