each irc computer was configured with 208 ports and so was able to support 200 simultaneous prestel users, the remaining 8 ports being used for test and control functions. access for the ordinary user was provided via the duplex asynchronous interface provided by banks of gec 16-port multi-channel asynchronous control units (mcaccu) known more simply as multiplexers. these devices in turn were accessed via banks of standard post office modems no. 20 operating at 1200/75 bit/s, which were connected directly to the public switched telephone network (pstn).