Wireless Computer Networking

Prepared by: Melanie Hanson

I. INTRODUCTION

This paper provides an introduction to some of the most common types of wireless computer networks. Several aspects of the networks are investigated. The different techniques that can be used to wirelessly send and receive data are discussed. Along with the data communication techniques, many other aspects of wireless computer networks are investigated.

II. OVERVIEW OF WIRELESS COMPUTER NETWORKS

There are several places where wireless computer networks are used. The most common applications include networks that reside within homes or office buildings or cellular phone networks. There are many reasons people use wireless networks rather than, or in addition to wired networks.1 Wireless networks are easier to install since no wires need to be placed. Due to the ease of installation, the cost of a wireless network is less than the cost of a wired network. Wireless networks are also much easier to expand due to the fact that additional wire does not need to be installed. The main advantage of wireless networks is the ability of the user to move around while he or she is connected to the network.

There are two main categories of wireless networks that are presently in use. The first type is the infrastructure network. This type of network is also referred to as a cellular network. The main features of this type of network include the fact that all communication is controlled by a base station, the area where that the network covers is divided into regions or cells, and that data can be sent from a network controlled by one base station to a network controlled by another base station using roaming techniques. Figure 1 depicts a typical infrastructure network.

Figure 1 Infrastructure Network1

The second type of wireless network is the ad hoc network. With this type of network, there is no base station controlling the communication and no specific structure. Figure 2 depicts a typical ad hoc network. The main goals of this type of network include multiple access and random access. Multiple access means that there can be many communication links existing at the same time. Random access means that a new device or node can be added to the network at any time.

Figure 2 Ad Hoc Network1

III. GENERATIONS OF CELLULAR WIRELESS NETWORKS

Within the cellular wireless network category, there are four generations.

1. First Generation

The first generation cellular wireless networks used analog technologies to communicate. This type of system is used for cordless telephones and analog cellular telephones.3 All transactions use FM modulation and go through a base station. Figure 3 illustrates a typical communication link within a first generation cellular wireless network. The user’s

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transmit data went through a base station to the mobile switching center (MSC) where the data was sent to another base station and to another user. All the network operations take place in the MSC.

Figure 3 Communication in a First Generation Cellular Wireless Network3

First generation systems were capable of transmitting analog speech and data. Transmission within these systems was inefficient and used a very low data rate. For this reason, few data transmissions took place. First generation wireless systems use the 900MHz frequency spectrum, use analog frequency division multiple access, and have a data rate of 2.4kilobits/second.4

2. Second Generation

Second generation cellular wireless networks use digital technologies to communicate. As with first generation systems, transactions within second generation systems also go through a base station. Where second generation systems differ from first generation systems is that many base stations are connected to base station controllers (BSCs) and the BSCs are connected to the MSC, rather than having the base stations connect to the MSC. By introducing the BSCs, much of the processing the first generation MSC was responsible for can be transferred to the BSCs. A standardized communication between the BSC and MSC was established to allow components produced by different manufacturers in the same system. Figure4 illustrates the configuration of a typical communication link within a second generation wireless network.

Figure 4 Communication in a Second Generation Wireless Network3

Second generation systems are not only used for voice, they are also used for data transmissions. Second generation systems can perform high rate data transfers and facsimiles. These systems use the 1800MHz frequency spectrum, use time division multiple access, and have a data rate of 9.6kilobits/second.

3. Third Generation

Third generation cellular wireless networks are in the final developmental stages and entering the usage stage. The goal of third generation systems is to provide standards that can be used for a wide variety of wireless applications. Third generations systems will also allow universal access throughout the entire world. These systems also aim to remove the difference between cordless telephones and cellular telephones. The goal is to have a universal personal communicator that can communicate using voice, data, and video information.

Third generation systems fell short of the original goal. There is not one system that can be used worldwide. There were very data rates expected for third generation systems, but the high data rates were not achieved. Third generation systems operate in the 2GHz frequency band, use code division multiple access, and have a data rate of 64kilobites/second.

4. Fourth Generation

Fourth generation cellular wireless networks are in the early developmental. The fourth generation systems will use the 40GHz and 60GHz frequency bands and will use orthogonal frequency division

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multiplexing. The actual data rate is unknown, but the target data rate is 100megabits/second.

IV. OVERVIEW OF AD HOC WIRELESS COMPUTER NETWORKS

Each ad hoc wireless network, from here out referred to as wireless network, is made up of several layers. The layers define the structure of the communication and define how the communication takes place. The layers of a typical wireless network include the physical layer, data link layer, network layer, transport layer, OS/middleware layer, and application layer.5 Figure 5 displays the relationship between the layers within a wireless network.

Figure 5 Layers of a Wireless Network5

The physical layer contains the radio frequency, modulation, and channel coding circuitry. The data link layer is responsible for error control, security, and mapping the data packets into frames. The network layer is responsible for routing packets and establishing the connection type. The transport layer provides transportation between network endpoints in a reliable and efficient manner. The OS/middleware layer manages disconnections and quality of service (QoS) issues. The application layer is responsible for tasks that take place between fixed and mobile hosts. The physical layer, data link layer, and network layer will be discussed in detail later.

Data that travels over a wireless network can be transmitted using a variety of methods. The most common methods include infrared and radio frequency. The frequencies that are typically used for wireless networks fall within the ISM band. ISM stands for industrial, scientific, or medical.6 The industrial band includes 902 megahertz (MHz) to 928MHz. The scientific band includes 2.4gigahertz (GHz) to 2.4853Ghz. The medical band includes 5.727GHz to 5.85GHz. The reason the ISM is typically used is that the FCC does not require a license to transmit or receive data that uses frequencies within these ranges.

There are regulations that govern the use of wireless data transmission for radio frequencies. All signals must use spread spectrum technology, there is a 1Watt power limit, and the signal must confine to a band.

