Passive and Active scanning questions

Hello!!!

I have several questions...

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.

1 Is there a reserved channel that probe packets are being sent on?
  1. If there is a reserved channel how do stations compete for sending probe signals (DCF or something else)?
  2. Which channel do APs reply on?
  3. 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)?

PASSIVE SCANNING

When MS passively scans the area I guess it "listenes" for the AP's beacon frames.

  1. Does NIC in the MS switch channels it "listens" beacon frames on?
  2. How often does it switch a channel?
  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?

Thanks in advance!!!

Reply to
Vedran
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"Vedran" hath wroth:

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.

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.

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.

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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.

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?

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.

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.

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.

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:

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Reply to
Jeff Liebermann

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