Subject: [manet] broadcasting in 802.11

From: Ali Hamidian <[email-masked]>
Date: Mon, 15 Nov 2004 18:01:29 +0100

Hi,

Using 802.11, no control frames are used for broadcast packets, so they 
are "unreliable" and a station never knows if a broadcast message was 
received by its neighbors or not. Now my question is: How can a station 
broadcast a message and be sure that all its neighbors received the message?

Is there any suggestions for extending 802.11 so that broadcast messages 
become "reliable"?

Kind regards
Ali

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From: "Grzegorz Jablonski" <[email-masked]>
Date: Thu, 18 Nov 2004 23:58:36 +0100

> Is there any suggestions for extending 802.11 so that broadcast messages 

I think this is technically impossible.
Sender can wait for an acknowledgement from all it neighbours, but how will 
it know taht the information he's got about neighbours is correct? I may be 
wrong, but as far as I know wireless node may only have approximated and 
usually slightly outdated information about his neighbours. Therefore you 
can never make sure that all neighbours have received your message.
The approximated information can as well be enough for your needs.

Greg

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From: Dave Maltz <[email-masked]>
Date: Fri, 19 Nov 2004 12:52:03 -0500

> Sender can wait for an acknowledgement from all it neighbours, but how
> will it know taht the information he's got about neighbours is correct? I
> may be wrong, but as far as I know wireless node may only have
> approximated and usually slightly outdated information about his
> neighbours. Therefore you can never make sure that all neighbours have
> received your message. The approximated information can as well be enough
> for your needs.

While not perfect and fairly expensive in overhead, the IMEP protocol will 
approximate reliable broadcast for you.  It's implemented in the Monarch 
extensions to ns-2 (and I recall Scott and/or Joe did an implementation as 
well).

There's also a trick that can help with reliability, if you're willing to 
keep your nodes in promiscuous listen mode and use a header flag or some 
convention to indicated broadcast packets.
The transmitting node must know the MAC address of any single neighbor 
(which it might learn via promiscuous listening).  The transmitting node 
then unicasts the "broadcast" packet to the neighbor, causing 802.11 to 
cover the packet with an RTS/CTS exchange.  If the other nodes are in 
promiscuous listen, they will overhear the packet, see the flag or 
convention, realize the packet is really intended as a broadcast, and pass 
it up the stack.

Cheers,
 -dam