Subject: Re: Node Movement Models in Ad hoc

From: Chip Elliott <[email-masked]>
Date: Fri, 16 Jul 1999 08:58:24 -0400

>I believe that an ad hoc network is considered to be a *homogeneous*
>wireless network. That means group of mobile wireless terminals with
>similar charactristics, regarding technical.  Therefore, I believe any
>simulation scenarios should assume that all terminals present the same
>behavioral characteristics.
>

I for one do not agree with this. In real ad hoc networks, anything
goes. One is allowed to use a variety of radios, power sources, antennas,
and comms links. BBN is certainly building ad hoc networks that have
very different kinds of nodes and links in them, ranging from fiber
optics and gigabit radios with unlimited power, down to handheld
devices with omni antennas at the extreme edge of radio range
(running at a dozen bits per second). All in the same ad hoc network.

Just to restate earlier thoughts on scenarios, I believe a scenario
can be accurately modeled by the following tuple:

* Pathloss matrix (terrain)
* Node movements
* Traffic flows

In short, one models a set of nodes moving across a terrain and
exchanging data with each other. If one wants to get fancy, one can
model time-varying interference as a combination of time-varying
pathloss matrix plus self-interference (which must be measured
on a per-transmission or finer basis).

I don't know of any reason why we think a single scenario will
accurately capture all the behavior of an ad hoc networking scheme.
Therefore it would be best to catalog a standard set of terrains,
of movements, and of traffic flows, and of traffic flows, and to
run a proposed ad hoc network across this entire range.
Sounds tedious but it's really just running a set of perl scripts
and collecting the results.

Miguel Sanchez has already made a first catalog of movements.
A little more work would give a reasonable set of traffic flows.
And then some grinding away with RF propagation tools would give
a set of pathloss matrices.

To briefly mount an old soapbox, PLEASE do not do simulations
that use free space propagation as the pathloss model. In general
these give RIDICULOUSLY MISLEADING results. We'll know that a
given protocol suite works ok in the Nevada salt flats, provided
there's no self-interference, but nothing more. Buildings,
trees, hills, walls, and elevator shafts do matter!

Cheers,

-- Chip

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From: Joe Macker <[email-masked]>
Date: Fri, 16 Jul 1999 12:25:47 -0400

> and comms links. BBN is certainly building ad hoc networks that have

I also disagree with the homogeneity statement above.  It is the
intention in the manet WG to allow and design for some heterogeneity. 
Nodes MAY have different physical wireless interfaces than others,
different power considerations,etc. 

-Joe

---

From: George Aggelou <[email-masked]>
Date: Fri, 16 Jul 1999 20:44:36 +0100

Joe & Chip,

> different power considerations,etc.

I do agree as well that mobile terminals may have diverse functional
characteristics and yes indeed a simulation environment should allow for
some degree of heterogeneity... BUT what I mentioned in my previous
message is that, to my knowledge, existing simulation studies on mobile
ad hoc networks assume that the wireless network is homogeneous and I
believe at this early stage of   protocol evaluation process we should
keep the study in this platform.  Do you think that heterogeneity is the
key factor in order to discriminate between two routing protocols? 
Certainly, heterogeneity is an attribute of communication networks but
what I am trying to push at the first place is why don;t we try to keep
simulation environments as simple as it allows in order to study the
perfomance of a routing protocol.
An example that comes to my mind is the fact that most of the proposed
simulation studies proposed so far do not even include functionalities
from MAC and physical layer.  How it is possible to talk about link
heterogeneity, for example, in this case?  (Late studies, however, make
an attempt to integrate functionalities from lower layers of the
protocol stack (i.e., DLL, MAC and PHY) in the routing protocol itself
(see Johnson's paper at MobiCom9 for example)).

In conclusion, although it is indeed the intention of the WG to allow
and design for some heterogeneity, there is not any study performed so
far mostly due to simplicity.  And above all, I am really concerned
whether we should attempt to measure performance of routing protocols in
heterogeneous mobile networks given that the WG is under standardisation
and there are still emerging issues on the  routing protocols and their
behavioral characteristics under diverse communication and mobility
scenarios. Heterogeneous schemes will open new horizons for research and
new challenging issues are expected to emerge.  Me, however, I am
particularly interested to see any work related to these issues in the
near future as mobile routing protocols continue to evolve.

Thank you all for your discriptive and useful comments!

Best Regards,
George A.

-- 
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George Aggelou			     	
Centre for Communications Systems Research   
Mobile Communications Research Group      		
University of Surrey  		     

E-mail:	[email-masked]	     	
Tel:	+44 (0)1483 300 800 ext. 3468/2292    
Fax:	+44 (0)1483 259504		     	
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