+D. A hardware block diagram of the digital subscriber switching stage for The switching system APT performs traffic handling functions and analogue accesses is shown in Figure 32.11. also operation and maintenance functions related to traffic handling. LICs, KRD and JTC/ETCs are connected to a time switch (TS). The division of APT into subsystems is determined by conditions The time switch is non-blocking and has full availability. Thus any and requirements that arise from traffic handling and operation and channel in the JTC/ETC within an LSM can be reached from any maintenance functions. These subsystems are implemented both in LIC within the same LSM. Via a time switch bus (TSB) any channel hardware and software. in other JTC/ETCs can also be reached. Consequently, the access In the following sections the basic APT subsystems are described. switch is completely insensitive to unequally distributed loading (mixture of high and low traffic subscribers). 32.9.1.1 APT subsystems When the subscriber switching stage is remotely located, built in emergency functions are activated in case of a break in the com- The subscriber Switching Subsystem, SSS, supervises the state of munication towards the host exchange. These functions allow the connected subscriber lines, sets up and releases connections in the subscriber to make internal calls with reduced facility level. It is also subscriber switching network and sends and receives signals to and possible to connect one of the subscriber lines to an emergency from subscribers. SSS is a mandatory subsystem in local exchange centre in order to maintain '999' service. applications. The Group Switch Subsystem, GSS, is used to set up speech paths The SSS hardware, the digital subscriber switching stage, consists between connected devices. of a number of Line Switch Modules (LSM) each of which contains The Digital Group Switch has a T-S-T (time-space-time) struc- line circuits for 128 analogue subscriber lines or alternatively 64 ture. It is built up of a number of time switch