Framing bits are also applied when a basic 1.544 Mbs stream is multiplexed with another stream or additional streams into higher order aggregate signal. Additional bits are added in specific timeslots and designated as framing bits to enable the receiving equipment to recover the original clock and separate the payload, first into the next lower order bitstream and then ultimately down through the multiplex hierarchy to the original 1.544 Mbs payload and 24 individual 64 Kbs voice channels or DS0 signals.

This makes the multiplexing bit oriented. That is, each stream is multiplexed into a specific pattern based on individual bits where each bit in each frame has a specific (theoretical) timing relationship to the same timeslot in peer bit streams. Because of the fact that each of the original 1.544 Mbs bit streams is generated from a clock that runs in the real world, and may not be precisely on the same frequency as any of the others generating the T1 signals being multiplexed, and because the timing of the signals being multiplexed may change due to propagation delay variation in the transmission media, the resulting aggregate signal multiplexing is said to be plesiochronous, meaning almost or nearly synchronous, but not asynchronous or non-synchronous. Multiplexing of signals from disparate clocks that are almost or nearly synchronous requires another technique called bit stuffing.

Bit stuffing ^[3] is exactly what it s name implies, adding or stuffing bits into a multiplexed stream to raise the speed, or...