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Introduction to IEEE 802

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					Introduction to IEEE 802.11
In 1997 the IEEE adopted IEEE Std. 802.11-1997, the first wireless LAN (WLAN)
standard. This standard defines the media access control (MAC) and physical (PHY)
layers for a LAN with wireless connectivity. It addresses local area networking where the
connected devices communicate over the air to other devices that are within close
proximity to each other. This paper provides an overview of the 802.11 architecture and
the different topologies incorporated to accomodate the unique characteristics of the
IEEE 802.11 wireless LAN standard.


The standard is similar in most respects to the IEEE 802.3 Ethernet standard.
Specifically, the 802.11 standard addresses:

      Functions required for an 802.11 compliant device to operate either in a peer-to-
       peer fashion or integrated with an existing wired LAN
      Operation of the 802.11 device within possibly overlapping 802.11 wireless
       LANs and the mobility of this device between multiple wireless LANs
      MAC level access control and data delivery services to allow upper layers of the
       802.11 network
      Several physical layer signaling techniques and interfaces
      Privacy and security of user data being transferred over the wireless media




Figure 1 - IEEE 802.11 standards mapped to the OSI reference model.

What makes a Wireless LAN unique?

There are a number of characteristics that are unique to the wireless environment (as
compared to a wired LAN) that the 802.11 standard must take into consideration. The
physical characteristics of a wireless LAN introduce range limitations and unreliable
media, dynamic topologies where stations move about, interference from outside sources,
and lack of the ability for every device to "hear" every other device within the WLAN.


These limitations force the WLAN standard to create fundamental definitions for short-
range LANs made up of components that are within close proximity of each other. Larger
geographic coverage is handled by building larger LANs from the smaller fundamental
building blocks or by integrating the smaller WLANs with an existing wired network.

More on mobility

Mobility of wireless stations may be the most important feature of a wireless LAN. A
WLAN would not serve much purpose if stations were not able to move about freely
from location to location either within a specific WLAN or between different WLAN
"segments".


For compatibility purposes, the 802.11 MAC must appear to the upper layers of the
network as a "standard" 802 LAN. The 802.11 MAC layer is forced to handle station
mobility in a fashion that is transparent to the upper layers of the 802 LAN stack. This
forces functionality into the 802.11 MAC layer that is typically handled by upper layers.

The IEEE 802.11 Wireless LAN Architecture

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.

Wireless LAN Station

The station (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.

Basic Service Set (BSS)

802.11 defines the Basic Service Set (BSS) as the basic building block of an 802.11
wireless LAN. The BSS consists of a group of any number of stations. The BSS is not a
very interesting topic until we take the topology of the WLAN into consideration.

802.11 Topologies

Independent Basic Service Set (IBSS)

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 Independent Basic Service Set (IBSS) or an Ad-hoc
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.

BSS




Figure 2 - Independent Basic Service Set (IBSS)

Infrastructure Basic Service Set

An Infrastructure Basic Service Set is a BSS with a component called an Access Point
(AP). 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.

Distribution System
Figure 3 - Infrastructure Basic Service Set

Distribution System (DS)
The distribution system (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.

Extending coverage via an Extended Service Set (ESS)
802.11 extends the range of mobility to an arbitrary range through the Extended Service
Set (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.




Figure 4 - Extended Service Set (ESS)

Station Services

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.

Authentication

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.

Open system authentication


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.

Shared key authentication

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 knowledge of the secret key. Use of shared key authentication requires
implementation of encryption via the Wired Equivalent Privacy or WEP algorithm.

De-authentication

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.

Privacy

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 tacking
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.

Data Delivery

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.

Distribution Services

Distribution services provide functionality across a distribution system. Typically, access
points provide distribution services. The five distribution services and functions detailed
below include: association, disassociation, re-association, distribution, and integration.

Association

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.

Disassociation

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.


An access point may force a station or stations to disassociate because of resource
restraints, the access point is shutting down or being removed from the network for a
variety of reasons. When a mobile station is aware that it will no longer require the
services of an access point, it may invoke the disassociation service to notify the access
point that the logical connection to the services of the access point from this mobile
station is no longer required.


Stations should disassociate when they leave a network, though there is nothing in the
architecture to assure this happens. Disassociation is a notification and can be invoked by
either associated party. Neither party can refuse termination of the association.

Re-association

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


Buy 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.

Distribution

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.

Integration

The integration service connects the 802.11 WLAN to other LANs, including one or
more wired LANs or 802.11 WLANs. A portal 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.

802.11 Media Access Control

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,
best-effort, 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 Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA). 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
Wireless Equivalent Privacy (WEP), which is an encryption service for data delivered on
the WLAN.

More on CSMA/CA

The fundamental access method of 802.11 is Carrier Sense Multiple Access with
Collision Avoidance or CSMA/CA. CSMA/CA works by a "listen before talk scheme".
This means that a station wishing to transmit must first sense the radio channel to
determine if another station is transmitting. If the medium is not busy, the transmission
may proceed.


The CSMA/CA protocol avoids collisions among stations sharing the medium by
utilizing a random backoff time if the station's physical or logical sensing mechanism
indicates a busy medium. The period of time immediately following a busy medium is
the highest probability of collisions occurring, especially under high utilization.

The CSMA/CA scheme implements a minimum time gap between frames from a given
user. Once a frame has been sent from a given transmitting station, that station must wait
until the time gap is up to try to transmit again. Once the time has passed, the station
selects a random amount of time (the backoff interval) to wait before "listening" again to
verify a clear channel on which to transmit. If the channel is still busy, another backoff
interval is selected that is less than the first. This process is repeated until the waiting
time approaches zero and the station is allowed to transmit. This type of multiple access
ensures judicious channel sharing while avoiding collisions.

802.11 Physical Layer (PHY)

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 Frequency Hopping Spread
Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) 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.

Spread Spectrum

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.

Frequency Hopping Spread Spectrum (FHSS)

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.

Direct Sequence Spread Spectrum (DSSS)

The principle of Direct Sequence 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 Direct Sequence Spread Spectrum (DSSS) PHY also uses the 2.4 GHz radio
frequency band.

Infrared (IR)

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.

Conclusion

Wireless networking has a promising future with 802.11 leading the way as the standard
for adoption in local networking environments. 802.11 addresses mobility, security,
reliability, and the dynamic nature of wireless LANS while keeping compatibility with
802-type legacy networks. Expect to see availability of 802.11 products increase
dramatically in the near future as businesses discover the increased productivity provided
by "untethered" networks.

				
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