Contents Telektronikk - Telenor

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change the routing for the next call.
The DAR-pointer is not moved.
- If the call attempt is blocked on the
second leg of the DAR-alternative,
(assuming E is the DAR-alternative,
both EB and EC is blocked for DARtraffic),
the call attempt will be lost. A
signal is returned to A, stepping the
pointer in the DAR list to the next
alternative, F. This alternative will be
the DAR-alternative for the next call
A-X.
DAR is a simple learning algorithm. It
was first described by British Telecom.
The principle is also used in a method to
be implemented by NTT, Japan in 1994.
Circuit reservation is mandatory for DAR
to obtain network stability and protection
of the direct routed traffic. In a simulation
study carried out by Norwegian
Telecom Research in 1988 [3] our top
level mesh network was simulated with
different routing strategies under different
load conditions like normal traffic
load, skew load, focused overload and
general overload. The main conclusions
were:
- The method gives a flexible network
with good traffic flow also when there
is considerable difference between
traffic actual offered and dimensioning
traffic/network resources. The network
will be robust towards traffic variations
and wrong dimensioning.
- The method distributes the congestion
justly.
- The network throughput will not
degenerate, but is robust towards general
overload. In this case the routing
converges towards direct routing.
- The method achieves most of the possible
benefits. Compared to more complex
adaptive methods, there are no
significant differences.
- The method is simple and does not
require major data collection from
other parts of the network. The additional
processor load will be small.
- The dimensioning of the network will
not be complicated. The network may
be dimensioned as for direct routing.
In [4] G.R. Ash from AT&T presents a
comparison between 4 routing strategies
- Learning with random routing (DAR)
- Real time state dependent routing
(RSDR – least loaded via route selection
based on real-time network status)
- Periodic state dependent routing (least
loaded via route selection based on
periodic network status)
- Sequential routing with flow optimisation
(time-variable engineered routes
plus overflow routes updated in realtime
(Example DNHR – Dynamic
Non-hierarchical Routing).
A large (103 node) and a small (10 node)
mesh network were simulated with high
day load, global overload, focused overload
and cable failure. The basic conclusions
were that
- real-time state dependent routing
(RSDR) significantly outperforms the
other strategies in large networks
- DAR and RSDR performs comparably
in small networks
- all dynamic routing strategies outperform
current hierarchical routing by a
large margin
- the cost/complexity of DAR may be
lower than the other strategies.
As the Norwegian network is a small network
these conclusions from 1994 are
consistent with our simulation study in
1988.
9 Dimensioning criterion
Telenor Nett distinguishes between
- Normal load dimensioning utilising
criterion for normal traffic load:
⋅ Congestion dimensioning, maximum
congestion at normal traffic load
(An ), e.g. 0.5 or 1 %
⋅ Standard dimensioning, in addition
to criteria for congestion dimensioning,
a criterion for maximum utilisation
(erlang/circuit), U, has to be fulfilled
(normally U = 0.8). The objective
is to make large circuit groups
more robust towards overload.
- Fault tolerant dimensioning (or reliability
dimensioning) utilises criteria
both for normal load and failure situation.
For failure situations the circuit
group is dimensioned to fulfil a
defined congestion level at dimensioning
traffic during failure, Af .
Af = An * Co * Cr where
Co = Overload factor
= traffic offered the circuit group
during failure / normal traffic
load on the same circuit group
Generally Co = 2.0 for dual homing,
1.5 for triple homing. Co has to be estimated
individually for each circuit
group in the top level mesh network
Cr = Traffic reduction factor
= Dimensioning traffic during failure
/ Normal traffic load
Some measurements have indicated that
in 94 % of the total number of hours in a
year, the traffic offered will be less than
80 % of the ITU-T normal traffic load.
We have decided to define the network
as unavailable when the call blocking is
higher than 20 %. As a first attempt our
dimensioning criteria were set to ensure
that the call blocking in the trunk network
should be less than 20 % (B =
20 %) during a failure situation if the
traffic offered does not exceed 80 % (Cr = 0.8) of the normal traffic load. The
modular dimensioning of circuit groups
in addition to yearly network expansion,
will, however, give some extra capacity
to most circuit groups.
When determining criteria for fault tolerant
dimensioning and in which parts of
the network the method should be used,
the models in ITU-T Rec. E.862, Dependability
planning of telecommunications
networks, have been very useful.
The use of high usage circuit groups will
be rather limited, mainly restricted to circuit
groups between large EOs in the
same LT-area with high mutual traffic
interest.
There are no plans to change the dimensioning
criteria when circuit reservation
is introduced as long as only a small proportion
of the traffic is given priority.
10 Optimal combination of
robustness methods
In the conventional PSTN network few
systematic methods have been used to
obtain a certain level of availability.
Automatic restoration in the transmission
level is, however, common for the main
long distance systems. Also alternative
routing in PSTN and occasional use of
diversity routing increase the availability.
In the target network the logical and
physical (SDH – Synchronous Digital
Hierarchy) network structures fit well
together. An example of a SDH-ring in
an LT-region is shown in Figure 10. In
the example the exchanges (EOs and
LTs) are connected to the transmission
ring by Add and Drop Multiplexors
(ADM). Systematic use of double hom-