User's Manual

Chapter 4 - Operation and Administration
BreezeACCESSS VL System Manual
4-82
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.
The decisions made by the Adaptive Modulation Algorithm relate to the
modulation level selected for transmission of new frames (first trial
attempt) as well as the modulation level for retransmissions. The
decisions are based on multiple parameters, including information on
received signal quality (SNR) that is received periodically from the
destination unit, the time that has passed since last transmission to the
relevant unit, and the recent history of successful and unsuccessful
transmissions/retransmissions. In the AU the decision algorithm is
performed separately for each SU.
The transmission/retransmission mechanism operates as follows:
A. Each new frame (first transmission attempt) will be transmitted at a
modulation level selected by the Adaptive Modulation algorithm.
B. If first transmission trial has failed, the frame will be retransmitted
at the same modulation level up to the maximum number of
retransmission attempts defined by the Number of HW Retries
parameter.
C. If the frame was not acknowledged it frame will be moved to the end
of the queue. All other frames in the queue that are intended to the
same destination will be moved as well, to preserve the original
order. This process is referred to as a Software Retry.
D. The frame will be retransmitted at the modulation level selected by
the algorithm for the new retrials sequence. Each retrial sequence
comprises a number of attempts defined by the Number of HW
Retries.
E. The process will be repeated up to a maximum number of Software
Retries defined by the Number of SW Retries parameter. The
Number of SW Retries parameter is not configurable and it is set to
the default of 3 SW Retries. (In units upgraded to SW version 2.0
from a lower version it will be set to the previously configured
value)..
NOTE
When Burst Mode Option is enabled, the SW Retry feature of the Adaptive Modulation
Algorithm is disabled. Otherwise it is enabled.
The Adaptive Modulation Parameters menu includes the following
parameters: