[e2e] Expected latency for a single hop: What about 802.11 networks?
detlef.bosau at web.de
Mon Aug 8 09:27:23 PDT 2005
I just had a very first glance on a paper by Christoph Lindemann et al.,
The paper deals with TCP in multihop wireless networks, as far as I see
particularly 802.11 networks.
The paper mentions the typical consideration: In wireless networks,
corruption based loss happens more often than corruption based drop.
Now, first of all: What is the MAC algorithm in 802.11 ad hoc (not
infrastructure!) networks / MANETs?
To the best of my knowledge, this is ALOHA. (BTW: I would greatly
appreciate a copy of Abramsons Paper. It´s on my reading list, but I
could not find it yet.)
AFAIK, ALOHA does _not_ detect collisions but relys upon positive
acknowledments: A packet is sent, repeated if necessary, until it is
acknowledged by the receiver.
Q: Is this correct?
If so, we have implict retransmissions on the MAC layer here.
Particularly, we would observe transport latencies as the temporal
distance between the first sending attempt and the final reception.
This seems to be similar to the latency estimation used in the ARPAnet
in the 80s and which is proven to be insufficient / divergent according
to Jains paper "Divergence of Timeout Algorithms....", refer to the
discussion concerning "Round Trip Delay with Retransmissions" in that paper.
Q: Does this mean, it is difficult to obtain correct latency estimates
by pure TCP/ACK observation in case of networks where local recovery is
Mail: detlef.bosau at web.de
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