IEEE 802.11 FAMILY

Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science

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IEEE 802.11 FAMILY Author:Nithya Suresh


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��,1752'8&7,21� The purpose of this document is to give its readers a basic overview of the 802.11 standards in

such a way that they will be able to understand the basic concepts ,the principles of operations

behind the features of the standard.This document was prepared as a partial requirement for

the course TDTS02.Obviously the document does not cover the whole of 802.11

standards.For a fuller and deeper understanding, the reader is encouraged to refer the

standards document itself.

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The 802.11 architecture is comprised of several components and services that interact to

provide station mobility transparent to the higher layers of the network stack.

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The VWDWLRQ (STA) is the most basic component of the wireless network. A station is any

device that contains the functionality of the 802.11 protocol, that being MAC, PHY, and a

connection to the wireless media. Typically the 802.11 functions are implemented in the

hardware and software of a network interface card (NIC).

A station could be a laptop PC, handheld device, or an Access Point. Stations may be mobile,

portable, or stationary and all stations support the 802.11 station services of authentication,

de-authentication, privacy, and data delivery.


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802.11 defines the %DVLF�6HUYLFH�6HW (BSS) as the basic building block of an 802.11 wireless

LAN. The BSS consists of a group of any number of stations.

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The most basic wireless LAN topology is a set of stations, which have recognized each other

and are connected via the wireless media in a peer-to-peer fashion. This form of network

topology is referred to as an ,QGHSHQGHQW�%DVLF�6HUYLFH�6HW (IBSS) or an $G�KRF network.

In an IBSS, the mobile stations communicate directly with each other. Every mobile station

may not be able to communicate with every other station due to the range limitations. There

are no relay functions in an IBSS therefore all stations need to be within range of each other

and communicate directly.

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An Infrastructure Basic Service Set is a BSS with a component called an $FFHVV�3RLQW��$3��

The access point provides a local relay function for the BSS. All stations in the BSS

communicate with the access point and no longer communicate directly. All frames are

relayed between stations by the access point. This local relay function effectively doubles the

range of the IBSS.

The access point may also provide connection to a distribution system


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'LVWULEXWLRQ�6\VWHP��'6� The GLVWULEXWLRQ�V\VWHP (DS) is the means by which an access

point communicates with another access point to exchange frames for stations in their

respective BSSs, forward frames to follow mobile stations as they move from one BSS to

another, and exchange frames with a wired network.

As IEEE 802.11 describes it, the distribution system is not necessarily a network nor does the

standard place any restrictions on how the distribution system is implemented, only on the

services it must provide. Thus the distribution system may be a wired network like 803.2 or a

special purpose box that interconnects the access points and provides the required distribution

services.

([WHQGHG�6HUYLFH�6HW��(66� 802.11 extends the range of mobility to an arbitrary range

through the ([WHQGHG�6HUYLFH�6HW (ESS). An extended service set is a set of infrastructure

BSS’s, where the access points communicate amongst themselves to forward traffic from one

BSS to another to facilitate movement of stations between BSS’s.

The access point performs this communication through the distribution system. The

distribution system is the backbone of the wireless LAN and may be constructed of either a

wired LAN or wireless network.

Typically the distribution system is a thin layer in each access point that determines the

destination for traffic received from a BSS. The distribution system determines if traffic

should be relayed back to a destination in the same BSS, forwarded on the distribution system

to another access point, or sent into the wired network to a destination not in the extended

service set. Communications received by an access point from the distribution system are

transmitted to the BSS to be received by the destination mobile station.

Network equipment outside of the extended service set views the ESS and all of its mobile

stations as a single MAC-layer network where all stations are physically stationary. Thus, the

ESS hides the mobility of the mobile stations from everything outside the ESS. This level of

indirection provided by the 802.11 architecture allows existing network protocols that have no

concept of mobility to operate correctly with a wireless LAN where there is mobility.

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The 802.11 standard defines services for providing functions among stations. Station services

are implemented within all stations on an 802.11 WLAN (including access points). The main

thrust behind station services is to provide security and data delivery services for the WLAN.

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Because wireless LANs have limited physical security to prevent unauthorized access, 802.11

defines authentication services to control access to the WLAN. The goal of authentication

service is to provide access control equal to a wired LAN.

The authentication service provides a mechanism for one station to identify another station.

Without this proof of identity, the station is not allowed to use the WLAN for data delivery.

All 802.11 stations, whether they are part of an independent BSS or ESS network, must use

the authentication service prior to communicating with another station.

IEEE 802.11 defines two types of authentication services:-

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This is the default authentication method, which is a very simple, two-step process. First the

station wanting to authenticate with another station sends an authentication management

frame containing the sending station’s identity. The receiving station then sends back a frame

alerting whether it recognizes the identity of the authenticating station.

