Subject: RE: [manet] TCP Retransmission and Goodput

From: "Farrag, Osama" <[email-masked]>
Date: Tue, 23 Nov 2004 17:23:40 -0500

Ehsan,

There could be many factors, in general 802.11 networks perform better
in terms goodput when RTS/CTS handshake is used.

a)       RTS/CTS mode allows nodes to detect collision faster than in
Data/ACK mode because Data messages are much longer than RTS messages.
So the channel revert to idle state faster and nodes will start
contending again in shorter time. 

b)       Probability of collisions from Hidden node problem is reduced
when RTS/CTS handshake is used. Less collision results in better
throughput.

Hope that helps.

Best Regards

Osama Farrag

Johns Hopkins University/ APL

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Ehsan Hamadani
Sent: Tuesday, November 23, 2004 4:34 PM
To: [email-masked]
Subject: [manet] TCP Retransmission and Goodput

Hi all,

I am simulating a 4 hop static network with one TCP flow between the
sender and receiver. There are 2 scenarios: One scenario (let's call it
scenario 1) is based on RTS/CTS message exchange for packets larger than
1024 Bytes and the other one (scenario 2) does not use any RTS/CTS to
send data over network.

When I monitor the behaviour of TCP in the network, I come across the
following issue:

While scenario 1 has less average congestion window size and flight size
(number of sent and unacknowledged data in network) with higher number
of TCP retransmission in comparison to scenario 2, the goodput of
scenario 1 is greater than scenario 2!!!    

For measuring the goodput, I am monitoring the sent sequence number by
sender. So, for instance, at time 50sec, the sender in scenario 1 is
sending packet with sequence number 10000 while the sender in scenario 2
is sending packet with sequence number 6000.

Does anyone know the reason behind this?

Thanks for your help

Ehsan

-----------------------------------------

Ehsan Hamadani, PhD student

School of Engineering and Math Science

City University

London, EC1V 0HB

United Kingdom

Office: +44 (0)207 040 3886

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---

From: Ram Ramanathan <[email-masked]>
Date: Tue, 23 Nov 2004 18:23:07 -0500

Is this really true in practice -- that 802.11 networks have better 
goodput when RTS/CTS is turned on?

Researchers doing experimental work with testbeds more often than not 
report better throughput with RTS/CTS turned off (at least those I've 
spoken to), hidden terminals notwithstanding.

While RTS/CTS solves the hidden terminal problem, it does introduce 
exposed terminals, and the overhead of the handshake. And in some 
cases, the negatives may be more than the positives. For instance, at 
high data rates (e.g. 54 Mbps) the time for RTS/CTS as a fraction of 
total packet transfer time gets to be significant, especially if 
RTS/CTS are sent at lower data rate as is typical.

I'm not disputing your statement, but do you (or anyone on this list) 
know of any study that corrobarates this statement? Are there 
comprehensive simulation or testbed based studies on this and does one 
differ from the other in conclusions? If so, I'd appreciate a pointer.

It would be good for the community to really understand the benefits of 
RTS/CTS for ad hoc networks.

cheers,

-Ram

On Nov 23, 2004, at 5:23 PM, Farrag, Osama wrote:

> From:[email-masked] [mailto:[email-masked]]On Behalf 
> OfEhsan Hamadani
> To:[email-masked]
> Subject:[manet] TCP Retransmission and Goodput
>
>  
>
> Hi all,
>
>  
>
> sender and receiver. There are 2 scenarios: One scenario (let’s call 
> it scenario 1) is based on RTS/CTS message exchange for packets larger 
> than 1024 Bytes and the other one (scenario 2) does not use any 
> RTS/CTS to send data over network.
>
> When I monitor thebehaviourof TCP in the network, I come across the 
> following issue:
>
>  
>
> While scenario 1 has less average congestion window size and flight 
> size (number of sent and unacknowledged data in network) with higher 
> number of TCP retransmission in comparison to scenario 2, the goodput 
> of scenario 1 is greater than scenario 2!!!   
>
>  
>
> sending packet with sequence number 10000 while the sender in scenario 
> 2 is sending packet with sequence number 6000.
>
>  
>
> SchoolofEngineeringand Math Science
>
> City University
>
> London,EC1V 0HB
>
> United Kingdom
>
>  
>
> Office:+44 (0)207 040 3886
>
>  
>
>