ogue exchanges are used in the U.K. for outward direction and four stages D, C, B and A in the inward small and large exchanges respectively. These systems employ a direction. switching matrix having a similar structure to crossbar systems but 2. The supervisory and register area comprising outgoing junc- using reed relays for the crosspoints. These have the advantage of tion, incoming junction and local supervisory relay sets, being sealed against corrosion and dirt and operate in only about together with a common pool of registers serving all three 2ms, making them more amenable to electronic control than Strow- types. ger or crossbar switches. They also have a separate operating coil The control area comprising calling number identification, for each crosspoint enabling switch sizes to be optimised for system called number marker and call control equipment. requirements. The higher speed of operation and the use of electronic control 32.3.1 The switching area permits greater use of self checking features to improve reliability and quality of service. The greater use of electronics enables these The switching network is a general purpose network as shown in systems to benefit from advancing technology in that field, in terms simplified form in Figure 32.1. The sizes of the various switches can of size and versatility in the provision of customer facilities. be varied to meet differing requirements of lines and traffic but are Junctions (via supy) A-B-C D switch switches Register Switch access selectors switch Calling number Registers generator Call control Figure 32.1 TXE2 system (Compare this with Figure 31.14) 32/4 Telephone exchanges multiples of a standard switching module comprising 25 crosspoints To originate a call, the customer loops his line and causes his arranged in a 5 X 5 reed relay matrix. The A switch serves to directory number (DN) to be inserted in a free register. If there are concentrate customers' lines onto B switch inlets and the number no free registers, the customer receives equipment en