In a Roaming network all Airports have the same Wireless Network name and Wireless security settings. If I am not mistaken in WDS also the same rule has to be followed.
That is basically correct. For Apple routers, there are three types of "extended" networks: 1) Extended which using wireless connections between base stations, 2) Roaming which uses wired connections between base stations, & 3) Wireless Distribution System (WDS). WDS also uses all wireless connections.
The first two superseded WDS which was designed to be used only with the older 802.11g base stations. It has one major advantage. You can extend a network in a serial fashion as it employed the use of a "main," "relay," and "remote" base stations. This feature was not incorporated with the newer Extended or Roaming designs.
Is it also applicable for an Airport Extreme Extended network(1 Primary base station connected to an Extended base station) ?
In essence, yes. The extended base station is exactly that, and extension of the primary's Wi-Fi network. Thus it would have the same network name and use the same security type & password.
If that is the case then an user can move around freely in between the Airport Extremes with the same user-id and password in all 3 scenario. What then is the difference apart from the AEs interconnected through ethernet and wireless in the 3 different topology?
The major difference is overall Wi-Fi bandwidth performance. Maintaining an extended network using only wireless connections requires additional network traffic overhead. This overhead reduces the available bandwidth for normal wireless traffic. This issue does not exist with a roaming network.
The other issue with an all wireless extended network is bandwidth loss. Each base station added to the extended network, cuts the overall bandwidth almost in half. Again, this is not an issue with the roaming type network.
Finally, with an all wireless extended network, the extending base stations can only extend the main at the same bandwidth that it receives from the main at that location. Thus wireless clients connected to one of the extending base stations will never perform as well as when then are connected to the main one.
