Management Frames prioritized ?

Jouni Malinen jkmaline
Wed Oct 15 18:51:48 PDT 2003

On Wed, Oct 15, 2003 at 01:48:22PM +0200, Sorin PAUN wrote:

> Do the management frames (Probe, Auth, Assoc Req/Resp) are prioritized 
> in rapport with data frames in the driver?

No, they are not. Probe Request frames are handled by the firmware code,
so it may prioritize Probe Response sending, but I do not know whether
this is done.

> More exactly, if an AP has data traffic to transmit to its associated 
> stations, does it have a queue in the driver or in the firmware? Do we 
> have some control on the size or can we perform operations on this 
> queue? And if it is receiving a Probe Request, does the Probe Response 
> frame bypass the data traffic queue?

There are number of queues involved in the transmit path.. Linux netif
has its queue. By default, this is 100 or 1000 frames depending on your
kernel version (if I remember correctly..). This can be changed with
'ip' command (iproute2).

The driver has a small queue for TX frame IDs. This can be limited by
changing PRISM2_TXFID_COUNT in hostap_wlan.h. By default, this is set to
8. However, this is not really a TX queue, but just a mechanism of
passing frames to the wlan card. Firmware has its own TX queue. I have
not seen any good documentation about the detailed implementation of
this queue. The driver has only limited way of controlling this queue by
not sending new frames for the firmware before receiving ACK/NAK for the
previous TX frames (i.e., this would limit the number of queued frames).

Probe Requests do bypass the Linux netif queue, but I do not know how
they are processed within the firmware TX queue.

> In the opposite direction, if a station has data traffic to transmit and 
> if I request it to do a scan, does it wait to transmit the data frames 
> before to send the Probe Requests?

Depends on implementation.. In case of Prism2 STA firmware, I have not
seen documentation for this.

> The second question is not relied to this driver specifically, but are 
> the management frames prioritized in some way at the MAC level? I found 
> some 802.11 tutorials that claimed that stations that have management 
> frames to transmit wait only a SIFS and in this way they are prioritized 
> in rapport with other stations that are issuing data traffic, but I 
> think that in the 802.11 protocol it is stated that all STA have to wait 
> a DIFS when transmitting a frame...

I don't think there is such a requirement/suggestion in IEEE 802.11
standard. In theory, it would of course be possible to configure
different EDCF parameters for management frames if the hardware supports
this kind of operation. IEEE 802.11e/WME would make this easier, but I
would still be somewhat surprised if AIFS=1 (only SIFS) would be used by
non-AP STAs for all management frames.

Jouni Malinen                                            PGP id EFC895FA

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