Subject: RE: [manet] MAC layer

From: "John Mullen" <[email-masked]>
Date: Thu, 8 Jan 2004 12:30:11 -0700

Hi Yannis,

Probably the greatest problems are the fact that control in Ad-Hoc nets
is distributed and information is very noisy.  At any distance, signal
strength can easily deviate 10 or 20 dB from its expected value
(assuming we could determine even that) and delay can be of a
significant duration.  If we get too tricky, the situation can get out
of hand.

Another difficulty is that there has to be, at least, a common control
and hailing channel.  Otherwise, a node would have to monitor all hop
sequences to be sure no one was trying to talk to it.  Also, what if two
nodes try to transmit to the same node using different hop sequences?
One solution is to have a control/hailing channel and then assign
sequences to different T/R pairs.  However, managing this is a real
bear, since no node is entirely sure where the other nodes are.

With fixed antennas, such as in cellular, this is not so great a
problem.  Nodes simply communicate with the tower that yields the best
link and the towers use prearranged sequence allocations to avoid
excessive interference.  In a MANET, however, there are no fixed nodes,
it is unclear which links are best, and decision-making is distributed.
Most of all, there is no fiber or wire to link the backbone.  Everything
is done through the MANET.

Nevertheless, these things are investigated and the
military/security/emergency response organizations are using them to
some extent.  However, in each of these applications, there is an
organizational structure imposed on the MANET which makes channel
assignment, etc. more practical.  In a general MANET, in which nodes may
not share any organizational ties, other than through the MANET itself,
the problem is far greater.

John Mullen

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Ioannis C. Avramopoulos
Sent: Friday, November 21, 2003 9:33 PM
To: [email-masked]
Subject: [manet] MAC layer

Dear fellows,

I have a question on the MAC layer used in ad hoc networks.

In 802.11 nodes share the medium, i.e. they use the same frequency
hopping pattern or chip sequence. Had nodes used orthogonal hopping
patterns, spectrum utilization would significantly increase and,
furthermore, the impact of malicious nodes with respect to jamming would
be severely 
mitigated.

I am interested in the reasons that schemes of the latter
type are not considered in the ad hoc community. I would appreciate it
if you could point me to any papers discussing such issues.

Kind regards,
Yannis

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