V. WIRELESS NETWORK STANDARDS

There are several standards that are used for wireless data communication. The main standard used in the United States is IEEE 802.11. Within this group of standards there are four different standards in use and several others in the developmental stages. The four standards in use are 802.11, 802.11, 802.11b, and 802.11g. The differences between these standards include the rate at which data is transferred, the frequency range, and the application where each standard is used. IEEE 802.11 is used for wireless Ethernet, uses the 2.4GHz frequency band, and has a data rate of 1-2Mega bits per second (Mbps).7 This standard is the oldest of the four, and is not normally used anymore. The other three standards are meant to improve on the original standard. IEEE 802.11a and IEEE 802.11 b devices are currently available. IEEE 802.11a is used for wireless asynchronous transfer mode (ATM) communication, uses the 5GHz frequency band, uses orthogonal frequency division multiplexing (OFDM) for its air interface, has a maximum range of 80m, and a throughput of 31Mbps.8 IEEE 802.11b is used for wireless Ethernet, uses the 2.4GHz frequency band, uses the direct sequence spread spectrum (DSSS) air interface, has a maximum range of 100m, and a throughput of 6Mbps. IEEE 802.11g devices will be available by the end of 2002 and will use the 2.4GHz frequency band, will use either OFDM or DSSS, will have a maximum range of 150m, and will have a throughput of 12Mbps. Some of the standards that are

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currently in the developmental stages include: IEEE 802.11d, 802.11e, 802.11f, 802.11h, 802.11i, and 802.11j. IEEE 802.11d is a standard that is similar to 802.11b, but aims to include other frequencies than 2.4GHz. IEEE 802.11e is a standard that aims to include QoS, which the other 820.11 standards do not include. IEEE 802.11f is a standard that aims to improve roaming capabilities. IEEE 802.11h is a standard that is similar to 802.11a but aims to improve the transmission power and channel selection capabilities. IEEE 802.11i is a standard that aims to have improved security. IEEE 802.11j is so early in the development stages that the goals have not yet been finalized.

Another other family of standards that is widely used in Europe is the HiperLAN group of standards. Within this group there are four different standards. The four standards include HiperLAN Type 1, HiperLAN Type 2, HiperAccess, and HiperLink. The various HiperLAN standards differ in ways similar to those of the IEEE 802.11 standards. HiperLAN Type 1 is used for wireless Ethernet, uses the 5GHz frequency band, and has a data rate of 23.5Mbps. HiperLAN Type 2 is used for wireless ATM, uses the 5GHz frequency band, and has a data rate of approximately 20Mbps. HiperAccess is used for wireless local loop applications, uses the 5GHz frequency band, and has a data rate of approximately 20Mbps. HiperLink is used for wireless point-to-point applications, uses the 17GHz frequency band, and has a data rate of approximately 155Mbps.

VI. OVERVIEW OF DATA COMMUNICATIONS TECHNIQUES

There are several different types of communication techniques used in wireless networks. The communication techniques are part of the physical layer of a wireless network. To fully understand the physical layer and how it works, one must first gain understanding of these techniques. Three of the most widely used techniques include frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA).

When using the FDMA technique, each user gets a specific frequency band. FDMA is the technique used in traditional telephone systems. The number of users determines the percentage of the frequency band that each user has at any given time. Random access is also

possible using FDMA techniques. Figure 6 shows how a frequency band is separated for several users; B is the total bandwidth and N is the number of users.9

Figure 6 FDMA System9

When using the TDMA technique, each user is assigned to one or more time slots. The only time a user may transmit data is during that time slot, which means that random access is not possible. Synchronization between users is one of the most crucial aspects of this technique, so a base station must be used to control this synchronization. Transmission within a TDMA system is first broken down into frames with a fixed length. The frames are divided into N equal length time slots. When a user transmits data within a time slot, the transmission is broken down into a preamble, the message data, and a postamble. Figure7 illustrates the time breakdown of several data transmissions.

Figure 7 TDMA Time Slot Breakdown9

When using the CDMA technique, each user is assigned to a specific code. The code allows many users to use the same frequency without causing confusion for the receiver. Random access is possible using CDMA techniques. CDMA is the most widely used technique, so it will be discussed in detail.

VII. CDMA AND SPREAD SPECTRUM TECHNOLOGIES

1. Overview

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The main use for the CDMA technology lies within spread spectrum systems. CDMA allows the spread spectrum systems to achieve multiple access. Some of the main advantages of CDMA include the fact that no matter how many users are using the system the entire frequency spectrum is used, privacy is achieved through the use of a specific code for each user, there is a good anti-jamming performance, and a high data rate exists for all users. Another feature of CDMA is that there is no maximum number of users. The performance per user is inversely proportional to the number of users in the system: as the number of users increases, the performance decreases equally for all users and as the number of users decreases, the performance increases equally for all users.

There are many benefits to using spread spectrum technologies when sending or receiving data. Spread spectrum systems can be present with other systems without disturbing or being disturbed by the other systems.10 The security built into spread spectrum systems and the resistance to signal degradation are also some of the main advantages.

The main function of spread spectrum technique is to make the bandwidth of the transmitted signal much larger than the bandwidth of the original signal. By increasing the bandwidth of the transmitted signal, the energy per frequency can be reduced since the total energy is spread into small segments rather than being concentrated at one frequency. By spreading the energy among a wider frequency band, the power density decreases. The second advantage of increasing the bandwidth is that redundancy is achieved. By spreading the frequency, the same message may appear on more than one frequency. When a message is present in more than one location, there is a better chance that the message will reach the receiver and there will be fewer errors. In order to perform the spread spectrum modulation, two steps are necessary. The first step is called spreading code modulation. In this step, the wider bandwidth is created. The second step is called message modulation. In this step, the message is prepared for transmission. There are two main spread spectrum techniques used: frequency hopping (FH) and direct sequence spread spectrum (DSSS).

2. Frequency Hopping

(1). Operation

As defined by the Institute for Telecommunication Sciences frequency hopping can be described as “a signal structuring technique employing automatic switching of the transmitted frequency. Selection of the frequency to be transmitted is typically made in a pseudo-random manner from a set of frequencies covering a band wider than the information bandwidth. The intended receiver would frequency hop in synchronization with the code of the transmitter in order to retrieve the desired information.”11 With FH, the spreading code modulation step can be defined as the frequencies on which the data will transfer. The dwell time, or amount of time that data is transmitted on a single frequency, is defined in this step. In the message modulation step, narrow band signals are generated with durations according to the dwell time. Figure 8 illustrates the original data (top waveform), the data after the spreading code modulation (middle waveform), and the data after the message modulation step (bottom waveform).

Figure 8 FH Modulation12

(2). MFSK Modulation

FH typically uses the minimum frequency shift keying (MFSK) modulation technique. When frequency shift keying (FSK) is performed, two signal tones are used.13 One signal is sent to represent a ‘1’ and the other signal is sent to represent a ‘0’. Figure 9 shows a data stream, the two individual signal tones, and the transmitted signal.

