arly won't fit in the limited space available in an STS-1. An STM-1, then, oper- ates at a fundamental rate of 155.52 Mbps, enough for the bandwidth requirements of the E4. This should be where the déjà vu starts to kick in: Perceptive readers will remember that 155.52 Mbps number from our dis- cussions of the SONET STS-3, which offers exactly the same bandwidth. An STM-1 frame is shown in Figure 3-60. It is a byte-interleaved, 9-row- by-270-column frame, with the first nine columns devoted to overhead and the remaining 261 devoted to payload transport. A comparison of the bandwidth between SONET and SDH systems is also interesting. The fundamental SDH signal is exactly three times the bandwidth of the fundamental SONET signal, and this relationship con- tinues all the way up the 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 Telephony Overhead Figure 3-60 An SDH STM-1 frame 9 rows Pointer 9 columns 261 columns STM Frame Overhead The overhead in an STM frame is very similar to that of an STS-1 frame, although the nomenclature varies somewhat. Instead of Section, Line, and Path Overhead to designate the different regions of the network that the overhead components address, SDH uses Regenerator Section, Mul tiplex Section, and Path Overhead, as shown in Figure 3-61. The Regen- erator Section (RSOH) occupies the first three rows of nine bytes, the Multiplex Section (MSOH) the final five. Row four is reserved for the pointer. As in SONET, the Path Overhead floats gently on the payload tides, rising and falling in response to phase shifts. Functionally, these overhead components are identical to their SONET counterparts. Overhead Details Because an STM-1 is three times as large as an STS-1, it has three times the overhead capacity-nine columns instead of three (plus Path Overhead). The first row of the RSOH is its SONET coun