Contents Telektronikk - Telenor

A B
tion which returns the call to the previous
exchange A. Exchange A can then continue
the routing on the next alternative
in the routing description, e.g. the other
load sharing alternative A-D in the
example of Figure 2. According to our
specification a call shall not be rerouted
more than once in order to reduce the
possibility of overloading signalling system
No. 7 in failure situations. Similarly,
the rerouting function can be turned off
manually.
5.4 Circuit reservation
Circuit reservation is a method of giving
priority to one ore more traffic types
when there is a lack of traffic capacity, or
to be more correct: The method determines
which traffic type to be congested.
The simplest one-level circuit reservation
with reservation level R makes all circuits
on a circuit group available for one
traffic type, while another traffic type is
blocked if the number of free circuits is
less or equal to R. The simplest functional
implementation of circuit reservation
is to have a free-circuit counter per circuit
group. The counter is counted down
when a circuit is seized and counted up
when a circuit is released.
The circuit reservation function may be
used to
- give preference to priority traffic, e.g.
emergency calls or calls from priority
subscribers
- give preference to one direction of a
bi-directional circuit group
- equalise the grade of service or give
downwards preference on a load sharing
group
- accept DAR calls only when there is
spare capacity to avoid the grade of
service for normal calls to be severely
reduced.
Some characteristics of the one-level circuit
reservation function are described in
C
D
Figure 2 Example of rerouting
chapter 7 through calculations and simulations.
The function being implemented
in the Norwegian network is similar
to the two-threshold selective circuit
reservation described in
CCITT E.412. However, there
are 4 traffic types: PRI (priority
traffic), DR (direct routed traffic),
AR (alternative routed traffic) and DAR
(dynamic alternative routed traffic). The
two-level circuit reservation may in most
practical situations be considered as a
combination of two one-level circuit
reservation systems. As a result it can be
used to combine the abilities of protecting
priority traffic, giving preference to
down going traffic and rejecting DAR
traffic when necessary.
6 Description of the
target network
There will be mainly 4 types of exchanges
(the expected number of each
type in brackets):
- International Exchanges, IE (2)
- Trunk Exchanges, TE (14)
- Local Tandem exchanges, LT (~ 50)
- End Offices, EO (~ 170).
For describing the network some definitions
are useful:
- An EO-area is defined as the area covered
by an end office including connected
concentrators and small
exchanges (analogue or digital).
- An LT-region is made of the EO-areas
which use the same LTs for transiting
traffic between themselves.
TE-region
LT-region 1 LT-region 2
TE LT
EO
- A TE-region is defined as the area
where the EO-areas are connected to
the same TEs.
EOs and LTs will be subscriber exchanges,
except in Oslo where the LTs
will carry only transit traffic. The network
for national traffic will in principle
consist of two two-level networks. For
simplicity we will here call the two networks
the LT-network and the TE-network.
The two networks are dimensioned
separately and built up hierarchically.
The LT-network carries traffic within one
LT-region. The TE-network carries traffic
between the LT-regions within one
TE-region, traffic to/from other TEregions
and traffic to/from the international
network.
Figure 3 shows an example of the two
two-level networks within one TEregion.
In the example the LT-network
consists of two LT-regions.
Each EO within an LT-region is connected
to the same LTs (two or in a few cases
three) with circuit groups in load sharing.
This type of connection is often called
double (or triple) homing. In Figure 3 the
thick lines from the LTs symbolise the
collection of circuit groups to the EOs.
In a similar way all EOs and LTs in a
TE-area will be connected to the same
pair (exceptionally triple) of TEs. These
exchanges make up one TE-region. Note
that the LT only functions as a normal
EO (subscriber exchange) towards the
TE-network. The circuit groups from an
EO to its superior TEs will in load sharing
carry all traffic to destinations outside
own LT-region. All traffic within the
Figure 3 An example showing the TE and LT networks within one TE-region
TEnetwork
LTnetwork
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