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This type of authentication assumes that each station has received a secret shared key through

a secure channel independent of the 802.11 network. Stations authenticate through shared


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knowledge of the secret key. Use of shared key authentication requires implementation of

encryption via the :LUHG�(TXLYDOHQW�3ULYDF\ or :(3 algorithm.

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The de-authentication service is used to eliminate a previously authorized user from any

further use of the network. Once a station is de-authenticated, that station is no longer able to

access the WLAN without performing the authentication function again.

De-authentication is a notification and cannot be refused. For example, when a station wishes

to be removed from a BSS, it can send a de-authentication management frame to the

associated access point to notify the access point of the removal from the network. An access

point could also de-authenticate a station by sending a de-authentication frame to the station.

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The privacy service of IEEE 802.11 is designed to provide an equivalent level of protection

for data on the WLAN as that provided by a wired network with restricted physical access.

This service protects that data only as it traverses the wireless medium. It is not designed to

provide complete protection of data between applications running over a mixed network.

With a wireless network, all stations and other devices can "hear" data traffic taking place

within range on the network, seriously impacting the security level of a wireless link. IEEE

802.11 counters this problem by offering a privacy service option that raises the security of

the 802.11 network to that of a wired network. The privacy service, applying to all data

frames and some authentication management frames, is an encryption algorithm based on the

802.11 Wired Equivalent Privacy (WEP) algorithm.

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Data delivery service is similar to that provided by all other IEEE 802 LANs. The data

delivery service provides reliable delivery of data frames from the MAC in one station to the

MAC in one or more other stations, with minimal duplication and reordering of frames.


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The association service is used to make a logical connection between a mobile station and an

access point. Each station must become associated with an access point before it is allowed to

send data through the access point onto the distribution system. The connection is necessary

in order for the distribution system to know where and how to deliver data to the mobile

station.

The mobile station invokes the association service once and only once, typically when the

station enters the BSS. Each station can associate with one access point though an access

point can associate with multiple stations.

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The disassociation service is used either to force a mobile station to eliminate an association

with an access point or for a mobile station to inform an access point that it no longer requires

the services of the distribution system. When a station becomes disassociated, it must begin a

new association to communicate with an access point again.

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Re-Association enables a station to change its current association with an access point. The


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re-association service is similar to the association service, with the exception that it includes

information about the access point with which a mobile station has been previously

associated. A mobile station will use the re-association service repeatedly as it moves through

out the ESS, loses contact with the access point with which it is associated, and needs to

become associated with a new access point.

By using the re-association service, a mobile station provides information to the access point

to which it will be associated and information pertaining to the access point which it will be

disassociated. This allows the newly associated access point to contact the previously

associated access point to obtain frames that may be waiting there for delivery to the mobile

station as well as other information that may be relevant to the new association.

The mobile station always initiates re-association.

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Distribution is the primary service used by an 802.11 station. A station uses the distribution

service every time it sends MAC frames across the distribution system. The distribution

service provides the distribution with only enough information to determine the proper

destination BSS for the MAC frame.

The three association services (association, re-association, and disassociation) provide the

necessary information for the distribution service to operate. Distribution within the

distribution system does not necessarily involve any additional features outside of the

association services, though a station must be associated with an access point for the

distribution service to forward frames properly

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The integration service connects the 802.11 WLAN to other LANs, including one or more

wired LANs or 802.11 WLANs. A SRUWDO performs the integration service. The portal is an

abstract architectural concept that typically resides in an access point though it could be part

of a separate network component entirely.

The integration service translates 802.11 frames to frames that may traverse another network,

and vice versa as well as translates frames from other networks to frames that may be

delivered by an 802.11 WLAN.


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(Refer to the formal specification in Clause 9 of 802.11 standard,detailed MAC state diagrams

in Annexe c)�

The 802.11 MAC layer provides functionality to allow reliable data delivery for the upper

layers over the wireless PHY media. The data delivery itself is based on an asynchronous,

EHVW�HIIRUW, connectionless delivery of MAC layer data. There is no guarantee that the frames

will be delivered successfully.

The 802.11 MAC provides a controlled access method to the shared wireless media called

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to the collision detection access method deployed by 802.3 Ethernet LANs.

The third function of the 802.11 MAC is to protect the data being delivered by providing

security and privacy services. Security is provided by the authentication services and by

:LUHG�(TXLYDOHQW�3ULYDF\��:(3�, which is an encryption service for data delivered on the

WLAN.

