Sincronismo

Avoiding synchronization
problems in SDH networks
Product note
HP 37778A
STM-16/OC-48 test set

Network synchronization is a
fundamental issue facing
installers and maintainers of SDH
equipment. Increasingly complex
SDH networks have placed even
greater demands on network-wide
synchronization, where
transmission quality is directly
related to the accuracy and
performance of networktiming.
A timing hierarchy where the
primary reference clock (master
clock) is poorly distributed to
network elements (NEs) lower
down the hierarchy can seriously

impact network performance. At
best, there may be an increase in
pointer activity resulting in
increased tributary jitter as
pointer hits filter down through
the network. At worst, excessive
pointer activity may lead tothe
corruption or even loss of payload
data.
This application note describes
the key measurements required to
ensure network synchronization is
maintained and discusses the
reasons for performing them.

Checking the timing distribution
and synchronization of SDH networks

Introduction
Failure to efficiently distribute
timing information around an SDH
network contributes to theintroduction of jitter and wander.
This inevitably undermines the
network’s synchronization
performance.
There are two key elements to
precise and reliable SDH network
synchronization. The first is an
accurate primary reference clock
(PRC); the second is ensuring that
timing information from the PRC
is routed accurately to all nodes in
the network.
Timing information in an SDH
network istypically distributed
around the network from the PRC
via a network reference chain (see
figure 1). The chain comprises

transit or local nodes with chains
of NEs connected between them.
Each clock throughout the chain
derives its timing information
from the optical carrier.
Checking that the PRC provides
the elements in the chain with
precise timing information
requires three separatetests:
q

Line frequency measurents to
identify the accuracy of an NEs
timing source.

q

Monitoring of pointer activity to
provide an early indication of
deterioration of timing
integrity.

q

Decoding of the synchronization status byte (S1 byte) to
help pinpoint breaks in the
timing chain.

A reading of 2.48832 Gb/s
± 4.6 ppm indicates that the NE is
within the timingconstraints that
the network requires. A reading of
>4.6 ppm could, however, indicate
a break in the timing chain from
the PRC. Decoding the synchronization status byte (the S1 byte in
the path overhead section of the
SDH signal) at the point of
frequency drift will provide more
insight into the reason for the drift
in accuracy.
Pointer analysis

PRC
G.811

Line frequency measurementsMeasuring the SDH line rate from
an NE provides an indication of
how accurate its timing source is.
A large frequency offset from the
nominal line frequency indicates
that the NE has lost tracability to
the PRC.

NE clock

NE clock

Slave
G.811
transit

Measuring the SDH line frequency
requires that an SDH test set is
connected to a suitable monitor
point in the network. If nomonitor
point is available, an optical
splitter must be used.

NE clock

NE clock

Slave
G.811
transit or
local

NE clock
NE clock

NE clock

Figure 1: SDH
synchronization
network reference
chain. Maximum
number of PRC
slaves allowed is
10. Maximum
number of NE
clocks between
slaves is 20 with
the total number in
the network limited
to 60.

To display the STM-16 linefrequency and line offset of the
NE’s signal using the HP 37778A
STM-16/OC-48 test set, select
'results' and 'frequency
measurement' on the results
screen shown above.

2

Although simple line frequency
checks are useful for identifying
timing problems, not all timing
problems are visible at the line
rate. For example, an STM-16
signal running at the nominal line
rate does not...