SYNCHRONOUS DIGITAL HERARCHY
INTRODUCTION :
It is an international standard networking principle and a multiplexing method.
The name of hierarchy has been taken from the multiplexing method which is
synchronous by nature. The evolution of this system will assist in improving the
economy of operability and reliability of a digital network.
HISTORICAL OVERVIEW :
In February 1988, an agreement was reached at CCITT study group XVIII in
Seoul, on set of recommendations, for a SDH representing a single world wide
standard for transporting the digital signal.The three different country standards of
PDH developed over a time period. The aim of these standards was to simplify
interconnection between network operators by allowing inter-connection of
equipment from different vendors to the extent that compatibility could be
achieved. It was achieved by SDH in 1990.
DEFINITION :
It is a standardized architecture for use in digital transmission network. It is based
on a synchronous multiplexing structure, which has several advantages. The
FIBCOM FOCUS AC1 product family is part of the FIBCOM product range for
access and regional telecom transmission network based on the standards and
recommendations on SDH from EISI and ITU , under technical collaboration
from TELLABS Denmark A/S , Denmark.
The FIBCOM FOCUS AC1 is a product family where STM-1
AND STM-4 Add/Drop multiplexers provides a cost efficient solution especially
in small nodes where the requirement is to add /drop a limited number of and
Terminal Multiplexers (TM) are implemented on a single module giving VC-4
,VC-3and VC-12 connectivity .This 2Mbits/signals.The number of tributary
signals can be increased to full capacity by adding additional tributary
modules.The possible protection schemes in the network includes SNC protection
of VC-4 ,VC-3 and VC-12 signals.Management of the FIBCOM FOCUS AC1
can be performed from a local craft terminal from network element manager or
from a network management system.
MERITS OF SDH :
I. Simplified multiplexing/demultiplexing techniques.
II. Direct access to lower speed tributaries, without need to
multiplex/demultiplex the entire high speed signal.
III. Enhanced operations, Administration, Maintenance & provisioning
capabilities .
IV. Capable of transporting existing PDH signals.
V. Capable of transporting future broadband channel bit rates.
VI. Capable of operating in a multi-vendor and multi-operator environment .
S.D.H. EVOLUTION :
S.D.H. evolution is possible because of the following factors :
1)FIBRE OPTIC BANDWIDTH : The bandwidth in Optical Fibre can be
increased and there is no limit for it. This gives a great advantage for using SDH.
2)INTELLIGENCE : The availability of cheaper memory opens new possibilities.
3)CUSTOMER SERVICE NEEDS :The requirement of the customer with
respect to different bandwidth requirements could be easily met without much
additional equipment.
ADVANTAGE :
SDH supports the following services :
• Low/High speed data.
• Voice
• Interconnection of LAN
• Computer links
• Broadband ISDN transport
PRINCIPLE :
• SDH defines a no. of “Containers”, each corresponding to an existing
plesiochronous rate.
• Each container has a “Path Overhead “ , which provides network management
capability.
• Virtual Container = Container + POH
• All equipment is syncronised to a national clock.
• According to recommendation G-709, different combinations of VCs which
can be accommodated in the “payload” of an STM-1 frame.
• When STM-1 payload is full, more network management capability is
added to form the “Section Overhead”.
• SOH remains with payload for the fibre section between synchronous
multiplexers.
• SOH bytes provide communication channels to cater for :
user channels.
Protection switching.
Section performance
Frame aligment
Other functions
