1 the time each vendor used proprietary methods of encoding infor- signal. Each of these signals has the same structure; floating pay- mation into an optical signal; the only common interface being at loads of tributary traffic and an overhead carrying pointers. The the electrical asynchronous ports. overhead actually carries other information besides the pointers. It As a result of this need for easy access to low order signals and carries information channels that allow the SDH products to com- mid span meet on fibres the new SONET (Synchronous Optical municate with each other throughout the network. They can ex- Network) standard was developed in North America. Before the change maintenance information (alarms, error rates, protection Self healing Self healing Self healing Transport network Switching centre Access network Figjure 53.32 Typical end to end Sonet/SDH network with integral OAM&P 53/26 Fibre optic communications switching etc.) that permits widespread network management and and optical conversion all in one unit thus reducing the number of control from a single point. network elements required. A simpler network combined with soph- Compared to plesiochronous (or asynchronous in North America), isticated control capability and the intelligence in each terminal to SDH brings five major benefits to the network that make deploy- report on its health (and correct it if necessary) results in a very ment very worthwhile: reliable network. 5. Transmission capacity. Because SDH is a standard for fibre 1. Multi-vendor. There is now a standard allowing for the mixing transmission it brings all the benefits of the bandwidth of fibre to of products from a variety of vendors without having to be con- the network. An STM16 product has a 2.4Gbit/s capacity, far greater cerned about interfaces. In fact this history of interface issues forced than todays plesiochronous products. And this is not the limit since the structure of the telephone network into categories of switching the standard could support up to 9.6Gbi