How is the 128 time-slot cross-connection calculated from 4-channel E1?

Each E1 channel runs with the PCM30/32 frame structure, with 32 time slots per frame, of which TS0 is used for frame alignment and alarms and TS16 carries channel-associated signaling (CAS), while the remaining 30 time slots carry 64 kbit/s services; the 4-channel E1 multiplied out gives 128 time slots, corresponding one-to-one with the built-in 128 time-slot cross-connection matrix of the IDM-120PN. The cross-connections of the IDM-120PN can be established at three levels: user-to-user, user-to-line and line-to-line, with data and signaling exchanged together. The electrical characteristics of the E1 interface comply with ITU-T G.703, with a bit rate of 2.048 Mbit/s, and cross-connection relationships are issued as time slot configuration, one slot at a time, so that the external optical cables and telephone wiring remain unchanged when circuits are adjusted.