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IEEE 802.11

IEEE 802.11 is a set of standards for implementing wireless local area network(WLAN)

computer communication in the 2.4, 3.6, 5 and 60 GHz frequency bands. They are created andmaintained by the IEEE LAN/MAN Standards Committee (IEEE 802). The base version of the

standard was released in 1997 and has had subsequent amendments. These standards provide thebasis for wireless network products using the Wi-Fibrand.

General description

The 802.11 family consist of a series of half-duplex over-the-airmodulation techniques

that use the same basic protocol. The most popular are those defined by the 802.11b and 802.11gprotocols, which are amendments to the original standard. 802.11-1997 was the first wireless

networking standard, but 802.11a was the first widely accepted one, followed by 802.11b and

802.11g. 802.11n is a new multi-streaming modulation technique. Other standards in the family(cf, h, j) are service amendments and extensions or corrections to the previous specifications.

802.11b and 802.11g use the 2.4 GHz ISM band, operating in the United States underPart 15 of the US Federal Communications Commission Rules and Regulations. Because of this

choice of frequency band, 802.11b and g equipment may occasionally suffer interference from

microwave ovens, cordless telephones and Bluetooth devices. 802.11b and 802.11g control their

interference and susceptibility to interference by using direct-sequence spread spectrum (DSSS)and orthogonal frequency-division multiplexing (OFDM) signaling methods, respectively.

802.11a uses the 5 GHz U-NII band, which, for much of the world, offers at least 23 non-

overlapping channels rather than the 2.4 GHz ISM frequency band, where adjacent channels

overlap - see list of WLAN channels. Better or worse performance with higher or lowerfrequencies (channels) may be realized, depending on the environment.

The segment of the radio frequency spectrum used by 802.11 varies between countries. In

the US, 802.11a and 802.11g devices may be operated without a license, as allowed in Part 15 of

the FCC Rules and Regulations. Frequencies used by channels one through six of 802.11b and802.11g fall within the 2.4 GHz amateur radio band. Licensed amateur radio operators may

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operate 802.11b/g devices underPart 97 of the FCC Rules and Regulations, allowing increased

power output but not commercial content or encryption.

History

802.11 technology has its origins in a 1985 ruling by the U.S. Federal Communications

Commission that released the ISM band for unlicensed use.

In 1991 NCR Corporation/AT&T (now Alcatel-Lucent and LSI Corporation) invented

the precursor to 802.11 in Nieuwegein, The Netherlands. The inventors initially intended to use

the technology for cashier systems. The first wireless products were brought to the market under

the name WaveLAN with raw data rates of 1 Mbit/s and 2 Mbit/s.

Vic Hayes, who held the chair of IEEE 802.11 for 10 years and has been called the

"father of Wi-Fi" was involved in designing the initial 802.11b and 802.11a standards within theIEEE.

[4]

In 1999, the Wi-Fi Alliance was formed as a trade association to hold the Wi-Fi

trademark under which most products are sold.

Protocols

802.11

protocol

Freq.

(GHz)

Bandwidth

(MHz)

Data rate per

stream

(Mbit/s)

Allowable

MIMO

streams

Modulation Approximate

indoor range

Approximate

outdoor

range]

(m) (ft) (m) (ft)

2.4 20 1, 2 1 DSSS,

FHSS

20 66 100 330

a 5 20 6, 9, 12, 18,

24,36, 48, 54

1 OFDM 35 115 120 390

3.7 5,000 16,000

b 2.4 20 1, 2, 5.5, 11 1 DSSS 35 115 140 460

g 2.4 20 6, 9, 12, 18,

24,36, 48, 54

1 OFDM,

DSSS

38 125 140 460

n 2.4/5 20 7.2,14.4,21.7,

28.9,43.3,57.8,65, 72.2

4 OFDM 70 230 250 820

40 15,30,45,60,90,120,135, 150

70 230 250 820

ac(DRAFT)

2.4/5 20 up to 87.6 840 up to 200

80 up to 433.3

160 up to 866.7

ad 2.4/5/60 up to 7000

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Frames

Current 802.11 standards define "frame" types for use in transmission of data as well asmanagement and control of wireless links.