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Figure 9 FSK13

The two signals must be orthogonal to make sure that the system works properly. Equation (1) is true if the signals are orthogonal.

(1)

In equation (1), s1 and s2 are the two signals and T is the duration of the pulse. There are two types of FSK, coherent and noncoherent. If the coherent type is used, pulses are generated and demodulated with phases known by the receiver. Orthogonality in a coherent system is achieved because the two signals are separated by an integer multiple of 1/(2T) Hz, where T is the number of seconds for each pulse. If the noncoherent type is used, the receiver does not know the phase of the signal. Orthogonality in a noncoherent system is achieved because the tones must be separated by an integer multiple of 1/T Hz.

MFSK uses the coherent type of FSK because it is the most bandwidth efficient. Equation (2) defines the signal transmitted for MFSK.

(2)

In equation (2), fc is the carrier frequency, dk is the sequence of data bits, and xk is defined in equation (3) and its value will always be either 0 or .

(3)

3. Direct Sequence Spread Spectrum

(1). Operation

The Institute for Telecommunication Sciences defines direct sequence spread spectrum as “A signal structuring technique utilizing a digital code sequence having a chip rate much higher than the information signal bit rate. Each information bit of digital signal is transmitted as a pseudorandom sequence of chips.”14

A chip is defined as “the longest duration signal in which signal parameters are approximately constant” 15

and the chip rate is defined as “the rate at which the informational signal bits are transmitted as a pseudorandom sequence of chips.”16 When using DSSS, the spreading code modulation step uses a chip sequence to represent the data that is to be transmitted. In the message modulation step, the chip pattern remains the same for all ‘0’ data bits and is inverted for all ‘1’ data bits. illustrates the original data (top waveform), the data after the spreading code modulation (middle waveform), and the data after the message modulation step (bottom waveform).

Figure 10 DSSS Modulation12

As with all spread spectrum techniques, the amount of bandwidth used to transmit data with the DSSS technique is much greater than the original bandwidth of the signal. With the DSSS technique, the spreading out of bandwidth equates to sending a small amount of the data over many different frequencies, commonly referred to as carrier frequencies. The receiver then brings the data from all the carrier frequencies together to regain the entire message.

(2). PSK Modulation

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DSSS typically uses the phase shift keying (PSK) modulation technique. One oscillator with a constant known phase is used. Data bits are represented by keeping the signal unchanged for ‘1’ data and shifting the signal by 180o for ‘0’ data. Figure 11 shows a data stream and the transmitted signal. Within the receiver, a phase locked loop is used to figure out the phase reference for the signal.

Figure 11 PSK12

4. Comparison of Frequency Hopping and Direct Sequence Spread Spectrum

(1). Maximum Number of Systems

The number of chips used for the DSSS spreading code is variable. As the number of chips increases, the number of systems that can use the same frequency band also increases. Although it may seem wise to use a large number of chips, as the number of chips increases the data rate must also increase due to the fact that there are more individual bits of information to send. A typical sequence uses 11 chips, which means that a 22MHz band is required. Since there must also be a minimum of 30MHz between frequencies that carry portions of the data, the actual maximum number of systems that may be used in one area without causing conflicts is 3.

Multiple systems in the same area are also allowed when using the FH technique. Due to the fact that there are 79 different frequencies in which the signal may hop, there are 78 different sequences that each use 79 hops. Based on the number of hopping sequences and carrier frequencies in which the data may hop, it is theoretically possible to have 26 systems, all using FH, operating in the same area. In most real situations, however, the maximum number of systems that can

operate in the same area without an overabundance of collisions is between 13 and 15.17

If a decision to use FH or DSSS is solely based on how many systems may operate in the same area, the choice is clearly FH.

(2). Interference

Interference is the presence of a signal or some other type of energy other than the desired one in the same frequency range of the desired signal. There are many types of interference that can affect the reception of data. Some types of interference include: narrowband interference, multipath fading, and adjacent channel interference. Narrowband interference is interference that is present only at one frequency. DSSS provides resistance to narrowband interference due to the fact that the signal is spread out over a wide frequency band. FH provides resistance to narrowband interference only if the frequency that the signal is being transmitted on is different than the frequency where the narrowband interference is present. If the narrowband interference is present at the same frequency that the signal is being transmitted on, the interference will cause problems. The only positive aspect with regard to the fact that the desired signal and the narrowband interference appear on the same frequency is that the desired signal will soon hop to another frequency, thus getting away from the interference.

Multipath fading is caused by interference between two or more versions of the same signal that are received at different times. Multiple versions of the same signal are generated when the signal reflects off a surface. FH is more resistant to multipath fading than DSSS since it has a wider signal pulse. The effect of multipath fading on both a FH signal and a DSSS signal is shown in Figure 12.

Figure 12 Effects of Multipath Fading10

Adjacent channel interference occurs when a receiver accepts data from nearby frequencies. A

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version of this, called the near-far affect, occurs when one system is transmitting a strong signal on a close frequency and is located near the receiver of another system. This type of interference affects DSSS more than FH because DSSS stays on one frequency band and FH hops between several frequencies.

(3). Data Retransmit

If a data packet becomes corrupt during the transmission process, a retransmit must take place. DSSS and FH handle the retransmission differently. When a data packet is retransmitted using the FH technique, a different set of frequencies is used to hop between than when the data packet was originally transmitted. In using a different group of frequencies at a different point in time, the chance that a data packet will become corrupt a second time is reduced. When a data packet is retransmitted using the DSSS technique, the packet is just resent at a later time. By transmitting the packet at a different point in time, the hope is that whatever caused the data to become corrupt is no longer present and the packet can be received error free. FH offers a better scheme for data retransmission due to the fact that there are two variables changed between the original transmission and the retransmission, compared to only one variable different with the DSSS technique.

VIII. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

1. Overview

The newest technique for data transmission is orthogonal frequency division multiplexing (OFDM). OFDM is a special type of multi-carrier modulation (MCM).18 MCM is a technology that breaks a channel up into several independent sub channels. The individual sub channels can then be transmitted in parallel at a lower bit rate and frequencies at which the sub channel data is transmitted can overlap. Figure 13 demonstrates the concept behind breaking a signal up into several sub channels.

