csma/ca

Unanswered Question
Dec 4th, 2007

hallo,

in a Basic Service Set the mobile Stations must exchange data throughout the AP.

Before transmitting a Station must sense the medium by radio and NAV.

Suppose we have 2 stations, sta1 and sta2 NON hidden each other.

Suppose sta1 transmit a frame to sta2, she writes the DURATION field of the frame, she set her MAC address as source and sta2 MAC as destination.

The radio signal from sta1 comes to sta2 and the AP (both).

Questions:

1) sta2 see the frame from sta1, probably before the AP.

sta2 refresh her NAV directly from this frame or MUST wait the frame from the AP ?

2) sta1 knows that there was no collision if receaves an ACK.

-That ACK frame comes from sta1 directly ?

or

-Comes from sta1 through the AP ?

or

- the ACK comes from the AP, before the AP relays the sta1 fram to sta2 ?

3) how does a sta knows that a data frame comes through AP and not directly from aanother sta ?

4) the AP use csma/ca or it can bypass it ?

best regards

francesco

I have this problem too.
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john.preves Sun, 12/09/2007 - 21:27

First, CSMA/CA is how wireless works. It's not optional.

Second the AP is just a radio...that is plugged into the switch; stations will not speak to each other without benefit of the AP unless the stations are in ad-hoc mode, which means they only connect to each other...

No magic here, the laws of physics and TCP/IP still apply.

fsoffia Sun, 12/09/2007 - 23:38

thanks john,

please could you answer at each question I posted ?

best regards

francesco

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