The most recent transport solution is OTN, which was defined initially for the submarine and core networks. It transports TDM circuits over long distances by providing Generic Forward Error Correction (GFEC). Lately, the standard has undergone major enhancements supporting extension into non-core networks and many operators are now looking to utilize the benefits of OTN in their metro and access networks. OTN is seen as the best transport solution because it offers fault management, performance monitoring, and protection mechanisms, coupled with a low cost-of-entry point and the ability to support current infrastructure with Ethernet, Fibre Channel, SDH/SONET and PDH/DSn networks.
Synchronous Digital Hierarchy (SDH) defined by ITU-T, and Synchronous Optical NETwork (SONET) defined by ANSI, was designed for optical transport at high bit rates. It provides simple multiplexing/demultiplexing with direct mapping of lower levels to higher levels. It is compatible with Plesiochronous Digital Hierarchy (PDH) because it can carry all PDH types in one universal hierarchy. SDH/SONET is synchronized to atomic clocks across the network and can provide traffic protection with backup lines.
PDH was a first-generation digital transmission system designed to carry voice and data. It represented the move from analog to digital transmission and was deployed widely but it is being replaced by other technologies now.