1.1 What is WLAN?
Wireless Local Area Network (WLAN) is a kind of local area network which established using a wireless link between the service providers and the clients using some wireless equipment. This network development is based on the IEEE 802.11 standard.
1.1.2 IEEE 802.11
IEEE 802.11 denotes a set of Wireless LAN/WLAN standards developed by working group 11 of the IEEE LAN/MAN Standards Committee (IEEE 802). The term 802.11x is also used to denote this set of standards and is not to be mistaken for any one of its elements. There is no single 802.11x standard. The term IEEE 802.11 is also used to refer to the original 802.11, which is now sometimes called "802.11 legacy" . The 802.11 family currently includes six over-the-air modulation techniques that all use the same protocol. The most popular techniques are those defined by the b, a, and g amendments to the original standard; security was originally included and was later enhanced via the 802.11i amendment. 802.11n is another modulation technique that has recently been developed; the standard is still under development, although products designed based on draft versions of the standard are being sold. Other standards in the family (c–f, h, and j) are service enhancements and extensions or corrections to previous specifications. 802.11b was the first widely accepted wireless networking standard, followed by 802.11a and 802.11g . 802.11b and 802.11g standards use the 2.40 GHz (gigahertz) band, operating (in the United States) under Part 15 of the FCC Rules and Regulations. Because of this choice of frequency band, 802.11b and 802.11g equipment can incur interference from microwave ovens, cordless telephones, Bluetooth devices, and other appliances using this same band. The 802.11a standard uses the 5 GHz band, and is therefore not affected by products operating on the 2.4 GHz band. Table 1.1: Protocol Summary of IEEE 802.11
Protocol Legacy 802.11a 802.11b 802.11g 802.11n
Release Date 1997 1999 1999 2003 2006
Operating Frequency GHz 2.4-2.5 5 2.4-2.5 2.4-2.5 2.4 and/or 5
Throughput (Typ) Mbps 0.7 23 4 19 74
Data Rate (Max) Mbps 2 54 11 54 248 = 2x2 ant
Range (Indoor) meters ~25 ~30 ~35 ~35 ~70
Range (Outdoor) meters ~75 ~100 ~110 ~115 ~160
1.2 Why it should be used?
Bangladesh entered the Internet world in 1993 using offline E-mail services. Online Dial-up services started in 1996 through VSAT based data connectivity. But it is not possible to give a Dial-up connection to all because; it uses the BTTB’s telephone line. While Dial-up is active the phone line is busy and it is not possible to give a client more than 4/5 Kbps speed. Using an ADSL modem it can be increased to more than 2 Mbps. But it is not enough for a corporate user and also it is very costly and there are many other problems which has described below. The Ethernet connectivity can give a maximum of 100 Mbps. But its range is too small. Wireless LAN has vast benefits over wired network in some aspects. In our country especially in big cities like Dhaka, it is very hard job to establish a wired network all over the city. Because, it is over populated, buildings were made with out any proper plan and also the roads. Generally the wire lines are established over head, which is not so secured. Wire can be broken due to any kind of natural or man made problem. It may be theft. Or it can be misused by any one by taking a parallel line from it. It may create leak of data security. It is also very expensive to establish a copper wire network road by road and maintenance of it. Besides that there are many rivers, cannels in our county, and also hill tracks in some parts. It is not possible to give a wired network over those. For all those reasons it is not a wise decision to use a wired network in our country. A Wireless LAN can be more reliable, low cost, convenient network considering above aspects. There are a number of Internet Service Provider (ISP) companies in our country giving Wireless LAN support to the clients. Those are known as Wireless ISP. These ISPs give internet or intranet service to the clients as their requirements. Those networks are reliable and also secured. It is easy to establish a connection in the company’s coverage area using a wireless device at the client end. The Wireless ISP Company should have proper resources to give that coverage. A model of a Wireless ISP company’s wireless part for Bangladesh is given below. The nation wide link can be a optical fiber or microwave link. Here the main coverage is shown in Dhaka city and thus BSSs are shown at here is more than one. It can be expand the network in other areas by adding additional equipments required to establish a BSS. And also it can give coverage on other areas by establish same network on that area.
