Patent GB-1501596-A (GB)
Title: Call discriminator and method of call discrimination
IPC: H04Q3/00
Filed: 1976-05-10
Abstract: 1501596 Automatic exchange systems COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL 10 May 1976 [13 May 1975] 19086/76 Heading H4K A call discriminator comprises a first input for receiving dialled digits from an exchange register, a second input for receiving class of service (COS) of an exchange inlet and/or outlet, a first analyser for receiving a predetermined number of the initial digits from the first input to determine the number of digits necessary for complete analysis of a call, a switch connected to the first input and controlled by the first analyser to select digits for further analysis and to route them to a second analyser and a comparator for comparing the COS from the second input with an output of the second analyser to produce a barring signal when a prohibited call is requested. In the embodiment, Fig. 1, the dialled digits are received in the discriminator by amplifiers and decoders AD from an exchange register. The first two dialled digits, routed through switch GA and decoder CBI are used to address a memory M 1  together with an input NE which indicates if the call is internal or external. Each word of memory M 1  is split into three parts: Z 1 , code controlling switch GA, Z2, code for COS and Z3, code for the number of digits to be analysed. The outputs of Z1 and Z2 are connected to a register RG1 and the outputs of Z3 are connected to a comparator CC which also receives the position of the last digit dialled through inputs NC and gives an output which clocks Register RG 1 through gate P2 which is also connected to an input T2, enabled at a time t2. Register RG 1 is reset to zero at a time t1 through T 1. The outputs of register RG 1 control switch GA and a second analyser which analyses three dialled digits as described below. The three digits, not necessarily the first three, are chosen by switch GA activated by a control signal from register RG 1 and are used to address a memory M2 which stores codes corresponding to geographic areas or for internal calls corresponding to the kind of service required. Each word is divided into two parts, one for internal and one for external calls. The selection of the half word to be used is effected by switch AI1, controlled by register RG1. The outputs of switch All are temporarily stored in a register RG2. The COS and barred access codes are received on inputs CA, IN respectively through switch AI2 and are used together with some outputs of register RG1 to address a memory M3 which has two parts the first containing data to be compared with that of memory M2 i.e. kinds of calls allowed and the second code numbers of various firms connected to the exchange. The outputs of memory M3 are compared with the data stored in register RG2 through a comparator comprising gates PE1-8 and P8 whose output A indicates if a call is to be allowed or barred. In the case where a leased (tie) line is used a further comparison is made, before the comparison described above, to determine whether the subscriber asking for the leased line belongs to the firm leasing the line.