SDH-Synchronous Digital Hierarchy

SDH: Synchronous Digital Hierarchy.

SDH is an international optical interface standard to transmit different types of signals on OFC. It accepts various native TDM signal formats and multiplexes-adds- or demultiplexes-drops- these signals without conversion. It provides improved network management- in band- and direct access to tributaries.

Plesiochronous Digital Hierarchy-PDH-includes all ITU TDM hierarchies including T-carrier and E-carrier systems. Signals are considered to be plesiochronous if their rates are nominally same, with any variations constrained to specified limits. PDH is defined by ITU G.702 and includes NADH-North American Digital Hierarchy-, EDH-European Digital Hierarchy- and JDH-Japanese Digital Hierarchy. All these systems share the basic rate of 64kbps that correspond to a PCM voice channel.

Synchronization

1.PRS / Master clock / Stratum1 GPS or Atomic clock.
2.Line timing. Derived from incoming SDH signal from a high-speed interface.
3.Loop timing. Similar to Line timing but incoming SDH is taken from a CPE or TM-Terminal Mux- than an ADM.
4.Stratum3. Free running internal oscillator.

Although synchronous networks display accurate timing, some variations occur between different network devices or between networks. This difference is known as phase variations. Short-term phase variations above 10 Hz are called Jitter and long tem phase variations below 10 Hz are called Wander. In digital networks, jitter and wander are handled by buffers found in the interfaces within different network devices. One example is a slip buffer, used to handle frequency difference between read and write operations.
Here bit stuffing is used and it is called controlled slip. The timing accuracy requirements increase as the stratum hierarchy increases as shown below.


Stratum clock Hierarchy
Mini. Free run Wander Bit slips controlled frame
Accuracy (0.12 hsec Increment) Slips

Stratum 1 +/- 1*10^-11 3.3hr 18hrs 20.6 wks
Stratum2 +/- 1.6*10^-8 7.5 sec 41sec 2.17hrs
Stratum3 +/- 4.6*10^-6 26ms 140ms 27sec
Stratum4 +/- 32*10^-6 4ms 20ms 3.9sec











SDH Layers

SDH has four optical interface layers as follows.
1.Path layer
2.Multiplex Section –MS- layer
3.Regenerator Section –RS- layer
4.Photonic layer.

Path layer multiplexes or demultiplexes the lower level VC-n payloads. POH is added or stripped at this layer and the alarm information contained in this layer represents the end-to-end status. Lower order multiplexers, subscriber loop access systems are examples of Path Terminating Equipments.
Signal at STM-N level is transported between NE’s in this layer. MSOH is added or stripped at this layer. The MSOH is used for communication between major nodes and for monitoring errors. High order mux, Optical Line Terminator are examples of MSTE-MS Terminating Equipments.
This layer is there between a TM and regenerator or between two regenerators.
RSOH is added or stripped at this layer. It is used for long haul transport.
Deals with transport of bits across physical fiber medium i.e. EàO and OàE conversion.

SDH Multiplexing

SDH Multiplexing follows a rigid hierarchy as follows.



1.Low level PDH signal:: Mapped by adding Justification bits à Container ( C11 for T1, C12 for E1, C2 for DS2, C3 for E3 and DS3, C4 for E4)
2.:: Mapped by adding POH à Virtual Circuits (VC11,VC12,VC2,VC3,VC4)
3.:: Aligned with TUPointers à Tributary Units ( TU11,TU12 and TU2)
4.::Multiplexed à TUG –Tributary Unit Group-( TUG2, TUG3)
5.:: Multiplexed à Higher Order VC’s.( VC3,VC4)
6:: Aligned with fixed byte stuffing and AUPointersà Administrative Unit (AU3,AU4)
7.:: Muxed à AUG(AUG1,AUG4,AUG16,AUG64,AUG256)
8.:: Muxed with MSOH and RSOHà STM-N


Higher level of the Synchronous hierarchy is formed by byte interleaving of the payload from a number N of STM-1 signals and then adding TOH of size N times that of an STM-1 and filling it with new management data and pointer values as appropriate. Before transmission the STM-N signal is scrambled to randomize the bit sequence for better transmission performance. A few bytes of overhead are left unscrambled to simplify subsequent de-multiplexing.