Frames are divided into very specific and standardized sections. Each frame consists of an MAC

header, payload and frame check sequence (FCS). Some frames may not have the payload. The

first two bytes of the MAC header form a frame control field specifying the form and function of

the frame. The frame control field is further subdivided into the following sub-fields:

Protocol Version: two bits representing the protocol version. Currently used protocol

version is zero. Other values are reserved for future use.

Type: two bits identifying the type of WLAN frame. Control, Data and Management are

various frame types defined in IEEE 802.11.

Sub Type: Four bits providing addition discrimination between frames. Type and Subtype together to identify the exact frame.

ToDS and FromDS: Each is one bit in size. They indicate whether a data frame isheaded for a distribution system. Control and management frames set these values to

zero. All the data frames will have one of these bits set. However communication within

an IBSS network always set these bits to zero.

More Fragments: The More Fragments bit is set when a packet is divided into multiple

frames for transmission. Every frame except the last frame of a packet will have this bit

set.

Retry: Sometimes frames require retransmission, and for this there is a Retry bit which isset to one when a frame is resent. This aids in the elimination of duplicate frames.

Power Management: This bit indicates the power management state of the sender afterthe completion of a frame exchange. Access points are required to manage the connection

and will never set the power saver bit.

More Data: The More Data bit is used to buffer frames received in a distributed system.

The access point uses this bit to facilitate stations in power saver mode. It indicates that at

least one frame is available and addresses all stations connected.

WEP: The WEP bit is modified after processing a frame. It is toggled to one after a

frame has been decrypted or if no encryption is set it will have already been one.

Order: This bit is only set when the "strict ordering" delivery method is employed.

Frames and fragments are not always sent in order as it causes a transmissionperformance penalty.

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The next two bytes are reserved for the Duration ID field. This field can take one of three forms:

Duration, Contention-Free Period (CFP), and Association ID (AID).

An 802.11 frame can have up to four address fields. Each field can carry a MAC address.

Address 1 is the receiver, Address 2 is the transmitter, Address 3 is used for filtering purposes bythe receiver.

The Sequence Control field is a two-byte section used for identifying message order aswell as eliminating duplicate frames. The first 4 bits are used for the fragmentation

number and the last 12 bits are the sequence number.

An optional two-byte Quality of Service control field which was added with 802.11e.

The Frame Body field is variable in size, from 0 to 2304 bytes plus any overhead from

security encapsulation and contains information from higher layers.

The Frame Check Sequence (FCS) is the last four bytes in the standard 802.11 frame.

Often referred to as the Cyclic Redundancy Check (CRC), it allows for integrity check ofretrieved frames. As frames are about to be sent the FCS is calculated and appended.

When a station receives a frame it can calculate the FCS of the frame and compare it to

the one received. If they match, it is assumed that the frame was not distorted duringtransmission.

Management Frames

Management Frames allow for the maintenance of communication. Some common 802.11

subtypes include:

Authentication frame: 802.11 authentication begins with the WNIC sending an

authentication frame to the access point containing its identity. With an open systemauthentication the WNIC only sends a single authentication frame and the access point

responds with an authentication frame of its own indicating acceptance or rejection. Withshared key authentication, after the WNIC sends its initial authentication request it will

receive an authentication frame from the access point containing challenge text. TheWNIC sends an authentication frame containing the encrypted version of the challenge

text to the access point. The access point ensures the text was encrypted with the correct

key by decrypting it with its own key. The result of this process determines the WNIC's

authentication status.

Association request frame: sent from a station it enables the access point to allocate

resources and synchronize. The frame carries information about the WNIC including

supported data rates and the SSID of the network the station wishes to associate with. Ifthe request is accepted, the access point reserves memory and establishes an association

ID for the WNIC. Association response frame: sent from an access point to a station containing the

acceptance or rejection to an association request. If it is an acceptance, the frame willcontain information such an association ID and supported data rates.

Beacon frame: Sent periodically from an access point to announce its presence and

provide the SSID, and other parameters for WNICs within range.

Deauthentication frame: Sent from a station wishing to terminate connection from

another station.

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Disassociation frame: Sent from a station wishing to terminate connection. It's an

elegant way to allow the access point to relinquish memory allocation and remove theWNIC from the association table.

