 Within each MCU information was handled in a "two-out-of-five" code which enabled errors to be detected, and the output of the program store was duplicated to give additional protection.The TXE 3 model gave satisfactory service and the experience gained from the model confirmed the validity of the basic design and led to the development of the TXE4.The TXE4 was a cost reduced development of the TXE3 system and catered for up to 40,000 subscribers with over 5,000 erlangs of both-way traffic and was normally staffed by several Technical Officers (TO).