GB1500274A - Digital switching systems - Google Patents

Abstract

1500274 Automatic exchange systems POST OFFICE 30 May 1975 [10 June 1974 (2)] 25564/74 and 25565/74 Heading H4K In a PCM, TDM concentrator system, each concentrator is connected by separate highways to at least two main switching centres which are interconnected by a data link via which messages may be exchanged so as to cause only one of the centres to effect control actions in respect of calls arising at a concentrator and to prevent double connections in respect of a same call from being set up on the two highways extending from the concentrator to the two main centres. The general arrangement is depicted in Fig. 1 where concentrators 1 each have an appearance on a concentrator control 28 in two different centres 5. Signalling between concentrators and centres takes place during the first and 16th channels of a 32 slot frame, 32 such frames forming a multiframe and 4 multiframes a superframe. The superframes are relatively staggered on the highways emanating from a single concentrator so as to prevent crosstalk and furthermore, an empty (hiccup) multiframe is inserted between super frames so as to reduce the possibility of double connections caused by two switching centres trying to control a same call. In response to a calling line, the control in one centre finds the line during an appropriate signalling period and then warns off the control in the other centre using the data link 32. If the first control does not then succeed in setting up a desired connection it requests the other control to attempt to do so. Details are provided of the signalling format in respect of line scan, channel allocation, tone supply, switching instructions, supervision and special services such as coin fee checking and howler tone supply. As to the latter services, a distinction is made between the hardware for special services permanently allotted to an individual line, e.g. coin freed (Fig. 6, not shown) and those of a transitory nature, e.g. howler (Fig. 7, not shown) wherein an additional switching network path is needed. Each control contains last look memory for call supervision (Figs. 13 and 14, not shown) while the overall control and interchange message signalling circuits are described with reference to Figs. 11 and 12 (not shown). Each concentrator contains a two-stage read-relay, switching network that provides connections between 1500 subscribers and four, 32 channel highways, each subscriber having access to 15 channels on each of the highways (Fig. 3, not shown).