The Transmission System is traditionally seen as the link between main WAN switching centres. These Transmission Systems consist of large bandwidth highways that form the backbone to the network. They typically serve many customers each with their own requirements so the systems have to be reliable, resilient and flexible.
Rather than have two wires for every voice or data conversation, Time Division Multiplexing is used. ITU-T G.704 defines 32 channels of 64Kb/s to form 2.048Mb/s where channel 0 is used for framing. You will often see the standard G.703 mentioned with G.704, this is because G.703 defines the unframed physical interface coaxial (75 ohm) or RJ48 (120 ohm) used for the E1/T1 connection at the client premises. Channel 0 is for timing used to synchronise the multiplexers at each end of the link. Channels 1 to 15 and 17 to 31 are for voice or data whilst channel 16 is used for Common Channel Signalling (CCS) or Channel Associated Signalling (CAS). Every 3.91 microseconds 8 bits from one channel is sent down the line followed by 8 bits from the next channel during the next 3.91 microseconds and so on in a round robin fashion throughout all the channels, thus 32 channels are used once every 125 microseconds.
The connection at the end is either a 75 ohm coax, 120 ohm coax or a 150 ohm UTP/STP.