Single and multi-hop networks
Telecommunications traffic continues to grow at a very rapid rate. This is being accelerated by the increase in the volume of data and mobile traffic, especially in India, thanks to the recent liberalization of the telecommunications market. A solution can be adopted to meet ever increasing traffic demands based on a combination of WDM, SDH and IP transport technologies.
Wavelength Division Multiplexing is used to multiplex multiple wavelength channels onto a single strand of fiber, thereby overcoming fiber congestion. SDH technology provides the granularity of capacity that customers demand today and the ability to protect these services from network failure. An IP transport network over WDM can provide high capacity Internet transit services to Internet service providers (ISPs).
Number hierarchythan synchronous
Synchronous digital hierarchy (SDH) networks have replaced PDH and have several key advantages.
ITU recommendations G.707, G.708 and G.709 form the basis of a global network.
Networks benefit from traffic resiliency to minimize traffic loss in the event of fiber failure or equipment failure.
Integrated monitoring technology enables network configuration and troubleshooting.
Flexible technology allows access to tributaries at any level.
Future-proof technology allows for a faster bit rate as technology advances.
European PDH networks could not interface with American networks, SDH networks can carry both types. The figure above shows how different PDH networks compare and which signals can be transported through the SDH network.
SDH - Network topologies
An online system is the system with PDH network topology. Traffic is added and removed only at network endpoints. Terminal nodes are used at the end of the network to add and remove traffic.
In any SDH network it is possible to use a node called regenerator . This node receives the high order SDH signal and retransmits it. No access to lower order traffic 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 the traffic.
A ring system consists of multiple add / remove muxes (ADMs) connected in a ring configuration. The traffic is accessible to any ADM around the ring and it is equalIt is not possible for traffic to be dropped at multiple nodes for broadcast. The ring network has the advantage of providing traffic resiliency, if there is a fiber break, the 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 over the network either on the SDH network or on a separate synchronization network.
Each node can fail over to sources of backup, if the primary source becomes unavailable. Different quality levels are set and the node will switch to the next higher quality source that it can find. In cases where the node uses line synchronization enterednte, the S1 byte in the MS overhead is used to denote the quality of the source.
The lowest quality source available for a node is usually its internal oscillator. In the event that a node switches to its own internal clock source, this should be corrected 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 carefully planned. If all the nodes in a network try to synchronize with its neighbor on the same side, you will get an effect called synchronization loop , as shown in the figure above. This network will quickly start generating errors as each node tries to synchronize.
The following figure shows how the payload is constructed, and it 's not so scary at first.