call attempts phonist). Similar principles were used on early telegraph exchanges. As the network grew and the operators were expected to handle An important parameter in the design of stored program control increasing volumes of traffic, the signalling and control elements systems, in particular, and common control systems in general, is were developed, which formed the basis of the first automatic the number of attempts to be processed, usually expressed in busy systems. hour call attempts (BHCA). Typically a 10,000 line local exchange In its more common form, the manual switchboard has subscriber- might require a processing capacity of up to 80,000 BHCA. s' (customers') terminations connected to jacks (sockets). The "links" consist of a pair of plugs on flexible cords (wires) which can 31.3.5 Blocking be used to interconnect any pair of jacks. Signalling consists of calling indicators (e.g. relays and lamps) associated with the jacks If two free trunks cannot be connected together because all suitable and clearing indicators associated with the cords and plugs. Call links are already engaged the call is said to be blocked. In Figure routing instructions are passed verbally. 31.8, if one call has already been set up from inlet 1 to outlet 1, an The simplest type is the magneto board on which the caller gains attempted call from inlet 2 to outlet 2 will be blocked. The effects the attention of the operator by using a hand generator to send an of the blocking are reduced here by allocation of the outlets to the a.c. signal which is detected by an electromechanical indicator at the three routes. switchboard. An improved design is the central battery (CB) board Blocking can be reduced by connecting certain outlets perma- which uses gravity switch contacts at the telephone to operate a nently to the inlet side of the network to permit a blocked call to seek relay at the exchange to light a lamp at the switchboard. In both a new path. Alternatively, a third stage of switching may be pro- cases the caller then tells t