SDH network four network elements: terminal multiplexer TM, regenerative repeater REG, spit-multiplexer ADM, and synchronous digital cross-connect device DXC,(is a combination of multiplexing, wiring, protection\/recovery, monitoring and management of multiple functions of the device, its common configuration: DXC4\/4 rate is 140Mb\/s or 155.52Mb\/s, DXC4\/1 rate is 2Mb\/s)
Note: SDH is a physical transmission mode, IP network is a network connection mode, IP ON SDH is POS is to let IP run on the network of SDH, these three concepts should be distinguished.