> Chris Day wrote:
>
>Would QoS / 802.11p fix this?
>
>

No... well.. not necessarily..

802.11pQ is an extension in the header ethernet frame header, which is
only relevant at the data link (2) layer .. a router will see an IP
datagram, and never the value from the 'p' field.

There are various ways of mapping 802.11p to DSCP/TOS, and that function
is usually done on a combination switch/router box. The rules governing
the mapping can be quite varied and configurable.

What normally happens is that for outbound packets, the switching
component of a said grunty router will set the 802.11p (CoS) field based
on the value of the DSCP , and for incoming will set DSCP based on CoS..
however it may also choose to override dscp (set it to zero), or set
cos/tos with fixed values for other reasons which may not be clear
without looking at the configuration of the specific router/switch in
question.

The default configuration of a Cisco 6500, for example, is to be rather
paranoid. It will clear the DSCP and CoS fields for packets passing
through it, and requires specific configuration commands to be entered
to negate that behaviour (for example for ip phones)

What it comes down to is that the moment you hit the first router or
non-ethernet link, 802.11p becomes irrelevant, and the only control you
will have over the priority of your packet will be in the IP DSCP field,
and once that packet leaves your network, you loose all control over
whether that priority will be changed/cleared/honoured.

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