This document is intended as an introductory guide to the Synchronous Digital Hierarchy (SDH) standard. The following is a representative sample of material from a much larger document.
Standards in the telecommunications field are always evolving. Information in this SDH primer is based on the latest information available from the ITU-T standardisation organization. Also included is a Technical Note on a facet of the SDH standards.
Use this primer as an introduction to the technology of SDH. Consult the actual material from ITU-T, paying particular attention to the latest revision, if more detailed information is required. For help in understanding the language of SDH telecommunications, a comprehensive List of Terms appears at the end of this document. Again, the list of terms is a representative sample.
Traditionally, digital transmission systems and hierarchies have been based on multiplexing signals which are plesiochronous (running at almost the same speed). Also, various parts of the world use different hierarchies which lead to problems of international interworking, for example, between those countries using the 1.544 Mbit/s system (U.S.A. and Japan) and those using the 2.048 Mbit/s system. To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, it is necessary to demultiplex the signal all the way down to the 2 Mbit/s level before the location of the 64 kbit/s channel can be identified. PDH requires "steps" (2-8, 8-34, and 34-140 demultiplex and multiplex) to drop out or add an individual speech or data channel.
Figure 1. PDH multiplexing by steps
The main limitations of PDH are:
The STM-1 frame is the basic transmission format for SDH. The frame lasts for 125 microseconds, therefore there are 8000 frames per second.
The STM-1 frame consists of overhead plus a virtual container capacity. The first 9 columns of each frame make up the Transport Overhead, and the last 261 columns make up the Virtual Container (VC) capacity. The VC plus the pointers (H1, H2, H3 bytes) is called the AU (Administrative Unit).
Carried within the VC capacity, which has its own frame structure of 9 rows and 261 columns, is the Path Overhead and the Container. The first column is for Path Overhead, it is followed by the payload container.
Virtual Containers can have any phase alignment within the Administrative Unit, and this alignment is indicated by the Pointer in row four, as will be described later in Pointers and Dynamic Alignment. Within the Section Overhead, the first three rows are used for the Regenerator Section Overhead, and the last 5 rows are used for the Multiplexer Section Overhead.
Figure 2. STM Frame Structure
The multiplexing principles of SDH follow:
The figure on the next page illustrates the ITU-T SDH multiplexing structure. The notations in the boxes, such as C-1, VC-3, and AU-4, are explained in the table after the figure.
At the lowest level, containers (C) are input to virtual containers (VC). The purpose of this function is to create a uniform VC payload by using bit-stuffing to bring all inputs to a common bit-rate ready for synchronous multiplexing. Various containers (ranging from VC-11 at 1.728 Mbit/s to VC-4 at 139.264 Mbit/s) are covered by the SDH hierarchy. Next, VCs are aligned into tributary units (TUs), where pointer processing operations are implemented.
These initial functions allow the payload to be multiplexed into TU groups (TUGs). As the figure illustrates, the xN label indicates the multiplexing integer used to multiplex the TUs to the TUGs. The next step is the multiplexing of the TUGs to higher level VCs, and TUG-2 and TUG-3 are multiplexed into VC-3 (ANSI mappings) and VC-4. These VCs are multiplexed with fixed byte-stuffing to form administration units (AUs) which are finally multiplexed into the AU group (AUG). This payload then is multiplexed into the Synchronous Transport Module (STM).
Figure 3. SDH Multiplexing Hierarchy
Table 5. SDH Multiplexing Structure
| Term | Contents | User |
| C-n | n = 1 to 4 | Contains payload at lowest multiplexing level |
| VC-n | n = 1,2 | Contains single C-n plus VC POH |
| VC-n | n = 3,4 | Contains C-n, TUG-2s, or TU-3s, plus POH for the specific level |
| TU-n | n = 1 to 3 | Contains VC plus tributary unit pointer |
| TUG-2 | (TU-n) = 1,3,4 | Contains various TU-n s |
| TUG-3 | TU-3, 7 TUG-2s | Contains TU-3, 7 TUG-2s |
| AU-n | n = 3,4 | Contains VCs plus AU pointer |
| STM-n | n = 1,3 AUGs | Contains n synchronously-multiplexed STM-1 signals |
| POH= | Path Overhead |
| C= | Container |
| TU= | Tributary Unit |
| AU= | Administrative Unit |
| VC= | Virtual Container |
| TUG= | Tributary Unit Group |
| STM= | Synchronous Transport Module |
Examples of 140 Mbit/s, 34 Mbit/s and 2 Mbit/s PDH access and mapping/demapping process follow in the next three figures.
Figure 4. 140 Mbit/s PDH Access and Multiplexing
Figure 5. 34 Mbit/s PDH Access and Multiplexing
Figure 6. 2 Mbit/s PDH Access and Multiplexing
Three sets of terms are arranged at the beginning of this section because it is important to see that they are not equivalent terms, which is how they get used in normal discussions. They are:
Add/ Drop The process where a part of the information carried in a transmission system is extracted (dropped) at an intermediate point and different information is inserted (added) for subsequent transmission. The remaining traffic passes straight through the multiplexer without additional processing.
Map/ Demap A term for multiplexing, implying more visibility inside the resultant multiplexed bit stream than available with conventional asynchronous techniques.
Multiplex/ Demultiplex Multiplex (MUX) - To transmit two or more signals over a single channel. Demultiplex (DEMUX) - To separate two or more signals previously combined by compatible multiplexing equipment. Demultiplexing - A process applied to a multiplex signal for recovering signals combined within it and for restoring the distinct individual channels of the signals.