T3 traditionally involves multiplexing 28 T1 lines on to a single higher-speed line (T3 line). Each of the 28 T1 lines in the T3 are asynchronous to one other. A T1 line accommodates 24 64-kilobit-per-second (Kbps) DS0 channels. While the DS0s are byte multiplexed to each T1, the T1 lines are bit multiplexed to the T3 in a framing format called M23 or C-BIT parity. M13 multiplexing is done in two steps. First, four T1 lines are bit multiplexed together to form a DS2 line using M12 multiplexing. These seven DS2 lines are then bit multiplexed together to form the DS3/T3 using M23 multiplexing.
M13 multiplexing uses bit stuffing to bring each asynchronous DS1/DS2 up to a common data rate for transmission.
