[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: [manet] TCP Expected Throughput
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
jomullen at nmsu.edu
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
__________________________________________________
Do You Yahoo!?
Tired
of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
_______________________________________________
manet mailing list
manet at ietf.org
https://www1.ietf.org/mailman/listinfo/manet