Subject: [manet] 802.11 Ad hoc mode?

From: "Su, William" <[email-masked]>
Date: Thu, 6 Feb 2003 14:39:30 -0500

Hi,

I posted this question couple of month ago, received no response so I'll try
it again...

Does anyone knows where I can find good source to answer these questions?
Couldn't seem to find them in the 802.11 spec (Part11:MAC-PHY
Specification):

1. In ad-hoc (IBSS) mode, how do the stations find each other in
multi-channel mode (if they are supported)?  The spec says all stations
generate beacons in random interval, but does not say if they do a full
sweep scan across all the channels for beacons if they don't hear anything
for a period of time.  In some configuration menus for 802.11 device (e.g.
D-Link cards) suggests that when turning on two ad-hoc mode stations they
shouldn't be turned on together but should allow some time to pass before
turning on the 2nd unit (otherwise 2 separate network could be formed).
What's the reason for this?  Why couldn't the station do a passive scan
periodically to avoid this?

2. Does IBSS mode support probes?  And are they used in the full sweep scan
(if multiple channel mode is supported)?  If so, how do stations coordinate
who generate probe-response and who doesn't (so there is no flooding of
replies)?

3. How does association work in IBSS mode?  Does a station send association
request to all the neighbors it hears on the beacons?  Also, does 802.11
IBSS use ARP to get the neighbor's IP address?  Does this process takes
place in association or it is separate?

Any information would be greatly appreciated,

William
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From: Yang Xiao <[email-masked]>
Date: Thu, 6 Feb 2003 11:54:31 -0800 (PST)

William

I believe that IEEE 802.11 never support "multiple channels" and has no
intention to support "multiple channels" so far.

Regards

Yang

> Any information would be greatly appreciated,

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From: David Young <[email-masked]>
Date: Thu, 6 Feb 2003 14:38:16 -0600

> Couldn't seem to find them in the 802.11 spec (Part11:MAC-PHY

  Answers below.

  BTW, this does not seem like a very IETFish question. Is there perhaps a
  better venue? Possibly I can persuade my workplace to set up a mailing
  list for practical discussions on 802.11 arcana.

> periodically to avoid this?

  Last I knew, IEEE 802.11 punted on the issue of merging two IBSSs. But
  for WiFi certification, an IBSS STA must join the oldest IBSS with
  the correct SSID. (Beacons for older IBSSs have greater TSFs.) In this
  way an IBSS merge is accomplished.

  Prism radios with STA firmware 1.4.9 and greater appear to do IBSS
  merges.  I think that Prism radios with firmware 1.3.6 do, also.
  Unless they are very new, your D-Link cards are Prism.

> place in association or it is separate?

  Regardless of what the standard may (or may not) say, I have never
  seen an IBSS STA send its neighbor an association request. I think
  that STAs will do pairwise authentication, however.

  An 802.11 IBSS is a function of layer 2, so it is not concerned with
  IP numbers.

Dave

-- 
David Young             OJC Technologies
[email-masked]      Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933
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From: Kevin Karcz <[email-masked]>
Date: Thu, 06 Feb 2003 17:22:42 -0500

More answers below.

>>who generate probe-response and who doesn't (so there is no flooding of

IBSS mode does use probe requests and responses.  In an IBSS the generation
of beacon frames is distributed.  Every time a beacon is to be sent out, all
the stations in the IBSS pick a random number.  Whichever station picked the
lowest number for that target beacon transmission time (TBTT) will be the first to
send their beacon.  All other stations upon hearing that beacon will discard
the beacon that they had queued for that TBTT.  Only the station that transmitted
the beacon will then be responsible for generating any Probe Responses during
that Beacon Interval.

>   that STAs will do pairwise authentication, however.

Association is only used in an Infrastructure BSS, it is never used in
IBSS mode.  It also is not required for an IBSS station to authenticate
before sending data either.  (Even when WEP is in use.)

>   An 802.11 IBSS is a function of layer 2, so it is not concerned with

Regards,
Kevin

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From: "Dr. Obscure" <[email-masked]>
Date: Sat, 08 Feb 2003 09:05:15 -0800

>   list for practical discussions on 802.11 arcana.

Yes.. might I suggest the newsgroup comp.std.wireless ?
(not quite as interactive, but it works :)

--ttfn
  --chane