The most characteristic feature of the exchange's central control unit design was that calls were processed serially. Consequently call set-up had to be fast. In particular, Call Control had to become free in less than the time of the inter-digital pause on calls incoming to the exchange. This time could be as low as 60 milliseconds. As the TXE2 call-set-up time was some 50 milliseconds, this design requirement was just met, but even so, the overall capacity of the system was determined by the probability of an incoming call being delayed too long in its initial connection to a register.