Data Communications Networking Devices - 4th Ed.pdf

656 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTIf you multiply the data rate of the lower level signal by the number of lower levelsignals used to make up a higher level signal, the sum will be slightly less than thedata rate of the higher level signal. This difference is used for framing the lowerlevel signals onto the higher level signal and is not employed for actual datatransmission. Thus, multiplying the data rate of a lower level signal by the numberof signals that make up a higher level signal can be used to obtain the actual datacarrying rate of a DS level, while the data rate in Table 6.6 is the signaling rate ofdata and frame information.Framing changesThe name of the service represented by the original DS1 system has changedseveral times since the early 1960s based upon changes in the composition of theframing structure. Today communications carriers offer D4 and ESF framing. D4framing is an older technique that is gradually being replaced by the ESF framingtechnique.D4 framingIn D4 framing a sequence of 12 frame bits is used to develop a precise patternemployed by T1 multiplexers to keep the bit stream in synchronization. Figure 6.23illustrates the D4 framing pattern, showing the value of the 193rd bit in each of the®rst 12 frames transmitted on a T1 circuit. This 12-bit frame pattern continuouslyrepeats itself, providing the synchronization signal used by equipment attached to aT1 circuit. Readers are referred to Section 4.11 for detailed information concerningthe D4 framing format.Figure 6.23D4 framing patternESF framingA more recently introduced T1 framing pattern called the Extended Super FrameESF) format contains 24 frame bits. Unlike the D4 pattern which repeats itself, theESF consists of three types of frame bits, two of which can vary. These three typesof frame bits include line control, cyclic redundancy checking, and a frame patternfor synchronization.In the ESF frame the bits in the odd frames from 1 through 23 are used by thetelephone company to perform networkmonitoring, set alarm conditions, andperform other control operations. Frame bits in frames 2, 6, 10, 14, 18, and 22 areused for cyclic redundancy checking. Finally, frame bits in frames 4, 8, 12, 16, 20,