Figure 13 Sub Channel Representation18

Orthogonality, as defined by the Merriam-Webster Dictionary, is “intersecting or lying at right angles.” Since it is hard to think of signals as lying at right angles, orthogonality can also be defined as signals that are completely independent of one another. When the signal is split into sub carrier signals, the data that is sent out on each sub channel, the orthogonality can be seen in both the time domain and the frequency domain. In the time domain, the frequency of each sub channel is a multiple of one another.19 Figure 14 illustrates an example of the time domain representation of sub carrier signals that originated from a single signal.

Figure 14 Time Domain Representation of Sub Carrier Signals19

The sub carrier signals can also be represented in the frequency domain as a sinc function. Figure 15 illustrates an example of the frequency domain representation of sub carrier signals that originated from a single signal.

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Figure 15 Frequency Domain Representation of Sub Carrier Signals19

As demonstrated by the frequency domain representation of the sub carrier signals, the maximum point of one sub carrier signal occurs where the other signals cross zero. By having this relationship, the signals are easy to receive, and other sub carrier signals do not introduce interference since the signals overlap when they are transmitted. By allowing many overlapping signals to be transmitted, the received signal is less likely to be subject to multipath fading.

The Fourier Transform (FT) is an important concept when dealing with OFDM. The FT is used to transform the sinusoidal signals that are represented in the time domain to a signal in the frequency domain. When performing the FT, the signal is broken down into components that have different frequencies. Figure 16 demonstrates the process in which a signal goes through in order for it to be converted from the time domain to the frequency domain.

Figure 16 Fourier Transformation18

Figure 16 also demonstrates the timing aspect of the signals. The sinusoidal signal that is represented in the time domain will have a finite length, T. When the FT is applied to that signal, there will be zero crossing points at intervals of 1/T apart. This 1/T interval determines how far apart the peaks for the sub carriers

will be placed since a peak is located at the zero crossing point of another sub carrier.

The inverse Fourier Transform (IFT) can be applied to reverse the process. The IFT brings the broken apart signals that are represented in the frequency domain together, back into one signal in the time domain. The IFT is used at the receiving side of an OFDM transmission to bring the signal back to its original representation.

2. OFDM Transmitters

There are several components to an OFDM transmitter. The components include: a serial to parallel conversion, a modulator, the inverse Fourier Transform, insertion of a guard interval, a parallel to serial conversion, and a digital to analog converter. Figure 17 illustrates the components of a typical OFDM transmitter.

Figure 17 OFDM Transmitter18

Within the serial to parallel conversion step, the original signal is divided into tens or hundreds of signals. The purpose for this division is to split one high-speed signal into many low speed signals. The modulation step uses quadrature amplitude modulation (QAM), which will be explained in detail later. These signals exist in the frequency domain. It is then necessary to perform the IFT to bring the frequency domain signals into the time domain. The guard interval step places each signal at the appropriate frequency so it can be transmitted and is used to suppress inter-symbol interference. The many parallel signals are then brought through a parallel to serial converter to reestablish one signal. The signal then passes through a digital to analog converter, a low pass filter, and an up converter before it is sent out for transmission.

3. OFDM Receivers

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An OFDM receiver is pictured in Figure 18. The receiver does the inverse steps of the transmitter and does them in the reverse order.

Figure 18 OFDM Receiver18

4. QAM

QAM is used in the modulation step for OFDM. In order to understand how QAM works, one must first understand how pulse amplitude modulation (PAM) works. PAM uses one oscillator that has a fixed phase that is known by the receiver. The amplitude of the signal has m discrete levels, where m is the number of encoding bits used for each symbol. There are three major steps required to transmit a signal using PAM. The first step is to use a lookup table to do the symbol encoding. The second step is to apply the encoded amplitude to a fixed pulse shape, which makes an amplitude-modulated pulse. The third step is to multiply the amplitude-modulated pulse by a carrier cosine signal and transmit the result.

With QAM, the same amplitudes are applied to both a sine and cosine carrier. The two channels are orthogonal, so they do not interfere with each other. The data rate can be calculated as the sum of the data rates of both channels. Since each channel is basically the same as the signal used with PAM, the bandwidth of each channel is the same as that of a PAM signal. The bandwidth efficiency of QAM is twice the bandwidth efficiency of PAM. Figure 19 depicts a QAM transmitter and receiver.

Figure 19 QAM Transmitter and Receiver13

IX. INFRARED

Infrared signals can also be used to transmit and receive data in a wireless network. Infrared communication is accomplished using light that is in the 850nm – 950nm range.20 There are no frequency regulations, but there are safety standards with regard to infrared signals. The safety standards are necessary to protect people from the light that they cannot see. Infrared communication is only used in indoor environments. The signals do not work outdoors, do not pass through walls, and are distorted when passed through windows.

1. Configurations

There are several different infrared configurations that can be used in wireless networks. The main distinctions include a directed or nondirected signal, and a line-of-sight or non-line-of-sight link.21 Directed links use a specially aimed transmitter and receiver to transfer data. Nondirected links use wide transmitters and receivers that transfer the data using a wide range technique. A combination of the directed and nondirected approach, called hybrid, is also used. In the hybrid configuration, transmitter or receiver is of the directed type and the other device is of the nondirected type. Line-of-sight links have the data travel directly between the transmitter and receiver; no objects lie in the path to interfere with the signal. Non-line-of-sight links have objects that lie in the path that the data must take to travel between the transmitter and receiver, so the data does not take a direct path between transmitter and receiver. Figure 20 pictorially demonstrates the differences between the various configurations.

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Figure 20 Infrared Configurations21

2. IM/DD

When transmitting data using an infrared connection, the intensity modulation with direct detection (IM/DD) technique is used. The IM/DD modulation technique improves the signal’s resistance to multipath fading because of the efficient spatial diversity of the signal. The signal has efficient spatial diversity due to the fact that the signal has a short carrier wavelength and there is a large-area, square-law detector. A square-law detector is one that has an output voltage that is proportional to the square of the input voltage.22 Within the intensity modulation part of the technique, the signal is modulated onto the instantaneous power of the carrier signal. The direct detection aspect describes the process of a photodetector producing a current that is proportional to the received power. Figure 21 illustrates a signal transmission and reception using the IM/DD technique.

Figure 21 Transmission and Reception for an Infrared Signal21

3. IrDA Standard

The Infrared Data Association (IrDA) has set some standards regarding wireless communication to help

provide uniformity among various manufacturers. These standards use the either direct or hybrid line-of-sight configuration. The main use of the IrDA standard is with short range, high speed, two way data links.23

There are several protocols that fall within the standards set by the IrDA.

