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48
Fig. 6
Master-slave network in Riyadh, Saudi-Arabia
Digital exchange
Analog exchange
Point for measuring the slip rate
Intermediate repeaters
Field experience of the
master-slave method
Experience of network synchronization
was gained when the first digital public
telephone network was taken into service
in the Kingdom of Saudi-Arabia on
December 13, 1978. The network consists
of three interconnected AXE 10 exchanges,
one transit exchange (Riyadh)
and two local exchanges (Al Malaz and
Al Ulaya). The local exchanges are
slaved to the transit exchange, see fig. 6.
After six months of stable operation, the
network synchronization was evaluated
by measurements on site. These showed
that all objectives had been fulfilled. For
example, the slip rate was recorded during
twelve days. The number of slips per
20 hours and channel was:
- between Riyadh and Al Malaz: 0
- between Riaydh and Al Ulaya: 4-10 3
- between Al Malaz and Al Ulaya: 7-10" 3
The allowable slip rate was one slip per
20 hours on a 6 kbit/s channel between
any pair of exchanges. Thus the recorded
slip rates were very much lower than
specified.
Since 1978 master-slave networks with
AXE 10 have been taken into service in
nine countries.
Analysis of mutual singleended
control
The mutual network is a linear and dynamically
simple system, but also a
large and unknown one, in which the
number of control and information
channels are restricted. The basic problem
is how to achieve stable linear control
of large systems, comprising a set of
buffer storages, which are continuously
tapped and replenished through a network
of channels. The problem therefore
belongs to the field of decentralized
control. A fundamental and well known
difficulty of decentralized control is that
the several autonomous control devices,
even if individually well tuned,
may each counteract or amplify the consequences
of the actions of the others.
This is because of lack of information
about the state of the whole system, and
this can cause instability.
This difficulty is aggravated by the many
unknown elements in the system. For
example, the network may be extended
by the addition of nodes or links, regulators
may fail temporarily etc. Consequently
all that has been aimed for is
to find the simplest local regulator that
can handle the disturbances and retain
network stability in the worst possible
case of network structure and regulator
breakdown. In order to find the most
suitable regulator, mutual control has
been investigated in two ways: by means
of theoretical analysis and by computer
simulations 8 .
Theoretical analysis of mutual control
The theoretical analysis provided the
answers to the following questions:
1. Is it possible to use mutual synchronization
in a network of oscillators
which are subject to disturbances
such as phase and frequency shifts,
linear frequency drift or jitter?
2. What would be the most suitable regulator
algorithm?
Theoretical analysis of mutual control
by time-discrete and integrating regulation
has been carried out by T. Bohlin 5 .
He proved that stable clock regulation is
feasible even in the event of unpredicted
changes in the network configuration.
The regulator algorithm found to give
the best result is used in the PI regulators.
They can handle clock frequency
differences in such a way that no permanent
phase errors occur.
An important prerequisite for the use of
a PI regulator, or any other regulator
having integrators, is that one of the oscillators
in the network must be unregulated.
Otherwise, if all oscillators are frequency
regulated and the sum of all
phase differences in the network is not
zero, the system frequency will drift.
With PI regulators the drift would be linear
It should be noted that while the
system frequency drifts there are no frequency
differences between oscillators
and therefore no slips occur
The mutual single-ended control, with
integrating regulators and a sink, is a
very stable method since it synchronizes
both the phase and the frequency
of a network without causing steadystate
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