Tuesday, March 24, 2009

Difference short and long retry limit in wlan (802.11)

The limits define the number of MAC retries for different
types of packets. The short retries are incremented
whenever a control frame or a short frame smaller than the
rts threshold) is retransmitted. Similarly long retries are
incremented when a long packet frame is retransmitted. If
either of these retries reach their respective limits (short
or long depending on the size of the data frame), the frame
is automatically discarded.

Madwifi drivers allow to set the retry limit for frames
with the command -
iwconfig ath0 retry XX

With the Atheros family of wireless cards, *nix based machines support a
nice utility by the name athstats. Athstats
allows you to measure both the short and the long retries for a node
on a per second basis.
Typical usage -
athstats duration

The duration value is in secs.

Check out our other post for differences


Anonymous said...


Anonymous said...

How does this have an impact on the performance of TCP?

Wen Gao said...

This post is a nice discussion on retry in WiFi networks. Thanks.

misfit said...

One more important point is the reason for having these two different retry counters is that the system designer can set different retry limits for these two different types of frames.

Such re-provisioning is useful for two things: 1. Deciding the importance of delivery of certain short (possibly mgmt) frames, and 2) doing appropriate buffer provisioning by deciding to how long to hold long frames.

Anonymous said...

Hi I Want know is retry limit related to EIFS ?

Tom said...

If there is an error in a previously sent frame, then in DCF the extended interframe space (EIFS) is the time to wait before re-transmission.

Wifiplanet says:
EIFS = aSIFSTime + (8 x ACKSize) + aPreambleLength + aPLCPHeaderLngth+ DIFS

The number of retries are related to the frame loss rate and I am guessing they should be independent of the EIFS.... not sure though.

Anonymous said...

Thank You Tom.

Anonymous said...

Contention Window usually doubles When transmission fails (after SIFS interval of time).
What happens in EIFS?

Thanks in advance

JosephA said...

There's a short/long retry field per DCU.

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