User manual

Chapter 2 - Managing a Single Device/Cell Air Interface General Page
AlvariCRAFT Device Manager 37 User Manual
microseconds per km/5 microseconds per mile.). The higher the distance from
the AU of the SU served by it, the higher the ACK timeout should be.
The ACK timeout in microseconds is: 20+Distance (km)*2*3.3 or 20+Distance
(miles)*2*5.
2 To ensure fairness in the contention back-off algorithm between SUs located at
different distances from the AU, the size of the time slot should also take into
account the one-way propagation delay. The size of the time slot of all units in
the cell should be proportional to the distance from the AU of the farthest SU
served by it.
The Cell Distance Option in an AU defines the method of computing distances.
When set to Manual, the Max Cell Distance parameter should be configured with
the estimated distance of the farthest SU served by the AU. When set to
Automatic, the AU uses a special algorithm to estimate its distance from each of
the SUs it serves, determine which SU is located the farthest and use the
estimated distance of the farthest SU as the maximum cell/link distance. The
value of the maximum cell distance parameter (either computed or configured
manually) is transmitted in the beacon messages to all SUs served by the AU, and
is used by all units to calculate the size of the time slot, that must be the same for
all units in the same sector. When the Per SU Distance Learning option is
enabled, the AU uses the re-association message to send to each SU its estimated
distance from the AU. The per-SU distance is used to calculate the ACK timeout to
be used by the SU. When the Per SU Distance Learning option is disabled (or if it
cannot be used because the SU uses a SW version that does not support this
feature), the SU will use the maximum cell distance to calculate the ACK timeout.
The AU always uses the maximum cell distance to calculate the ACK timeout.
It should be noted that if the size of the time slot used by all units is adapted to
the distance of the farthest unit, then no unit will have an advantage when
competing for services. However, this reduces the overall achievable throughput of
the cell. In certain situations, the operator may decide to improve the overall
throughput by reducing the slot size below the value required for full fairness.
This means that when there is competition for bandwidth, the back-off algorithm
will give an advantage to SUs that are located closer to the AU
The Cell Distance parameters includes the following:
Cell Distance Option Applicable only for AU. The Cell Distance Option defines whether the
maximum distance of the AU from any of the SUs it serves will be
determined manually (using the Maximum Cell Distance parameter)
or automatically.
The Options are Automatic or Manual.