11-03-2014, 03:00 PM
802.11 networks
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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.
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.
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.
Interaccess point communication as part 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