Subject: RE: [manet] TCP Expected Throughput

From: "Mullen, John" <[email-masked]>
Date: Fri, 2 Sep 2005 14:41:00 -0600

If you have a MANET, expected throughput will be considerably less than
the channel capacity.  How much less will depend on several factors.
Here are two:
 
1. Retransmissions for forwarding. Suppose you have A B C D and A is
sending a message consisting of two packets to D via B and C.  First, A
sends the first packet  to B.  Then B sends it to C.  During this time,
A will sense B's carrier and not send the second packet.  Then, C sends
to D.  Even if A does not sense C's signal, any attempt to send the
second packet would fail, due to a data collision at B.  Hence, A will
not be able to send the second packet for about three times the nominal
transmission time for the first packet.  Also, while this is going on,
other nodes in the vicinity won't be able to use the channel, either.
Depending on traffic patterns, node density and node configuration, this
could easily cut your effective channel capacity by two thirds.
 
2. Retransmission for errors.  In a MANET, the effective range is the
result of a stochastic balance between the likelyhood of success
transmittion and the probability of selecting a node during a search.
Because of fine-grained variations in signal strength due to fading, a
packet may be lost at any distance.  However, as distance increases, the
probability of having to retransmit a randomly dropped packet increases.
If the maximum number of MAC retries is fairly high, the protocol is
likely to stick with a poor link, compensating by retransmitting
packets, rather than switching to a better path.  If this is the case,
you could lose a significant fraction of your capacity due to
retransmission of dropped packets.  
 
In short, the throughput is what you get, depending on a number of
factors.  I would say that 500 Kb/s is not a surprising figure, though
it might be possible to do a bit better.

John P. Mullen, Ph.D.
(505) 646-2958
[email-masked]

________________________________

From: [email-masked] [mailto:[email-masked]] On Behalf
Of Hasan Holandi
Sent: Friday, September 02, 2005 1:51 PM
To: [email-masked]
Subject: [manet] TCP Expected Throughput

Dear All,
 
I have a very trivial (but important to me) question:
 
Basically, I would like to know when we have a 4 hop ad hoc network with
a 2 Mb/s channel, what is the expected throughput?? Will that be also
2Mb/s (which of course in reality would be much less due to MAC
contention) or it would be 500 Kb/s?? In other words if i have file size
of 16Mb, theoretically will it take 8 sec or 32 sec?? 
 
I would appreciate if can clarify this issue with your response.
 
Hass

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From: [email-masked]
Date: Fri, 2 Sep 2005 23:09:57 +0200 (CEST)

Hi;

> it might be possible to do a bit better.

If you see the work of  Gupta and kumar, as the number of nodes per unit
area increases, the throughput per source-to-destination (S–D) pair
decreases approximately like 1/sqrt(n). This is the best performance
achievable even allowing for optimal scheduling, routing, and relaying of
packets in the networks and is a somewhat pessimistic result. The number
of hops in a typical route is of order sqrt(n). So, 500Kb/s is conform to
the model. But this is not a

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From: [email-masked]
Date: Fri, 2 Sep 2005 23:17:17 +0200 (CEST)

Hi;

If you see the work of  Gupta and kumar, as the number of nodes per unit
area increases, the throughput per source-to-destination (S–D) pair
decreases approximately like 1/sqrt(n). This is the best performance
achievable even allowing for optimal scheduling, routing, and relaying of
packets in the networks and is a somewhat pessimistic result. The number
of hops in a typical route is of order sqrt(n). So, 500Kb/s is conform to
the model. But this is not a rule, throughput depends on many factors:
interference, buffer size, traffic patterns, node density , etc. and it is
is difficult to determine the realy throughput.

> John P. Mullen, Ph.D.

Hakim

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From: Li Li <[email-masked]>
Date: Wed, 07 Sep 2005 09:01:20 -0400

I think these good insights on the drop of throughputs not only pertain to 
the TCP transmissions, but are actually applicable to MANET in general, 
including UDP transmissions? TCP probably makes it even worse on the 
throughput due to the flow control and the retransmissions at the transport 
layer?

li li

At 11:17 PM 9/2/2005 +0200, [email-masked] wrote:
>area increases, the throughput per source-to-destination (SD) pair

Li Li, Ph.D
Communications Research Centre, Canada
3701 Carling Ave., P.O. Box 11490, Station H
Ottawa, Ontario K2H 8S2
Tel: 613 990 5246
Email: [email-masked]