In case of multiple SSID's on some AP, how is the information forwarded?
I mean, the simplest AP's have just one wireless interface so its basically receiving trough one antenna and send it trough the same antenna which does not make much sense to me since in this case they end up beeing in the same collision domain.
The SSID is only part of the information used between an AP and a station, and many software clients will aggregate or simplify the networks they find into one entry in the wireless networks. However with most enterprise wireless solutions, each AP will have a separate BSSID (looks like a MAC address) that is associated with a logical interface on the physical interface.
With some software clients (Intel used to do this, but I haven't used it in a while so not sure if they still do), you will see each unique AP/BSSID listed in the wireless network list, so you end up with multiple entries with the same SSID. You can also use a software tool like InSSIDer to view the individual devices broadcasting the same SSID.
You are correct in that they share the same "collision domain" and resources, however there are many reasons to do so. For instance, you may want to provide different security mechanisms such as WPA2-Enterprise for your employees and an "Open" network with a captive portal for guests.
You could do this with two different sets of APs, each doing one or the other. However in the 2.4GHz space, you really only have three channels to work with in your channel plan, so unless you only need one AP for each, you are going to have channel overlap and still be sharing the "collision domain" (and this also complicates the channel planning significantly, but that's another discussion). Better to do so with multiple logical interfaces on one device as this means less devices, a lower cost (both capital and operational), and a simpler network to manage.
Now there is a caveat, and that is with each additional BSSID/SSID on a physical device, the more you do lower the efficiency of the RF use, mainly because you are creating additional management traffic in the air. While dated, you can find a good explanation on this effect in this document, if you are interested.
Yes, multiple SSIDs on an AP will share the same collision domain. Think of the SSIDs like a VLAN in that the SSID that the client associates to is a way to identify "I'm part of this network" and thus allow the AP to apply different security policies, for example.
In terms of actual broadcast collisions and throughput, 20 devices on one SSID is functionally the same as 5 devices on 5 different SSIDs, all on the same AP. The different SSIDs allow you to segregate the clients and treat them differently.
SSIDs will share the same collision domain only if they are configured using the same channel. If SSID1 use channel 1 and SSID6 use channel6, they will not share the same collision domain. Because they are communicating over different non-overlapping channels.
