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18
QOS AND OVERPROVISIONING
If the traffic coming to the IDU is very bursty and time-variable, so called "overprovisioning" (or
"overbooking") of the radio is a possible method for cost savings. Due to the statistical variation and the
fact that the traffic peaks seldom occur simultaneously. The combined traffic has a peak value less than the
sum of the peak values of contributing interfaces.
The random nature of real traffic will sometimes cause the radio channel to be overloaded. This will happen
more often when unfavourable weather conditions force the use of lower modulation formats (when
adaptive modulation, AMR, is used). Overprovisioning must take into account overloading conditions and
the priority of frames must be considered. Obviously less important traffic and non-realtime traffic should
be dropped first.
iPasolink can use statistical multiplexing very effectively because it understands the incoming frame
priority, it may shape the traffic and there is a queuing mechanism to the radio path.
The operator should design the radio capacity based on the traffic statistics and SLA requirements and
define the QoS parameters required in the radio.
ADAPTIVE MODULATION
As was the case already with the previous generation PASOLINK NEO HP AMR, iPasolink may use adaptive
modulation (AMR) which improves the reliability of high priority traffic or alternatively increases the
available capacity for lower priority traffic during majority of time. AMR is especially important when using
high modulation formats with lower sensitivity and lower fade margin resulting in higher equipment costs
such as larger antenna. With AMR different traffic classes may have a different fade margin and availability.
Figure 9. Adaptive modulation.
An example: the most cost-efficient solution could be that a nominally 366 Mbit/s hop is designed for
99,9993% availability for 16QAM 183 Mbit/s for "Business Critical and Real Time" traffic. For Best Effort
traffic, the full 366 Mbit/s 256QAM availability could be 99,993% of time. In this manner, the last 25% of
traffic (e.g. Real Time) would have practically 100% availability (91 Mbit/s QPSK). This kind of availability
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