g Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Chapter 3 our attention to higher bit rate services. As we wander our way through this explanation, pay particular attention to the complexity involved in creating higher rate payloads. This is one of the great advantages of SONET and SDH. The next level in the North American Digital Hierarchy is called DS2. And while it is rarely seen outside of the safety of the multiplexer in which it resides, it plays an important role in the creation of higher bit rate services. It is created when a multiplexer bit interleaves four DS1 signals, inserting as it does so a control bit, known as a C-bit, every 48 bits in the payload stream. Bit interleaving is an important construct here, because it contributes to the complexity of the overall payload. In a bit-interleaved system, multiple bit streams are combined on a bit-by-bit basis as shown in Figure 3-45. When payload components are bit inter- leaved to create a higher-rate multiplexed signal, the system first selects bit 1 from channel 1, bit 1 from channel 2, bit 1 from channel 3, and so on. Once it has selected and transmitted all of the first bits, it goes on to the second bits from each channel, then the third, until it has created the super-rate frame. Along the way it intersperses C-bits, which are used to perform certain control and management functions within the frame. Once the 6.312 Mbps DS2 signal has been created, the system shifts into high gear to create the next level in the transmission hierarchy. Seven DS2 signals are then bit interleaved along with C-bits after every 84 payload bits to create a composite 44.736 Mbps DS3 signal. The first part of this process, the creation of the DS2 payload, is called M12 mul- tiplexing; the second step, which combines DS2s to form a DS3, is called M23 multiplexing. The overall process is called M13, and is illustrated in Figu