side. Each call requires transmission in each direction and these two nalling. paths can occupy the same crosspoint in the space switch but at The advantages are that the variety of hardware may be mini- different time slots, usually 180 degrees out of phase. Separate time mised, SO easing production and stocking problems, and that facility switches must be provided in each direction of transmission. changes may be readily accomplished. Disadvantages are that soft- ware development can be exceedingly complex and that real time 31.5.3 Digital local exchange problems may occur. The switching networks in Section 31.5.2 provide only for the 31.6.2 Software distribution of calls from the inlets to an equal number of outlets through the same number of links. In practical exchanges, it is only The essence of spc is that it is software driven. The functionality of necessary to provide as many paths through the switching network the exchange control processors can be changed to suit different as the number of calls which are expected to be in progress at one hardware configurations (number and type of lines, traffic etc.) for time. (See also Section 31.3.) different installations and growth during the life of the system. The The concentration of lines on to a smaller number of paths or features offered can also develop as requirements change over time trunks is provided in the form of an additional time switching stage. (e.g. tariff structures, dialling codes) without major hardware re- The number of available time slots to the main switch is less than placements. the total number of input time slots and store locations in the This software must, of course, meet the same standards of relia- concentrator speech stores they serve. This might consist typically bility and availability as those demanded of the switch and proces- of up to 16 groups of 256 lines, each written cyclically into a 256 sor hardware. Software does not wear out like hardware. Failures word store but with only 256 timeslots available for allocation to t