Subject: RE: [manet] Tx Range on Contention

From: "S.Sivavakeesar" <[email-masked]>
Date: Tue, 22 Mar 2005 10:02:12 -0000

Dear John, Tan et al :
 
Thanks for your information. I was reading a paper recently in which the authors try to adjust the tx range and hence the network topology based on the contention (delay). Their argument is ambiquous as what they say is contradictory to the whole lot of previous work on power control. Initially they claimed that the minimally connected topology is not optimal for typical ad hoc networks - this is aginst the whole lot of previous works. Then the paper made another statement that lower transmission range/power cannot always provide optimal throughput performance. Their proposed method is based on the argument that increasing the tx range (for all nodes at the same time) would lead to reduced contention and thus increased throughput. This is where I had a problem, which I wanted to check with you - MANET colleques. They had simulation results where they analysed the throughput as a function of Tx Range. They considered three different load scenarios - low-, modetely-, high-loaded. From the graphs it is clear that for low-loaded and highly-loaded scenarios, the throughput doesn't depend much on Tx Range. However, for a moderately loaded network, the throughput increases upto a saturation point (this is vague - however, probably true for low node-density network). So even from their own result, it becomes clear that increasing the Tx Range wouldn't always improve the throughput performance. However, the paper doesn't make this distinction, instead makes a geeral statement that increasing the Tx Rnage always bring in reduced contention and increased throughput. 
 
As you mentioned in your mail, increasing the tx range/power in a saturated network would cause more problems than it solves. 
 
Further comment from you guys are most welcome.
 
Cheers
 
Siva

________________________________

From: Dr John P Mullen [mailto:[email-masked]]
Sent: Tue 3/22/2005 3:46 AM
To: Sivavakeesar S Mr (PG/R - CCSR)
Cc: manet; tanhweex
Subject: Re: [manet] Tx Range on Contention

Hi Siva,

As Tan pointed out, it depends.  As transmit range is increased, 
throughput could either increase or decrease, depending on the nature
of the load, etc. 

However, it is more likely to decrease throughput in a situation in
which  many node pairs are communicating with each other.

If I double power, terrain permitting, I can double range and decrease
the number of links required to complete a transmission, but this will
not help paths that are already one hop long.  On the other hand,
although I double my range, I more than quadruple the area over which I
might interfere with other traffic.  This occurs because 1) while range
is linear, interference involves area and 2) at LOS frequencies and
moderate bit rates, the maximum interference range is significantly
greater than the maximum communications range. So, in most cases, I
would ecpect such a tactic to  cause more problems than it solves.

John Mullen

S.Sivavakeesar wrote:

>Dear Huixian :
>
>Thanks for your mail. I think it is better for me to put my question in
a different way. Given a static MANET having a constant traffic load,
node-density, and terrain-range, what happens to the throughput if we
change the tx range only. ie. I need a graph of throughput V tx range.
Can you say something about the pattern of this graph (every thing is
unchanged except the tx range) ?
>
>Thanks in advance for your invaluable time
>
>Cheers
>
>Siva
>
>
>
>
>
>
>
>
>
>Date: Mon, 21 Mar 2005 08:30:56 +0800
>From: Huixian <[email-masked]>
>Subject: Re: [manet] Tx Range on Contention
>To: [email-masked]
>Message-ID: <[email-masked]>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Hi Siva,
>
>There are a few factors to take into consideration, when evaluating the
>effects of the Tx range on throughput. Some of these factors include
>node mobility, node density and traffic load.
>
>Having a higher Tx range will tend to increase the contention at the MAC
>layer, lead to higher probability of collisions and reduce the spatial
>reuse of the channel. However, higher Tx range will also lead to shorter
>path lengths and therefore, less frequent link breakages.
>
>I think you need to consider the above mentioned factors when evaluating
>the effects of adjusting the Tx power on throughput. In general,
>increasing the Tx power (and hence the range) will tend to increase the
>throughput during high node mobility. If you increase your node density
>gradually, you will find that your throughput starts to drop due to the
>increased contention and decreased spatial reuse.
>
>Hope this helps!
>
>regards
>Hwee-Xian Tan
>School of Computing
>National University of Singapore
>
>
>
>

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From: Christian de Waal <[email-masked]>
Date: Tue, 22 Mar 2005 11:49:36 +0100

Hello,

just for the record, contradicting a whole lot of previous work doesn't 
make something wrong in itself. In our "Influence of tx power 
control..." paper which I cited in my previous message, we have also 
made the observation that minimally connected topologies can be very 
unfavourable for multihop communication due to unnecessarily long paths.

I would conclude with the commonplace that "things aren't always 
simple". Transmission powers can be too low despite resulting in a 
connected topology, and they can certainly be too high if the network is 
large. The way to go is somewhere in between.

Regards,
Christian de Waal

S.Sivavakeesar wrote:

> Initially they claimed that the minimally connected topology is not 
> optimal for typical ad hoc networks – this is aginst the whole lot of 
> previous works.

-- 
____________________________________________________________________
Christian de Waal

University of Bonn                            Phone: +49 228 73-4548
Institute of Computer Science IV                Fax: +49 228 73-4571
Roemerstr. 164                         E-mail: [email-masked]
53117 Bonn, Germany                    http://www.cs.uni-bonn.de/IV/