[e2e] Expected latency for a single hop: What about 802.11 networks?
David P. Reed
dpreed at reed.com
Mon Aug 8 10:31:25 PDT 2005
The MAC protocol in 802.11 is not ALOHA. You'd best get the spec if you
really want to understand it, because it's pretty complex.
It doesn't detect collisions, however. Nor does it depend on positive
acks. It relies on collision avoidance techniques to reduce collision
losses to a low enough level, and end-to-end acks to clean up the rest.
There is a "polled" mode (point coordination function) that is hardly
ever implemented. Instead, the "distributed coordination function"
(DCF) is typically employed, but modified in many cases by RTS/CTS
exchanges, this latter being the means to reduce collisions in most
cases (CTS is a positive ack for RTS).
Many networks are set up so that CTS/RTS applies only to long frames
(i.e. file transfers).
Ultimately, it means that what TCP/ACK observation sees when an 802.11
link is involved depends on how well the CTS/RTS works.
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