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Chapter: Computer Networks : Data Link Layer

IEEE 802.11

IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, which covers the physical and data link layers.

Wireless LANs:


Wireless communication is one of the fastest-growing technologies. The demand for connecting devices without the use of cables is increasing everywhere. Wireless LANs can be found on college campuses, in office buildings, and in many public areas.


IEEE 802.11


IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, which covers the physical and data link layers.


1. Architecture: The standard defines two kinds of services: the basic service set (BSS) andthe extended service set (ESS).


Basic Service Set


IEEE 802.11 defines the basic service set (BSS) as the building block of a wireless LAN. A basic service set is made of stationary or mobile wireless stations and an optional central base station, known as the access point (AP).


The BSS without an AP is a stand-alone network and cannot send data to other BSSs. It is called an ad hoc architecture. In this architecture, stations can form a network without the need of an AP; they can locate one another and agree to be part of a BSS. A BSS with an AP is sometimes referred to as an infrastructure network

Extended Service Set


An extended service set (ESS) is made up of two or more BSSs with APs. In this case, the BSSs are connected through a distribution system, which is usually a wired LAN. The distribution system connects the APs in the BSSs. IEEE 802.11 does not restrict the distribution system; it can be any IEEE LAN such as an Ethernet. Note that the extended service set uses two types of stations: mobile and stationary. The mobile stations are normal stations inside a BSS. The stationary stations are AP stations that are part of a wired LAN.



2. MAC Sublayer


IEEE 802.11 defines two MAC sublayers: the distributed coordination function (DCF) and point coordination function (PCF). Figure 14.3 shows the relationship between the two MAC sublayers, the LLC sublayer, and the physical layer.


a.   Distributed Coordination Function


One of the two protocols defined by IEEE at the MAC sublayer is called the distributed coordination function (DCF). DCF uses CSMAICA as the access method.


Wireless LANs cannot implement CSMA/CD for three reasons:


1.        For collision detection a station must be able to send data and receive collision signals at the same time. This can mean costly stations and increased bandwidth requirements.


2.        Collision may not be detected because of the hidden station problem.


3.        The distance between stations can be great. Signal fading could prevent a station at one end from hearing a collision at the other end.


Network Allocation Vector


When a station sends an RTS frame, it includes the duration of time that it needs to occupy the channel. The stations that are affected by this transmission create a timer called a network allocation vector (NAV) that shows how much time must pass before these stations are allowed to check the channel for idleness. Each time a station accesses the system and sends an RTS frame, other stations start their NAV.


b.   Point Coordination Function (PCP)

The point coordination function (PCF) is an optional access method that can be implemented in an infrastructure network (not in an ad hoc network). It is implemented on top of the DCF and is used mostly for time-sensitive transmission. PCF has a centralized, contention-free polling access method. The AP performs polling for stations that are capable of being polled. The stations are polled one after another, sending any data they have to the AP.


3. Frame Format


The MAC layer frame consists of nine fields.


Frame control (FC): The FC field is 2 bytes long and defines the type of frame andsome control information.


D. In all frame types except one, this field defines the duration of the transmission that isused to set the value of NAY. In one control frame, this field defines the ID of the frame

Addresses. There are four address fields, each 6 bytes long. The meaning of each addressfield depends on the value of the To DS and From DS subfields.


Sequence control. This field defines the sequence number of the frame to be used inflow control.


Frame body. This field, which can be between 0 and 2312 bytes, contains informationbased on the type and the subtype defined in the FC field.


FCS. The FCS field is 4 bytes long and contains a CRC-32 error detection sequence.


4. Frame Types


A wireless LAN defined by IEEE 802.11 have three categories of frames: management frames, control frames, and data frames.


Management Frames: Management frames are used for the initial communicationbetween stations and access points.


Control Frames Control frames are used for accessing the channel and acknowledgingframes

Data Frames: Data frames are used for carrying data and control information.


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