The IrDA Serial Infrared Physical Layer protocol provides guidelines. The data rate must be up to 4Mb/s. The modulation technique for 4Mb/s data must be four-pulse-position modulation. This technique encodes one of four possibilities in a pulse.24 The information then comes from the position of the pulse, rather than from the pulse itself. Within the IrDA standard, the only wavelengths allowed are 850nm – 900nm. Receivers are required to have a field of view of at least 15o.

The IrDA Infrared Link Access Protocol (IrLAP) defines one primary device and all others as secondary devices. The IrLAP defines how link initialization, connection start, data exchange, disconnections, and link shutdown will be handled. The IrLAP also encapsulates frames.

The IrDA Infrared Link Management Protocol (IrLMP) handles three tasks. The IrLMP must discover the services that the link has available. The IrLMP must handle multiplexing several communication applications over one link. The IrLMP must also manage the link.

4. Problems with Infrared Communication

There are two main problems that are associated with using infrared signals for wireless networks. The first problem is that the noise level is high compared to the signal level. The comparison between the noise intensity and signal intensity is commonly referred to as the signal to noise ratio. For infrared signals, the signal to noise ratio is high. Since infrared signals are light signals, other lighting can introduce noise that affects the infrared signal. Most noise that affects infrared communication comes from sunlight, incandescent lights, or fluorescent lights. One common solution for this problem is to use optical filters to remove unwanted signals.

The other problem that is the light must be safe for the eyes of the people in the room. When working with light with wavelengths less than 1400nm, the light can

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pass through a person’s cornea and cause damage to the retina. There has been discussion about changing the wavelength for infrared communication to 1550nm, larger than the wavelength that can cause harm to a person’s eyes. Due to the fact that the photodiodes used with this longer wavelength are very expensive, the change has not taken place.

These two problems come in conflict within a second solution to the first problem. This solution could be to make the infrared signal stronger, so the signal to noise ratio is improved. The reason this solution is a problem with respect to the second problem is that when the infrared signal is made stronger, there is a more likely chance that the signal could cause harm to a person’s eyes. The two problems should not bee analyzed individually, and careful attention must be made when seeking a solution.

5. Allowing Multiple Users

There are several techniques that could allow multiple users on a wireless network that uses infrared communication. The two main multiplexing techniques include optical multiplexing and electrical multiplexing.

Within optical multiplexing schemes, many users are allowed to transmit data at the same time, using the same space. There is no loss per user when multiple users transmit at the same time. Optical multiplexing techniques also have a high average power efficiency. There are two main types of optical multiplexing: wavelength division multiple access (WDMA) and space division multiple access (SDMA). WDMA systems allow users to use different infrared wavelengths. The receivers in WDMA systems use a bandpass optical filter to select the correct signal. SDMA systems use angle diversity receivers. An angle diversity receiver uses many receiving elements, each of which is facing a different direction. The receiver then separates the desired signal based on the direction it came from.

Electrical multiplexing schemes are similar to those used with radio signals in that many users use the same optical channel. The types of electrical multiplexing include time division, code division and subcarrier frequency division. These types of multiplexing are also very similar to the multiplexing technciques used

with radio signals. Each of these types suffers from loss of per user capacity when multiple users transmit data. Time division multiplexing has a high power efficiency, but code division multiplexing and subcarrier frequency division multiplexing have lower power efficiencies. Code division multiplexing and subcarrier frequency division multiplexing allow multiple users to transmit data at the same time, but time division multiplexing does not.

6. Infrared Communication Compared to Radio Communication

Depending on the situation, infrared communication or radio communication is the superior choice. The benefits of infrared communication include high-speed data transfer at a relatively low cost, a virtually unlimited unregulated bandwidth, few occurrences of interference with other signals since infrared is confined to a single room, data transmission is more secure since it is confined to a single room, and infrared communication is not subject to multipath fading as radio communication is. There are also advantages to using radio communication. These advantages include communication that can extend beyond the confines of one room without wired connections and there is no interference due to lighting when using radio signals. Infrared communication is typically chosen in short range applications or when the complexity of the receiver signal processing must be minimized. Radio communication is typically chosen for long range applications or when transmission power must be minimized.

X. THE PHYSICAL LAYER

The physical layer is described in terms of the standard used for the network. The two main standards that will be discussed are IEEE 802.11 and HiperLAN.

1. IEEE 802.11 Physical Layer

Within the IEEE 802.11 physical layer, there are two sub layers: the physical layer convergence procedure (PLCP) and the physical media dependent (PMD) sub layers. The PLCP sub layer defines the framing formats for sending and receiving data. The PMD sub layer defines the methods used to send and receive the data.

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The methods described in the PMD sub layer include frequency hopping spread spectrum (FH), direct sequence spread spectrum (DSSS), infrared, high rate direct sequence spread spectrum (HR/DSSS), and orthogonal frequency division multiplexing (OFDM). The PMD also includes modulation techniques, which must be used along with one of the previously listed methods. Some modulation techniques include phase shift keying (PSK), frequency shift keying (FSK), and quadrature amplitude modulation (QAM).

The general frame defined by the PLCP is pictured in Figure 22. The four main components include the preamble, the PLCP header, the data and the check data (CRC). There are two parts to the preamble, the synchronization (SYNC) and the start frame delimiter (SFD). The SYNC field is used to ensure that the data goes to the correct antenna and the SFD field is used to establish timing for the frame. The PLCP header contains all information necessary for decoding the frame. The data rate for transmitting the PLCP header is 1Mbps. The preamble and PLCP header is different for FH, DSSS, OFDM, and infrared.

Figure 22 General Format for PLCP Frames7

The PLCP frame for FH is pictured in Figure 23. The preamble is 96 bits long, 80 bits for the SYNC field and 16 bits for the SFD field. The SYNC field consists of an alternating zero and one pattern, where the leftmost bit is a zero. The SFD field is equal to 0x0CBD. The preamble is transmitted in a left to right pattern. The PLCP header consists of 32 bits, 12 bits for the PSDU length word (PLW), 4 bits for the PLCP signaling field (PSF), and 16 bits for error checking bits for the data contained in the PLCP header. The PLW field contains the number of octets that are in the PSDU. The PSF contains a code indicating the transmission rate. Valid transmission rats are between 1Mbps and 4.5Mbps, with increments of 0.5Mbps. The PSDU contains the data.

