The basic principle of SDH

The basic principle of SDH

SDH level information structure is referred to as Synchronous Transport Module using STM-N (Synchronous Transport Mode, N = 1,4, 16,64), the basic module STM-1, four STM-1 synchronous multiplexing constitute STM- 4, sixteen STM-1 or four STM-4 synchronous multiplexing constitute STM-16, four STM-16 synchronous multiplexing constitute STM-64, or even four STM-64 synchronous multiplexing constitute STM-256; SDH using massive frame structure to carry information, each frame consists of longitudinal and transverse row 9 column 270 × N bytes, each byte containing 8bit, the entire frame structure is divided into a section overhead (Section OverHead, SOH) area, STM-N payload area and a management unit pointer (AU PTR) three regional areas, where section overhead area is mainly for network operation, management, maintenance and finger together to ensure that information can be transmitted properly flexible, it is divided into regenerator section overhead ( Regenerator Section OverHead, RSOH) and multiplex section overhead (Multiplex Section OverHead, MSOH); with a snap-pointer; payload area used to store bits of real information for business and a small amount of overhead bytes for channel maintenance channel management to indicate when the message payload area at the exact location of the first byte of STM-N frame so that the reception can correctly extract payload. SDH frame transmission when the press from left to right, arranged in order clusters of type stream from top to bottom sequentially transmitted every frame transmission time is 125μs, transmitted per second 1/125 × 1,000,000 frame of STM-1 in terms of per frame bit number 8bit × (9 × 270 × 1) = 19440bit, the STM-1 transfer rate is 19,440 × 8,000 = 155.520Mbit/s; STM-4 and the transfer rate is 4 × 155.520Mbit/s = 622.080Mbit/s; the transmission rate of STM-16 is 16 × 155.520 (or 4 × 622.080) = 2488.320Mbit/s.


When the SDH transmission traffic signal traffic signal to enter a variety of SDH frames to go through mapping, positioning and multiplexing three steps. ITU-T G.707 standard recommendations multiplex mapping structure as shown.

ITU-T G.707 standard recommendations multiplex mapping structure

Mapping is the signal to go through a variety of rate justification loaded into the appropriate standard containers (C), then add the path overhead (POH) forming a virtual container (VC) of the process, the frame phase deviation occurs is called frame offset.

Positioning the frame offset information that is incorporated into the tributary unit (TU) or management unit (AU) process, through tributary unit pointer (TU PTR) or administrative unit pointer (AU PTR) function to achieve.

The concept is relatively simple reuse, reuse is a plurality of signal adaptation into the lower-order channel layer order path layer, or a plurality of higher-order channel layer signal into the multiplexing process of adaptation by layer. Multiplexing is staggered by byte interleaving manner TU organized into higher order VC or the AU organized into STM-N process, since TU and AU pointer after each VC signal processing branch has phase synchronization, so that the complex process is synchronous with string theory and data multiplexing and conversion are similar.