Synchronous Digital Hierarchy:-
Synchronous Digital hierarchy is the international standard for transmitting digital information over optical networks. It ensures compatibility between high speed digital telephony systems based on optical fiber rather than copper cabling.SDH standards are known as SONET in United State but are compatible with those used everywhere else.
SDH transmission method multiplexes many lower order signals to higher order signal,and vice versa.It operates at speed of between 155 Mbps and 2.54Gbps.
Advantages of SDH:-
1. Network simplification.
2. High flexibility.
3. In band management.
4.PDH integration.
5. Availability of high speed standards.
6. Efficient multiplexing or demultiplexing.
7. Interconnection between multiple vendor equipment.
SDH synchronization:-
All the SDH elements in a network is synchronized to a same highly accurate clock called the Primary Reference Clock(PRC).This is a major difference between PDH and SDH.PDH signals can be transported on an SDH frame, but they are multiplexed independently of the SDH timing.
SDH Multiplexing Structure:-
SDH multiplexing is a technical step by which several lower order path layer signals of SDH are adapting into a higher order path or the several higher order path layer signals of SDH are adapting into a multiplex section.The basic building block of an SDH frame is a STM signal.It is also the first level of SDH at 155.52Mbps and is known as a synchronous transport module(STM).The SDH hierarchy is as follows-
STM-1 155.52Mbps 155Mbps
STM-4 622.08Mbps 622Mbps
STM-16 2488.32Mbps 2.4Gbps
STM-64 9953.28Mbps 10Gbps
STM-256 39813.12Mbps 40Gbps
2.Virtual container(VC-n) where n=1 to 4
3.Tributary unit group(TUG-n) where n=2 or 3
4.Administrative unit(AU-n) where n=3 or 4
5.Administrative unit group (AUG)
6.STM-1
Electrical Signal Bit rate Abbreviated as
STM-0 51.84Mbps 51MbpsSTM-1 155.52Mbps 155Mbps
STM-4 622.08Mbps 622Mbps
STM-16 2488.32Mbps 2.4Gbps
STM-64 9953.28Mbps 10Gbps
STM-256 39813.12Mbps 40Gbps
SDH Multiplexing Elements:-
1.Container(C-n) where n=1 to 42.Virtual container(VC-n) where n=1 to 4
3.Tributary unit group(TUG-n) where n=2 or 3
4.Administrative unit(AU-n) where n=3 or 4
5.Administrative unit group (AUG)
6.STM-1
What is STM-1?
An STM is the information structure used to support section layer connections in the SDH.It consists of information payload and section overhead information fields organized in a block frame structure which repeats every 125 micro seconds.A basic STM is defined at 155 520 kbit/s. This is termed STM-1.Higher capacity STMs are formed at rates equivalent to N times this basic rate.STM capacities for N=4,N=16 and N=64 are defined,higher values are under consideration. The STM-1 comprises a single Administrative Unit Group together with the SOH.The STM-N contains N AUGs together with SOH.
1.Section overhead carrying control and managements functions
------regenerator section overhead.
------multiplex section overhead
2.Administrative unit pointers cater to problems with SDH timing.
3.Information payload is the area carrying subscriber data along with the path overhead information.
SDH frame structure:-
The SDH frames is built up from a 9 rows and 270 columns byte matrix.the order of transmission is row by row ,from left to right and top to bottom.The frame repetition rate is 125 micro seconds. Each byte in payload represents a 64kbps channel.The three main areas of the STM frame are;-1.Section overhead carrying control and managements functions
------regenerator section overhead.
------multiplex section overhead
2.Administrative unit pointers cater to problems with SDH timing.
3.Information payload is the area carrying subscriber data along with the path overhead information.
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