Probe request frame: Sent from a station when it requires information from another

station.

Probe response frame: Sent from an access point containing capability information,

supported data rates, etc., after receiving a probe request frame. Reassociation request frame: A WNIC sends a reassociation request when it drops from

range of the currently associated access point and finds another access point with astronger signal. The new access point coordinates the forwarding of any information that

may still be contained in the buffer of the previous access point.

Reassociation response frame: Sent from an access point containing the acceptance orrejection to a WNIC reassociation request frame. The frame includes information

required for association such as the association ID and supported data rates.

Information Elements

2. In terms ofICT, an Information Element (IE) is a part of management frames in the IEEE802.11 wireless LAN protocol. IEs are a device's way to transfer descriptive information about

itself inside management frames. There are usually several IEs inside each such frame, and each

is built ofTLVs mostly defined outside the basic IEEE 802.11 specification.

The common structure of an IE is as follows:

1 1 3 1-252

------------------------------------------------

|Type |Length| OUI | Data |

------------------------------------------------

Whereas the OUI (organizationally unique identifier) is only used when necessary to the protocolbeing used, and the data field holds the TLVs relevant to that IE.

Control Frames

Control frames facilitate in the exchange of data frames between stations. Some common 802.11control frames include:

Acknowledgement (ACK) frame: After receiving a data frame, the receiving station willsend an ACK frame to the sending station if no errors are found. If the sending station

doesn't receive an ACK frame within a predetermined period of time, the sending station

will resend the frame. Request to Send (RTS) frame: The RTS and CTS frames provide an optional collision

reduction scheme for access points with hidden stations. A station sends a RTS frame to

as the first step in a two-way handshake required before sending data frames.

Clear to Send (CTS) frame: A station responds to an RTS frame with a CTS frame. Itprovides clearance for the requesting station to send a data frame. The CTS provides

collision control management by including a time value for which all other stations are to

hold off transmission while the requesting stations transmits.

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Data frames carry packets from web pages, files, etc. within the body, using RFC 1042

encapsulation and EtherType numbers for protocol identification.

Standard and amendments

The data rates supported by the original 802.11standardsare too slow to support mostgeneral business requirements and slowed the adoption of WLANs. So several standards of

802.11 was developed.

IEEE 802.11-1997: The WLAN standard was originally 1 Mbit/s and 2 Mbit/s, 2.4 GHz

RF and infrared (IR) standard (1997), all the others listed below are Amendments to this

standard, except for Recommended Practices 802.11F and 802.11T.

IEEE 802.11a: 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)(OFDM)

IEEE 802.11b: Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999) (also known

as 802.11 High Rate)

IEEE 802.11c: Bridge operation procedures; included in the IEEE 802.1D standard(2001)

IEEE 802.11d: International (country-to-country) roaming extensions (2001)

IEEE 802.11e: Enhancements: QoS, including packet bursting (2005)

IEEE 802.11F: Inter-Access Point Protocol (2003) Withdrawn February 2006

IEEE 802.11g: 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)

IEEE 802.11h: Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)

IEEE 802.11i: Enhanced security (2004)

IEEE 802.11j: Extensions for Japan (2004)

IEEE 802.11-2007: A new release of the standard that includes amendments a, b, d, e, g,

h, i and j. (July 2007)

IEEE 802.11k: Radio resource measurement enhancements (2008)

IEEE 802.11n: Higher throughput improvements using MIMO (multiple input, multiple

output antennas) (September 2009)

IEEE 802.11p: WAVEWireless Access for the Vehicular Environment (such asambulances and passenger cars) (July 2010)

IEEE 802.11r: Fast BSS transition (FT) (2008)

IEEE 802.11s: Mesh Networking, Extended Service Set (ESS) (July 2011)

IEEE 802.11T: Wireless Performance Prediction (WPP)test methods and metricsRecommendation cancelled

IEEE 802.11u: Improvements related to HotSpots and 3rd party authorization of clients,

e.g. cellular network offload (February 2011)

IEEE 802.11v: Wireless network management (February 2011)

IEEE 802.11w: Protected Management Frames (September 2009)

IEEE 802.11y: 36503700 MHz Operation in the U.S. (2008)