Figure 1.1: Model of a Wireless ISP
1.3 Why one should be interested in WLAN field?
The telecom industry is changing with breathtaking speed. There are a lot of telecommunication and Wireless ISP companies working in our country and there are a lot of companies to come. At present telecommunication is the most challenging and interesting field out of all other engineering fields. All the telecom company has some common structure. So, there are many similarities between a mobile or PSTN (Public Switched Telephone Network) operator and a Wireless ISP. The skills one gather from a Wireless ISP can use in the telecom companies. The man can be skilled on installing different devices, surveying a site, proposing a link budget. He can face the practical problems occur in installing radio networks and can be skilled in solving those problems and also troubleshoot the devices and the radio link. In the mobile operators, there are many restrictions. One can not work with all things. But as still Wireless ISP companies are smaller in our country one can get opportunity to work in different sections which will increase his experiences and skills. Lastly it can be say that, as it is a challenging field, the person likes facing challenges will enjoy working in this field
1.4 Organization of this report
This Internship report has seven chapters in total. The second chapter contains theory about the radio frequency properties and different modulation techniques In third chapter, different RF antennas and it accessories are described. Fourth chapter contains the Wireless LAN’s theory and architecture in brief. Chapter five analyzes to survey a site, and how to budget a link. The sixth chapter describes the device installation process for the APERTO and CANOPY devices. The seventh and final chapter is the concluding chapter where limitations of this works are reported and few suggestions of our work are provided along with the concluding remarks.
1.5 Aims and objectives
RF Properties and Modulation Techniques
RF Properties and Modulation Techniques
2.1 Radio Frequency
2.2.1 Radio Frequency
Radio frequencies are high frequency alternating current (AC) signals that are passed along a copper conductor and then radiated into the air via an antenna. An antenna converts/transforms a wired signal to a wireless signal and vice versa. When the high frequency AC signal is radiated into the air, it forms radio waves. These radio waves propagate (move) away from the source (the antenna) in a straight line in all directions at once.
2.2.2 RF Behaviors
RF is sometimes referred to as "smoke and mirrors" because RF seems to act erratically and inconsistently under given circumstances. Things as small as a connector not being tight enough or a slight impedance mismatch on the line can cause erratic behavior and undesirable results. The following sections describe these types of behaviors and what can happen to radio waves as they are transmitted. Gain Gain, illustrated in Figure 2.1, is the term used to describe an increase in an RF signal' amplitude . Gain is usually an active process; meaning that an external s power source, such as an RF amplifier, is used to amplify the signal or a high-gain antenna is used to focus the beam width of a signal to increase its signal amplitude.
Figure 2.1: Power gain
However, passive processes can also cause gain. For example, reflected RF signals combine with the main signal to increase the main signal' strength. Increasing the RF s signal' strength may have a positive or a negative result. Typically, more power is s better, but there are cases, such as when a transmitter is radiating power very close to legal power output limit, where added power would be a serious problem.
Loss Loss describes a decrease in signal strength (Figure 2.2). Many things can cause RF signal loss, both while the signal is still in the cable as a high frequency AC electrical signal and when the signal is propagated as radio waves through the air by the antenna. Resistance of cables and connectors causes loss due to the converting of the AC signal to heat. Impedance mismatches in the cables and connectors can cause power to be reflected back toward the source, which can cause signal degradation. Objects directly in the propagated wave' transmission path can absorb, reflect, or s destroy RF signals. Loss can be intentionally injected into a circuit with an RF attenuator. RF attenuators are accurate resistors that convert high frequency AC to heat in order to reduce signal amplitude at that point in the circuit. 
