Two 802.11 access modes can be used in a WLAN:
- Ad hoc mode
- Infrastructure mode
Ad hoc mode is based on the Independent Basic Service Set (IBSS). In IBSS, clients can set up connections directly to other clients without an intermediate AP. This allows you to set up peer-to-peer network connections and is sometimes used in a SOHO. The main problem with ad hoc mode is that it is difficult to secure since each device you need to connect to will require authentication. This problem, in turn, creates scalability issues.
Infrastructure mode was designed to deal with security and scalability issues. In infrastructure mode, wireless clients can communicate with each other, albeit via an AP. Two infrastructure mode implementations are in use:
- Basic Service Set (BSS)
- Extended Service Set (ESS)
In BSS mode,
clients connect to an AP, which allows them to communicate with other clients or LANbased resources. The WLAN is identified by a single SSID; however, each AP requires a unique ID, called a Basic Service Set Identifier (BSSID), which is the MAC address of the AP’s wireless card. This mode is commonly used for wireless clients that don’t roam, such as PCs.
In ESS mode,
two or more BSSs are interconnected to allow for larger roaming distances. To make this as transparent as possible to the clients, such as PDAs, laptops, or mobile phones, a single SSID is used among all of the APs. Each AP, however, will have a unique BSSID.
A WLAN coverage area includes the physical area in which the RF signal can be sent and received Two types of WLAN coverage’s are based on the two infrastructure mode implementations:
- Basic Service Area (BSA)
- Extended Service Area (ESA)
The terms BSS and BSA, and ESS and ESA, can be confusing. BSS and ESS refer to the building topology whereas BSA and ESA refer to the actual signal coverage
With BSA, a single area called a cell is used to provide coverage for the WLAN clients and AP
With ESA, multiple cells are used to provide for additional coverage over larger distances or to overcome areas that have or signal interference or degradation. When using ESA, remember that each cell should use a different radio channel.