User manual

Chapter 2 - Managing a Single Device/Cell Performance Parameters Page
AlvariCRAFT Device Manager 152 User Manual
2.23.11 Statistics-Based Rate Control
When enabled, the Statistics-Based Rate Control algorithm, constantly evaluates
the achievable throughput for a particular modulation by counting the number of
packets that are successfully transmitted and the packets that are not received
and that need to be retransmitted. Based on these statistics, and on each
modulation’s specific data rate, it will calculate the real throughput that the unit
can support in the current conditions for a particular modulation.
When choosing between modulations, up to a limit, a small number of
retransmissions on a higher modulation is compensated by the overall better
performance ensured by that modulation. When a critical retransmissions
percentage is reached however, it is necessary to decrease the modulation to
achieve better throughputs as shown in Table 2-4.
When the above mentioned retransmission percentages are reached, the
Statistics-Based Rate Control algorithm will lower the modulation.
If the connection is stable on a particular modulation, the unit will periodically
check whether it can further increase the modulation (except on modulation 8), by
sending a number of test packets using this higher modulation and checking the
retransmission rate.
NOTE
Statistics-Based Rate Control is only supported in units with HW Revision C or higher.
Table 2-4: Retransmission Percentage Equivalence
Modulation
Level
PHY Rate
(Mbps)
PHY Rate Difference Compared
to Previous Modulation (Mbps)
Retransmission Percentage
Equivalent to PHY Rate Difference
8 54 54-48=6 6/54=11%
7 48 48-36=12 12/48=25%
6 36 36-24=12 12/36=33%
5 24 24-18=6 6/24=25%
4 18 18-12=6 6/18=33%
3 12 12-9=3 3/12=25%
2 9 9-6=3 3/9=33%
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