Adhoc networking, was Re: compat-wireless and minstrel

Derek Smithies
2009-11-16T21:01:25Z

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
>>
>
>
>

--
Derek Smithies Ph.D.
IndraNet Technologies Ltd.
ph +64 3 365 6485
Web: http://www.indranet-technologies.com/

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