IEEE 802.11z: Extensions to Direct Link Setup (DLS) (September 2010)

IEEE 802.11-2012: A new release of the standard that includes amendments k, n, p, r, s,

u, v, w, y and z (March 2012)

IEEE 802.11aa: Robust streaming of Audio Video Transport Streams (June 2012)

IEEE 802.11ad: Very High Throughput 60 GHz (December 2012) - see WiGig

IEEE 802.11ae: Prioritization of Management Frames (March 2012)

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In process

IEEE 802.11ac: Very High Throughput

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(AP). An access point is a station, thus addressable. So, data moves between the BSS and the

DS with the help of these access points.

Creating large and complex networks using BSS's and DS's leads us to the next level of

hierarchy, the Extended Service Set or ESS. The beauty of the ESS is the entire network looks

like an independent basic service set to the Logical Link Control layer (LLC). This means that

stations within the ESS can communicate or even move between BSSs transparently to the LLC.

Infrastructure Mode

One of the requirements of IEEE 802.11 is that it can be used with existing wired

networks. 802.11 solved this challenge with the use of a Portal. A portal is the logical integrationbetween wired LANs and 802.11. It also can serve as the access point to the DS. All data going to

an 802.11 LAN from an 802.X LAN must pass through a portal. It thus functions as bridge

between wired and wireless.

The implementation of the DS is not specified by 802.11. Therefore, a distribution system

may be created from existing or new technologies. A point-to-point bridge connecting LANs in two

separate buildings could become a DS.

While the implementation for the DS is not specified, 802.11 does specify the services, which

the DS must support. Services are divided into two sections

1. Station Services (SS)

2. Distribution System Services (DSS).

There are five services provided by the DSS

1. Association

2. Reassociation

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3. Disassociation

4. Distribution

5. Integration

Association supports no-transition mobility but is not enough to support BSS-transition.

Enter Reassociation. This service allows the station to switch its association from one AP to

another. Both association and reassociation are initiated by the station. Disassociation is when theassociation between the station and the AP is terminated. This can be initiated by either party. A

disassociated station cannot send or receive data. ESS-transition are not supported. A station can

move to a new ESS but will have to reinitiate connections.

Distribution and Integration are the remaining DSS's. Distribution is simply getting the data

from the sender to the intended receiver. The message is sent to the local AP (input AP), then

distributed through the DS to the AP (output AP) that the recipient is associated with. If the sender

and receiver are in the same BSS, the input and out AP's are the same. So the distribution service

is logically invoked whether the data is going through the DS or not. Integration is when the output

AP is a portal. Thus, 802.x LANs are integrated into the 802.11 DS.

Station services are:

1. Authentication

2. Deauthentication

3. Privacy

4. MAC Service Data Unit (MSDU) Delivery.

With a wireless system, the medium is not exactly bounded as with a wired system. In order to

control access to the network, stations must first establish their identity. This is much like trying to

enter a radio net in the military.

Before you are acknowledged and allowed to converse, you must first pass a series of tests to

ensure that you are who you say you are. That is really all authentication is. Once a station has

been authenticated, it may then associate itself. The authentication relationship may be between

two stations inside an IBSS or to the AP of the BSS. Authentication outside of the BSS does not

take place.

There are two types of authentication services offered by 802.11. The first is Open System

Authentication. This means that anyone who attempts to authenticate will receive authentication.

The second type is Shared Key Authentication. In order to become authenticated the users must

be in possession of a shared secret. The shared secret is implemented with the use of the Wired

Equivalent Privacy (WEP) privacy algorithm. The shared secret is delivered to all stations aheadof time in some secure method (such as someone walking around and loading the secret onto

each station).

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Deauthentication is when either the station or AP wishes to terminate a stations

authentication. When this happens the station is automatically disassociated. Privacy is an

encryption algorithm, which is used so that other 802.11 users cannot eavesdrop on your LAN

traffic. IEEE 802.11 specifies Wired Equivalent Privacy (WEP) as an optional algorithm to satisfy

privacy. If WEP is not used then stations are "in the clear" or "in the red", meaning that their traffic

is not encrypted. Data transmitted in the clear are called plaintext. Data transmissions, which are

encrypted, are called ciphertext. All stations start "in the red" until they are authenticated. MSDUdelivery ensures that the information in the MAC service data unit is delivered between the

medium access control service access points.

