SDH Networks - Frame Structure and Channel Mappings

SDH Networks - Frame Structure & Channel Mappings

Figure below  shows a simple SDH network and summarizes some Channel mapping and multiplexing in SDH. 

PDH equipment with various interfaces (2, 45 and 140 Mb/s in this example) may use SDH muxes and regenerators for transport. 

The PDH signals are adapted to the synchronous frequency by justification and forms a C-n. Adding lower-or higher-order path overhead (LO/HOPOH) forms a VC-n. 

Pointers are used to indicate the start of the signal in the VC-ns. Lower order VC-ns are multiplexed to form a VC-4, which finally forms an STM-n (n=1, 4, 16, 64, 256) signal.

The SDH regenerator terminates a regenerator section and the multiplex section is formed between the endpoint SDH muxes. The location of the overhead of the different paths and sections are indicated in the SDH frame in the middle.

Fig. below shows the structure of a a SDH frame with payload area that carries data and the overhead sections that are used for proving information to payload data, alarms, protection and signaling.

Section Overhead (SOH): The first 9 bytes in each of the 9 rows are called the overhead. 

G.707 makes a distinction between the regenerator section overhead (RSOH) and the multiplex section overhead (MSOH). 

The reason for this is to be able to couple the functions of certain overhead bytes to the network architecture. The table below describes the individual functions of the bytes.

Path overhead: The path overhead (POH) plus a container forms a virtual container. The POH has the task of monitoring quality and indicating the type of container. 

The format and size of the POH depends on the container type. A distinction is made between two different POH types:

The heterogeneous nature of modern network structures has made it necessary that  all PDH and ATM signals are transported over the SDH network. 

The process of matching the signals to the network is  called mapping. 

The container is the basic package unit for tributary channels. A special container(C-n) is  provided for each PDH tributary signal. These containers are always much larger than the payload to be transported. 

There main capacity is used partly for justification (stuffing) in order to equalize out timing inaccuracies in the PDH signals. Where synchronous tributaries are mapped, fixed fill bytes are inserted instead of justification bytes. 

A virtual container (VC-n) is made up from the container thus formed together with the path overhead(POH). This is transmitted unchanged over a path through the network. The next step towards formation of a complete STM-N signal is the addition of a pointer indicating the start of the POH. 

The unit formed by the pointer and the virtual container is called an administrative unit (AU-n) or a tributary unit (TU-n). 

Several TUs taken together form a tributary unit group (TUG-n); these are in turn collected together into a VC. One or more Aus form an administrative unit group(AUG). 

Finally, the AUG plus the section over-head(SOH) forms the STM-N.

Transmission at the higher hierarchy levels: To achieve higher bit rates, AU-3/4s are multiplexed into STM-N frames.

The following hierarchy levels are defined in SDH:

STM-1 155.52Mbit/s

STM-4 622.08Mbit/s

STM-16 2488.32Mbit/s

STM-64 9953.28Mbit/s

The STM-N frame structures are basically N times the STM-1 structure. For example, the STM-4 overhead is four times the size of the STM-1 overhead. The SOH content is specified for each stage individually.