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Selecting Test Features
Adding Pointer Adjustments
Adding Pointer Adjustments
Introduction
Pointers perform a critical role in the error free transmission of payload data
(subscriber data) through a SDH network. They also enable individual payload
channels to be inserted or extracted from a high speed STM-n line signal (for
example the functionality provided by ADM's).
Pointer adjustments are often necessary to compensate for asynchronous operation
between different nodes within an SDH network.These adjustments however can
result in jitter being added to a PDH signal output from an SDH network element.
Jitter caused by Pointer Adjustments
Pointer adjustments are the mechanisms within SDH used to compensate for
frequency and phase differences between VCs and the outgoing SDH frames. These
pointer adjustments are byte wide and since they can occur randomly, they may
cause significant amounts of payload signal jitter. It is therefore necessary to control
the jitter on payload signals that is due to pointer adjustments.
Pointer adjustment activity within a network can be randomly spaced individual
pointer adjustments, pointer bursts or periodic pointer adjustments.
The ITU-T G.783 and ETSI TM-1015 standards defines a set of pointer sequences
to be used when evaluating an NE's pointer adjustment jitter performance.
The OmniBER 718 generates a set of test sequences which can be used to simulate
network pointer adjustment activity. This allows the amount of tributary jitter due to
different types of pointer adjustment to be measured in the OmniBER 718.
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