Electronic crosspoints for switching analogue transmission circuits

Crosspoints are switches which interconnect line and trunk circuits in exchanges. The paper is concerned with switching analogue transmission circuits. Digital-circuit switching has much in common and is briefly mentioned. Exchange switches comprise assemblies of crosspoints. Electronic crosspoints are assembled by wiring which, compared with the mechanical assembly of metal-contact crosspoints, provides greater latitude in the construction, and greater economy in the number of crosspoints needed. The nature of electronic crosspoints compels the use of isolating transformers between the lines terminating at the exchanges and the trunks within the exchanges, which permits the trunks to have unbalanced transmission and also helps to economise on crosspoint costs. There are some transmission difficulties associated with 2-wire switching, but these are reduced by 4-wire switching which also provides opportunities for transmission improvements. The best use is made of the available bandwidth by time-division-multiplexing (t.d.m.) the transmission, whereby each crosspoint serves a multiplexed channel instead of one unmultiplexed channel. Other advantages of multiplexing include lower crosstalk, more signalling channels at practically no extra cost; and guide-wire path search and connection and multiplexed supervisories, both of which cost much less in t.d.m. form than in space-division form.