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Wireless LAN IEEE

2.1 IEEE 802.11 Wireless LAN Standards

This chapter gives the overview of the IEEE 802.11 Standard (WLAN). The IEEE 802.11 standard (WLAN) is designed under the assumption that all nodes use Omni directional antennas. Most of the research to date has concentrated on developing MAC protocols under this assumption [2][4].This chapter will give the understanding of basic concepts , the principle of operations ,the features and components of 802.11 standard [2].

2.1.1 IEEE 802.11 standard Architecture

The architecture of the IEEE 802.11 WLAN is designed to support a network where most decision-making is distributed across the mobile stations [4]. The basic components of the IEEE 802.11 WLAN are described below

Station: In IEEE 802.11 WLAN network a station is the component that connects to the wireless medium. The station can be portable, mobile or stationary. Every station supports all the services which include security, privacy and delivery of data [4].

Basic Service Set (BSS): IEEE 802.11 WLAN is based on cellular architecture where the system is divided into cells. Each cell is called Basic Service Set (BSS) [2]. A BSS is set of stations that communicate with each other. There are two mode or types of communications the Infrastructure mode or Ad Hoc network mode [2][4].

Ad Hoc network mode: In Ad Hoc network mode all the station in the BSS can communicate with each other directly and there is no connection to a wired network or Access point (AP)[2][4], and part of its functionality is performed by the end-user stations (like Beacon Generation, synchronization, etc.), and other functions are not supported (like frame-relaying between two stations not in range, or Power Saving).This is also known as Independent BSS (IBSS) [4].

Infrastructure mode: On the other hand when a BSS includes an access point (AP)or base station, the BSS is no longer independent and is called Infrastructure mode [4].In infrastructure mode, base station (Access Point-AP) can coordinate the access of stations (STA) to the wireless medium and form the cells.[2] The Access Points are connected through some kind of backbone called Distribution System (DS), typically Ethernet, and in some cases wireless itself. [2]

Extended Service Set (ESS). An ESS is a set of combination of all Infrastructure BSSs, their respective Access Points (AP) and the Distribution System (DS), where the APs communicate among themselves to forward traffic from one BSS to another. The APs perform this communication via a Distribution System (DS) [2][4].

The DS is the backbone of the IEEE 802.11 WLAN and can be composed of wireless or wired network [4]. The Extended Service Set (ESS) is shown in Fig.2.1. [2]

Fig 2.1 IEEE 802.11 WLAN (The Extended Service Set) [2]

2.1.2 IEEE 802.11 Layers Description

The 802.11 protocol covers the MAC and Physical Layers [2]. The IEEE 802.11 Standard currently defines a single MAC which interacts with different types of physical layers (PHY). The IEEE Layers description is shown in Fig.2.2. [2]

Fig.2.2. IEEE Layers description is shown in [2][5]

2.1.3 IEEE 802.11 MAC Layer functional description

The IEEE 802.11 Medium Access control (MAC) provides the functionality needed to provide a reliable delivery mechanism for user data over wireless medium [4].

There are some other functions performed by the 802.11 MAC that are typically related to upper layer protocols, such as Fragmentation, Packet Retransmissions, and Acknowledgements [1][2]. The functionality performed by IEEE802.11 MAC Layer is described below:

The first function of the IEEE 802.11 MAC is to provide a reliable data delivery service to the users. This is achieved through a frame exchange protocol at MAC level [4].which consists of minimum two frames, the Data frame set from source to destination and acknowledgement frame set (Ack) from destination to source [4]. The protocol also suggests an Optional use of request to send (RTS) and clear to send (CTS) frame exchange between source and destination.[4]

The Second function of IEEE 802.11 MAC is to provide a fair mechanism to control access to shared wireless medium [2][4]. IEEE 802.11 MAC achieve this through implementing two different access mechanisms[4]:

The distributed coordination function (DCF) ,which is a contention-based mechanism

The point coordination function (PCF) ,which is a centrally controlled mechanism

The third function of IEEE 802.11 MAC layer is to protect the Data it delivers. This is done through a privacy service called Wired Equivalent Privacy (WEP) which encrypts the data before sending over the wireless medium [4].