Figure 2.2: Power loss
Being able to measure and compensate for loss in an RF connection or circuit is important because radios have a receive sensitivity threshold. A sensitivity threshold defined as the point at which a radio can clearly distinguish a signal from background noise. Since a receiver’s sensitivity is finite, the transmitting station must transmit signal with enough amplitude to be recognizable at the receiver. If losses occur between the transmitter and receiver, the problem must be corrected either by removing the objects causing loss or by increasing the transmission power. Reflection Reflection, (as illustrated in Figure 2.3) occurs when a propagating electromagnetic wave impinges upon an object that has very large dimensions when compared to the wavelength of the propagating wave . Reflections occur from the surface of the earth, buildings, walls, and many other obstacles. If the surface is smooth, the reflected signal may remain intact, though there is some loss due to absorption and scattering of the signal.
Figure 2.3: Reflection
RF signal reflection can cause serious problems for wireless LANs. This reflecting main signal from many objects in the area of the transmission is referred to as multipath. Multipath can have severe adverse affects on a wireless LAN, such as degrading or canceling the main signal and causing holes or gaps in the RF coverage area. Surfaces such as lakes, metal roofs, metal blinds, metal doors, and others can cause severe reflection, and hence, multipath. Reflection of this magnitude is never desirable and typically requires special functionality (antenna diversity) within the wireless LAN hardware to compensate for it. Refraction Refraction describes the bending of a radio wave as it passes through a medium of different density. As an RF wave passes into a denser medium (like a pool of cold air lying in a valley) the wave will be bent such that its direction changes. When passing through such a medium, some of the wave will be reflected away from the intended signal path, and some will be bent through the medium in another direction, as illustrated in Figure 2.4. 
Figure 2.4: Refraction
Refraction can become a problem for long distance RF links. As atmospheric conditions change, the RF waves may change direction, diverting the signal away from the intended Diffraction Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities or an otherwise rough surface . At high frequencies, diffraction, like reflection, depends on the geometry of the obstructing object and the amplitude, phase, and polarization of the incident wave at the point of diffraction. Diffraction is commonly confused with and improperly used interchangeably with refraction. Care should be taken not to confuse these terms. Diffraction describes a wave bending around an obstacle (Figure 2.5), whereas refraction describes a wave bending through a medium. Taking the rock in the pond example from above, now consider a small twig sticking up through the surface of the water near where the rock. As the ripples hit the stick, they would be blocked to a small degree, but to a larger degree, the ripples would bend around the twig. This illustration shows how diffraction acts with obstacles in its path, depending on the makeup of the obstacle. If Object was large or jagged enough, the wave might not bend, but rather might be blocked.
Figure 2.5: Diffraction
Diffraction is the slowing of the wave front at the point where the wave front strikes an obstacle, while the rest of the wave front maintains the same speed of propagation. Diffraction is the effect of waves turning, or bending, around the obstacle. As another example, consider a machine blowing a steady stream of smoke. The smoke would flow straight until an obstacle entered its path. Introducing a large wooden block into the smoke stream would cause the smoke to curl around the corners of the block causing a noticeable degradation in the smoke' velocity at that point and a significant s change in direction. Scattering Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength of the signal, and the number of obstacles per unit volume is large . Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the signal path, as can be seen in Figure 2.6.
