From: "Jean-Pierre Tosoni" <jp.tosoni@acksys.fr>
To: <linux-wireless@vger.kernel.org>
Subject: mesh questions/suggestion about airtime metric and device constant in mesh_hwmp.c
Date: Mon, 15 Apr 2013 15:39:47 +0200
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Hi all,

The airtime_link_metric_get()  function follows closely section 13.9 of the
standard, except for the "channel access overhead" which is said to depend
on the PHY, headers, etc. 
Hence I understand that a complete implementation is not trivial.

The 'device_constant' variable is given the constant value '1<<ARITH_SHIFT',
which means 1 microsecond if I am correct. But the smallest possible
duration for the PPDU header is 20 microseconds in non-HT or 24 us in HT, as
shown in section 20.3.2

For the test frame of 8192 bits, say at 300Mbps (MCS15), Bt/r = the
currently computed airtime is 28 instead of 51, underevaluated by -45% !
Since the value accumulates on the mesh path, it may make a big difference
for the path selection.

Questions:
Even if device_constant is kept as a constant, shouldn't we set it to
something more realistic, like 20 or 24 ?
(I mean 20<<ARITH_SHIFT)
Does the standard really mean to include the PPDU header ?

(By the way, I understand that this value should also include the DIFS, and
maybe the ACK, so it should be much greater than 20)

Regards,

Jean-Pierre Tosoni