he operator verbally what call he wants to vided. make. It is possible to design a network having an odd number of The great strength of the system is that the 'control' is human and, switching stages in which blocking never occurs. Non-blocking therefore, intelligent and the variety of connections that can be made arrays need more crosspoints than acceptable blocking networks is virtually unlimited. Services such as advice of duration and and are uneconomic for most commercial telephone switching ap- charge, transfer of calls when absent, wake up calls etc., which are plications. SO complex to provide automatically, present no problem at all on manual exchanges. 31.3.6 Network routing The weakness of the manual exchange, which has led to its almost complete disappearance, was essentially its slowness. Unless oper- Telephone networks are normally based on a hierachical arrange- ators could set up calls faster it would have been impossible to find ment and the higher levels are often fully interconnected, e.g. the enough operators to handle the volume of traffic on the modern UK transit network. Dimensioning is simplified by the use of fixed network. routing but the overall Grade of Service is sensitive to faults and overloads on individual links. All routes must, therefore, be dimen- 31.4.2 Step by step sioned to overload conditions. It is possible to use Automatic Alternative Routing based on Ratchet driven switches, remotely controlled directly by pulses overflow, although the dimensioning is more difficult since the from the calling telephone, are used to simulate the operator's overflow traffic is not random and, therefore, Erlangs loss formula actions on a plug and cord switchboard. The wipers (moving con- does not apply. tacts) are connected by short flexible cords and perform a similar If unlimited overflow is allowed, there is a danger of affecting the function to the switchboard plugs. The place of the jack field is performance of traffic which normally uses the secondary route. taken by a bank of fixed 