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– Power save - poll (PS-poll)

– Request to send (RTS)

– Clear to send (CTS)

– Acknowledgment (ACK)

– Contention-free (CF)-end

– CF-end + CF-Ack

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– Data

– Data + CF-Ack

– Data + CF-poll

– Data + CF-Ack + CF-poll

– Null Function

– CF-Ack

– CF-Poll

– CF-Ack + CF-Poll

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– association request and association response

– reassociation request and reassociation response

– probe request and probe response

– beacon

– announcement traffic indication message

– disassociation

– authentication and deauthentication


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PCF provides contention free services.Special stations called point coordinators are used o

ensure that the medium is provided with contention.Coordinators reside in Aps,so PCF is

restricted to Infrastructre Networks.PCF is not widely implemented�

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DCF is the basis for CSMA/CA mechanism.DCF allows multiple stations to interact without a

central control.DCF is used in IBSS and Infrastructure networks.

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802.11 uses 4 different InterFrame Spaces,among which three are used to determine medium

access.Varying IFS create different priority levels for different types of traffic.IFS is a fixed

amount of time independent of transmission speed.Different physical layers specify different

IFS times.

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Hidden node/Exposed node Problem

RTS/CTS clearing mechanism is used to overcome this problem.But then we have the

tradeoff between overhead and retransmission costs.


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Physical carrier sensing is expensive and difficult to build ,not to mention the problem of

hidden nodes.Hence another carrier sensing mechanism is employed,viz,virtual carrier

sensing.This is accomplished by NAV(Network Allocation Vector).Most 802.11 frames carry

a Duration field which is used to reserve the medium fr a fixed time period.NAV is a timer

that indicates the amount of the time that the medium will be reserved.Stations set NAV to the

time they expect to use the medium while others count down from NAV to 0.A non-zero

NAV indicates a busy medium.

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�If medium is idle for DIFS interval(based on physical or virtual sensing),transmission can

begin immediately.�

�If medium is busy,access is deferred until medium is idle for DIFS and exponential backoff.

�Backoff counter is decremented by 1 if a timeslot is determined to be idle.

�Unicast data must be acknowledged as part of an atomic exchange

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Each frame is associated with a retry counter based on frame size as compared to RTS/CTS

threshold�

• short retry counter

• long retry counter

Fragments are given a maximum lifetime by MAC before discarding them.

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Scanning is the process of identifying existing networks. The 802.11 standard defines both

passive and active scanning; whereby, a radio NIC searches for access points. Passive

scanning is mandatory where each NIC scans individual channels to find the best access

point signal. Periodically, access points broadcast a beacon, and the radio NIC receives

these beacons while scanning and takes note of the corresponding signal strengths. The

beacons contain information about the access point, including service set identifier,

supported data rates, etc. The radio NIC can use this information along with the signal

strength to compare access points and decide upon which one to use.

Optional active scanning is similar, except the radio NIC initiates the process by

broadcasting a probe frame, and all access points within range respond with a probe

response. Active scanning enables a radio NIC to receive immediate response from

access points, without waiting for a beacon transmission. The issue, however, is that

active scanning imposes additional overhead on the network because of the

transmission of probe and corresponding response frames.

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All station clocks within a BSS are synchronized by periodic transmission of time stamped

EHDFRQV. In the infrastructure mode, the AP serves as the timing master and generates all

timing beacons. Synchronization is maintained to within 4 microseconds plus propagation

delay.Timing beacons also play an important role in power management. There are two power

saving modes defined: DZDNH�and GR]H. In the DZDNH�mode, stations are fully powered and

can receive packets at any time. Nodes must inform the AP before entering doze. In this

mode, nodes must “wake up” periodically to listen for beacons which indicate that AP has

queued messages.


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The 802.11 physical layer (PHY) is the interface between the MAC and the wireless media

where frames are transmitted and received. The PHY provides three functions. First, the PHY

provides an interface to exchange frames with the upper MAC layer for transmission and

reception of data. Secondly, the PHY uses signal carrier and spread spectrum modulation to

transmit data frames over the media. Thirdly, the PHY provides a carrier sense indication

back to the MAC to verify activity on the media.

802.11 provides three different PHY definitions: Both )UHTXHQF\�+RSSLQJ�6SUHDG�6SHFWUXP�

�)+66� and 'LUHFW�6HTXHQFH�6SUHDG�6SHFWUXP��'666� support 1 and 2 Mbps data rates. An

extension to the 802.11 architecture (802.11a) defines different multiplexing techniques that

can achieve data rates up to 54 Mbps. Another extension to the standard (802.11b) defines 11

Mbps and 5.5 Mbps data rates (in addition to the 1 and 2Mbps rates) utilizing an extension to

DSSS called High Rate DSSS (HR/DSSS). 802.11b also defines a rate shifting technique

where 11 Mbps networks may fall back to 5.5 Mbps, 2 Mbps, or 1 Mps under noisy

conditions or to inter-operate with legacy 802.11 PHY layers .

