As previously discussed, even though the distribution system and the wired medium could be different, usually there are only two infrastructures: a wireless environment/infrastructure consisting of STAs/clients that is connected to a wired environment/infrastructure through APs.
Figure 4-3 shows one possible simplified topology as a perspective to compare and contrast functionalities of DS and WLAN.
The authentication, authorization, and accounting (AAA) infrastructure is part of the distribution system. Usually the RADIUS server is specific to the WLAN environment, and once authenticated, the WLAN users are authorized by the corporate AAA system. As you will see in the "Enterprise Guest Access" section of Chapter 11, "Operational and Design Considerations for Secure WLANs," this topology could allow guest users access to the Internet but not to any corporate network resources. This topology works well not only for big corporate organizations but also for universities and small businesses.
Figure 4-4 shows another possible scenario: an aggregator or wireless service provider (WSP).
In this scenario, there is a WSP cloud and an access provider layer. This scenario is usually employed in what is called public WLAN (PWLAN). The WSP provides multiple establishments, such as conferences, hotels, or airports, with Internet access. The establishments provide the WLAN infrastructure and an access layer to the WSP. The WSP usually handles the AAA functions in addition to billing and related services.
Recently, all the carriers and a few service providers have emerged in the WSP layer, whereas the airports, coffee houses, and burger joints have started providing the WLAN access. The attractive feature (at least to the SPs) is that if a user has an account or service with the service provider, the user can ubiquitously access the Internet anywhere he or she travels.
Please bear in mind that Figures 4-3 and 4-4 are provided simply to gain a perspective. Chapter 11 and Chapter 13, "WLAN Deployment Examples," cover more detailed designs.