Figure 2.6: Scattering
Some outdoor examples of objects that can cause scattering in a mobile communications system include foliage, street signs, and lampposts. Scattering can take place in two primary ways. First, scattering can occur when a wave strikes an uneven surface and is reflected in many directions simultaneously. Scattering of this type yields many small amplitude reflections and destroys the main RF signal. Dissipation of an RF signal may occur when an RF wave is reflected off sand, rocks, or other jagged surfaces. When scattered in this manner, RF signal degradation can be significant to the point of intermittently disrupting communications or causing complete signal loss. 10
Second, scattering can occur as a signal wave travels through particles in the medium such as heavy dust content. In this case, rather than being reflected off an uneven surface, the RF waves are individually reflected on a very small scale off tiny particles. Voltage Standing Wave Ratio (VSWR) VSWR occurs when there is mismatched impedance (resistance to current flow, measured in Ohms) between devices in an RF system. VSWR is caused by an RF signal reflected at a point of impedance mismatch in the signal path. VSWR causes return loss which is defined as the loss of forward energy through a system due to some of the power being reflected back towards the transmitter. If the impedances of the ends of a connection do not match, then the maximum amount of the transmitted power will not be received at the antenna. When part of the RF signal is reflected back toward the transmitter, the signal level on the line varies instead of being steady. This variance is an indicator of VSWR.  As an illustration of VSWR, imagine water flowing through two garden hoses. As long as the two hoses are the same diameter, water flows through them seamlessly. If the hose connected to the faucet were significantly larger than the next hose down the line, there would be backpressure on the faucet and even at the connection between the two hoses. This standing backpressure illustrates VSWR, as can be seen in Figure 2.7. In this example, you can see that backpressure can have negative effects and not nearly as much water is transferred to the second hose as there would have been with matching hoses screwed together properly.
Figure 2.7: VSWR-like water through a hose
VSWR Measurements VSWR is a ratio, so it is expressed as a relationship between two numbers. A typical VSWR value would be 1.5:1. The two numbers relate the ratio of impedance mismatch against a perfect impedance match. The second number is always 1, representing the perfect match, where as the first number varies. The lower the first number (closer to 1), the better impedance matching your system has. For example, a VSWR of 1.1:1 is better than 1.4:1. A VSWR measurement of 1:1 would denote a perfect impedance match and no voltage standing wave would be present in the signal path. Effects of VSWR Excessive VSWR can cause serious problems in an RF circuit. Most of the time, the result is a marked decrease in the amplitude of the transmitted RF signal. However, 11
since some transmitters are not protected against power being applied (or returned) to the transmitter output circuit, the reflected power can burn out the electronics of the transmitter. VSWR' effects are evident when transmitter circuits burn out, power s output levels are unstable, and the power observed is significantly different from the expected power. The methods of changing VSWR in a circuit include proper use of proper equipment. Tight connections between cables and connectors, use of impedance matched hardware throughout, and use of high-quality equipment with calibration reports where necessary are all good preventative measures against VSWR. VSWR can be measured with high-accuracy instrumentation such as SWR meters, but this measurement is beyond the scope of this text and the job tasks of a network administrator.
2.2 Spread Spectrum
2.2.1 Spread Spectrum
Spread spectrum is a communications technique characterized by wide bandwidth and low peak power. Spread spectrum communication uses various modulation techniques in wireless LANs and possesses many advantages over its precursor, narrow band communication . Spread spectrum signals are noise-like, hard to detect, and even harder to intercept or demodulate without the proper equipment. Jamming and interference have a lesser affect on a spread spectrum communication than on narrow band communications. For these reasons, spread spectrum has long been a favorite of the military.
2.2.2 Narrow Band Transmission
A narrowband transmission is a communications technology that uses only enough of the frequency spectrum to carry the data signal and no more, spread spectrum is in opposition to that mission since it uses much wider frequency bands than is necessary to transmit the information. This brings us to the first requirement for a signal to be considered spread spectrum. A signal is a spread spectrum signal when the bandwidth is much wider than what is required to send the information.  Figure 2.8 illustrates the difference between narrowband and spread spectrum transmissions. One of the characteristics of narrow band is high peak power. More power is required to send a transmission when using a smaller frequency range. In order for narrow band signals to be received, they must stand out above the general level of noise, called the noise floor, by a significant amount. Because its band is so narrow, and high peak power ensures error-free reception of a narrow band signal.
