Subject: Re: Scenarios for simulations

From: "Sanket S. Nesargi" <[email-masked]>
Date: Thu, 11 Feb 1999 17:25:27 -0600

Hi,
     I was just wondering, if the Manhattan model used for traditional
cellular networks made sense here. The nodes would be modeled 
as moving along a grid representing streets in a downtown and can 
turn only at right angles. This could lead to the breakage of links, if 
the turns were around buildings leading to loss of sight of the 
antennas and due to absorption/multi-path fading due to buildings. 
Such a grid based model could also make sense, say for a group 
of fire-fighters equipped with low-power communication devices 
(forming an ad-hoc network) when they may be trying to fight a fire 
in an urban setting: both inside and outside buildings.

Also, Thomas had suggested scenarios for
"a disaster relief operation, where some semi-fixed units can
provide routing at strategic places, but where "end nodes" may be
much more mobile."
This appears to be more of an application of virtual cellular networks,
where there are mobile base stations, and more mobile end nodes. 
In such a model we have the added luxury of the mobile base stations
having more resources than the mobile terminals. However, issues
relevant to mobility and resultant changes in topology still apply in
such a model. Also, if connection oriented communication is desired,
as in cellular networks, this virtual cellular model poses some 
interesting challenges.
A description of this system model can be found in our Infocom '99 paper
"Distributed Wireless Channel Allocation in Networks with Mobile Base
Stations"
available at http://www.utdallas.edu/~ravip/paper.list.html.

- Sanket

------------------------------------------------------------------------
Sanket S Nesargi
Research Assistant, Distributed Systems Lab
UT Dallas, Richardson, TX
Ph: (Off) 972-UTD-2420  (Res) 972-437-1740
------------------------------------------------------------------------

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From: Joe Macker <[email-masked]>
Date: Thu, 11 Feb 1999 19:05:02 -0500

>turn only at right angles. This could lead to the breakage of links, if 

How about the Paris/DC Lafayette model where core streets are radial and
travelers are forced to occassionally turn down chaotic pseudo-grid
sidestreets that may deadend at a moments notice... Thus causing them to
call for assistance quite frequently over their mobile ad hoc network...;->

-joe

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From: "C.-K. Toh" <[email-masked]>
Date: Thu, 11 Feb 1999 20:38:38 -0500

> call for assistance quite frequently over their mobile ad hoc network...;->

I think using a confined migration model is less likely 
to be appealing. Ad hoc networks are less rigid than cellular
in terms of structure and hence it should not be restricted to
specific environments. 

-- 
Prof. C-K. Toh, Assistant Professor,
Mobile Multimedia & HiSpeed Network Lab, 
School of Electrical and Computer Engineering
Georgia Institute of Technology, Atlanta,
Georgia GA 30332, USA. Fax: 404 894 7883
---------------------------------------------

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From: Joe Macker <[email-masked]>
Date: Fri, 12 Feb 1999 10:18:49 -0500

>in terms of structure and hence it should not be restricted to
CK:

I agree with you, I was trying to spread a little humor based upon the
history of poor city planning (how choatically-structured some city side
streets can become).  I should have explicitly indicated a sarcasm tag in
my posting.

The point on overly confined models is well taken and I wholeheartedly
agree.  Another additional issue is if one does use a highly detailed
confined urban model, reflection/fading/blockage/topography and other
considerations also affect wireless topology..its not just a flat grid
location/range problem.  The detailed 3D environment plays a role.

While these issues are important for real system deployment and planning, I
believe more open mobility and traffic models help determine the generic
value of a mobile routing algorithm(s) across a range of conditions.
Nonetheless, its nice to get some data and discussion on particular
scenario conditions that might represent urban usage and also real world
movement patterns in other than urban environments.

-joe

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From: Ravi Prakash <[email-masked]>
Date: Fri, 12 Feb 1999 10:21:56 -0600 (CST)

I agree with Joe and Sanket. And I did see the sarcasm tag! So, no problem
there. 

A flat grid may be alright to obtain generic data, and it may satisfy 
several of us (including me on several occasions) who model mobility as a 
random movement of nodes in this flat region. However, in reality, mobility 
patterns may be quite different and we do need to consider reality sooner 
or later. :-)

Also, I think Sanket was trying to raise issues of multi-path fading, 
blocking, absorption when he referred to the Manhattan model, which BTW is
used in the cellular telephony community to mean urban settings with 
lots of buildings separated by streets running parallel and perpendicular to 
each other. 

If we consider an ad-hoc network to be composed of devices carried by
pedestrians in the downtown area we need to consider blocking/fading/
reflection, etc. One may be surprised by the frequency with which links
appear and disappear. Signals don't fade gracefully. There are sudden
drops. This has the potential to affect the performance of
all the routing protocols being discussed by MANET. 

For example, if it turns out that routes between pairs of nodes are
short-lived, we may have to take a hard look at the caching policies of 
routing protocols like AODV and DSR. Even if they are not short-lived, 
the duration an entry should stay in the cache before being purged should
be influenced by the terrain. Any protocol that assumes that paths
remain fairly stable may have poor performance in such a situation. 

What if one wants to use some resource reservation/admission control 
based policy for QoS guarantees? If the paths change rapidly due to the
peculiarities of the environment, the game may not be worth the candle: 
the effort of setting up reservations (as in RSVP, for example) may be
more than the subsequent performance gains. 

Also, if we consider in-building ad-hoc networks, the flat grid model
is simply not acceptable. The architects may place "stuff" that is 
aesthetically pleasing, but plays havoc with the propagation 
characteristics. Apparently, the nice looking tall glass windows at the 
Denver airport absorb signals like a sponge. I'm told it was a nightmare for 
the service providers. 

Thanks for your indulgence!

----Ravi.
========================================================================
Ravi Prakash ([email-masked])		www.utdallas.edu/~ravip
Computer Science Program
The University of Texas at Dallas
Richardson, TX 75083-0688.  Phone: (972) 883-2289,   Fax: (972) 883-2349

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From: Joe Macker <[email-masked]>
Date: Fri, 12 Feb 1999 13:36:56 -0500

>lots of buildings separated by streets running parallel and perpendicular to 

Yes, Sanket raised the issues..I was being redundant.  I think the notion
of taking lessons from real world scenarios and data is to be applauded
(using the anticipated characteristics as a guide in parameterizing simpler
evaluation models is one idea).  It would great if someone compiled a list
of such references and ongoing work.  Perhaps this is already done to a
degree?

In some scenarios, things may not have to be that mobile to be quite dynamic.
This is an interesting thought that is being alluded to in several postings.

-joe