2.1.3.1 MAC architecture

The IEEE 802.11 MAC architecture can be described as shown in Figure 2.3 as providing the PCF through the services of the DCF.[1]

Fig 2.3 IEEE 802.11 MAC architecture [1]

2.1.4 The Basic Access Mechanism: DCF and CSMA/CA

The basic access mechanism called Distributed Coordination Function (DCF) [1][2].when DCF is employed ,IEEE 802.11 basically uses a protocol called Carrier Sense Multiple Access with Collision Avoidance mechanism ( CSMA/CA) with binary exponential algorithm [5][4].CSMA protocols are well known ,In Ethernet another type of CSMA protocol is used ,which is Carrier Sense Multiple Access with Collision Detection (CSMA/CD) [2].

In this protocol, both physical carrier sensing and virtual carrier sensing are used [5].In CSMA/CA, if a station desiring to transmit senses the medium, if the medium is busy (i.e. some other station is transmitting) then the station will defer its transmission to a later time. If the medium is sensed free then the station is allowed to transmit [2].It does not sense the channel while transmitting but emits its entire frame [5]. This kind of protocols are very effective when the medium is not heavily loaded, since it allows stations to transmit with minimum delay, but there is always a chance of stations transmitting at the same time (collision), caused by the fact that the stations sensed the medium free and decided to transmit at once [2].

In the case of collision the MAC layer can coordinate the retransmission of the packet which will cause significant delay. In Ethernet, the retransmission phase is based on the Exponential Random Back off algorithm [2]. Collision Detection mechanism is a good idea on a wired LAN but it cannot be used on a Wireless LAN environment, because of two main reasons:

Implementing a Collision Detection Mechanism would require the implementation of a Full Duplex radio, capable of transmitting and receiving at once, an approach that would increase the price significantly [2]. It is very hard and expensive to implement simultaneous transmission and reception in nearby frequency bands. For this reason IEEE 802.11 MAC implements a Network Allocation Vector (NAV) [4][5].

We cannot assume in wireless environment that all stations hear each other (which is the basic assumption of the Collision Detection scheme), and the fact that a station willing to transmit and senses the medium free, doesn't necessarily mean that the medium is free around the receiver area [2].

In order to overcome these problems, the 802.11 uses a Collision Avoidance mechanism together with a Positive Acknowledge scheme in DCF mode, as follows[2][5]: A station willing to transmit senses the medium, if the medium is busy then it defers. If the medium is free for a specified time (Distributed Inter Frame Space ,DIFS ) then the station is allowed to transmit. The receiving station will check the CRC of the received packet and send back the acknowledgment packet (ACK). Receipt of the acknowledgment will indicate the transmitter that no collision has occurred. If the sender does not receive the acknowledgment then it will retransmit the fragment until it gets acknowledged or thrown away after a given number of retransmissions [2].

IEEE 802.11 use both physical carrier sensing and virtual carrier sensing [5].Because physical carrier sensing cannot prevent collision as in some cases stations may not hear each other. IEEE 802.11 standard defines a Virtual Carrier Sense mechanism in order to reduce the probability of collision of two stations. If a station willing to transmit a packet, it first transmit a short control packet called RTS (Request To Send), which include the source, destination, and the duration of the following transaction (i.e. the time required to transmit packet and the respective ACK), the destination station responds if the medium is free with a response control Packet called CTS (Clear to Send), which will include the same duration information [2].

Fig 2.4 Transactions between two stations ( RTS/CTS/Data/ACK ), and the NAV setting[1]

All stations receiving either the RTS and/or the CTS set their Virtual Carrier Sense indicator known as Network Allocation Vector (NAV) for the given duration [2], and use this information together with the Physical Carrier Sensing while sensing the medium. The above Figure 2.4 shows a transaction between two stations, and the NAV setting of their neighbors [1].

Because of this mechanism the probability of collision reduces on the receiver end where stations is "hidden" from the transmitter, since that station will hear the CTS and "reserve" the medium as busy until the end of the transaction. The duration information on the RTS also protects the transmitter area from collisions on the ACK packet. It should also be noted RTS and CTS are short frames, it also reduces the overhead of collisions, since these are recognized and recorded faster than it would be recognized if the whole packet was to be transmitted, (this is true if the packet is significantly bigger than the RTS, so the standard allows for short packets to be transmitted without the RTS/CTS transaction, and this is controlled per station by a parameter called RTS Threshold) [2].On a Wireless LAN environment it would be preferable to use smaller packets because of the higher Bit Error Rate of a radio link , the probability of a packet to get corrupted increases with the packet size. And in case of packet corruption (either because of collision or noise), small packets reduces the overhead of retransmission [2].

2.1.4 Inter Frame space (IFS)

The time interval between frames is called the Inter Frame Spaces(IFS). The 802.11 standard defines Four different IFSs to provide priority levels for access to the wireless media, they are listed in order, from the shortest to the longest [1] [2].

