NGN Synchronous Digital Hierarchy - NGN

What is Synchronous Digital Hierarchy?

SDH Networks replaced PDH and had several key advantages.

  • G.707, G.708, and G.709 ITU recommendations provide basis for global networking.
  • Networks benefit from traffic resilience to minimize traffic loss in the event of fiber break of equipment failure.
  • Built in monitoring technology allows remote configuration and troubleshooting of network.
  • Flexible technology allows for tributary access at any level.
  • Future proof technology allows for faster bit rates as technology advances.


Unlike European PDH networks SDH networks of US networks can carry both types. Let’s find the below example to know how the different PDH networks compare and which signals can be carried across the SDH network.

SDH – Network Topologies

Line System


A lone system is system considered for the PDH network topology. Here traffic is added and dropped at the points of end of the network. To add and drop the traffic at the terminal nodes are used.

It is possible to use a node known as a regenerator in any SDH network. 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 is also possible for traffic to be dropped at several nodes for the broadcast purposes.


The ring network also has the benefit of offering traffic resilience, if there is a fibre break traffic I not lost. Network resilience is discussed in further detail later.

SDH Network Synchronisation

While PDH networks were not centrally synchronized, SDH networks are (hence the name synchronous digital hierarchy). Somewhere on the operators 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 above. This network will quickly start to generate errors as each node tries to synchronize off each other.

SDH Hierarchy

Let’s see the below figure that shows how the payload is constructed, and it isn’t as scary as it looks at first. Let’s explain this with next couple of slides how the SDH signal is constructed from the lower level payloads.


STM-1 Frame

The frame is made up of rows of 9 overheads and 261 payload bytes.


Here SIM-1 frame is transmitted row by row as explained below. The 9 overhead bytes in a row are transmitted, followed by the 261 bytes of payload, the next row is then transmitted in same way till the complete frame has been transmitted. The whole frame is transmitted in 125 micro seconds.


STM-1 Overheads

The first 3 rows of the overhead are known as the repeater section overheads. The 4th row form the AU pointers, and the last 5 rows hold the multiplex section overheads.


Let’s explain this scenario with different types of overheads in a system where the payload is passed through several intermediate regenerators before reaching the ADM that it is being added/dropped from.


Here use the repeater section overheads for communications and monitoring between any two neighboring nodes.


Use the multiplex section overheads for communications and monitoring among the two nodes which will have add/drop facilities such as ADMs.


At a lower level, there are also path overheads that are added at a tributary level, these will be discussed in further detail later.


Let’s think about different overhead alarms to pinpoint trouble on the network. An RS alarm indicate a problem on the HO SDH side between two nodes, while if investigating an MS alarm you can rule out problems at the regenerator nodes.

SDH Path Trace

SDH path trace is useful to pinpoint interconnection problems between nodes. Let’s find different types of physical interconnections like splices and patches within optical frames between two nodes. Each node is configured by the network operator to send a unique string that identifies it.

The another way of configuration of each node is done with the string that it should receive from its neighboring node.

It will be OK if the path trace is received by the node matches the things that they are expecting.

Incase if the received path trace does not match the trace that the node is expecting, then this indicates a problem with the connection between the nodes.


SDH Management

The DCC channels includes among the section overheads which allow for easy management of the SDH network. A network management system connected to a node on the network can communicate with other nodes on the network using the DCC channels. A gateway node is known as the node that is connected to the DCN network. There will be more than one gateway node on the network for resilience purposes.


SDH Network Resilience

Usually a ring configuration traffic is passed among the both routes around the ring from the originating ADM (Add/Drop Multiplexer). At any ADM where the signal is not dropped it simply passes through. Even though if any traffic passes around the ring from both routes then only one route is used to extract traffic from at the receiving ADM which is known as the active route or path. The second route is known as the standby route or path.


Incase if any fiber break on the active path, then use other ADM with the alternative signal as the active path. This allows fast and automatic restoration of traffic flow to customers. Incase if the fiber break is repaired, then the ring does not automatically switch back which causes further traffic “hit,”. Let’s use this as the standby path in case of future failure on the new active path. The MUX that loses traffic will use the K bytes to signal the protection switch back to the originating MUX.


Manual ring switches can also be performed either from the network management center or from local terminals operated by engineers.


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