nter- part. It is a nine-row by 270-column frame that can be configured into 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 one of five container types, typically written C-n (where "C" means "con- tainer"). N can be 11, 12, 2, 3, or 4; they are designed to transport a vari- ety of payload types. When an STM-1 is formatted for the transport of virtual tributaries (known as "virtual containers" in the SDH world), the payload pointers must be modified. In the case of a payload that is carrying virtual con- tainers, the pointer is known as an Administrative Unit type 3 (AU-3). If the payload is not structured to carry virtual containers, but is instead intended for the transport of higher rate payloads, then the pointer is known as an Administrative Unit type 4. Generally speaking, an AU-3 is typically used for the transport of North American Digital Hierarchy payloads; AU-4 is used for European signal types. The SDH Frame To understand the SDH frame structure, it is first helpful to understand the relationship between SDH and SONET. Functionally, they are iden- tical: In both cases, the intent of the technology is to provide a globally standardized transmission system for high-speed data. SONET is indeed optimized for the T1-heavy North American market, while SDH is more applicable to Europe; beyond that, however, the overhead and design con- siderations of the two are virtually identical. There are, however, some key differences. Perhaps the greatest difference between the two lies in the physical nature of the frame. A SONET STS-1 frame has 810 total bytes, for an overall aggregate bit rate of 51.84 Mbps-perfectly adequate for the North American 44.736 Mbps DS3. An SDH STM-1 frame, however, designed to transport a 139.264 Mbps European E4 or CEPT-4 signal, must be larger if it is to accommodate that much bandwidth-it cle