Subject: RE: [manet] Interlayer Interactions

From: [email-masked]
Date: Wed, 2 Apr 2003 18:26:47 -0800

Thanks Bruce for starting the thread again.

I fully agree with you that we need to have forward thinking about this.

So, let's start discussion on this once again and if the outcome of our
discussion will prove that cross-layer interaction is not of great use,
we will drop this idea...

What are the opinions of other members of manet community? I am sure
many people won't like this idea but there are some who will like it..

Regards
Ankur

-----Original Message-----
From: A. Bruce McDonald [mailto:[email-masked]] 
Sent: Tuesday, April 01, 2003 2:08 PM
To: [email-masked]
Cc: [email-masked]
Subject: Re: [manet] Interlayer Interactions

Hi All;

IMHO the concept of interlayer interaction is not simply a matter of 'ad
hoc' communications across a control plane. The idea is much more to the
core of understanding the issues involved with the design of ad hoc
networks. The example I like to give involves the inherent 'coupling'
between the MAC-layer and routing. This is fundamental property in the
case of a random access MAC---the problem is that channel contention at
the MAC-layer spans multiuple hops; moreover, the *solutions* to
problems
such as the hidden-terminal actually *cause* a phenomena we refer to as
'path-coupling'. In effect, apparently disjoint paths can experience
'induced-load'; in some situations this is necesssary to prevent
collisions, however, there are many scenarios when the induced load is
an
artifact that prevents a node from transmiting even if the transmission
would not cause a collission.

This is a concrete example of a more
fundamental issue. That is, the question as to whether or not a graph
model (time varying, random, or whatever) is an appropriate model for an
ad hoc network. Can anyone justify it?  Show me links and explain to me
how a topological routing algorithm would use knowledge of these link
even
if it were possible to gather complete accurate global topology
information?  The concept of a "link" and, hence, the graph model itself
is inhernetly flawed.  The question is what effect this has on the
design
of routing algorithms which are themselves based on a graph model?  Do
they make sense?  Perhaps a better model exists, perhaps not.
Understanding the nature of the cross-layer interaction could perhaps
allow us approximate the ad hoc network using a graph. Just a thought.

The essence of my argument is that 'cross-layer interaction' should not
be
just a buzz word for sharing information between arbitrary layers, or
using layer-X information to control some aspect of layer-(X-2).
Instead,
it should be thought of in terms of the systematic study the how the
characteristics, properties and algorithms we associate with layer X
affect the performance of all other layers.  The outcome of this line of
thinking is to enhance our understanding of these complex relationships
so
that we can then---either build new models and algorithms, or, add an
inter-layer control/management plane.  It seems likely that both will be
required to some extent. However, carving a solid understanding first
will
lead to more effective and robust solutions in the future.

As a community
we need to be forward thinking---I know there is a fly or die attitude
in
some corners that has difficulty with the snail pace. Just turn to the
cell phone/PCS industry---the fundamental technical ideas are 30 years
old. The reason for the success today is due to advances in DSP
technology
and chip density.

Best regards,

Bruce

--------------------------------------------------------
A. Bruce McDonald	f:617.373.8970
Assistant Prof.		v:617.373.3028
ECE Dept., 417 Dana     [email-masked]
360 Huntington Ave.     www.ece.neu.edu/faculty/mcdonald
Northeastern University
Boston, MA 02115
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---

From: Joe Macker <[email-masked]>
Date: Thu, 03 Apr 2003 10:03:44 -0500

FYI, this is an old thread....see some very old minutes/briefs...
My observation is that most WG members agree that some level of 
cross-layer interaction is useful to improve performance.
Present specifications even have placeholders for it.

The non-existence of standard mechanisms is the main problem.
Many implementors I speak with are investigating such interactions,
but we would still like code that works in the absence of such mechanisms.

-joe

---

From: Joe Macker <[email-masked]>
Date: Thu, 03 Apr 2003 11:31:35 -0500

At 10:38 AM 4/3/2003 -0500, A. Bruce McDonald wrote:

>Hi Joe,
>
>It seems there are two ways to look at this which differ semantically.
>
>My perspective is that 'cross-layer interaction' is a property,
>characteristic, or side-effect that is inherent in the layered system---in
>this case the ad hoc network. My example was that certain attributes of
>the MAC that are unique (more-or-less) to ad hoc networks have a certain

I agree with your semantic argument and I think different MAC layers will differ in 
what they can or cannot offer and routing protocols will differ in what interaction may
be useful.

>inherent impact on higher layers, and, hence, perhaps should be considered
>in the design of routing algorithms.  In short, the cross-layer
>interaction is something to be studied and understood--it is not a
>mechansim or something to be defined and implemented.

Agreed, that is the right scientific approach.  Certainly if a MAC layer has robust information like 
"neighbor link up/down" that would may be useful right away to most wireless routing protocols.  
The broader interaction issues are more complex.

>Once we have better models and deeper understanding of the nature of these
>inherent interactions that may couple the performance of different layers
>in ways that are fundamentally different than other networks, then we can
>investigate how the mechansims for inter-layer control and/or design of
>algorithms can reflect this understanding to perform better.

Agreed this is a great ongoing research topic worth pursuing further, but I feel the potential value
of it is nothing that hasn't been discussed already (both in research forum and IETF circles).

I , for one, would encourage more investigation in the appropriate circles.

Some previous posts have been implying its something new that has not been discussed 
before or that people aren't interested in it.  I don't think that is true. Its a matter of scope
and the appropriate forum.
 
The ANS IRTF list may want to comment and may be an appropriate venue for 
broadening the knowledge base further over time.

---

From: Martin Drozda <[email-masked]>
Date: Fri, 04 Apr 2003 17:30:12 +0200

We have used ANOVA (Analysis of Variance) to argue about existence or
non-existence of (interlayer) interactions in various classes of ad hoc
networks. Employing a statistical approach allows for a very
straightforward definition of interactions between setup parameters.
This is very helpful if one needs to compare across different setups and
very useful for protocol engineering. Also the method is not limited to
interlayer (semantic) interaction (between protocols) but, in general,
can evaluate interactions between any input parameters and their
combinations. Our results show that 2-, 3-, or 4-way interactions are
present. A 4-way interaction would be an interaction among 4 input
parameters that can be any of MAC, routing protocols, speed of nodes,
topology, injection rate etc. Thus, certain forms of interactions can be
extremely COMPLEX. A 2-way interaction between routing and MAC protocols
was present for almost any class of ad hoc networks that we studied.
Currently, we are trying to extend the results to non-synthetic
topologies with high enough number of nodes to draw more general
conclusions.

Take a look at the two references below for details.
md

Christopher L. Barrett, Martin Drozda, Achla Marathe, and Madhav V.
Marathe. Analyzing Interaction Between Network Protocols, Topology and
Traffic in Wireless Radio Networks. Proc. IEEE Wireless Communications
and Networking Conference (WCNC'03), New Orleans, Louisiana, 2003.

http://www.sim.uni-hannover.de/~drozda/papers/wcnc_2003.pdf

Christopher L. Barrett, Martin Drozda, Achla Marathe, and Madhav V.
Marathe. Characterizing the Interaction Between Routing and MAC
Protocols in Ad-Hoc Networks. Proc. The Third ACM International
Symposium on Mobile Ad Hoc Networking and Computing (MOBIHOC 2002), June
2002, Lausanne, Switzerland.

http://www.sim.uni-hannover.de/~drozda/papers/Mobihoc_2002.pdf
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