Figure 2.8: Narrow band verses Spread Spectrum on a frequency domain
A compelling argument against narrowband transmission-other than the high peak power required to send it-is that narrow band signals can be jammed or experience interference very easily. Jamming is the intentional overpowering of a transmission using unwanted signals transmitted on the same band. Because its band is so narrow, other narrow band signals, including noise, can completely eliminate the information by overpowering a narrowband transmission; much like a passing train overpowers a quiet conversation.
2.2.3 Spread Spectrum Technology
Spread spectrum technology allows taking the same amount of information than previously using a narrow band carrier signal and spreading it out over a much larger frequency range. For example, 1 MHz at 10 Watts with narrow band, but 20 MHz at 100 mW with spread spectrum. By using a wider frequency spectrum, we reduce the probability that the data will be corrupted or jammed. A narrow band jamming attempt on a spread spectrum signal would likely be thwarted by virtue of only a small part of the information falling into the narrow band signal' frequency range. s s Most of the digital data would be received error-free . Today' spread spectrum RF radios can retransmit any small amount of data loss due to narrowband interference. While the spread spectrum band is relatively wide, the peak power of the signal is quite low. This is the second requirement for a signal to be considered spread spectrum. For a signal to be considered spread spectrum, it must use low power. These two characteristics of spread spectrum (use of a wide band of frequencies and very low power) make it look to most receivers as if it were a noise signal. Noise is a wide band, low power signal, but the difference is that noise is unwanted. Furthermore, since most radio receivers will view the spread spectrum signal as noise, these receivers will not attempt to demodulate or interpret it, creating a slightly more secure communication.
2.2.4 Frequency Hopping Spread Spectrum (FHSS)
Frequency hopping spread spectrum is a spread spectrum technique that uses frequency agility to spread the data over more than 83 MHz. Frequency agility refers to the radio’s ability to change transmission frequency abruptly within the usable RF frequency band . In the case of frequency hopping wireless LANs, the usable portion of the 2.4 GHz ISM band is 83.5 MHz, per FCC regulation and the IEEE 802.11 standard.
How FHSS Works In frequency hopping systems, the carrier changes frequency, or hops, according to a pseudorandom sequence. The pseudorandom sequence is a list of several frequencies to which the carrier will hop at specified time intervals before repeating the pattern. The transmitter uses this hop sequence to select its transmission frequencies. The carrier will remain at a certain frequency for a specified time (known as the dwell time), and then use a small amount of time to hop to the next frequency (hop time). When the list of frequencies has been exhausted, the transmitter will repeat the sequence. Figure 2.9 shows a frequency hopping system using a hop sequence of five frequencies over 5 MHz band. In this example, the sequence is: 1. 2.449 GHz 2. 2.452 GHz 3. 2.448 GHz 4. 2.450 GHz 5. 2.451 GHz
Figure 2.9: Single frequency hopping system
Once the radio has transmitted the information on the 2.451 GHz carrier, the radio will repeat the hop sequence, starting again at 2.449 GHz. The process of repeating the sequence will continue until the information is received completely. The receiver radio is synchronized to the transmitting radio' hop sequence in order to s receive on the proper frequency at the proper time. The signal is then demodulated and used by the receiving computer. Effects of Narrow Band Interference Frequency hopping is a method of sending data where the transmission and receiving systems hop along a repeatable pattern of frequencies together. As is the case with all spread spectrum technologies, frequency hopping systems are resistant-but not immune-to narrow band interference. In example in Figure 2.9, if a signal were to interfere with our frequency hopping signal on, say, 2.451 GHz, only that portion of the spread spectrum signal would be lost. The rest of the spread spectrum signal would remain intact, and the lost data would be retransmitted.
