Contents Telektronikk - Telenor

ing will to some extent ensure a level of
grade of service also during failure situations.
The ring structure of SDH makes it
possible to choose between diversity
routing in the ring (50 % redundancy)
and path protection (100 % redundancy),
or a combination giving a resulting transmission
capacity K in a failure situation.
The grade of service during a failure situation
is determined by the dimensioning
criteria, the profile of the traffic stream
and the transmission capacity K. Failures
in transit exchanges will be taken care of.
However, EOs and ADMs connecting the
EO to the SDH ring, will have no replacement.
Diversity routing may be used either
within each circuit group or by routing
the two circuit groups from an EO to the
LTs opposite directions in the ring. However,
the properties will not be the same
for all traffic cases. With diversity routing
of each circuit group, 50 % of the
capacity will be kept on all circuit groups
independent of where the fibre cable is
cut. If the two circuit groups from an EO
to the LTs are routed different ways in
the ring and there is a cable fault between
EOA and EOB in Figure 10, we have the
following situation: The circuit groups
EOA - LT1 and EOB - LT2 are not affected
by the failure, while EOA - LT2 and
EOB - LT1 are both broken. Therefore,
both traffic routes between EOA and EOB are broken. If path protection is not fully
implemented (or another type of restoration),
diversity routing of each circuit
group should be recommended.
The optimal choice of robustness level in
the physical and logical network is not
obvious. In a fault tolerant network an
increase in the dimensioning criteria for
circuit groups will improve the robustness
both against failures in transmission
systems and transit exchanges. However,
in the LT network, exchange capacity is
normally much more expensive than
marginal transmission capacity. Therefore,
a combination of fault tolerant
dimensioning of the circuit groups and
path protection of the transmission system
may be optimal depending upon the
importance of the traffic, the unavailability
of the exchange and transmission systems
and the actual traffic profile.
Some preliminary investigations utilising
the principles of dependability planning
of ITU-T Rec. E. 862, indicate that the
differences between the traffic profiles of
business and residential exchanges (see
Figure 11) are important. The residential
exchange has a 2-hour traffic peak from
20 to 22 in the evening, while the business
exchange has a traffic peak lasting
6–7 hours during the day. As the failure
rate is highest during normal working
hours and Rec. E. 862 puts more emphasis
on business traffic, the following preliminary
conclusions are not surprising:
- The traffic profile for business
exchanges may imply that corresponding
circuit groups should be dimensioned
with higher capacity (fault tolerant
dimensioning). With today’s
dimensioning level for these circuit
groups path protection in SDHrings
seems advisable.
- The short evening peak for
residential exchanges implies
that the proposed dimensioning is
sufficient. In many cases no extra
path protection of transmission
systems is needed in addition to
diversity routing.
- Path protection of 60 % of
the transmission capacity
in addition to 50 %
capacity due to diversity
routing would result in a EOA transmission capacity during
failure of K = 0.8. This would
be sufficient to cover nearly all single
transmission failures with reasonable
grade of service. Even 40 % path protection
(K = 0.7) will give very good
robustness. However, the administration
costs may favour a solution with
full protection or in some cases no path
protection at all.
Traffic/Busy/Hour Traffic
1
0.8
0.6
0.4
0.2
0
00-01
Business
Residential
09-10
11 Concluding remarks
The PSTN/ISDN target network of
Telenor is now under implementation.
The network structure with load sharing
and dual/triple homing and fault tolerant
dimensioning will be completed by the
end of 1995. However, not all subscriber
exchanges will at that time have two
complete independent transmission routes
for the circuit groups to the local tandems
and trunk exchanges.
ADM ADM
ADM ADM
15-16
ADM
20-21
23-24
Time of day
Figure 11 Typical traffic profiles for exchanges with mainly residential or business
subscribers
LT 1
LT 2
EO C
EO B
Figure 10 An example of a SDH-ring
connecting the exchanges within a LTregion
101