Re: Adhoc networking, was Re: compat-wireless and minstrel

Adam Wozniak
2009-11-16T22:39:31Z

In your example where A&C can pass data, but not hear each other's
beacons, how is rate information passed between them? ProbeReq/Resp ?

Derek Smithies wrote:
> Hi,
> Statistics is a wonderful thing.
> Every node is required to send a beacon at time
> X+r, X+x+r, X+2x+r, X+3x+r, ......
>
> All nodes are agreed on the value of x (which is the beacon interval).
> All nodes are agreed on the value of X
> r is a random value, and is (from memory) 20 slots long.
>
> given this, all nodes work (to borrow an analogy from music) in time,
> or beat in sync with each other.
>
> Now, if a node hears a beacon on its BSSID inside that r period, the
> node will not transmit a beacon. This way, for a 20 node network in a
> room, you should only get 1 (or sometimes 2) beacons transmitted every
> beacon interval.
>
> If we assume that every node correctly attempts to send a beacon
> somewhere in that period r, and that somewhere is randomly
> distributed, then we will hear a beacon from most nodes, which is good
> enough.
>
> Consider the case of three nodes, A, B, C.
>
> A and B are turned on, and create an Adhoc network. A and B agree on
> a)supported rates
> b)the value of X, the value of x
> c)the channel
> d)the ESSSID
> and so start sending a beacon. Inside this beacon is BSSID. The BSSID
> is a random value. The particular random value used implies acceptance
> of a,b,c,d. Look at the name. Basic Service Set ID. The basic service
> set is the collection of those values a,b,c,d.
>
> Now, node C is turned on. A is positioned such that A&C cannot
> communicate. However, B can communicate with A&C.
>
> C is turned on, detects the presence of B, likes B's beacons, and
> agrees on all the settings in B's beacons. In other words, C likes and
> agrees with a,b,c,d.
> So C starts sending beacons with the same BSSID as B.
> At this point, it does not matter that A cannot set C's beacons. A and
> C have agreed on the BSSID.
>
> Change the story a little bit.
> In this locality, there is often a burst of 1ms of noise every 2ms.
> This means that most beacons are shotdown. However, data packets at
> 54Mbit/sec get through.
> A&B saw each others beacons and agreed on a BSSID. C was turned on,
> and agreed with the BSSID of B.
>
> C sends out data packets, with the BSSID of B. A sees the datapackets
> of C. Since the datapackets of C have A's BSSID, A has to accept them.
>
> Now, you see where this is all going. What is the meaning of a beacon
> containing a BSSID of all zero ?
> Further, you see that all nodes do need to send a beacon. This makes
> node discovery a little easier.
> Even though A and C cannot see each others beacons, they should still
> communicate as they have the same agreed on BSSID.
>
> Derek.
>
>
> On Mon, 16 Nov 2009, Adam Wozniak wrote:
>
>> This assumption seems too stoichastic. Reading 802.11-2007 section
>> 11.1.2.2, it doesn't seem that we're guaranteed to always receive
>> beacons from all stations. Stations will cancel their pending beacon
>> transmission if they receive a beacon before their random delay times
>> out. In the extreme case where the number of stations is very large,
>> it seems possible that you may never hear beacons for some stations.
>>
>> Johannes Berg wrote:
>>> On Mon, 2009-11-16 at 09:25 -0800, Adam Wozniak wrote:
>>>
>>>> If we have only three stations in an ad-hoc network, where all
>>>> three can hear the other two, only one of them should be beaconing,
>>>> correct?
>>>
>>> No, if they all behave correctly beaconing will be distributed.
>>>
>>> johannes
>>>
>>
>>
>>
>