Figure 23 PLCP Frame Format for FH Systems7

The PLCP frame for DSSS is pictured in Figure 24. The preamble consists of 144 bits, 128 bits for the SYNC field and 16 bits for the SFD field. The SYNC field consists of a scrambled pattern of ones. The SFD field is equal to 0xF3A0. The PLCP header field consists of 48 bits, 8 bits for the signal field, 8 bits for the service field, 16 bits for the length field, and 16 bits for error checking. The signal field provides information about the data rate: for 1Mbps, 0x0A is the code; for 2Mbps, 0x14 is the code; for 5.5Mbps, 0x37 is the code; and for 11Mbps 0x6E is the code. The service field is reserved for future use. The length field contains the number of microseconds necessary to transmit the MPDU. The MPDU contains the data.

Figure 24 PLCP Frame Format for DSSS Systems7

The PLCP frame for OFDM is pictured in Figure25. The preamble is 12 symbols long; there are 10 short symbols and 2 long symbols. The length field contains the length of the packet. The rate field contains information about the modulation technique used and the coding rate.25 The entire signal field is 16 bits long and is transmitted least significant bit first. The parity bit is calculated using even parity. The contents of both tail fields are zero. The first seven bits of the service field are zero and the last 9 bits of the field are reserved for future use. The PSDU contains the data. Since all frames must contain 48, 96, 192, or 288 bits, the pad field is used to achieve the correct number of bits.

Figure 25 PLCP Frame Format for OFDM Systems7

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The PLCP frame for infrared system is pictured in Figure 26. Within an infrared system a ‘1’ represents the presence of a pulse and a ‘0’ represents the absence of a pulse. The SYNC field contains 57 to 73 slots that have an alternating zero-one pattern with a zero being the last value. The SYNC is used for set up purposes such as recovering the clock. The SFD field is four slots long and has 1001 pattern. The DR field is three slots long and defines the data rate; a 000 pattern indicates 1Mbps and a 001 pattern indicates 2Mpbs. The DCLA field is the DC level adjustment field and is 32 slots long. The value of the DCLA field is 0x00800080 for 1Mbps and is 0x22222222 for 2Mbps. The length field is 16 bits long and tells the number of octets to be transmitted in the PSDU. The CRC field contains 16 bits for check data protects the length field.

Figure 26 PLCP Frame Format for Infrared Systems20

2. HiperLAN Physical Layer

The physical layer HiperLAN uses OFDM.26 There are 52 sub carriers used; 48 sub carriers are for data and 4 sub carriers are for phase tracking. There are several different modulation and coding alternatives that can be used within HiperLAN systems. The seven modes are defined in Table 1.

Table 1 HiperLAN modes28

Mode Modulation Code Rate

Bit rate Bytes/ symbol

1 BPSK ½ 6 Mbps 32 BPSK ¾ 9 Mbps 4.53 QPSK ½ 12 Mbps 64 QPSK ¾ 18 Mbps 95 16QAM 9/16 27 Mbps 13.56 16QAM ¾ 36 Mbps 187 64QAM ¾ 54 Mbps 27

BPSK and QPSK are variants of the PSK modulation technique discussed in a previous section. 16QAM and 64QAM are variants of the QAM technique previously discussed.

XI. DATA LINK LAYER

The layer directly above the physical layer is the data link layer. There are several components of the data link layer. As with the physical layer, the components differ based on what standard is being used.

1. IEEE 802.11 Data Link Layer

The main component of the data link layer is the medium access control layer (MAC). There are several requirements for the MAC. There must be one MAC that can support many physical layers. If one MAC will support many physical layers, it must be able to support one or more channels and channels with different characteristics. It must be permitted that an overlap of many networks may occur in the same space. The MAC must be resistant to interference, able to deal with hidden nodes, provide provisions for time bounded services, and provide provisions for privacy and access control.27

To support multiple physical layers and allow many networks to overlap spacially, distributed coordination function (DCF) is used. The DCF consists of a carrier sense multiple access protocol with collision avoidance (CSMA/CA). The main goal of the CSMA/CA is to reduce the number of collisions that take place. To reduce the effect of interference on the system, an acknowledgment (ACK) was added to the CSMA/CA system. Figure 27 illustrates a transfer between and source and destination using the CSMA/CA and ACK system. The DIFS is a distributed interframe space and the SIFS is a short interference space. These two spaces are used to time the transactions appropriately.

Figure 27 CSMA/CA and ACK Data Transfer27

The problem of hidden nodes can be described as a transmitter and receiver not being able to communicate with one another but both can communicate with an

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access point. The optimum solution is to use a separate request to send (RTS) and clear to send (CTS) message. A net allocation vector (NAV) is also used to control the transmission. Figure 28 illustrates the additions to the previous system to allow for separate RTS and CTS messages.

Figure 28 CSMA/CS + ACK + RTC/CTS Data Transfer27

Frames are used to provide privacy and access control. The formats for the frames are specified, just as the frames for the PLCP are specified. The MAC frame is pictured in Figure 29. The numbers on top of each field indicate the length of the field in bytes.

Figure 29 MAC Frame27

The frame control field is broken down into sub fields and is pictured in Figure 30. The numbers on top of the sub fields are the number of bits within each sub field. The duration ID field is two bytes long. If the field is a control frames, the duration ID field is used for association identity and power save characteristics. For other frames, the duration ID field contains the duration value. The address fields contain the basic service set identifier (BSSID), the destination address, source address, receiver address, and transmitter address. The sequence control field contains a 12 bit sequence number and a 4 bit fragment number if the frame is a fragment of a large data transmission. The CRC field contains 32 bits for check data that apply to the entire frame.

The components of the frame control field can be described as the following. The protocol version sub field is set to zero. The type and subtype sub fields work together to identify the function of the frame. Possible frame functions include control, data, and management. The to DS sub field is set to 1 for frames

sent from stations associated with an endpoint and is set to 0 for all others. The from DS sub field is set to 1 for all frames leaving the distribution system (DS) and 0 for all others. The more fragments sub field is set to 1 if there are more fragment sub fields, and 0 if there are not more fragment sub fields. The retry sub field is set to 1 if the current transmission is a retransmission and 0 if it is not a retransmission. The power management sub field is set to 1 if the station is in power saving mode and 0 if the station is not in power saving mode. The more data sub field is set to 1 to indicate that there is additional data that what is present in the frame. The WEP sub field is set to 1 to indicate that the body of the frame has been processed by a wired equivalent privacy (WEP) algorithm and set to 0 to indicate that a WEP algorithm has not processed the body of the frame. The order sub field is set to 1 if the frame contains MSDU and is set to 0 if the frame does not contain MSDU.

