ution and network synchronization is covered in detail in Section 5 of Chapter 5, entitled "Clocking Considerations" as part of overall network architecture. MULTIPLEXING AND FRAMING 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 im- plied 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). 4 NETWORK TECHNOLOGY AND METHODOLOGY 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 chan- nels, or DSOs, 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. Twenty-four clock samples of 8 bits each result in 192 bits of payload. After each 192-bit sample is completed, it is serialized and placed immediately following a framing bit making a 193-bit frame. The resulting frame rate is maintained at the same 8-kHz clock rate, resulting in a 1,544,000 bps stream. The 