Short inter frame space (SIFS)

Point coordination inter frame space (PIFS)

Distributed inter frame space (DIFS)

Extended inter frame space (EIFS)

The different IFSs are independent of the station bit rate Figure2.5 shows some of these relationships [1].

Fig 2.5 IFS Relationships

Slot Time: It is defined in such a way that a station will always be capable of determining if another station has accessed the medium at the beginning of the previous slot [2].

Slot Time formula

SlotTime = CCATime + RxTxTurnaroundTime + AirPropagationTime + MACProcessingDelay [1]

CCATime: It is the minimum time (in μs) the Clear Channel Assessment (CCA) mechanism has available to assess the medium within every time slot to determine whether the medium is busy or idle [1].

RxTxTurnaroundTime:It is the maximum time (in μs) that the PHY requires to change from receiving to transmitting the start of the first symbol[1].

AirPropagationTime: It is the anticipated time (in μs) it takes a transmitted signal to travel from the transmitting station to the receiving station. MACProcessingDelay is the nominal time (in μs) that the MAC uses to process a frame and prepare a response to the frame [1].

Short Inter Frame Space (SIFS):

It is used to separate transmissions belonging to a single dialog (e.g. Packet-Ack). SIFS is the minimum Inter Frame Space, and there is always at most one single station to transmit at this given time hence having highest priority over all other stations. This value is a fixed value per PHY and it includes the delay for the transmitting station to switch back to the receive mode and get ready for decoding the incoming packet [2].

SIFS Formula

SIFSTime = RxRFDelay + RxPLCPDelay + MACProcessingDelay + RxTxTurnaroundTime [1]

RxRFDelay: It is the nominal time (in μs) between the end of a symbol at the air interface to the issuance of a PMD-DATA indicate to the PLCP [1].

RxPLCPDelay:It is the nominal time (in μs) that the PLCP uses to deliver a bit from the PMD receive path to the MAC [1].

The definitions for MACProcessingDelay and RxTxTurnaroundTime are given above in the Slot Time formula.

Point Coordination Inter Frame Space (PIFS):

It is used by the Access Point in Point Coordination Function (PCF) mode, to gain access to the medium before any other station. This value is SIFS+Slot Time[2].

PIFS Formula

PIFSTime =SIFSTime+ SlotTime [1]

Distributed Inter Frame Space (DIFS):

It is the Inter Frame Space used for a station willing to start a new transmission, or the station defer for DIFS duration to make sure the medium is idel.It is calculated as PIFS+ slot Time[2].

DIFS Formula

PIFSTime =SIFSTime+ 2*SlotTime [1]

EIFS - Extended IFS

It is a longer IFS used by a station that has received a packet that couldnot understand or a corrupted packet. This is needed to prevent a station (could not understand the duration information for the Virtual Carrier Sense mechanism) from colliding with a future packet belonging to the current dialog[2].

EIFS Formula

EIFSTime = SIFSTime + (8*ACKSize) + PreambleLength + PLCPHeaderLngth+ DIFSTime [1]

PreambleLength: The current PHY's Preamble Length (in μs) [1].

PLCPHeaderLngth :The current PHY's PLCP Header Length (in μs). [1]

Fig 2.5 DIFS timing relationships [1]

2.1.5 Exponential Backoff Algorithm

Backoff is a well known method to resolve the conflict between different stations willing to access the medium. This method requires each station to choose a Random Number(n) between 0 and a given number, and wait for this number of Slots before accessing the medium, while checking the medium (i.e. checking whether a different station has attempted to transmit), which reduces the collision probability by half. In exponential backoff, each time a station chooses a slot and it happens to collide, it increases the maximum number for the random selection exponentially [2].The 802.11 standard defines an Exponential Backoff Algorithm, which must be executed in the following cases:

If the station senses the medium as busy before the first transmission of a packet.

After each retransmission.

After a successful transmission.

The only case when this mechanism is not used is when the station decides to transmit a new packet and the medium has been free for more than DIFS [2]. The above figure 2.4 shows a schematic of the access mechanism.

The working of Exponential backoff algorithm of 802.11 is as follows: Before transmission a station must sense the medium, if the medium is Busy at its first attempt the station defers until the end of current transmission [1].Next time before attempting to transmit or immediately after a successful transmission, the station selects the random interval time or backoff interval, which is the Backoff Time [1].