The bottom line is this, authentication is basically a network wide password. Privacy iswhether or not encryption is used. Wired Equivalent Privacy is used to protect authorized stations

from eavesdroppers. WEP is reasonably strong. The algorithm can be broken in time. The

relationship between breaking the algorithm is directly related to the length of time that a key is in

use. So, WEP allows for changing of the key to prevent brute force attack of the algorithm. WEP

can be implemented in hardware or in software.

Security

In 2001, a group from the University of California, Berkeleypresented a paper describing

weaknesses in the 802.11 Wired Equivalent Privacy (WEP) security mechanism defined in the

original standard; they were followed by Fluhrer, Mantin, and Shamir's paper titled "Weaknesses

in the Key Scheduling Algorithm of RC4". Not long after, Adam Stubblefield and AT&Tpublicly announced the first verification of the attack. In the attack, they were able to intercept

transmissions and gain unauthorized access to wireless networks.

http://en.wikipedia.org/wiki/University_of_California,_Berkeleyhttp://en.wikipedia.org/wiki/802.11http://en.wikipedia.org/wiki/Wired_Equivalent_Privacyhttp://en.wikipedia.org/wiki/Fluhrer,_Mantin_and_Shamir_attackhttp://en.wikipedia.org/wiki/RC4http://en.wikipedia.org/wiki/AT%26T_Corporationhttp://en.wikipedia.org/wiki/AT%26T_Corporationhttp://en.wikipedia.org/wiki/RC4http://en.wikipedia.org/wiki/Fluhrer,_Mantin_and_Shamir_attackhttp://en.wikipedia.org/wiki/Wired_Equivalent_Privacyhttp://en.wikipedia.org/wiki/802.11http://en.wikipedia.org/wiki/University_of_California,_Berkeley

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The Security in 802.11 is increased by,

Authentication on1. New nodes issue a Request for authentication.2. Network sends a block of random text.3. The node encrypts it with network password and returns.

One shared secret key per network The same encryption algorithm is used for privacy. WEP Algorithm is based on RC4

PRNT algorithm developed by RSA Data Security, Inc is widely used.

PHYSICAL LAYER:

The purpose of this document is to explain the basic ideas laying in the foundation of the

technologies adopted by IEEE 802.11 standards for wireless communications at the physicallayer. It is designed for audience working with or administrating the devices complying to the

named standards, and willing to know their principles of operation believing that such

knowledge can help to make educated decisions regarding the related equipment, choose andutilize the available hardware more efficiently.

Using Radio Waves For Data Transmission

Designing a wireless high speed data exchange system is not a trivial task to do. Neither is

the development of the standard for wireless local area networks. The major problems at thephysical layer here caused by the nature of the chosen media are:

Bandwidth allocation;

External interference;

Reflection.

802.11 First Standard For Wireless

LANs

The Institute of Electronic and

Electrical Engineers (IEEE) hasreleased IEEE 802.11 in June 1997.

The standard defined physical and

MAC layers of wireless local area

networks (WLANs).

The physical layer of the

original 802.11 standardized three

wireless data exchange techniques:

Infrared (IR);

Frequency hopping spread spectrum (FHSS);

Direct sequence spread spectrum (DSSS).

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The 802.11 radio WLANs operate in the 2.4GHz (2.4 to 2.483 GHz) unlicensed Radio

Frequency (RF) band. The maximum isotropic transmission power in this band allowed by FCCin US is 1Wt, but 802.11 devices are usually limited to the 100mWt value.

The physical layer in 802.11 is split into Physical Layer Convergence Protocol (PLCP) andthe Physical Medium Dependent (PMD) sub layers. The PLCP prepares/parses data units

transmitted/received using various 802.11 media access techniques. The PMD performs the datatransmission/reception and modulation/demodulation directly accessing air under the guidance ofthe PLCP. The 802.11 MAC layer to the great extend is affected by the nature of the media. For

example, it implements a relatively complex for the second layer fragmentation of PDUs.

