WaveRider LMS4000 User Manual page 148

11 Monitoring the Network
Similarly, the percentage of payloads not delivered on the first transmission, but delivered on
the second transmission
= 1,761 / (68,235 - 66,001) = 78%
It is generally a good indication if most payloads that fail on the first try are then successful
with only one retry.
The percentage of payloads that are not able to be delivered
= 102 / 68,680 = 0.15%
A very low undeliverable payload rate implies that user service has a high level of integrity,
and that the radio link is not significantly impacting higher-level TCP/IP applications.
The impact of the retransmissions can be calculated by looking at the total number of
transmissions requiring acknowledgments:
= 1xD + 2xE + 3xF + 4xG + 4xH = 71,138.
Adding to this value the non-acknowledged broadcast payloads (txPayloadsBCast = 445)
results in total txPayloads - 71,583.
A simple metric of overall sector link quality is the effective utilization of the channel, which can
be readily calculated as desired payloads transmitted/actual payloads transmitted, or:
(Tx Data Payloads + Tx Ctrl Payloads - txPayloadsBCast - txPayloadsFailAssocDeleted) /
(TxPayloads - txPayloadsBCast)
= (67,790 + 901 - 445 - 11) / (71,583 - 445) = 68,235 / 71,138 = 96%
which suggests that 4% of the radio traffic is used to retransmit packets, which is referred to in
this document as the Retransmission Rate.
From an operational point of view, it is important to keep the number of retransmissions to a
minimum since they reduce the total air time available and the total network throughput.
Although these calculations can appear tedious since all of the referenced statistics are
available through MIBs, SNMP management tools, such as SNMPc can directly collect the
statistics, calculate the above metric, and track and report its value over time.
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