+ o11s-devel On Mon, Apr 15, 2013 at 3:39 PM, Jean-Pierre Tosoni <jp.tosoni@xxxxxxxxx> wrote: > 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 The airtime is in units of 0.01TU, so a device_constant of 1 is more like 10us. > 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. Since O (channel access overhead) is constant, changing it just scales the metric equally across all links, so I'm not sure what difference this would make? Getting device_constant as a function of current protection parameters and channel type (ie. actual channel access overhead) however, makes sense. > 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) -- Thomas -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
