Subject: [manet] controlling virtual carrier sense in 802.11 radios

From: David Young <[email-masked]>
Date: Tue, 29 Oct 2002 01:10:04 -0600

If researchers were able to exercise fine-grained control over the
virtual carrier sense in 802.11 radios, that would make possible a lot
of interesting experiments in arbitrating media access.

Just for an example of a desirable control, say it is possible to
upload to an 802.11b radio, at regular intervals of time, a bitmap
which describes the time slots where the radio should regard the virtual
carrier sense as "positive," regardless of what transpires on the medium.
Then certain sophisticated MAC algorithms can be implemented in the
radio's host computer, and the results transformed into a virtual carrier
sense bitmap which is uploaded to the radio. Now the radio can transmit
at its full speed under influence of the virtual carrier sense uploaded
by the host.

If consumer radios allowed this sort of control over the virtual carrier
sense, would that make them more desirable for research?

Dave

-- 
David Young             OJC Technologies
[email-masked]      Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933
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From: Ram Ramanathan <[email-masked]>
Date: Tue, 29 Oct 2002 09:44:37 -0500

David,

That is a great thought, and yes, I believe researchers
can make use of it. An obvious application is to use
802.11 radios to do TDMA -- by computing the schedules
in the host computer and downloading it into the radio.
At least for stationary ad hoc networks (the kind most
applicable currently commercially), this should sort of
work.

If such a bitmap could be AND'ed or OR'd with real
VCS, then there could be other applications too. For
example, you could create a protected channel for
sending network control messages (Hello's, route
query's etc.). E.g, during a particular "slot" all
nodes in a neighborhood except one is forced to desist.

I am sure there are other sophisticated uses, and
there is no dearth of fertile minds in ad hoc networking:)

In general, radio vendors have been poor at providing
controls over radio parameters, and I wholeheartedly
applaud and encourage any effort in this direction
as eminently healthy for ad hoc networking.

But there is a big "If" here. Do I dare hope that
you are actually a radio vendor and might actually
be considering this?:)

-Ram.

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From: Ram Ramanathan <[email-masked]>
Date: Tue, 29 Oct 2002 09:44:37 -0500

David,

-Ram.

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From: Rex Buddenberg <[email-masked]>
Date: 29 Oct 2002 13:28:45 -0800

David,

First, a caveat: we're outside the scope of MANET and into the data link
layer.

That out of the way, you're right.  The MAC (CSMA/CA) algorithm locked
into 802.11 chips has a lot of room for improvement for wireless LANs. 
And a whole lot of need for improvement as soon as we get to larger
scale such as satellite systems with significant propagation delay.  You
need either scheduling (all cases) or polling (where you have good
interactivity) MAC.  Algorithm aside, there appears to be no good way to
insert such a tweak ... if you find one, let me know.

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From: Rex Buddenberg <[email-masked]>
Date: 29 Oct 2002 13:28:45 -0800

David,

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From: David Young <[email-masked]>
Date: Tue, 29 Oct 2002 17:17:46 -0600

> First, a caveat: we're outside the scope of MANET and into the data link

  Ok.  I brought the topic to the list because I know for certain that
  many subscribers are concerned with it.  Does someone know a more
  suitable venue for a MAC discussion?

Dave

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From: David Young <[email-masked]>
Date: Tue, 29 Oct 2002 17:17:46 -0600

Dave

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From: Daniel Oldham <[email-masked]>
Date: Wed, 30 Oct 2002 12:23:10 GMT

Rex:

If you need "either scheduling (all cases) or polling" then please 
explain why you need an ad hoc algorithm for satellite communications. 
Once a satellite establishes a link layer connection then the information 
for routing is implied in the connection. The ad hoc algorithm then seems 
to be just extra overhead on the satellite.
--
Daniel R. Oldham
NASA Glenn Research Center
[email-masked]

>>>>>>>>>>>>>>>>>> Original Message <<<<<<<<<<<<<<<<<<

On 10/29/02, 4:28:45 PM, Rex Buddenberg <[email-masked]> wrote

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From: Daniel Oldham <[email-masked]>
Date: Wed, 30 Oct 2002 12:23:10 GMT

Rex:

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From: Rex Buddenberg <[email-masked]>
Date: 30 Oct 2002 15:35:07 -0800

Daniel,

My vision of a scheduling algorithm would be a dual-queue affair where
one queue is contention-based and is the means of entry into the second
queue which is disciplined by the net controller publishing a schedule
telling emitters how to take turns (i.e. no contention).  

So the net controller (satellite in your apparent vision, commsta ashore
in mine (satellite is a bent pipe) obviously has some state knowledge at
the data link layer -- knows who the subscribers in the second queue
are.  Pushing that data up to the routing information protocol is
certainly a feasible action and an economical one because it entails no
additional bits on the wire.

The 'manet' aspect is that departures from the second (scheduled) queue
might be less than graceful:
 - the graceful exit would be issuance of the data link equivalent of a
FIN message and an Ack.  No issue here.
 - in a dynamic topology (the point of MANET), exits would simply be
registered as a station not transmitting at its scheduled time.
Unfortunately, this silence could come from several causes many of which
would be transient (momentary antenna shading, for example).  How
'serious' should an outage be to be recognized as a topology change? 
And would this be a piece of data that the network scheduler would have
in the normal course of business (I think the datum 'station x has
missed n schedules' would be readily available)?  

Of the network segments that, say, an Army division might employ, I'd
expect that the satellite segments to be among the most stable -- far
more so than terrestrial LOS segments.  This tends to mean to me that
the MANET issues are less pressing in satcom network segments but the
silent host problem is endemic.  

Any where near what you thought you were asking?

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From: Rex Buddenberg <[email-masked]>
Date: 30 Oct 2002 15:35:07 -0800

Daniel,