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Synchronization of Digital
Telecommunication Networks
Harald Brandt and Aleksander Marlevi
Precise time control of a digital telecommunication network is the basis for
disturbance-free operation. Insufficient control can result in an unacceptable
high rate of slips, which impairs the transmission quality.
The authors describe how these slips arise and how they can be reduced with the
aid of buffers and very accurate or automatically controlled clocks. Different
methods for network synchronization and the application of these in a telephone
network are also discussed.
and occur when the incoming bit rate is
higher or lower than the exchange clock
rate.
Slips are counted per channel as slip of
an 8-bit time slot.
The number of slips per day in the traffic
between two digital exchanges is proportional
to the frequency difference of
the local clocks. For example, if two
atomic clocks with a frequency inaccuracy
of 10 11 are used, a slip occurs
every 72 days With two crystal controlled
clocks having a frequency inaccuracy
of 10 6 the slip rate is one per
minute. Additional slips can also occur
because of jitter on the transmission
links.
UDC 621.395.74:681.116
Fig. 1
The path of a message through the digital network
To/from the longdistance
network
The need for network
synchronization
How slips occur
Following a message through a digital
telecommunication network makes it
easier to understand the need for network
synchronization. The description
is based on fig. 1, which shows an idealized
network. Digital exchanges, here
considered as the sources of information,
inject messages into the system in
the form of evenly spaced electric
pulses, bits. Each message is sent
through the transmission system and is
occasionally switched to a new route in
order to reach the called subscriber. In
each exchange bits are received with
their own incoming bit rate and stored in
a buffer. The buffers can only store a
limited number of bits, for example 8 or
256 bits, corresponding to a time slot
and a frame respectively. The stored bits
are switched at a frequency determined
by the exchange clock.
When the incoming bit rate is higher
than the clock rate there is no time to
send on the stored bits before new bits
arrive. When the incoming bit rate is
lower than the clock rate the stored bits
will instead be sent twice before new
bits arrive. Thus, it is obvious that the
incoming bit rate and the exchange
clock rate must havethesame long-term
mean value, otherwise the transmission
through the exchange becomes distorted.
The distortions are called slips
Automatic clock control can be used to
avoid slips caused by frequency differences.
Slips caused by transmission
links can be avoided with the aid of buffers.
The effect of slips
The effect of a slip can be illustrated by
the simile of a commuter who relies entirely
on his own watch. Unfortunately
his watch is slow compared with the railway
station clock, and the commuter
therefore misses his train. By synchronizing
clocks it is possible to communicate
without losing essential information.
Slips have different effects in different
telecommunication services, such as
speech, data and facsimile. One reason
forthis isthedifference in coding redundancy.
The higher the redundancy the
smaller the effect.
Facsimile constitutes a good example.
In this field the amount of disturbance
caused by a slip is greatly dependent on
the encoding technique used. A slip can
result in a displacement or in black or
white streaks in the line being scanned.
At worst a single slip can ruin the whole
picture and make retransmission necessary.
Fig. 2 shows the effect a slip can
have on a certain type of facsimile transmission.
Different types of clocks
Two types of clocks are used in telecommunication:
cesium and quartz crystal
clocks.