Backoff Time = Random () * Slot Time [1]

where

Random() = integer drawn from a uniform distribution over the interval [0,CW],

Where CW (contention window) is an integer within the range of values[1]:

CWmin ≤ CW ≤ CWmax.

Contention Window (CW):

It is also called collision window which the time required by the network to detect a collision between packets. Whenever a collision occurred, on every attempt CW is increased by the factor of two. And the backoff time interval counter is decremented while the medium is idle and it is frozen whenever the medium detected busy (transmission detected). CW is set to CWmin after a successful transmission [1]. Fig 2.6 is an example of exponential increase of CW.

Fig 2.6 An example of exponential increase of CW [1]

2.1.6 IEEE 802.11 Frame Types

In IEEE 802.11 standard, there are three main types of frames [1][2]:

Data Frames: These are used for data transmission.

Control Frames: These are used to control the access to the medium (i.e. RTS, CTS, and ACK).

Management Frames: These are transmitted in the same way as data frames to exchange management information, but they are not forwarded to upper layers.

Each of these types is as well subdivided into different subtypes, according to their specific function.

2.1.7 IEEE 802.11 Frame Formats

All IEEE 802.11 frames are composed by the following components [2]:

Fig 2.1.7 the General MAC frame Component [2]

Preamble: This field includes symbols for synchronization, channel estimation and start frame for frame timing.

PLCP Header: It contains information that will be used by the PLCP layer to decode the frame (data rates, etc.).

MAC Data: This is Mac Frame which is described in next section.

CRC: this is a 32 bit field containing a 32-bit Cyclic Redundancy Check (CRC)

2.1.7.1 MAC Frame Format:

The MAC frame is used for data and management frames and its general format is shown in fig 2.8[1][2].Its field are described in next section.

Fig 2.8 MAC frame format [1]

MAC Frame Fields[1][2]

Frame Control Field: The Frame Control field contains the following information [1] [2]:

Fig 2.9 Frame Control field [1]

Protocol Version: This field will be used to recognize possible future versions. In the current version of the standard the value is fixed as 0.

The Type and Subtype fields: Identify the function of the frame. There are three frame types: control, data, and management. Each of the frame types has several defined subtypes. Appendix A Table 1 defines the valid combinations of type and subtype.

ToDS: This bit is set to 1 when the frame is addressed to the AP for forwarding it to the Distribution System. The Bit is set to 0 in all other frames.

FromDS: This bit is set to 1 when the frame is coming from the Distribution System.

More Fragments: This bit is set to 1 when there are more fragments belonging to the same frame following this current fragment.

Retry: This bit indicates that this fragment is a retransmission of a previously transmitted fragment; this will be used by the receiver station to recognize duplicate transmissions of frames that may occur when an Acknowledgment packet is lost.

Power Management: This bit indicates the Power Management mode that the station will be in after the transmission of this frame. This is used by stations which are changing state either from Power Save to Active or viceversa.

More Data: This bit is also used for Power Management and it is used by the AP to indicate that there are more frames buffered to this station. The station may decide to use this information to continue polling or even changing mode to Active.

WEP: This bit indicates that the frame body is encrypted according to the WEP algorithm

Order: This bit indicates that this frame is being sent using the Strictly-Ordered service class

Duration/ID: Duration value used for the NAV calculation.

Address Fields: A frame may contain up to 4 Addresses depending on the ToDS and FromDS bits defined in the Control Field, as follows:

Address-1 is always the Recipient Address (i.e. the station on the BSS who is the immediate recipient of the packet), if ToDS is set this is the Address of the AP, if ToDS is not set then this is the address of the end-station.

Address-2 is always the Transmitter Address (i.e. the station who is physically transmitting the packet), if FromDS is set this is the address of the AP, if it is not set then it is the address of the Station.

Address-3 is in most cases the remaining, missing address, on a frame with FromDS set to 1, then the Address-3 is the original Source Address, if the frame has the ToDS set then Address 3 is the destination Address.

Address-4 is used on the special case where a Wireless Distribution System is used, and the frame is being transmitted from one Access Point to another, in this case both the ToDS and FromDS bits are set, so both the original Destination and the original Source Addresses are missing.

Sequence Control: Used to represent the order of different fragments belonging to the same frame and to recognize packet duplications.

Format of Common Individual Frame Types

Request To Send (RTS) Frame Format

The RTS frame looks as follows:

Fig 2.10 RTS frame [1]

Duration: Time required for transmitting the next Data or Management frame, plus one CTS frame, plus one ACK frame, plus three SIFS intervals.

