Telecommunications traffic continues to grow at a very fast pace. This is accelerated through the increasing volume of data and mobile traffic, especially in India, through the recent liberalization of the telecommunications market. A solution can be adopted to meet the ever-increasing traffic requirements based on a combination of WDM, SDH, and IP transport technologies.

Wavelength-division multiplexing is used to multiplex several wavelength channels on a single strand of fiber, thus overcoming fiber congestion. SDH technology offers the capacity granularity, which the customers demand today and offers the possibility to protect these services against network outages. An IP-over-WDM transport network can offer high capacity Internet transit services to Internet Services Providers (ISPs).

Synchronous Digital Hierarchy

Synchronous Digital Hierarchy (SDH) networks replaced PDH and have several key advantages.

• G.707, G.708, and G.709 ITU recommendations provide the basis for global networking.

• Networks benefit from traffic resilience to minimize traffic loss in the event of fiber break or equipment failure.

• Built-in monitoring technology allows remote configuration and troubleshooting of network.

• Flexible technology allows tributary access at any level.

• Future proof technology allows for faster bit rates as technology advances.

European PDH networks could not interface with US networks, SDH networks can carry both types. The above figure shows how the different PDH networks compare and which signals can be carried across the SDH network.

SDH – Network Topologies

A line system is the system to the PDH network topology. Traffic is added and dropped only at the endpoints of the network. Terminal nodes are used at the end of the network for adding and dropping the traffic.

Line System

Within any SDH network, it is possible to use a node known as a regenerator. This node receives the high order SDH signal and retransmits it. No lower order traffic access is possible from a regenerator and they are only used to cover long distances between sites, where the distance means that the received power would be too low to carry traffic.

Ring System

A ring system consists of several add/drop muxes (ADMs) connected in a ring configuration. Traffic can be accessed at any ADM around the ring and it also possible for traffic to be dropped at several nodes for broadcast purposes. The ring network has the benefit of offering traffic resilience, if there is a fiber break traffic is not lost. Network resilience is discussed in detail in a subsequent chapter.

SDH Network Synchronization

While PDH networks were not centrally synchronized, SDH networks are (hence, the name synchronous digital hierarchy). Somewhere on the operator’s network will be a primary reference source. This source is distributed around the network either over the SDH network or over a separate synchronization network.

Each node can switch to backup sources, if the main source becomes unavailable. Various quality levels are defined and the node will switch the next best quality source it can find. In cases where the node uses the incoming line timing, the S1 byte in the MS overhead is used to denote the quality of the source.

The lowest quality source available to a node is generally its internal oscillator. In a case where a node switches to its own internal clock source, this should be remedied as soon as possible, as the node may start to generate errors over time.

It is important that the synchronization strategy for a network is planned carefully. If all the nodes in a network try to synchronize off its neighbor on the same side, you will get an effect called a timing loop, as shown in the above figure. This network will quickly start to generate errors as each node tries to synchronize off each other.

SDH Hierarchy

The following figure shows how the payload is constructed, and it isn’t as scary as it looks at first.