540 CHAPTER 29: CPOS INTERFACE CONFIGURATIONSDH Frame Structure The frame structure of SDH signal STM-N is described in the following part.Low-speed tributary signals should distribute in one frame regularly and evenly forthe convenience of adding/dropping them in high-speed signal. ITU-T specifiesthat STM-N frames adopt the structure of rectangle blocks in bytes, as illustratedin the following figure:Figure 150 STM-N frame structureSTM-N is a rectangle-block frame structure of 9 rows x 270 x N columns, wherethe N in STM-N equals the N columns. N takes the value 1, 4, 16, and so on,indicating the number of STM-1 signals that form SDH signal.The STM-N frame structure consists of three parts: section overhead (SOH), whichincludes regenerator section overhead (RSOH) and multiplex section overhead(MSOH); administration unit pointer (AU-PTR); and payload. AU-PTR is the pointerthat indicates the location of the first byte of payload in an STM-N frame so thatthe receiving end can correctly extract payload.Terms ■ Multiplex Unit: A basic SDH multiplex unit includes multiple containers (C-n),virtual containers (VC-n), tributary units (TU-n), tributary unit groups (TUG-n),administrative units (AU-n) and administrative unit groups (AUG-n), where n isthe hierarchical sequence number of unit level.■ Container: Information structure unit that carries service signals at differentrates. G.709 defines the criteria for five standard containers: C-11, C-12, C-2,C-3 and C-4.■ Virtual container (VC): Information structure unit supporting channel layerconnection of SDH. It terminates an SDH channel. VC is divided intolower-order and higher-order VCs. VC-4 and VC-3 in AU-3 are higher-ordervirtual containers.■ Tributary unit (TU) and tributary unit group (TUG): TU is the informationstructure that provides adaptation between higher-order and lower-orderchannel layers. TUG is a set of one or more TUs whose location is fixed inhigher-order VC payload.RegeneratorsectionoverheadAU-PTRMultiplexsectionoverheadPayload1234567899 x N 261 x N9 x 270 x N (bytes)