RA: Address of the STA that is the intended immediate recipient of the next Data or Management frame.

TA: Address of the STA transmitting the RTS frame.

Clear To Send (CTS) frame format

The CTS frame looks as follows:

Fig 2.11 RTS frame [1]

Duration: It is obtained from the Duration field of the immediately previous RTS frame, minus the time, in microseconds, required to transmit the CTS frame and its SIFS interval.

RA: It is copied from the TA field of the immediately previous RTS frame to which the CTS is a response. It is actually the destination of this packet.

Acknowledgment (ACK) frame format

The ACK frame looks as follows:.

Fig 2.12 ACK frame [1]

Duration: It is obtained from the Duration field of the previous frame, minus the time, in microseconds, required to transmit the ACK frame and its SIFS interval.

RA: It is copied from the Address-2 field of the immediately previous frame. It is actually the destination of this packet.

2.1.8 Point Coordination Function (PCF)

Beyond the basic Distributed Coordination Function, there is an optional Point Coordination Function, which may be used to implement time-bounded services, like voice or video transmission. The Point Coordination Function makes use of the higher priority that the Access Point may gain by the use of a smaller Inter Frame Space (PIFS)[2]. By using the higher priority access, the Access Point issues polling requests to the stations for data transmission, hence controlling the medium access. In order to allow regular stations the capability to still access the medium, there is a provision that the Access Point must leave enough time for Distributed access in between the PCF [2].

2.1.9 IEEE 802.11 Physical Layer

The physical layer is the interface between the MAC and the wireless media where frames are transmitted and received. It performs three major functions:

Provides an interface to exchange frames with the MAC layer for transmission and reception of data.

Uses signal carrier and spread spectrum modulation to transmit data frames over the wireless medium.

Provides a carrier sensing mechanism to the MAC to verify activity on the media.

Fig 2.13 IEEE 802.11 PHY layer Transmission Techniques

The IEEE 802.11 PHY specification is consist of the characteristics such as type of modulation, carrier frequency of operation, channel bandwidth and transmission power. [1] The 1997 IEEE 802.11 PHY standard specifies three transmission techniques, the Infrared (IR) and two short range radio, known as Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS).Both techniques are part of Spread spectrum that does not required licensing (the 2.4-GHz ISM band). All of these techniques operate at 1 or 2 Mbps and at low enough power that they do not conflict too much.[1][5]

In 1999 the two new techniques were introduced to achieve higher bandwidth. These are called orthogonal frequency division multiplexing (OFDM) (IEEE 802.11a) and High Rate Direct Sequence Spread Spectrum (HR-DSS) (IEEE 802.11b). They operate at up to 54 Mbps and 11 Mbps. The two standards, 802.11a and 802.11b are not compatible with each other. In order to use both standards together in 2001 a second OFDM modulation IEEE 802.11g was introduced, but in a different frequency band from the first one [3][5]. Following is the detail of each technology.

Infrared (IR): The Infrared uses diffused (i.e. not line of sight) transmission at 0.85 or 0.95 microns. It can use two speeds options 1 Mbps (basic access rate) and 2Mbps (enhance access rate) [5]. 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 walls, So cells in different rooms are well isolated from each other. But due to low bandwidth this is not a popular option [5].

Spread Spectrum: Spread spectrum is a technique which uses more bandwidth for reliable communication. In this technique more bandwidth is used than the system really needs for transmission to reduce the interference on the media. Spread spectrum spreads the transmitted bandwidth of the signal, reduce the peak power but keep same total power.

Frequency Hopping Spread Spectrum (FHSS): FHSS utilizes a set of narrow channels and "hops" through all of them in a predetermined sequence .It uses the 79 channels ,each with 1MHz wide ,starting at the low end of the 2.4 GHz ISM Band [5]. Every 20 to 400 msec the system "hops" to a new channel following a predetermined cyclic pattern [5]. FHSS uses the 2.4 GHz radio frequency band, operating with at 1 or 2 Mbps data rate [5].

Direct Sequence Spread Spectrum (DSSS): DSSS also uses the 2.4 GHz radio frequency band, operating with at 1 or 2 Mbps data rate [5] .The DSSS spread the signal on a larger frequency band by multiplexing it with a code to minimize interference and noise. In the receiver, the original signal is recovered by receiving the whole spread channel and demodulating with the same code used by the transmitter.

