tlets through the same lines are those serving two customers on a system known as shared number of links. In practical exchanges, it is only necessary to service. The case of one customer per line is known as exclusive provide as many paths through the switching network as the number service. of calls which are expected to be in progress at one time. The Structure of telephone systems 31/7 9 inlets 4 outlets 9 outlets 6 links inlets 81 crosspoints 9 inlets 9 outlets 9 links 3 X 6 2 X 6 = 30 crosspoints Group Group Figure 31.9 Trunking principle of concentration stage for Group crossbar and electronic exchanges 31.2.5 Types of exchanges The early exchanges were all manually operated and many are still in service around the world. The exchange equipment consists of a switchboard at which an operator (telephonist) connects calls on demand. 3X9+3X9= 54 crosspoints The term 'automatic' is used to describe exchanges where the switching is carried out by machine under the remote control of the caller who could be either a customer or an operator. Figure 31.8 Trunking principle of distribution stage for The first public automatic exchange opened in 1892 at La Porte, crossbar and electronic exchanges Indiana (USA), and used rotary switches invented in 1891 by Almon concentration of lines on to a smaller number of paths or trunks can be achieved using the principle illustrated by another simple example in Figure 31.9. A complete local exchange then consists of a combination of a distribution (group selection) stage and one or more line concentra- Junctions to and from tion units, as shown in Figure 31.10. other exchanges The separate switches are interconnected through yet another switching stage which does not provide concentration or expansion, i.e. a square switch as shown in Figure 31.8. This group switching (distribution) stage can also be used to make connections to other exchanges. (See also Section 31.4.3.) Group To complete the exchange, two other functional blocks are selection needed. The call connection (and di