enance. Called number 23541 Even on these systems there are some calls which customers either cannot or will not set up for themselves. To provide for these, numbers are allocated (e.g. 100) which can be dialled for assistance. Figure 31.12 Strowger (non-director) exchange trunking Such calls are connected through the automatic exchange to a switchboard where the operator can establish the required connec- Analogue switching 31/11 Junctions MARKERS REGISTER Figure 31.14 Crossbar exchange tion also through the automatic equipment. These switchboards are dinates. Typical mechanisms contain 200 or more crosspoints per called automanual exchanges. switch. The switches are generally used in combinations to form a group selection (distribution) stage and a combined concentra- 31.4.3 tion/expansion stage, as shown in Figure 31.14. Additional crossbar Register control (Crossbar) switches may be used for establishing other connections necessary for the operation of the exchange e.g. RF, which connects registers, In crossbar and analogue electronic systems, the equipment operates as required, to callers via the concentration stage CS. at high speed, it functions on an end-to-end instead of the step-by- Fig 31.15 shows a much simplified diagram of a single crosspoint step basis used by Strowger exchanges and the trunking pattern does and a general view of a complete switch. The flexible selecting not conform with the decimal notation used by the telephone dial. For these reasons, it is necessary to interpose some equipment during the setting up process which will convert the customers' dialling signals into information which can be used by the switching To other equipment. When a call is originated, therefore, the control sets crosspoints in row crosspoints to connect the calling line to a register which has functions broadly similar to those of an operator. The register Hold bar detects and stores the dial (or MF key) pulses and signals them to the control. The control, which is duplicated or sectioned in some Flexible way