IEEE 802.11 The IEEE standard 802.11 specifies the most famous family of WLANs in which many products are available. As the standard‟s number indicates, this standard belongs to the group of 802.x LAN standards, e.g., 802.3 Ethernet or 802.5 Token Ring. This means that the standard specifies the physical and medium access layer adapted to the special requirements of wireless LANs, but offers the same interface as the others to higher layers to maintain interoperability.
SYSTEM ARCHITECTURE Wireless networks can exhibit two different basic system architectures. : infrastructure-based and ad-hoc. infrastructure-based network
Several nodes, called stations (STAi), are connected to access points (AP). Stations are terminals with access mechanisms to the wireless medium and radio contact to the AP. The stations and the AP which are within the same radio coverage form a basic service set (BSSi). The example shows two BSSs – BSS1 and BSS2 – which are connected via a distribution system. A distribution system connects several BSSs via the AP to form a single network and thereby extends the wireless coverage area. This network is now called an extended service set (ESS) and has its own identifier, the ESSID.
Stations within the same ESS may communicate with each other , even though these stations may be in different basic service areas and may even be moving between basic service areas.
The ESSID is the „name‟ of a network and is used to separate different networks. Without knowing the ESSID it should not be possible to participate in the WLAN. The distribution system connects the wireless networks via the APs with a portal, which forms the interworking unit to other LANs. Stations can select an AP and associate with it. The APs support roaming (i.e., changing access points). The distribution system handles data transfer between the different APs. APs provide synchronization within a BSS, support power management, and can control medium access to support time-bounded service.
Ad-hoc network In addition to infrastructure-based networks, IEEE 802.11 allows the building of ad-hoc networks between stations, thus forming one or more independent BSSs (IBSS). In this case, an IBSS comprises a group of stations using the same radio frequency.
Stations STA1, STA2, and STA3 are in IBSS1, STA4 and STA5 in IBSS2. This means for example that STA3 can communicate directly with STA2 but not with STA5. Several IBSSs can either be formed via the distance between the IBSSs or by using different carrier frequencies.
MULTI BSS ENVIRONMENTS: “ VIRTUAL AP” Early 802.11 radio chips had the ability to create a single BSS. An AP can connect users to only one wireless network. In early developments with limited user counts , a single logical network was sufficient. As wireless network grew, one network no longer sufficed. Most organization get regular visitors , many of whom have 802.11 equipment and need internet access. Guests are not trusted users. One way of coping with guest access is to create 2 extended service sets on the same physical infrastructure.
Current 802.11 chipset can create multiple networks with the same radio. Using this chipsets , each AP hardware device can create 2 BSS, one for the network named guest and one for the network named internal. Wireless device see two separate networks in the radio domain, and can connect to whatever one suits their needs. This illustrates the development of virtual access points. Each BSS acts like its own self-contained AP, with its own ESSID and other relative contains.
THE DISTRIBUTED SYSTEM The DS is responsible for tracking where a station is physically located and delivering frames appropriately . When a frame is sent to a mobile station , the DS is charged with the task of delivering it to the access point serving the mobile station. Most access points operates as bridges. They have atleast one wireless network interface and one ethernet network interface. Frames between the two network media is controlled by a bridging engine.
Frames may be sent by the bridge to the wireless networks; any frames sent by the bridge‟s wireless port are transmitted to all associated stations. The station A can send station B a frame by relaying the frame through the bridging engine in AP.
INTERACCESS POINT COMMUNICATION ASPART OF THE DS This is a method to manage association. A wireless station is associated with only one AP at a time. All the other Aps in the ESS need to learn about that station. If a wireless station associated with AP4 sends a frame to a station associated with AP1, the bridging engine inside AP4 must send the frame over the backbone ethernet to AP1 so it can be delivered to its ultimate destination. Many access points on the market use an interaccess point protocol (IAPP) over the backbone medium
NETWORK BOUNDARIES 802.11 network has fuzzy boundaries. As with mobile telephone networks, allowing basic service areas to overlap increases the probability of successful transitions between basic service areas and offers the highest level of network coverage.
Station moving from BSS2 to BSS4 is not likely to move coverage. On the other hand , if ap2 fails the network is cut into 2 disjoint parts, and station in BSS1 lose connectivity when moving out of BSS1 into BSS4. Coping with coverage holes from access point failure is a task that requires attention during the network design phase.
OVERLAPPING NETWORK TYPES Different types of 802.11 networks may also overlap. Independent BSSs may be created within the basic within the basic service area of an access point. This five stations are assigned to two different BSS, they may share same wireless medium. Stations may obtain access to the medium only by using the rules specified in the 802.11 MAC; these rules were carefully designed to enable multiple 802.11 networks to coexit in the same Spatial area.