Orthogonal Frequency division Multiplexing (OFDM): It is the first high speed WLAN known as 802.11a, deliver up to 54 Mbps in a wider 5 GHz ISM band. It uses different 52 frequencies, 48 for data and 4 for synchronization. This technique is considered as a form of Spread Spectrum but it is different from CDMA and FHSS. Splitting a signal over narrow bands has key advantages over using single wider band. This Technique has good spectrum efficiency in terms of bits/Hz and good immunity to multipath fading [5].

High Rate Direct Sequence Spread Spectrum (HR-DSS): It is called IEEE 802.11b but its not a fellow up of IEEE 802.11a it was approved and come to the market first.HR-DSS is also a spread spectrum technique which uses 11 million chips/sec to achieve 11 Mbps in 2.4 GHz band. Data rate supported by 802.11b are 1, 2, 5.5 and 11 Mbps. The two slow rates run at 1Mbaud, with 1 and 2 bits per baud, using phase shift modulation [5].In practice the operating speed of 802.11b is always nearly 11Mbps. It is slower than 802.11a but its range is 7 times greater [5].

2.1.10 Omnidirectional, Directional and Smart Antennas:

2.1.10.1 Omnidirectional Antennas

Since the early days of wireless communications, there has been the simple dipole antenna, which radiates and receives equally well in all directions. To find its users, this single-element design broadcasts Omnidirectional [8][10] . It is also called the "non-directional" antenna because it does not favor any particular direction [10]. While adequate for simple RF environments where no specific knowledge of the users' whereabouts is available, this unfocused approach scatters signals, reaching desired users with only a small percentage of the overall energy sent out into the environment [8] . Omnidirectional antennas badly impact spectral efficiency and have limiting frequency reuse .By using omnidirectional antennas there is neither increase of the desired signal, nor suppression of undesired signals [11]. These limitations forced system designers to the design of directional and smart antenna system to overcome these problems.

Fig 2.14 Omni Directional Antenna and coverage pattern[8]

2.1.10.2 Directional Antennas

A directional antenna can transmit a signal in any direction, using an array of antennas called array of elements [10]. The directivity of an antenna is a statement of how the RF energy is focused in one or two directions, resulting in increase of signal strength in one or more directions and elimination in the others[10][11]. Because the amount of RF energy remains the same, but is distributed over less area, the apparent signal strength is higher, This apparent increase in signal strength is the antenna gain [11]. The more the elements of a directional antenna are, the more the increase of the signal in the desired direction. As the number of antenna elements increases, the beam width and the signal gain can be controlled more effectively. There are directional antennas with 2, 4,8,16 etc. elements [10].

Fig 2.15 T Directional antennas and coverage pattern [8][10]

2.1.10.3 Smart Antenna System

Smart antennas are proposed as a physical layer device that can increase the capacity of wireless networks [7]. As defined by IEEE, it is the antenna system that has circuit elements associated with its radiating elements such that one or more of the antenna properties are controlled by the received signal [9]. A smart antenna system combines multiple directional antenna elements with a signal processing capability can automatically change the directionality of its radiation patterns in response to its signal environment. This can dramatically increase the performance characteristics (i.e capacity) of a wireless system.[8] The Smart Antenna system is further divided into three levels the switched beam, dynamic phased array and adaptive array [7].where adaptive beam forming and dynamic phased array are more complex and intelligent than switched beam systems.

Switched Beam Antennas: In switched beam antennas, there is a predefined set of directions in which an antenna can be pointed. Use of these antennas in wireless networks requires MAC and possibly routing protocols to track which antenna sectors point toward other nodes [7].

Dynamically Phased Arrays Antennas: An array of antenna elements can be pointed in a direction by changing the phase of the signals emitted from each element so that they arrive on the wave front in the preferred direction at the same time thus constructively interfering in the pointing direction and destructively interfering elsewhere. They must be calibrated to support beam forming. Dynamically phased arrays have the ability to determine the direction of the arrival (DOA) of signals so that the antennas can adapt and immediately point toward the source. The Dynamic phased arrays antennas do not require MAC and routing protocols to track network state [7].

Adaptive Antenna Arrays Antennas: Adaptive antenna arrays antennas use adaptive beam forming and a algorithm is used to determine the direction of the received signal [13].The adaptive Antenna arrays do not require MAC and routing protocols to track state[7]. Adaptive antenna arrays antennas include algorithms for reducing interference, which cancelled by adjusting the radiation pattern nulls and increase the Signal to Interference ration (SIR) [7][13]. These techniques adapt to the environment accounting for the multipath arrival of signals [7].

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