Patent GB-1364173-A (GB)
Title: Digital transmission systems
IPC: H04L12/46
Filed: 1972-02-24
Abstract: 1364173 Digital transmission; closed loop systems WESTERN ELECTRIC CO Inc 24 Feb 1972 [1 March 1971] 8478/72 Heading H4P In a system as described for example in Specification 1,350,800, comprising a plurality of interconnected loops each loop having a number of stations B connected thereto capable of communicating with any other station on the same, or another, loop through switching stations, each loop having an assigned address code, apparatus is provided in combination with a developed address pattern to minimize the number of loops traversed between any two stations on different loops. The apparatus provides a first signal representative of the Hamming distance between the address of the loop in which the message is circulating and the address of the destination loop, and a second signal representative of the Hamming distance between an interconnecting loop and the address of the destination loop together with a means for providing a control signal when the Hamming distance of the second signal is less than the first which causes transfer of the message appropriately. In one embodiment the number of loops traversed is equal to the Hamming distance between source and destination. If the loop system has a hierarchical structure, it is possible to partition the address into parts i.e. national, regional, local &c., and make successive comparison of each part at each level i.e. the first comparison of the succession will determine the route along "national" portions of the network whereas a second comparison will be made when the message at a "regional" level loop et seq. This method combines the feature of a Hamming distance algorhythm described in the Specification with savings in address length. In an example addresses otherwise having a length of 59 bits could with small additional computing costs be reduced to approximately 11 bits. In addition local stations may be added to a regional network by only simple modifications of neighbouring addresses. Selection is performed at interconnecting (C) stations, see Fig. 5 (not shown), also abridgment of Specification 1,350,800, the relevant parts of Fig. 5, being described in greater detail with respect to Figs. 6, 7 (also not shown). The specific feature of the addressing method described comprises inserting dummy "d" bits into the address which do not contribute to the calculation of the Hamming distance. In Fig.7, a register (54) receives the address, an eight stage register 73 stores the address of the loop, and a Hamming distance detector (71) comprises a plurality of sub detectors (71- 1) &c. each of which develops a signal proportional to the Hamming distance between two pairs of binary bits, which signals are totalled in a counter. 0, 1, d are coded as 00, 01, 10 respectively as shown in Fig. 8A in which a message is assumed to be circulating in a loop dd00, (see register 73) and the destination loop is identified as 1011 in register 54. In a comparison of the stored codes of 54, 73, if the first digit of each pair is considered as a 1 no contribution is made since "d" is also identified in this manner. Considering the last two pairs the Hamming distance is 2. On the other hand, comparison of 54, 74 determines a Hamming distance of 1, thus the message will be transferred to the next loop. Fig. 8 illustrates a sub detector 71-1. If the first bit of each pair is 0 indicating a 0 or 1, NOR 41 gives a 1 output. If however a "d" is present NOR 41 gives 0 which inhibits AND 43. Half adder comprising exclusive OR 42 is responsive to the second bit of each pair and develops 1 only when applied bits differ thus AND 43 inverts an output only when addresses differ. Development of a general addressing algorithm for use with systems having various configurations is described in the Specification. As with the prior application, the various loops may have different channel capacities, and slightly different speeds hence need not be synchronous with one another.