User manual

Chapter 2 - Managing a Single Device/Cell Performance Parameters Page
AlvariCRAFT Device Manager 150 User Manual
2.23.9 AIFS (AU and SU Only)
The time interval between two consecutive transmissions of frames is called
Inter-Frame Spacing (IFS). This is the time during which the unit determines
whether the medium is idle using the carrier sense mechanism. The IFS depends
on the type of the next frame to be transmitted, as follows:
 SIFS (Short Inter-Frame Spacing) is used for certain frames that should be
transmitted immediately, such as ACK and CTS frames. The value of SIFS is
16 microseconds.
 DIFS (Distributed coordination function Inter-Frame Spacing) is typically used
for other frame types when the medium is free. If the unit decides that the
medium is not free, it will defer transmission by DIFS plus a number of time
slots as determined by the Contention Window back-off algorithm (see
Section 2.23.1) after reaching a decision that the medium has become free.
DIFS equal SIFS plus AIFS, where AIFS can be configured to a value from 1 to 50
time slots. A unit with a lower AIFS has an advantage over units with a higher
AIFS, since it has a better chance to gain access to limited wireless link resources.
Typically, AIFS should be configured to two time slots. A value of 1 should only be
used in one of the two units in a point-to-point link, where in the other unit the
AIFS remains configured to two time slots. This ensures that the unit with AIFS
configured to one has an advantage over the other unit, provided that the
Minimum Contention Window (See Section 2.23.1) parameter in both units is
configured to 0 to disable the contention window back-off algorithm.
The AIFS parameter is not applicable when the Wireless Link Prioritization Option
is enabled.
The available options are 1to 50 (time slots).
2.23.10 Adaptive Modulation Parameters
The Adaptive Modulation Algorithm enables adapting the modulation level of
transmitted data to the prevailing conditions of the applicable radio link. The
algorithm provides Access Units with simultaneous, adaptive support for multiple
Subscriber Units at different modulation levels, as transmission's modulation
level decisions are made separately for each associated SU.
Link quality fluctuates due to various environmental conditions. Dynamically
switching between the possible modulation levels increases the probability of
using the maximum modulation level suitable for the current radio link quality at
any given moment.