IR Layer:

Baseband Transmission

850 t0 950 nm range IR

1 Mbps 0r 2 Mbps

Diffuse IR

Up to 10m in typical receivers

FHSS Layer:

2.4 GHz ISM Band

1 and 2 Mbps

3 sets of frequency hopping patterns. Each set has 22 hopping sequences. Total 66channels.

Consecutive frequencies in each sequence are at least 6 MHz apart to avoid a narrowbandinterferer

Adjacent or overlapping cells use different patterns

DSSS Layer:

2.4 GHz band

11 chip spreading factor

11 Channels

Only 3 channels without overlap

10 mW to 100 mW transmitted power

1 and 2 Mbps

Terminal Problem in WLAN:

Terminal problem is peculiar to wireless because it is no found in wired modetransmission. There are two major problems in WLAN. They are

1. Hidden Node Problem2. Exposed Node Problem

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HIDDEN NODE PROBLEM

Inwireless networking, the hidden node problem orhidden terminal problem occurs

when a node is visible from a wireless access point (AP), but not from other nodes

communicating with said AP. This leads to difficulties inmedia access control.

Hidden nodes in awireless networkrefer to nodes that are out of range of other nodes or

a collection of nodes. Take a physical star topology with an access point with many nodes

surrounding it in a circular fashion: Each node is within communication range of the AP, but the

nodes cannot communicate with each other, as they do not have a physical connection to eachother. In a wireless network, it is likely that the node at the far edge of the access point's range,

which is known as A, can see the access point, but it is unlikely that the same node can see a

node on the opposite end of the access point's range, C. These nodes are known as hidden. The

problem is when nodes A and C start to sendpacketssimultaneously to the access point B. Since

the nodes cannot sense the carrier, Carrier sense multiple access with collision avoidance(CSMA/CA) does not work, and collisions occur, corrupting the data at the access point. To

overcome this problem, handshaking is implemented in conjunction with the CSMA/CA scheme.

The hidden node problem can be observed easily in widespread (>50m radius)WLANsetups with many nodes that usedirectional antennasand have high upload. This is why IEEE

802.11 is suited for bridging the last mile for broadband access only to a very limited extent.

Newer standards such as WiMAX assign time slots to individual stations, thus preventing

multiple nodes from sending simultaneously and ensuring fairness even in over-subscriptionscenarios.

Solution:

IEEE 802.11 uses 802.11 RTS/CTS acknowledgment and handshake packets to partly

overcome the hidden node problem. RTS/CTS is not a complete solution and may decreasethroughput even further, but adaptive acknowledgments from the base station can help too.

http://en.wikipedia.org/wiki/Wireless_networkinghttp://en.wikipedia.org/wiki/Wireless_networkinghttp://en.wikipedia.org/wiki/Wireless_networkinghttp://en.wikipedia.org/wiki/Node_%28networking%29http://en.wikipedia.org/wiki/Node_%28networking%29http://en.wikipedia.org/wiki/Wireless_access_pointhttp://en.wikipedia.org/wiki/Wireless_access_pointhttp://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/Wireless_networkhttp://en.wikipedia.org/wiki/Wireless_networkhttp://en.wikipedia.org/wiki/Wireless_networkhttp://en.wikipedia.org/wiki/Star_networkhttp://en.wikipedia.org/wiki/Star_networkhttp://en.wikipedia.org/wiki/Packet_%28information_technology%29http://en.wikipedia.org/wiki/Packet_%28information_technology%29http://en.wikipedia.org/wiki/Packet_%28information_technology%29http://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidancehttp://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidancehttp://en.wikipedia.org/wiki/CSMA_CAhttp://en.wikipedia.org/wiki/CSMA_CAhttp://en.wikipedia.org/wiki/CSMA_CAhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/Last_milehttp://en.wikipedia.org/wiki/Last_milehttp://en.wikipedia.org/wiki/Last_milehttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/802.11_RTS/CTShttp://en.wikipedia.org/wiki/802.11_RTS/CTShttp://en.wikipedia.org/wiki/802.11_RTS/CTShttp://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/WiMAXhttp://en.wikipedia.org/wiki/Last_milehttp://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/IEEE_802.11http://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/CSMA_CAhttp://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidancehttp://en.wikipedia.org/wiki/Packet_%28information_technology%29http://en.wikipedia.org/wiki/Star_networkhttp://en.wikipedia.org/wiki/Wireless_networkhttp://en.wikipedia.org/wiki/Media_Access_Controlhttp://en.wikipedia.org/wiki/Wireless_access_pointhttp://en.wikipedia.org/wiki/Node_%28networking%29http://en.wikipedia.org/wiki/Wireless_networking

