ms. This arrange- sory is given an identifying code known as the equipment number ment illustrated in Figure 32.2 also ensures that exchange security (EN) which indicates its location on the equipment racks. is safeguarded in the event of failure of any subsystem without the The TXE4 system 32/5 Six security planes Switching units Junctions Traffic growth Scanner Markers (per plane) Supervisory Registers processors control units Cyclic stores Processing growth Enhancement Connection processors growth Feature growth Figure 32.2 TXE4 system (Compare this with Figure 31.16) need for interchange of information or external monitoring of the terminations can be completed by the operation of seven cross- working of each subsystem. points. The BC stages of each switching unit are sectionalised to provide 32.4.1 The switching network a number of identical sub-units (6 or 8) which can be considered as being in planes on top of one another. This concept of sectionalisa- The switching network consists of a number of separate but identi- tion provides security in the switching network, since a fault affect- cal units up to a maximum of 48. Each unit handles a fixed amount ing any one sub-unit will not mean the loss of service to any of bothway traffic and the number of units provided for any particu- customers but only a slightly reduced grade of service. lar application is, therefore, determined by the total traffic (origina- Customers, junctions and other circuits are connected to the net- ting, terminating and tandem) to be handled. The network is work via A switches, separate switches being provided for cus- arranged as a three stage link system folded upon itself via an tomers and junctions. The traffic concentration within the A switch additional switching stage, SO that connection between any two can be adjusted to match the calling rate of the terminations to the 32/6 Telephone exchanges fixed capacity of the switching unit. On the opposite side of the BC connected to the periphery of the network via junction A switch