re 3-46. The problem with this process is the bit-interleaved nature of the mul- tiplexing scheme. Because the DS1 signal components arrive from dif- ferent sources, they may be (and usually are) slightly off from one another in terms of the overall phase of the signal-in effect, their Figure 3-45 Bit 1, frame 4 Bit-interleaved Bit 1, frame 2 system Bit 1. frame 1 Bit 1. frame 3 Downloaded from Digital Engineering 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 Telephony Figure 3-46 Overhead 1.544 Mbps Added multiplexing process 24-DS-Os Overhead + framing bit 6.312 Mbps Added 4-DS-1s 44.736 Mbps 7-DS-2s Figure 3-47 M12 frame has four subframes and 48-bit payload fields-1 1, 176 "speeds" differ slightly. This is unacceptable to a multiplexer which must rate align them if it is to properly multiplex them, beginning with the head of each signal. In order to do this, the multiplexer inserts additional bits, known as "stuff bits,' into the signal pattern at strategic places that serve to rate align the components. The structure of a bit-stuffed DS2 frame is shown in Figure 3-47; a DS3 frame is shown in Figure 3-48. The complexity of this process should now be fairly obvious to the reader. If we follow the left-to-right path shown in Figure 3-49, we see the rich complexity that suffuses the M13 signal-building process. Twenty- four 64 Kbps DSOs are aggregated at the ingress side of the T14 multi- plexer, grouped into a T1 frame, and combined with a single frame bit to 4The process is similar for the E1 hierarchy. Downloaded from Digital Engineering 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 Figure 3-48 M13 frame has seven sub- frames and 84- bit payload fields- 760 Figure 3-49 The M13 Overhead multiplexing 1.54