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procedure is followed for subscribers on
the lower.
3.3.1 Normal configurations
Traffic limits for the normal trunk configurations,
which are the two link and
four link configurations, are as follows:
Two DTRH PBAs: 43.0 E
Two DTRF PBAs: 82.5 E
In the case of two DTRH PBAs the MD
links are the bottleneck, i.e. Erlang’s formula
for 58 channels is used. In the case
of two DTRF PBAs the direct and crossover
cluster link in the IRIM are the
bottleneck, i.e. Erlang’s formula for 56
channels multiplied by two is used.
These limits are based on the use of the
same trunk PBAs in the IRSUs in the
MD, i.e. that all IRSUs have 2 DTRHs or
2 DTRFs, respectively. If several IRSUs
with DTRHs are connected to a DTRF in
the IRIM, the DTRH PBAs should be
distributed equally on the two MD links
for capacity reasons.
Furthermore, it is assumed that each
IRSU has subscribers on both the upper
and lower side of the system, i.e. at least
two subscriber groups per IRSU. In an
MD configuration, the total traffic from
all IRSUs has to be within the above
given limits.
3.3.2 Special configurations
If the traffic from the IRSUs is much
smaller than 43.0 Erlang or a little more
than 43.0 Erlang, some special configurations
may be considered. These configurations
are normally not recommended
due to reliability reasons or traffic reasons
(because of an asymmetric configuration).
3.3.2.1 One trunk PBA configurations
The economical one trunk PBA configuration
can be used if the traffic is low and
the reliability aspect is not considered
that important.
Figure 4 shows a one trunk PBA configuration
with DTRF PBA in the IRIM and
the IRSUs. The IRSUs have subscribers
on both upper and lower side.
Traffic limits for the one PBA configurations
are as follows:
One DTRH PBA: 18.2 E
One DTRF PBA: 34.8 E
In the case of DTRH PBA the MD link is
the bottleneck, i.e. Erlang’s formula for
29 channels is used. Very unbalanced
traffic is well tolerated. For example there
can be 17.0 Erlang from subscribers on
the upper side and 1.2 Erlang from subscribers
on the lower side and vice versa.
In the case of DTRF PBA the bottleneck
is the direct and cross-over cluster link in
the IRIM, i.e. Erlang’s formula for 28
channels multiplied by two is used. It
must here be assumed that the traffic on
the upper and lower is well balanced.
If the trunk PBA in the IRIM or the trunk
PBA in the IRSU fails, the connection
between the IRIM and the IRSU will be
disrupted, whereas for a normal configuration
the traffic will use the alternative
cross-over link when the direct link is
disrupted.
3.3.3 Asymmetric configuration
This configuration has one DTRF PBA
and one DTRH PBA in the IRIM and in
the IRSU as well, i.e. three MD links.
Figure 5 shows an asymmetric configuration.
The asymmetric configuration may seem
a good solution for traffic needs between
the traffic capacity for a configuration
with two DTRHs and two DTRFs in the
IRIM. But it should only be used with the
alternative channel allocation algorithm
which chooses the MD link pair with
most free channels.
Without this alternative channel allocation
algorithm the capacity can be improved
by placing more subscribers on
the side (upper or lower) where the
DTRH is placed, which at first may seem
surprising. When the traffic on the
DTRH side is twice the traffic on the
DTRF side the traffic capacity is 61
Figure 4 Figure 5 Figure 6
Erlang. In this case the high traffic from
the DTRH will be blocked on the cluster
link, but different from the opposite traffic
situation the overflow traffic together
with the direct traffic will now have two
MD links available on the DTRF side.
There will be very little overflow traffic
from the DTRF side to the DTRH side.
The point is to distribute the traffic so
that the cluster links carry equal traffic as
much as possible.
If the alternative channel allocation algorithm
is used the performance of the balanced
upper and lower traffic case is
improved. This algorithm has the effect
of distributing the traffic more evenly
and thus reducing the more bursty overflow
traffic. With balanced traffic on the
upper and lower side the traffic capacity
is 66.3 Erlang and is dimensioned according
to Erlangs formulae with 87
channels minus PS channels. So, if this
alternative channel allocation algorithm
is used the asymmetric configuration
may be considered.
3.4 Mini IRSU
The mini IRSU is an inexpensive, simple
IRSU. The mini IRSU is special in two
respects. It contains only one subscriber
group and it has only one trunk PBA
(DTRH or DTRF) which means it has no
cross-over link. This means that a mini
IRSU subscriber is a ‘worst case’ subscriber
when the mini IRSU is in an MD
with other non mini IRSUs. To fulfil the
GOS requirement for all subscribers in
an MD it is necessary to decrease the
total traffic if the mini IRSU traffic increases.
The traffic capacity for the mini
IRSU is 17.4 Erlang. The bottleneck is
the cluster link, hence Erlang’s formula
with 28 channels is used. However, the
bottleneck is quickly changed to the MD
link with 29 channels when other non
mini IRSUs are connected to the same
MD and there are DTRHs in the IRIM. If
there are DTRFs in the IRIM the bottleneck
is again the cluster link.
Figure 6 shows a configuration with
DTRH PBAs in the IRIM and the IRSUs
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