TABLE OF CONTENTS Another key to understanding digital communications networks is multiplexing and framing. If you understand layer 1 clocking and data recovery, then clocking and data recovery in layer 2 will come naturally. Many things happen at layer 2, but the overall function is to keep the bits organized framed, timed, and synchronized. Lacking that, the higher layer functions don t do very well because they assume these functions to be intact and functioning at the lower layer. Put another way, clocking and synchronization are only implied at layer 3 and above. Two major multiplexing and framing approaches in current use include T-carrier and synchronous optical network/synchronous digital hierarchy (SONET/SDH).
Communications network-multiplexing techniques grew out of the T-carrier concept created as a family of trunking techniques. Dubbed pair gain, the goal was to double trunk capacity from 12 to 24 channels or talk paths on a single trunk facility. In a few short years, the technology morphed its way into the network architecture. Its key parameters include 8-kHz sampling and 8-bit quantization. This results in a single voice channel rate of 64 Kbs. Sampling 24 voice signals simultaneously, or within the time frame of one sample, results in a framing time of 125 microseconds. When these 24 channels, or DS0s, are bundled into a serial stream with a single framing bit, it aggregates to a T-carrier at 1.544 Mbs. Figure 4-3 shows the duration and timing parameters of a channelized T1 or DS1 signal.
