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Would be great if someone clarifies this doubt.
Lets assume two Wireless Access Points AP1 & AP2 with these conditions
1. both in the same 802.11 standard
2. same channel
3. using different SSIDs
(just like in adjacent apartment houses).

In this case, do these two Access points (and the clients associated to them) coordinate via CSMA/CA ? ie., if one of the AP's or a client station is about to transmit, does it wait & observe the other AP's & its clients' transmission before sending the frame in air ?

Also, do the clients associated with these different APs coordinate via CSMA/CA ?

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2 Answers 2

Yes, with a caveat.

The way CSMA/CA works is that every station (whether it is a client station or the AP station does not matter) listens to the channel for a while, and transmits only if it is clear. Also, if the station hears a valid 802.11 packet, it will read its header (to see if it is destined for that station); in the header, there is a value which, if set, instructs all the listening stations to consider the channel as "busy" for a certain duration (for instance, a simple data packet could have it set to the time to transmit the packet itself plus the time for the packet's receiver to transmit the ACK).

The caveat is what happens when there are two transmitters which cannot hear each other, but the receiver for one of the transmissions can hear both transmitters (the hidden node problem). In this case, it is possible that both transmitters think the channel is "clear", but if both transmit at the same time, their transmissions will "collide" at the receiver. There is a way to avoid this problem (RTS/CTS), but it is rarely used due to its high overhead (since both the RTS and CTS will use the lowest rate).

Note that at no time in the explanation above the SSID was mentioned. For CSMA/CA, whether the stations are using the same SSID or not makes no difference at all. However, clients using the same AP are more likely to be located near each other, so there is less chance of the hidden node problem between them.

To confuse the situation a bit more:

  • The range varies with the automatically selected transmission speed. So two stations might be able to "see" each other (at 1Mbps), but be too far to interfere with each other during actual data transmission to nearer clients (which will probably be at higher speeds). The extra attenuation from the apartment walls in your example also helps here.
  • The stations do not need to be in the same channel to interfere. The channel width is around 20MHz, but the channels are spaced 5MHz apart. And to make things even less obvious, the spectrum mask makes the signal level at >11MHz from the channel center frequency much lower.
  • As a counterpart to the previous point, the signal does not completely disappears outside its channel. The spectrum mask only makes the signal level much lower outside the channel. However, this should only matter if the receiver is extremly near the transmitter (on the order of less than a metre).
  • The stations do not have to be using the same 802.11 standard; there are backwards compability tricks on the standard. For instance, if a 802.11g AP detects a client station which only knows 802.11b, it can set a flag on its beacons to make all clients begin their packets with something a 802.11b-only station can read. However, since this time the AP gets involved, having two different APs starts to matter more.
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The answer is no. An AP will see other RF as "interference" whether it is another AP, microwave, bluetooth, etc. They figure it out by using spread spectrum and frequency hopping to do the best they can to use the RF bandwidth that is available.

Now clients between the same AP will use CSMA, but only amongst themselves, they do not coordinate with the other AP's clients. (I'm pretty sure this takes place at the MAC layer, not at the physical layer. The MAC layers are not shared between the AP's, but the physical layers are)

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This answer is wrong on so many levels, I do not even know where to start. So, starting from the beginning: an AP will not see another AP as interference if they are near enough, since it can read the packet headers (and has to, else it cannot know they are for the other AP!). "They figure it out" is hopelessly vague. I do not believe anyone uses the frequency-hopping from the original 802.11 standard anymore. And the MAC layer is shared between all the stations (AP or not), after all it is its purpose to negotiate access to the media (MAC = Media Access Control). –  CesarB Mar 21 '10 at 20:22

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