Figure 30 Frame Control Field27

2. HiperLAN Data Link Layer

The data link layer in HiperLAN systems consists of 3 sub layers: the medium access control (MAC) protocol, the error correction (EC) protocol, and the radio link control (RLC) protocol.

The MAC protocol provides access to the radio link. Time-division duplex and dynamic time division multiple access are used for the interface to the air by assigning time slots to data packets. Figure 31 illustrates the frame that the MAC uses for the air interface.

Figure 31 MAC Frame for HiperLAN26

The components of the frame include the broadcast control (BCH), frame control (FCH), access control (ACH), downlink (DL) phase, uplink (UL) phase, and random access (RCH). The BCH is used only for downlink and contains control information. The FCH is also only used for download and contains resource

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allocation information. The ACH contains information regarding previous attempts. The downlink and uplink phases contain the data that is transferred. The RCH is only used for uplink and contains information requesting transmission resources. Each frame has a 2ms duration.

The EC protocol used in HiperLAN systems is base on the selective repeat idea. The basic operation of the EC includes detecting bit errors and retransmitting the data that contained the error.

The RLC contains three major parts: the association control function (ACF), the radio resource control function (RRC), and the data link control user connection control function (DCC). The functions of the ACF include listening to the BCH portion of the MAC frame and choosing the access point that will provide the best quality. The DCC controls the resources for a MAC entry by controlling the data link control user connections. The RRC deals with handover, power saving, and frequency selection. When performing a handover operation, measurements are taken on nearby access points and the best one is chosen to continue the communication. Powers saving features involved with RCC include controlling the power of the transmitter. Frequency selection is performed automatically to make sure that signals do not interfere and that the entire spectrum is used.

XII. NETWORK LAYER

The layer directly above the data link layer is the network layer. The responsibilities of the network layer include packet routing, congestion control, mobility management, and routing under mobile constraints. There are two main ways for packet routing to take place. The first algorithm requires updating the topology of the network frequently. This algorithm has the advantage that the routing is better, but less bandwidth is available since some is used to frequently update the topology. The second algorithm requires updating the topology of the network infrequently. This algorithm has the advantage of more bandwidth available since less is used for topology updates, but the routing is less efficient since the topology is not as up to date. There are two types of traffic that the network layer must handle: unicast traffic and broadcast traffic. Unicast traffic includes all traffic that goes to one receiver. Broadcast traffic includes all that goes to all the receivers in the network.

Routing for these types of traffic can be very different, but the network layer handles both.

XIII. POWER CONSUMPTION

Power consumption is an important topic in wireless networking. Many of the devices that are used within wireless networks operate on battery power. Since it is desired to have the device operate as long as possible without having a large battery, power saving techniques are important. Most of the power consumed in these devices for the wireless network functions include the power used for the transmitter and receiver; the transmitter uses more power than the receiver. There are two main ways to reduce the total power used for transmissions. The first way is to reduce the number of collisions, decreasing the total number retransmissions necessary. The second way is to choose a smart, direct route for the data to travel so that there are few redirections.

In most systems, the receiver remains on at all times. Reducing the time that the receiver remains is a second way to reduce the total power consumption. The best way is to broadcast a receive schedule for all receivers in the network. Each receiver would know when to turn on to expect data and would turn off when data was not expected.

XIV. TCP

In any network, wireless or wired, there must be a transmission control protocol (TCP) that is used to provide a reliable and in-sequence transport service.28

There are four main tasks that TCP is responsible for. TCP must break up messages at the transmitting end, reassemble the messages at the receiving end, handle lost packets, and rearrange the packets if they are in the wrong order. The steps that any TCP must execute include breaking data into small chunks, placing a header at the beginning of each chunk that includes the source and destination port numbers and sequence number, handle acknowledgements that the sender should receive when it receives a data packet, resend the data packet if the acknowledgement is not received, and use a time window to control the amount of data sent.

TCP works much better for wired networks than for wireless networks.29 There are two main reasons for

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the difference in operation. The first reason is that every time a packet is lost, the data rate is reduced. In wired networks, there are far fewer packs lost than in wireless networks, so the data rate remains fairly constant for wired networks and variable for wireless networks. The second reason is that timeouts are more likely to occur when multiple packets are lost. With the continual decrease in speed caused by the loss of packets, the data rate will be so slow for the wireless network that timeouts will occur. Since timeouts can be classified as lost packets, the data rate continues to slow.

One solution to the problem with the TCP problem for wireless networks is to use TCP decoupling. Within this approach, data packets and header packets are sent separately rather than together. Each header packet contains no data. The traditional TCP approach is only used on the header packets only, not the data packets. The data packets associated with a header packet will traverse the network in the same way that the header packet did. It is assumed that the data packets will encounter the same types of congestion as the header packets since take the same path.

There are nine goals of the TCP decoupling approach. The first goal is to only send header packets if there is data that needs to be sent. The reason for this goal is to save bandwidth. If header packets are sent when there is no data, bandwidth is wasted. The second goal is to not automatically retransmit data packets that are lost. The reason for this goal is that it is more reliable for something, other than TCP, to handle packet retransmission. The third goal is to not introduce packet reordering. This goal is needed to reduce the size. Including packet reordering information increases the amount of data and increases the bandwidth required to transmit the packet. The fourth goal is to not introduce added transmission delay due to congestion control. A congestion control processes may unnecessarily, or mistakenly decrease the data rate. The fifth goal is to keep the data within each data packet original, not make modifications. Speed is increased if the data packets remain in the original state because modifying data packets takes time and slows down transmission. The sixth goal is to not have long data packets. Long data packets increase the chances that fragmentation will occur causing the data to be incorrectly received. The seventh goal is that the header packets use as little bandwidth as possible. Using small header packets saves bandwidth for data

packets. The final goals include having the TCP decoupling approach be simple, efficient, easy to set up, easy to configure, and easy to use.

XV. WIRELESS NETWORK SECURITY

There are three major ways to achieve secure wireless network: wireless application protocol (WAP), wireless transport layer security (WTLS), and virtual private network (VPN).

WAP provides data encryption.30 There are two types of data encryption, 64-bit and 128-bit. The 64-bit type consists of a 40-bit encryption key and a 24-bit initialization vector. The 128-bit type consists of a 104-bit encryption key and a 24-bit initialization vector.

