Fundamentals of SDH

Giới thiệu tổng quát về hệ thống SDH được sử dụng trong mạng cáp quang

Sampling Theorem

 A bandlimited signal can be reconstructed exactly if it is sampled at a rate atleast twice the maximum frequency component in it.

Fs ≥ 2Fm

PULSE CODE MODULATION

 Developed by A.M.Reaves in 1938

 Uses TDM technique

 Voice Frequency ranges upto 4 Khz

 Sampling the Voice Signal @ 8 Khz (Ts=125 µsec)

 8 bits per sample

 Digital Bit Rate: 8000 X 8 = 64 Kbps

BUILDING UP THE BASE STREAM (2MB)……

One sample duration (approx) = 4 µs Duration between samples = 125 µs

30 Channel PCM System

 30 voice channels

 One channel for synchronisation

 One channel for signalling

 Totol no.of channels =32

 One frame (125 µs) is divided into 32 time slots

PCM bit rate

 32 time slots in a frame

 Each slot having 8 bits

 Total no.of bits per frame= 32*8 =256 bits/frame

 Total no.of frames per sec =8000

 Total no.of bits per sec =256 * 8000 =2048 Kbps

 Plesiochronous Digital Hierarchy

 a technology used in telecommunications network to transport large quantity of data over digital transport equipment such as fibre optic and microwave radio wave systems.

 the term “plesiochronous” is derived from greek plesio which

means near, and chronous, time.

 it means that pdh networks run in a state where different parts

of the network are almost, but not quite perfectly synchronised

 The basic data rate is 2.048 Mbps

PDH BIT RATES

(European standard)

 E1- 2048 Kbps (2Mb) [30 Voice Channel]

 E2- 8448 Kbps (8Mb) [120 Voice Channel]

 E3- 34368 Kbps (34Mb) [480 Voice Channel]

 E4- 139264 Kbps (140Mb) [1920 Voice Channel]

PDH HIERARCHY

 Insufficient capacity for network management

 Higher bit rates are difficult to achieve

 Supports only linear topology

 no common standards among vendors.

SDH-Synchronous Digital Hierarchy

 SDH is an ITU-T standard for a high capacity telecom network

 SDH is a synchronous digital transport system, aim to provide

a simple, economical and flexible telecom infrastructure

 This is the information structure used to support information payload and overhead information organized in a block frame structure which repeats every 125 micro seconds

 The basis of Synchronous Digital Hierarchy (SDH) is

synchronous multiplexing - data from multiple tributary

sources is byte interleaved.

MERITS OF SDH

 Simplified multiplexing / demultiplexing techniques

 Direct access to lower speed tributaries

 Enhance O peration , A dministration & M aintenance

 Easy growth to higher bit rates in steps with evolution of transmission technology

 Capable of transporting existing PDH

 Capable of transporting future ATM

 Capable of operating multi vendor and multi –operator environment

SDH Rates

 SDH is a transport hierarchy based on multiples of 155.52

Mbit/s

 The basic unit of SDH is STM-1

 STM-Synchronous Transport Module

 Higher rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.

STM-N

Where N =1,4,16 ,64 n is a multiples of four

STM-1 Frame

 Synchronous Transport Module –

 A frame with a bit rate of 155.52 Mbit/s is defined in ITU-T Recommendation G.707

 It is made up from a byte matrix of 9 rows and 270 columns

9 261

PAY LOAD

SOH

• As indicated in the figure, the STM – n signal is multiples of

frames consisting of 9 rows with 270 bytes in each row

• The order of transmission of information is first from left to right and then from top to bottom

• The first 9 bytes in each row are for information and used by

the SDH system itself.This area is divided into 3 parts

• Regenerator Section Overhead(RSOH)

• Multiplex Section Overhead(MSOH)

• Pointers

STM-1 frame structure (contd )

THE TRUCK ANALOGY

PAYLOAD

OVERHEAD

THE TRUCK ANALOGY

PAYLOAD

OVERHEAD

THE TRUCK

PAYLOAD

OVERHEAD

RSOH: Regenerator section overhead

MSOH: Multiplex section overhead

Payload: Area for information transport

Transport capacity of one Byte: 64 kbit/s

Frame capacity: 270 x 9 x 8 x 8000 = 155.520 Mbit/s

Frame repetition time: 125 µs

9TH ROW

FRAME REPRESENTATION

SDH ACCOMMODATES EXISTING SIGNALS

S D H

M U X

31

SDH Multiplexing

 The multiplexing principles of SDH follows these terms

 Mapping - A process used when tributaries are adapted into

Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information

 Aligning - This process takes place when a pointer is included in

a Tributary Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located

 Multiplexing - This process is used when multiple lower-order path layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section

Multiplexing Structure

• Tributary Unit (TU)

• Tributary Unit Group (TUG)

• Administrative Unit (AU)

• Administrative Unit Group (AUG)

• Synchronous Transport Module - N (STM – N)

VIRTUAL CONTAINER

 35 BYTES - ONE 2MB

CONTAINER POH

TRIBUTARY UNIT

 When pointer is added to the virtual container ,it is called tributary unit ( ONE 2 MB)

VC POINTER

4 X 9

36 BYTES

Tributary unit group -3

 Homogeneous assembly of identical tributary units

Tug-2

TUG-3 TUG-3 CONTAIS 21X2MB

VC-4 FORMATION

 Virtual container formation

VC-4=261

P O H

 AU POINTER IS ADDED TO THE VC

PAYLOAD

AU Pointer

ADMINISTRATIVE UNIT GROUP

 SOH BYTES ARE ADDED

The following are the different steps in the mapping of 2Mbps stream

 Formation of container C12

 Formation of virtual container VC12

 Formation of tributary unit TU12

 Multiplexing of TU12 ‘s to form TUG3

 Multiplexing of TUG3‘s to form VC4

 Formation of administrative unit AU4

 Formation of administrative unit group AUG

 Adding SOH to form STM1

MUX PRINCIPLE: STM-1(from

C-12)

TUG-3

P O H

TUG-2 TUG-2 TUG-2 TUG-2 TUG-2 TUG-2 TUG-2

TU-12 TU-12 TU-12

CONTAINER-12

P O H

P T R

E1: 2.048Mb/s

STM-1 AU-4 VC-4

C-12 VC-12

TUG-3

TUG-2

TU-12

x3 x7

x3

VC-n

AU-n

AUG

STM-n Synchronous Transport Module

Administrative Unit Group: One or

more AU(s)

Administrative Unit: VC + pointers

Virtual Container: payload + path

overhead

AUG

STM-1 AU-4 VC-4

C-3

VC-3 TUG-3

E3: 34.368Mb/s DS3: 44.736Mb/s

STM-n Synchronous Transport Module

Administrative Unit Group: One or

more AU(s)

Administrative Unit: VC + pointers

Virtual Container: payload + path

overhead

AUG

Mapping of 34 Mbps stream

STM-1 AU-4 VC-4

VC-n

AU-n

AUG

STM-n Synchronous Transport Module

Administrative Unit Group: One or

more AU(s)

Administrative Unit: VC + pointers

Virtual Container: payload + path

overhead

AUG

Mapping of 140 Mbps stream

C-4 E4: 139.264Mb/s

Section Overhead

 RSOH –Regenerator Section Overhead

 MSOH –Multiplex Section Overhead

SOH BYTE ALLOCATION

A1,A2 Frame alignment

B1B2 Error monitoring

D1 D3 Data communication channel for RSOHD4 D12 Data communication channel for MSOHE1-E2 Order wire channel