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Spread spectrum is a technique trading bandwidth for reliability. The goal is to use more

bandwidth than the system really needs for transmission to reduce the impact of localized

interference on the media. Spread spectrum spreads the transmitted bandwidth of the resulting

signal, reducing the peak power but keeping total power the same.

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Frequency Hopping utilizes a set of narrow channels and "hops" through all of them in a

predetermined sequence. For example, the 2.4 GHz frequency band is divided into 70

channels of 1 MHz each. Every 20 to 400 msec the system "hops" to a new channel following

a predetermined cyclic pattern.

The 802.11 Frequency Hopping Spread Spectrum (FHSS) PHY uses the 2.4 GHz radio

frequency band, operating with at 1 or 2 Mbps data rate.

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The principle of 'LUHFW�6HTXHQFH is to spread a signal on a larger frequency band by

multiplexing it with a signature or code to minimize localized interference and background

noise. To spread the signal, each bit is modulated by a code. In the receiver, the original signal

is recovered by receiving the whole spread channel and demodulating with the same code

used by the transmitter. The 802.11 'LUHFW�6HTXHQFH�6SUHDG�6SHFWUXP (DSSS) PHY also uses

the 2.4 GHz radio frequency band.

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The Infrared PHY utilizes infrared light to transmit binary data either at 1 Mbps (basic access

rate) or 2 Mbps (enhanced access rate) using a specific modulation technique for each. For 1

Mbps, the infrared PHY uses a 16-pulse position modulation (PPM). The concept of PPM is

to vary the position of a pulse to represent different binary symbols. Infrared transmission at 2

Mbps utilizes a 4 PPM modulation technique.


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The term "Roaming" means that a client can switch between access points seamlessly while

physically moving or as a result of load balancing between access points. As the client

physically gets closer to another access point, the signal strength from the first will dropp

while the signal strength from the other will increase. At one point the signal strenghts of the

two will be equal but then the other will have the strongest signal and the client should roam

to the next access point. To avoid interference the channels selected should be properly

spaced apart. The graphics illustrate how one can reuse a channel and still avoid interference.

The client will seamlessly roam between these access points.

For the client to be able to roam seamlessly, it is necessary for the access points to:

• Be connected to the same IP subnet so the client won’t have to change IP address

• Have the same SSID to identify the wireless network

• Have the same WEP keys so that the client knows how to encrypt the data

If one or more of these requirements are not met, network communication for the client will

halt. The client will not regain communication before it configures the correct parameters for

the new access point or untill the client reaches another access point which has the same

configuration as the first one.

The client does not have to physically move to roam. Roaming could also happen as

the result of the access points negotiating load sharing or load balancing. Wireless network is

a shared medium and more clients means less resource for each. The sensible thing would be

for the access points to share the load. The decision to roam a client is the result of the access

points evaluating factors like the number of associated clients on the different access points,

the signal strength and quality of the client and the traffic load of the different access points.

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The task groups of the 802.11 standard are:

��D - Created a standard for WLAN operations in the 5 GHz band, with data rates of up

to 54 million bits per sec (Mbps). Published in 1999.

��E - Created a standard (also known as WiFi) for WLAN operations in 2.4 GHz band,

with data rates of up to 11 Mbps. Published in 1999.

��F - Provided documentation of 802.11-specific MAC procedures to the ISO/IEC

(International Organization for Standardization/International Electrotechnical Commission)

10038 (IEEE 802.1D) standard.

��G - Publish definitions and requirements to allow the 802.11 standard to operate in

countries not currently served by the standard.

��H - Attempts to enhance the 802.11 MAC to increase the quality of service possible.

Improvement in capabilities and efficiency are planned to allow applications such as voice,

video, or audio transport over 802.11 wireless networks.

��I - Develop recommended practices for implementing the 802.11 concepts of Access

Points and Distribution Systems. The purpose is to increase compatibility between Access

Point devices from different vendors.

��J - Developing a higher-speed PHY extension to the 802.11b standard, while

maintaining backward compatibility with current 802.11b devices with target data rate of at

least 20 Mbps

��K - Enhancing the 802.11 MAC and 802.11a PHY to provide network management and

control extensions for spectrum and transmit power management in the 5 GHz band to allow

regulatory acceptance of the standard in some European countries.

��L - Enhancing the security and authentication mechanisms of the 802.11 standard.

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http://grouper.ieee.org/groups/802/11/ (IEEE 802.11 Wireless LAN Working Group)

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IEEE 802.11 FAMILY