12-11-2012, 04:47 PM
Synchronizing network elements on SONET/ SDH rings
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Introduction
The high-speed transport area is becoming more and more important
as the need for high bandwidth increases day by day in
this Information Technology oriented world. SONET / SDH
technologies are becoming the backbones of most of the networks,
in spite of the type of the traffic that are carried.
Unlike the Plesiochronous Digital Hierarchy (PDH) systems
which uses independent, accurate clocks at switching nodes,
the Synchronous Digital Hierarchy (SDH) systems follow a
combination of Master-Slave and Mutual synchronization techniques.
The synchronization mechanism become more complex
since SDH is designed to accommodate the legacy PDH
network traffic also.
This paper describes the complexity of the high-speed networks
by analyzing different popular architectures and takes a
look at the challenges in synchronizing various systems such
as Add-Drop Multiplexers (ADM), Terminal Multiplexers,
Cross Connects, line cards and other Network Elements. There
are a number of standards, which are relevant in this area, and
the paper concentrates on the Timing characteristics of SDH
Equipment Slave Clocks (SEC). The International Telecommunication
Union (ITU) recommendation for SEC timing specification
is G.813. The long-term stability of the clocks, jitter
and wander transfer and tolerance, the transient and holdover
responses are the main specifications. Under the transient
behaviour specification, there is provision for reference switching
which is an inevitable feature in multiple reference systems.
There are excellent PLLs available in the industry, which
can satisfy the TIE (Time interval error) requirements of the
specification along with the other features.
Complex Network architectures
Most of the high-speed networks engineered today are based
on optical transmission through optical fibers. With the requirement
for extreme reliability and availability, the networks are
getting more and more complex. The systems are becoming
bigger and the fault tolerance mechanisms are more prominent.
Mechanisms to survive network failures are built into the architectures.
The ability of a network to withstand and recover
form failures is one of the most important requirements of
networks.
Synchronization Standards
The current generation high speed networks like SONET and
SDH systems are designed for master-slave as well as mutual
synchronization architecture, unlike the PDH networks where
network clocks are independent and the buffers within the
transceivers as well as bit stuffing mechanisms take care of
the allowable frequency drift between two networks. With the
SDH and SONET networks, all the clocks are traceable to a
master clock, or a bunch of master clocks, the requirements
for which are defined in G.811 (PRC) by the International
Telecommunication Union (ITU). These are equivalent to the
Stratum 1 clocks or PRS, which are defined in the North
American Standards.
There are second levels of clock regenerators, which are
deployed in the network, that are called the node clocks or
Synchronization Supply Units, which are defined by the G.812
from ITU-T. These are equivalent to the stratum 2 clocks from
the North American standard. These are also called SETS and
are having the same level as the BITS clocks (Building Integrated
Timing Supply). These are synchronization supply units,
which provides clocks to an entire system or building and the
performance of which are very close to the stratum 1 clocks.
As part of SDH equipment is the SETS (Synchronization
Equipment Timing Supply) Unit is defined in G.783, which
describes the building blocks of SDH equipment. The clock
equipment function is commonly called as the Slave Equipment
Clock (SEC); equivalent to the SONET Minimum Clock
(SMC) standards defined by the North American standards
(Stratum 3E).