Quality of Service Contention Window Control:
The Quality of Service Contention Window Control tunes the aggressiveness of CSMA backoffs when
collisions occur. The MAC takes random backoffs in the range [0, 2^cw_min]. Every time there is a
collision/noise it will increase this cw_min by 1, until it is capped by cw_max.
E.g. 4,10 translates to random backoffs in the range [0,16] in the beginning for a packet. If the first try
results in a collision, it will pick another backoff in the range [0,32], then [0,64], until [0,1024]. After
successful transmission, backoff is reset to [0,16]. The default is 4,10 for low priority, and 3,6 for high
priority. For larger networks, it is recommended to increase the Low Priority minimum to reduce the
chance of collisions occurring.
End-to-End Automatic Repeat request (Beta Feature – License Enabled)
The End-to-End ARQ feature provides packet re-ordering capability to the radio. This feature is useful in
applications that are sensitive to out of order packets (i.e. video applications where the decoder does not
have the ability to re-order packets).
To enable packet reordering, specify the port number of the subject data in the same format as the QoS
ports on the source and destination radio. The settings should match on both radios and do not need to
be set on any relays.
Retransmissions Disabled – Only packet re-ordering with no end-to-end retransmissions if packets are
lost. The receiver timeout is the length of time the receiver waits for out-of-order packets before giving
up and delivering the data it has in its buffer. This is similar in concept to the jitter buffer in common video
decoders (e.g. VLC). The worst-case end-to-end delay will be incremented by the receiver timeout value.
Retransmissions Enabled – Packet re-ordering and end-to-end retransmissions enabled in case of packet
loss. The retransmission timeout is the time the transmitter will wait before re-sending a lost packet. The
worst-case end-to-end delay will be incremented by the retransmission timeout value.
Determining Timeout Value – Both receiver timeout and retransmission timeout should be set to roughly
3 times the end-to-end latency. The end-to-end latency can be found by disabling e2e and doing a ping
between the transmitter and receiver.
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