Specifications

Chapter 4 - Operation and Administration Menus and Parameters
BreezeACCESS 4900 125 System Manual
the AU and the SU (the one-way propagation delay at 5 GHz is 3.3
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.
 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 Mode parameter in the AU defines the method of computing
distances. When set to Manual, the Maximum 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 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 previous 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
(using the Fairness Factor parameter). 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 menu includes the following parameters: