Scanning in ieee80211 softmac layer and setting essid (newbie)

Scanning in ieee80211 softmac layer and setting essid (newbie)

[Ian Brown]

Hello,
I see that setting the essid of a wireless card
(by iwconfig eth1 essid homenet)
triggers scanning;
namely , it calls ieee80211softmac_assoc_work() method which in
turn calls ieee80211softmac_start_scan().

I use zd1211 driver which works with the softmac layer
of the last git kernel.

I saw that this scanning call occurs wheter the card is down
or up (namely, ifconfig eth1 up was called).

Only in the first case (card is down) ,it only says:
Associate: Scanning for networks first.
SoftMAC: Associate: failed to initiate scan. Is device up?

and when card is up it says:

SoftMAC: Associate: Scanning for networks first.
SoftMAC: Start scanning with channel: 1
SoftMAC: Scanning 13 channels
SoftMAC: Scanning finished

several times.

My question is : why is this scanning needed when set essid is called?
and why is the repeatition?

I am a little newbie in this...

Regards,
IB

Re: Scanning in ieee80211 softmac layer and setting essid (newbie)

[Dan Williams]

On Tue, 2006-05-09 at 16:38 +0300, Ian Brown wrote:
> Hello,
> I see that setting the essid of a wireless card
> (by iwconfig eth1 essid homenet)
> triggers scanning;
> My question is : why is this scanning needed when set essid is called?
> and why is the repeatition?

Because the wireless card needs to know information about the access
point before it can associate, like channel, BSSID, supported rates,
etc.  None of these things are indicated by the SSID you've just set on
the card. Normally, when you set an SSID and the card knows nothing
about that SSID, the card will scan all channels looking for access
points that happen to match the SSID you've just set.  It can then get
all the necessary information that it needs to connect to the access
point, but only _after_ it's scanned and found one with that SSID.

If the card does background scanning and keeps a list of access points
on its own, that makes things a little faster.  But the scan is still
necessary in many instances.

Dan

Re: Scanning in ieee80211 softmac layer and setting essid (newbie)

[Uli Kunitz]

Ian Brown wrote:

> My question is : why is this scanning needed when set essid is called?
> and why is the repeatition?

ESSID doesn't tell you the channel nummber, so you have to scan
for a beacon containing the ESSID in passive scanning. Even if you
do active scanning you need to check all channels because your
ESSID could be on anyone of the channels.

Uli

-- 
Uli Kunitz - kune@deine-taler.de