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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
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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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Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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.
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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.
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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.
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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
&DUULHU�6HQVH�0XOWLSOH�$FFHVV�ZLWK�&ROOLVLRQ�$YRLGDQFH��&60$�&$�. CSMA/CA is similar
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.
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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.
Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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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Linköpings University Nithya Suresh Department of Computer IEEE 802.3 Family and Information Science
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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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