"Vedran" <(E-Mail Removed)> hath wroth:
>ACTIVE SCANNING
>
>When station initiates active scanning it sends out probe frame, that tells
>nerby APs to send probe reply or beacon or something like that packets back
>to the MS. I guess that station can not send or recieve user data during
>that period.
Bad guess. A probe frame is just like any other management packet.
The difference is that the client will scan through all the available
channels looking for a suitable response. If the software that
initiates the probe request isn't smart enough to put the channel back
to where it was before it starting scanning, then it will break the
connection and session.
>1 Is there a reserved channel that probe packets are being sent on?
No. Probes scan through all the channels. Actually, that's not quite
true. There has to be an initiating program or driver that tells the
wireless client to initiate a probe request and on what channel. It
also had to deal with the responses.
Please note that ALL 802.11b management frames are sent at 1Mbits/sec,
the slowest speed.
>2. If there is a reserved channel how do stations compete for sending probe
>signals (DCF or something else)?
No. 802.11b/g is not cellular. There's no control channel.
DCF and EDCF are a partial solution to the hidden node problem. The
access point impliments token passing (WiCCP) and assigns dedicated
time slots to specific client radios. This has nothing to do with
conventional 802.11b/g.
http://patraswireless.net/software.html
>3. Which channel do APs reply on?
The same channel that the probe request was heard. AP are usually
glued to one channel and do NOT move around. Therefore, it will reply
on the channel to which it is configured. However, there are access
points that automagically search for an empty channel. These also
respond on whatever channel they land upon but it might vary somewhat
depending on local interference and traffic.
>4. If all stations use one channel, how do they synchronize with each other
>(are they competing for the medium like in DCF or something else)?
What's a "station"? The FCC does not allow sychronization between
multiple clients and multiple access points. If this were allowed,
one system could monopolize all the time slots and all the channels.
When running asychronously, there are collisions, but it gives other
users a fair share of the airtime.
I'm not sure I understand your question. Try again?
>PASSIVE SCANNING
>
>When MS passively scans the area I guess it "listenes" for the AP's beacon
>frames.
I'm not sure exactly how WZC works. I'll guess(tm) that WZC sniffs
for broadcasts and then uses probe request/response when it's time to
connect.
>1. Does NIC in the MS switch channels it "listens" beacon frames on?
Yes. The client will scan through all the channels. However, WZC
only lists SSID's, not channels. If there's a pair of access points
with the same SSID, but on different channels, it will only list one.
>2. How often does it switch a channel?
I don't know. It goes fairly quickly. However, when faced with a
huge number of SSID's heard, WZC can take an inordinate amount of time
to list them all. Therefore, I suspect that the switching time
between channels varies with congestion.
>3. If it switches a channel, it can't recieve data when it listenes another
>channel, so what impact does this passive channel scanning have on data
>transfer rate?
None. When connected to an access point (i.e. session in progress),
it isn't scanning for new connections.
There's a bit on how WZC works at:
http://www.microsoft.com/technet/com...uy/cg1102.mspx
--
Jeff Liebermann
(E-Mail Removed)
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Similar Threads
Linear Mode
