SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media. It is the international equivalent of SONET. Both technologies provide faster, more reliable and less expensive network interconnection than their predecessor PDH (Plesiochronous Digital Hierarchy) equipment. SDH services are inherently secure and provide point to point connectivity both between PoPs and, via a variety of termination methods, can be extended out to customer premises.

SDH uses the following Synchronous Transport Modules (STM) and rates: STM-1 (155 megabits per second), STM-4 (622 Mbps), STM-16 (2.5 gigabits per second), and STM-64 (10 Gbps).

Services are sub multiplexed from these rates down to E1 (2Mbps), E3 (34Mbps) and DS3 (45Mbps)

SDH - a resilient infrastructure

Deployment of synchronous transmission systems is straightforward, requiring only the availability of either dark fibre or optical wavelengths and suitable power and premises.

SDH is by design a resilient transport. SDH networks are based upon fibre rings. There may be multiple Points of Presence on each ring. (In the TANet network, each Point of Presence is located in a major business district).

Traffic between any two points is sent in both directions around the ring. Should there be a failure on one side of the ring, no matter how catastrophic, traffic simply reaches its destination via the other route.

SDH - the TANet implementation

Fibernet offers its customers connection to it's TANet SDH services at 2Mbit/sec (E1), 34Mbit/sec (E3), 45 Mbit/sec (DS3), 155Mbit/sec (STM-1) and 622Mbit/sec (STM-4).

The TANet national SDH network is based upon 14 interconnected regional rings. Each ring is hosted upon one or more Wave Division Multiplexing wavelengths. Smaller metropolitan rings exist in London, Birmingham, Manchester, Leeds and Glasgow and are implemented directly on dark fibres.

Each SDH ring is interconnected with its national or metropolitan neighbour at two or more locations ensuring no individual point of failure. Traffic from customer connections is simultaneously transmitted in both directions around each ring. One of the key benefits of this approach compared to that of traditional wide area networks is the removal of any single point of failure from the network. Customer connectivity and throughput is maintained even in the event of catastrophic network failures.

The sophisticated network management capabilities of the TANet SDH network give a vast improvement in the control of transmission networks. Improved network restoration and reconfiguration capabilities result in better availability, and faster provisioning of services. The Network Operation Centre is based in Hampshire.