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Protocol Description(RTS/CTS)

Anodewishing to send data initiates the process by sending a Request to Send frame

(RTS). The destination node replies with a Clear To Send frame (CTS). Any other node

receiving the RTS or CTS frame should refrain from sending data for a given time (solving thehidden node problem). The amount of time the node should wait before trying to get access to

the medium is included in both the RTS and the CTS frame. This protocol was designed underthe assumption that all nodes have the same transmission ranges.

The other methods that can be employed to solve hidden node problem are :

Increase Transmitting Power From the Nodes

Useomnidirectional antennas

Remove obstacles

Move the node

Use protocol enhancement software

Useantenna diversity

EXPOSED NODE PROBLEM

Inwireless networks, the exposed node problem occurs when a node is prevented from

sending packets to other nodes due to a neighboring transmitter. Consider an example of 4 nodes

labeled R1, S1, S2, and R2, where the two receivers are out of range of each other, yet the twotransmitters in the middle are in range of each other. Here, if a transmission between S1 and R1

is taking place, node S2 is prevented from transmitting to R2 as it concludes aftercarrier sense

that it will interfere with the transmission by its neighbor S1. However note that R2 could still

receive the transmission of S2 without interference because it is out of range of S1.

Solution:

IEEE 802.11 RTS/CTS mechanism helps to solve this problem only if the nodes are

synchronized and packet sizes and data rates are the same for both the transmitting nodes. When

a node hears an RTS from a neighboring node, but not the corresponding CTS, that node can

deduce that it is an exposed node and is permitted to transmit to other neighboring nodes.

http://en.wikipedia.org/wiki/Node_%28networking%29http://en.wikipedia.org/wiki/Node_%28networking%29http://en.wikipedia.org/wiki/Node_%28networking%29http://en.wikipedia.org/wiki/Frame_%28networking%29http://en.wikipedia.org/wiki/Frame_%28networking%29http://en.wikipedia.org/wiki/Hidden_node_problemhttp://en.wikipedia.org/wiki/Hidden_node_problemhttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/Antenna_diversityhttp://en.wikipedia.org/wiki/Antenna_diversityhttp://en.wikipedia.org/wiki/Antenna_diversityhttp://en.wikipedia.org/wiki/Wireless_networkshttp://en.wikipedia.org/wiki/Wireless_networkshttp://en.wikipedia.org/wiki/Wireless_networkshttp://en.wikipedia.org/wiki/Carrier_sensehttp://en.wikipedia.org/wiki/Carrier_sensehttp://en.wikipedia.org/wiki/Carrier_sensehttp://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTShttp://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTShttp://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTShttp://en.wikipedia.org/wiki/Carrier_sensehttp://en.wikipedia.org/wiki/Wireless_networkshttp://en.wikipedia.org/wiki/Antenna_diversityhttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/Hidden_node_problemhttp://en.wikipedia.org/wiki/Frame_%28networking%29http://en.wikipedia.org/wiki/Node_%28networking%29

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If the nodes are not synchronized (or if the packet sizes are different or the data rates are

different) the problem may occur that the sender will not hear the CTS or the ACK during thetransmission of data of the second sender.

Applications:

Wireless LANs have a great deal of applications. Modern implementations of WLANs

range from small in-home networks to large, campus-sized ones to completely mobile networks

on airplanes and trains. Users can access the Internet from WLAN hotspots in restaurants, hotels,and now with portable devices that connect to 3G or 4G networks. Oftentimes these types of

public access points require no registration or password to join the network. Others can be

accessed once registration has occurred and/or a fee is paid.

Future:

More Bandwidth in future by

1. Better encoding: Multilevel modulation 8Mbps2. Fewer channels with more bandwidth 4 MHz channels3. Find another band. May get 150 MHz band in 5 GHz band.