WLAN

Wireless LAN

• A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier.

• WLAN Reference Architecture

3

Reference Architecture of WLANs

•Station (STA)– terminal with access mechanisms to

the wireless medium and radio contact to the access point

•Basic Service Set (BSS)– Group of stations using the same

radio frequency•Access Point (AP)

– station integrated into the WLAN and the distribution system

•Portal– bridge to other (wired) networks

•Distribution System– Interconnect network to form one

logical network (ESS: Extended Service Set) based on several BSS

Distribution System

Portal

Access Point

BSS2

BSS1

Access Point

STA1

STA2

STA3

ESS

4

Reference Architecture

• The smallest building block of a WLAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and competing for access to the same shared medium.

• A BSS may be isolated or it may connect to a backbone distribution system through an access point.

• The access point functions as a bridge. • The BSS generally corresponds to what is referred to as a cell

in the literature.

5

Reference Architecture

• An extended service set (ESS) consists of two or more BSSs interconnected by a distribution system.

• Typically, the distribution system is a wired backbone LAN. • The ESS appears as a single logical LAN to the logical link

control (LLC) level.• The standard defines three types of stations based on

mobility: No transition BSS transition ESS transition

Reference Architecture

• No transition: A station of this type is either stationary or moves only within

the direct communication range of the communicating stations of a single

BSS.

• BSS transition: This is defined as a station movement from one BSS to

another BSS within the same ESS. In this case, delivery of data to the station

requires that the addressing capability be able to recognize the new location

of the station.

• ESS transition: This is defined as a station movement from a BSS in one ESS to

a BSS within another ESS. This case is supported only in the sense that the

station can move.

7

WLAN ConfigurationInfrastructure Network

Ad-hoc Network

APAP

AP

Distributed System

AP: Access Point

IBSSIBSS

BSS

BSS

BSS

Ad hoc Network

• In an ad hoc WLAN, wireless nodes, or stations, transmit directly to each other via wireless NICs without an intervening connectivity device.

• An ad hoc arrangement would not work well for a WLAN with many users or whose users are spread out over a wide area, or where obstacles could stand in the way of signals between stations.

Infrastructure Network

• WLANs use the infrastructure mode depends on an intervening connectivity device called an access point (AP).

• The AP accepts wireless signals from multiple nodes and retransmits them to the rest of the network.

• AP may also be known as base stations.

Infrastructure Network

• To cover its intended range, an access point must have sufficient power and be strategically placed so that stations can communicate with it.

• It’s common for a WLAN to include several access points.

• The number of access points depends on the number of stations. The maximum number of stations each access point can serve varies from 10 to 100, depending on the wireless technology used.

WLAN and Wired LAN

• WLANs operate in almost the same way as wired LANs, using the same networking protocols and supporting the most of the same applications.

• WLANs support the same protocols (for example, TCP/IP) as wired LANs.

• This compatibility ensures that wireless and wired transmission methods can be integrated on the same network.

How are WLANs Different?

• They use specialized physical and data link protocols

13

mobile terminal

access point

fixedterminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructurenetwork

LLC LLC

• They integrate into existing networks through AP which provide a bridging function