In reality, an interfering narrow band signal may occupy several megahertz of bandwidth. Since a frequency hopping band is over 83 MHz wide, even this interfering signal will cause little degradation of the spread spectrum signal. Frequency Hopping Systems The IEEE and Open-Air standards regarding FHSS systems describe: 1. The frequency bands which may be used 2. Hop sequences 3. Dwell times 4. Data rates The IEEE 802.11 standard specifies data rates of 1 Mbps and 2 Mbps and Open-Air (a standard created by the now defunct Wireless LAN Interoperability Forum) specifies data rates of 800 kbps and 1.6 Mbps. In order for a frequency hopping system to be 802.11 or Open-Air compliant, it must operate in the 2.4 GHz ISM band (which is defined by the FCC as being from 2.4000 GHz to 2.5000 GHz). Both standards allow operation in the range of 2.4000 GHz to 2.4835 GHz. Channels A frequency hopping system will operate using a specified hop pattern called a channel. Frequency hopping systems typically use the FCC’s 26 standard hop patterns or a subset thereof. Some frequency hopping systems will allow custom hop patterns to be created, and others even allow synchronization between systems to completely eliminate collisions in a co-located environment.
Figure 2.10: Co-located frequency hopping system
Though it is possible to have as many as 79 synchronized, co-located access points, with this many systems, each frequency hopping radio would require precise synchronization with all of the others in order not to interfere with (transmit on the same frequency as) another frequency hopping radio in the area. The cost of such a set of systems is prohibitive and is generally not considered an option. If synchronized radios are used, the expense tends to dictate 12 co-located systems as the maximum. 15
If non-synchronized radios are to be used, then 26 systems can be co-located in a wireless LAN; this number is considered to be the maximum in a medium-traffic wireless LAN. Increasing the traffic significantly or routinely transferring large files places the practical limit on the number of co-located systems at about 15. More than 15 co-located frequency-hopping systems in this environment will interfere to the extent that collisions will begin to reduce the aggregate throughput of the wireless LAN. Dwell Time In frequency hopping systems, it must transmit on a specified frequency for a time, and then hop to a different frequency to continue transmitting. When a frequency hopping system transmits on a frequency, it must do so for a specified amount of time. This time is called the dwell time. Once the dwell time has expired, the system will switch to a different frequency and begin to transmit again. Suppose a frequency hopping system transmits on only two frequencies, 2.401 GHz and 2.402 GHz. The system will transmit on the 2.401 GHz frequency for the duration of the dwell time100 milliseconds (ms), for example. After 100ms the radio must change its transmitter frequency to 2.402 GHz and send information at that frequency for 100ms. Hop Time When considering the hopping action of a frequency hopping radio, dwell time is only part of the story. When a frequency hopping radio jumps from frequency A to frequency B, it must change the transmit frequency in one of two ways. It either must switch to a different circuit tuned to the new frequency, or it must change some element of the current circuit in order to tune to the new frequency. In either case, the process of changing to the new frequency must be complete before transmission can resume, and this change takes time due to electrical latencies inherent in the circuitry. There is a small amount of time during this frequency change in which the radio is not transmitting called the hop time. The hop time is measured in microseconds (µs) and with relatively long dwell times of around 100-200 ms, the hop time is not significant. A typical 802.11 FHSS system hops between channels in 200-300 µs. With very short dwell times of 500 – 600µs, like those being used in some frequency hopping systems such as Bluetooth, hop time can become very significant. If we look at the effect of hop time in terms of data throughput, we discover that the longer the hop time in relation to the dwell time, the slower the data rate of bits being transmitted.
2.2.5 Direct Sequence Spread Spectrum (DSSS)
Direct sequence spread spectrum is very widely known and the most used of the spread spectrum types, owing most of its popularity to its ease of implementation and high data rates. The majority of wireless LAN equipment on the market today uses DSSS technology. DSSS is a method of sending data in which the transmitting and receiving systems are both on a 22 MHz-wide set of frequencies. The wide channel enables devices to transmit more information at a higher data rate than current FHSS systems.
How DSSS Works DSSS combines a data signal at the sending station with a higher data rate bit sequence, which is referred to as a chipping code or processing gain. A high processing gain increases the signal’s resistance to interference. The minimum linear processing gain that the FCC allows is 10, and most commercial products operate under 20. The IEEE 802.11 work...