WTLS is a method that provides authentication between a client and server.31 There are three types of authentication. Class 1 authentication is anonymous, and is the least secure since the users are connected with an unknown server. Class 2 authentication is server authentication. With this type, users know that they are connected to the correct server. Class 3 authentication is client and server authentication. This type is the most secure because both the client and server know who is connected.

VPN is the interference between a secure and non-secure network that is used to gain authentication and provide access control and privacy.32 In order to use VPN, software must be installed on all devices that will connect to the network, which provides interference between secure and insecure networks.

XVI. CONCLUSIONS

This paper provides an overview of the various techniques used to transmit data using wireless networks including direct sequence spread spectrum, frequency hopping spread spectrum, orthogonal frequency division multiplexing, and infrared. The two main types of wireless networks, ad hoc and cellular, are discussed. The paper provides information regarding the layers that comprise an ad hoc network and the communication protocol (TCP) that can be used. An overview of the types of security for wireless networks is also discussed.

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Reference

[1] Introduction to Wireless LANs 15 Mar 2002. <http://www.wlana.com/learn/intro.pdf>.

[2] Glas, Jack P. Non-Cellular Wireless Communications. 3 Mar 2002. < http://cas.et.tudelft.nl/~glas/thesis/>.

[3] Rappaport, Theodore S. Wireless Communications: Principles and Practice. Upper Saddle River: Prentice Hall PTR, 1996.

[4] Dornan, Andy. “Fast Forward to 4G?” Network Magazine 17.3 (2002): 34-39.

[5] Jones, Christine E. “A Survey of Energy Efficient Network Protocols for Wireless Networks.” Wireless Networks 7 (2001): 343-58.

[6] Prem, Edward C. Wireless Local Area Networks. 1 Apr 2002 <http://www.cis.ohio-state.edu/~jain/cis788-99/wireless_lans/index.htm>.

[7] Zuo, Zhengping. In-building Wireless LANs. 1 Apr 2002 <http:// www.cis.ohio-state.edu/~jain/cis788-99/wireless_lans/index.htm>.

[8] Dornan, Andy. “LANs with No Wires, but Strings Still Attached” Network Magazine 17.2 (2002): 44-47.

[9] Glisic, Savo and Branka Vucietic. Spread Spectrum CDMA Systems for Wireless Communications. Boston: Artech House: 1997.

[10] Schwartz, Sorin M. Frequency Hopping Spread Spectrum (FHSS) vs. Direct Sequence Spread Spectrum (DHSS) in the Broadband Wireless Access and WLAN Arenas. 5 Mar 2002 <http://www.cvcwireless.com/c3_freq_hop_vs_direct_seq.pdf>.

[11] Frequency-Hopping Spread Spectrum . 8 Apr 2002. <http://www.its.bldrdoc.gov/fs-1037/dir-016/_2346.htm>.

[12] Meel, ir. J. Spread Spectrum (SS) introduction. 14 Mar 2002. < http://www.sss-mag.com/pdf/Ss_jme_denayer_intro_print.pdf>.

[13] Levesque, Allen and Kaveh Pahlavan. Wireless Information Networks. New York: John Wiley and Sons, 1995.

[14] Direct-Sequence Spread Spectrum . 8 Apr 2002. <http://www.its.bldrdoc.gov/fs-1037/dir-012/_1688.htm>.

[15] Chip . 8 Apr 2002. <http://www.its.bldrdoc.gov/fs-1037/dir-007/_0951.htm>.

[16] Chip Rate . 8 Apr 2002. <http://www.its.bldrdoc.gov/fs-1037/dir-007/_0952.htm>.

[17] Spread Spectrum (SS) applications . 14 Mar 2002. <http://www.sss-mag.com/pdf/Ss_jme_denayer_appl_print.pdf>.

[18] Zamanian, Ali. “Orthogonal Frequency Division Mulitplex Overview.” Microwave Journal 44.10 (2001): 122-134.

[19] Chen, James C. LAN Technology Promises Real Mobile Internet.” Wireless System Design Feb. 2001: 49-54.

[20] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. 11 Feb 2002 <http://www.mit.edu/afs/sipb/contrib/doc/specs/restricted/ieee/802.11-1999.pdf>.

[21] Barry, John R. and Joseph M. Kahn. “Wireless Infrared Communications.” Proceedings of the IEEE 85.2 (1997): 265-298.

[22] Lab 7: Radio Astronomy I: Square-Law Detector. 16 Apr 2002. <http://www-astro.physics.uiowa.edu/~rlm/astro137/lab7/lab7.html>.

[23] Technical Summary of “IrDA DATA” and “IrDA CONTROL”. 16 Apr 2002 <http://www.irda.org/standards/standards.asp>.

[24] Myslik, Vladimir. Introduction to IrDA. 16 Apr 2002 <http://www.hw.cz/english/docs/irda/irda.html>.

[25] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications High-speed Physical Layer in the 5GHz Band. 11 Feb 2002 <http://www.mit.edu/afs/sipb/contrib/doc/specs/restricted/ieee/802.11a-1999.pdf>.

[26] Johnsson, Martin. HiperLAN2 – The Broadband Radio Transmission Technology Operating in the 5GHz Frequency Band. 22 Apr 2002 <http://www.hiperlan2.com/mjbroadbandoverview.asp>.

[27] Chayat, Naftali. Frequency Hopping Spread Spectrum PHY of the 802.11 Wireless LAN Standard. 12 Feb 2002 <http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdf>.

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[28] Newton, Steven E. General Description of the TCP/IP Protocols. 21 Mar 2002 <http://oac3.hcs.uth.tmc.edu/staff/snewton/tcp-tutorial/sed2.html>.

[29] Wang, S. Y. and Kung, H. T. “Use of TCP Decoupling in Improving TCP Performance over Wireless Networks.” Wireless Networks 7.3 (2001): 221-236.

[30] Ellison, Craig. Exploiting and Protecting 802.11b Wireless Networks. 11 Mar 2002.

<http://www.extremetech.com/print_article/0,3428,a=13880,00.asp>.

[31] Fratto, Mike. Tutorial: Wireless Security. 11 Mar 2002 < http://www.networkcomputing.com/1202/1202f1d1.html>.

[32] Molita, Dave. “WLAN Security on the Rise.” Network Computing. 13.3 (2002): 86-89.

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