BASIC FRAMING | Network Technology

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 is exactly what it’s name implies, adding or ‘‘stuffing’’ bits into a multiplexed stream to raise the speed, or number of bits per unit of time, so there are enough bits to fill the timeslots in the higher order channel. For example, when 4 DSI signals are multiplexed to make up a DS2 signal, one of the signals is sent at exactly, 1,544,000Bps, one has 364Bps, the third gets 314Bps, while the fourth gets 414Bps, making an aggregate for the DS2 of 6,306,272Bps.

When 7 DS2 signals are used to build up a DS3, still more bits are added to each DS2 to enable the network to accommodate the disparate nature of the various DSI clocks and multiplexing operation.

The T-carrier concept originated in the United States, but was followed in due course in other countries. Initially designed for four-wire media, it found its way to coaxial cable, wireless, and optical fiber media. An international version of the DS1 is called an E1. It uses the same 8 Kbs sampling and 64 Kbs DS-0 channel rates, and 125-microsecond framing. However, 30 timeslots are placed in the 125-microsecond frame, resulting in a payload of 240 bits per frame, a payload rate of 1.920 Mbs, and a total channel rate of2.048 Mbs.

Another major difference and significant improvement of E1 over T1 structure is increased overhead. From the start, this was a troubling characteristic of T1, not because it was too much, but because it wasn’t enough. There was never a standard method, nor enough bits to deal with the many overhead requirements for voice service. Besides, when data transport came on the scene, US DS0 channels could reliably deal with only 56 Kbs instead of the entire 64 Kbs bandwidth. So the designers of E1 digital facilities added 2 to 64 Kbs channels providing 128 Kbs. Adding these two timeslots in the 125-microsecond frame resulted in 256 bits in each frame, 240 for payload and 16 for overhead.