Editing System X (telephony) (section)

===Digital local exchange===
The Digital Local Exchange (DLE) hosts a number of concentrators and routes calls to different DLEs or DMSUs depending on the destination of the call. The heart of the DLE is the Digital Switching Subsystem (DSS) which consists of Time Switches and a Space Switch. Incoming traffic on the 30 channel PCM highways from the Concentrator Units is connected to Time Switches. The purpose of these is to take any incoming individual Time Slot and connect it to an outgoing Time Slot and so perform a switching and routing function. To allow access to a large range of outgoing routes, individual Time Switches are connected to each other by a Space Switch. The Time Slot inter-connections are held in Switch Maps which are updated by Software running on the Processor Utility Subsystem (PUS). The nature of the Time Switch-Space Switch architecture is such that the system is very unlikely to be affected by a faulty time or space switch, unless many faults are present. The switch is a 'non-blocking' switch.