Unit protection utilising synchronous digital hierarchy (SDH) communication systems-why a new design approach is necessary

Synchronous digital hierarchy (SDH) is now the favoured network transmission infrastructure for the majority of communication service providers. Some large legacy networks are in the process of being converted from plesiochronous digital hierarchy (PDH) to SDH but many green field networks are built using entirely SDH technology. SDH communication paths can be protected from failure in different ways but the operation of the SDH protection mechanism inevitably results in the appearance of differential delay. Differential or asymmetric delay is defined as the difference in delay between the transmit and receive paths. Traditional unit protection designs cannot accommodate significant differential delay and security is impaired. This paper looks at a strategy for dealing with the application of unit protection on SDH and concludes that one of the solutions is to modify the design of the unit protection