Subject: Ad Hoc Network Scalability

From: Phillip Neumiller <[email-masked]>
Date: Wed, 27 Jun 2001 15:36:48 -0400

Hello,

In the paper found at http://www.pdos.lcs.mit.edu/papers/grid:mobicom01/
"Capacity of Ad Hoc Networks" from MIT labs for CS, the paper concludes
that the "key factor" determining whether large ad hoc networks are feasible

is the locality of traffic (which results in fewer hops on average between
communicators).

Have others found this to be the "key factor" in ad hoc network feasibility?

It would seem to me, that "WHAT" the key factor is would depend somewhat
on the network architecture being analyzed and specifically it would depend
heavily on the MAC implementation.  It seems to me that this paper may
have been more appropriately titled "Capacity of Ad Hoc 802.11 Wireless
Networks".  Is it generally agreed that the 802.11 MAC can adequately
represent what the "key scalability factors" are?  I think the 802.11 MAC
is expedient but not necessarily that useful for generalized analysis.  Is
there such a thing as a perfect MAC model?, i.e. a MAC that achieves X%
efficiency (my guess is its just a circuit switch big enough to connect 
everyone to everyone else n(n+1)/2!!!).  This would not be very practical
in an RF MANET so there must be some happy medium (pun intended)!

Thanks,

Phil Neumiller

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From: "Rajan, Hridesh (Hridesh)" <[email-masked]>
Date: Thu, 28 Jun 2001 10:33:11 +0530

Hi Phil,

> In the paper found at 
> http://www.pdos.lcs.mit.edu/papers/grid:mobicom01/
> "Capacity of Ad Hoc Networks" from MIT labs for CS, the paper 
> concludes that the "key factor" determining whether large ad hoc 
> networks are feasible is the locality of traffic (which results 
> in fewer hops on average between communicators).
> 

I think in the paper they have just substantiated the widely
held belief by experimental evidences. It is pretty much 
obvious that if all the nodes of an Ad Hoc networks will communicate
with a path length equal to network diameter there will lot of 
contentions and as a result decrease in the throughput.

> 
> It would seem to me, that "WHAT" the key factor is would 
> depend somewhat on the network architecture being analyzed and 
> specifically it would depend heavily on the MAC implementation. 

I think the fact can't be denied that in the Ad Hoc networks other 
factors like network architecture become somewhat irrelevant
(Strictly my belief/Comments invited/Guidance towards any research 
work which substantiates my belief also invited). Isn't it the
traffic flow which is most important in this scenario??

Thank you,

Hridesh

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From: Phillip Neumiller <[email-masked]>
Date: Thu, 28 Jun 2001 07:12:53 -0400

Hi Rajan,

> contentions and as a result decrease in the throughput.

OK, but they also showed some pretty bad performance of 802.11
in a "generic" ad hoc topology.  My feeling is that one could
probably do a better job in the MAC and this would flush out
some other limiting factors.

My other "feeling" on the subject is that MANET's will "probably"
not be deployed as pure MANET's but may rather be allowable extensions
of existing infrastructures.  As a practical matter, other limiting
factors like the MAC become much more important than the locality
of communication.  Also in situations of high mobility the loss
of packets (lack of seamlessness) will become a problem for various
types of QoS classes.  

I would agree that in a pure ad hoc network, with a CSMA/CA MAC
that locality of reference in traffic will be a big factor IFF
the routing is ignorant of network bottlenecks.  Then again I have
been a big advocate of building some kind of "generic" triggers 
between the MAC/Link protocols and the IP layer so that we can
be smarter and faster about routing without relying on an ARQ in
the transport (i.e. Nagle, etc).

> traffic flow which is most important in this scenario??

Traffic flow is EXTREMELY sensitive to the type of routing that is
being done and the assumed mobility model.

I guess I will respectfully disagree.  It seems you have too many
built in assumptions to say that "network architecture is irrelevant".
Are you assuming some type of routing with this statement or is that
also irrelevant in your opinion?

Best regards,

Phil Neumiller

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From: Phillip Neumiller <[email-masked]>
Date: Thu, 28 Jun 2001 07:12:53 -0400

Hi Rajan,

Best regards,

Phil Neumiller