It seems like the code I pointed out earlier in rx.c is certainly wrong
then; bitrates used in absence of other information should not be
guessed based on the received rate, but should come instead from the
BSSID. Correct?
Although, once you start these silly examples, it seems to me like the
best thing to do is just assume the receiver can handle anything and let
minstrel sort out what works and what doesn't. This has obvious problems
for PID and similar rate algorithms.
Derek Smithies wrote:
Hi,
Referring back to my letter below, you see that when A&B agree on
their BSSID, this implies that A&B have agreed on:
a)supported rates
b)the value of X, the value of x
c)the channel
d)the ESSSID
When B&C start talking, and C adopts the same BSSID as B, then we have
(by inference) that C has agreed to the same rates as A. There is no
need to pass rate information between C and A.
===========================
Ok, now here is a silly example. Suppose C supports bitrates b4,b5 and
b6.
B does not support bitrates b4,b5 and b6.
Further, A does support bitrates b4,b5,b6
This could be that B only does 1, 2, 5.5 and 11MBits/sec. But A&C do
everything up to 54Mbit/sec.
B has caused a problem cause he is deficient (not doing G-Rates)
In one view, B should report that he has all the rates of A. B would
then hope that A's rate control algorithm will detect that B cannot do
the G-rates. Minstrel will do this fine. Stepup-Stepdown rate
algorithms will struggle here if their table is constructed in the
wrong order.
For this network, the BSSID defines the basic service set. you cannot
have nodes on the same BSSID who report as handling different rate sets.
Derek.
On Mon, 16 Nov 2009, Adam Wozniak wrote:
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
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