terpart, with the exception of the last two bytes, which are labeled as being reserved for national use and are specific to the PTT administration that Regenerator Section Overhead Segments Figure 3-61 SDH Overhead Multiplex Section Overhead Multiplex Section Overhead Path Overhead 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 implements the network. In SONET they are not yet assigned. The sec- ond row is different from SONET in that it has three bytes reserved for media-dependent implementation (differences in the actual transmis- sion medium, whether copper, coaxial, or fiber) and the final two reserved for national use. As before, they are not yet definitively assigned in the SONET realm. The final row of the RSOH also sports two bytes reserved for media- dependent information, while they are reserved in SONET. All other regenerator section/section overhead functions are identical between the two. The MSOH overhead in the SDH frame is almost exactly the same as that of the SONET Line Overhead, with one exception: Row nine of the SDH frame has two bytes reserved for national administration use. They are reserved in the SONET world. The pointer in an SDH frame is conceptually identical to that of a SONET pointer, although it has some minor differences in nomenclature. In SDH the pointer is referred to as an AU pointer, referring to the stan- dard naming convention described earlier. SONET and SDH were originally rolled out to replace the T1 and E1 hierarchies, which were suffering from demands for bandwidth beyond what they were capable of delivering. Their principal deliverable was voice-and lots of it. Let's take a moment now to describe the process of voice digitization, still a key component of network transport. Voice Digitization The goal of digitizing the human voice for transport across an all-digital network g