[e2e] Expected latency for a single hop: What about 802.11 networks?

Detlef Bosau detlef.bosau at web.de
Mon Aug 8 14:46:35 PDT 2005

David P. Reed wrote:
> 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.

Oh :-(

You just have destroyed my view of life..........

I knew about the CA stuff before, but not that 802.11 in fact does not 
care, when collision actually _occurs_.

(Call me lazybones, call me coward, but I avoid reading IEEE standards 
whenever possible =8-0
It´s nevertheless inevitable sometimes, but I rather read 20 RFCs than 1 
  IEEE stanard. O.k., it´s a standard, not a cartoon.....)

However, what you say here totally changes my way of thinking. I 
typically compare WLAN and Ethernet, which is still possible for low 
loads and when single, independent segments are compared. I.e., 
collusion does hardly occur and in a single segment e2e recovery should 
not behave that different than ALOHA, moreover there is hardly any 
network capacity at all and CWND etc. is small.

In case of increasing load, and therefore an increasing number of 
collisions), and if the 802.11 network is the last link in a number of 
subsequent links, there should be quite a difference to Ethernet when 
all collision losses must be cured end to end....

O.k., bearing this in mind, local recovery protocols like snoop appear 
totally different to me than before.

I think, it will take some days for me to understand all the consequences.

Thanks a lot.

I´ve learned something new today.

BTW: (Of course I will find it in the standards, it´s only I fear it´s 
on page 345 of 800....) What is the _reason_ for this decision _not_ to 
handle actual collisions locally but leave it to the e2e protocol?
To my understanding (up to now...) CA does _avoid_ collisions but does 
not totally prevent them. Or is CA that successfull that actual 
collisions can nearly be neglected?


Detlef Bosau
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70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
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