Digital switching network for telecommunications exchange

Abstract

A number of methods are used in the prior art to provide speech sample security in a duplicated switch plane telecommunications switching system and these fall into three categories; (i) use the switching planes in a main/standby mode with errors detected by a check code accompanying the speech sample on the plane in use detected errors cause the standby to be switched-to, (ii) transmit speech across both planes simultaneously and compare and (iii) as per (ii) accompanied by a check code to identify the plane at fault. Categories (i) and (iii) require 'extra' routes to transmit the check code whereas category (ii) can not detect which sample is in error. In serial transmission switch planes there are no 'spare' routes available without creating extra routes for the check code alone. The basic principle of the invention is to pass the speech through one plane only which is known as the 'biased to' plane. A checkcode is passed through the other 'biased from' plane. Identical routes are used by the speech data and the check code through these respective planes. The check code is generated in the Rx DLT and both the checkcode and the speech date are checked in the Tx DLT. The DLT selects the biased-to plane under the control of the switch control system. The checkcode format used comprises a 4 bit speech checkcode and a 4 bit checkcode check allowing faults in the 'biased from' plane to be detected.