SDH Principles Highlighted

Hello, everyone!

This post will introduce you the principles of SDH.

What is the SDH

SDH: Synchronous Digital Hierarchy

Advantages of SDH

1. Interfaces

• Electrical interfaces:-Can be connected to all existing PDH signals.

• Optical interfaces:-Can be connected to multiple vendors’ optical transmission equipment.

2. Multiplexing methods

• Basic rate is STM-1, other rates are multiples of the basic rate

3.OAM function

• Abundant overheads bytes for automation, network monitoring and maintenance

4. Compatibility 

• Working with all kinds of signals like PDH, SDH, ATM & FDDI 

From ITU-T G.707:

• STM-1 is the basic transmission format

• One frame lasts for 125 microseconds (8000 frames/s)

• Rectangular block structure 9 rows and 270 columns

• Each unit is one byte (8 bits)

• Transmission mode: Byte by byte, row by row, from left to right, from top to bottom

SDH Frame Structure has 3 parts:

1. Information Payload

2. Section Overhead

3. Pointer

Information Payload

• Also known as Virtual Container level 4 (VC-4)

• Used to transport low speed tributary signals

• Contains low rate signals and Path Overhead (POH)

• Location: rows #1 ~ #9, columns #10 ~ #270

Section Overhead

Pointer

SDH Multiplexing Structure

1. Mapping

• A process used when tributaries are adapted into VCs by adding justification bits and POH information

2. Aligning

• This process takes place when a pointer is included in a Tributary Unit (TU)or an Administrative Unit (AU), to allow the 1st byte of the VC to be located

3. Multiplexing

• This process is used when multiple low-order path signals are adapted into a higher-order path signal, or when high-order path signals are adapted into a Multiplex Section

4. Stuffing

• As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SDH frame to provide enough space for all various tributary rates.Therefore, at certain points in the multiplexing hierarchy, this space capacity is filled with “fixed stuffing” bits that carry no information, but are required to fill up the particular frame

• TUG = Tributary Unit Group

• AUG = Administrative Unit Group

• STM = Synchronous Transfer Module

SDH Rates

• STM-1 = 155.520 Mbit/s

• STM-4 = 622.080 Mbit/s

• STM-16 = 2,488.320 Mbit/s (~2.5 Gbit/s)

• STM-64 = 9,953.280 Mbit/s (~10 Gbit/s)

More: HCIA-Transmission | Chapter 2 SDH Principles-1

Thanks!