Wireless Networks PDF

10/9/2024 Wireless Networks 1 Reference: Wireless Communications and Networks Second Edition by William Stallings. Evaluation Tools 1. Mid Exam : 30 Marks 2. Class Work (Quiz/...

10/9/2024 Wireless Networks 1 Reference: Wireless Communications and Networks Second Edition by William Stallings. Evaluation Tools 1. Mid Exam : 30 Marks 2. Class Work (Quiz/ seminar) : 20 Marks 3. Final Exam : 50 Marks 2 1  10/9/2024 Introduction  Wireless technology has become the most exciting area in telecommunications and networking.  The rapid growth of mobile telephone use, various satellite services, and now the wireless Internet and wireless LANs are generating tremendous changes in telecommunications and networking. 3 Wireless Comes of Age  Guglielmo Marconi invented the wireless telegraph in 1896. In 1901, he sent telegraphic signals across the Atlantic Ocean, enabling communication over a distance of about 3200 km. This paved the way for radio, television, mobile phones, and communications satellites, revolutionizing global communication.  First launched in the 1960s, communication satellites now handle significant voice and television traffic, with modern low-orbit satellites providing data services with minimal delay.  Wireless networking, supported by IEEE 802.11 and Bluetooth, enables the creation of WANs, MANs, and LANs without cables. 4 2  10/9/2024  Cellular phones offer better reception, security, and Internet connectivity. These advancements highlight the profound impact of wireless communication, which continues to shrink the world and move towards a global wireless network delivering various services.  Figure 1.1 highlights some of the key milestones in the development of wireless communications. 5 6 3  10/9/2024 The Cellular Revolution: Key Points Market Growth: -1990: 11 million mobile phone users. -Today: Billions of users. - 2002: Mobile phones outnumbered fixed-line phones. Modern Mobile Devices - Internet access and digital cameras - Communication with fixed base stations d i 1 7 Technical Innovations - Smaller, lighter, and more efficient handsets - Decreased costs, especially in competitive markets since 1996 Beyond Phones - New wireless devices with internet access (e.g., personal organizers, web-enabled phones) - Automotive wireless devices (e.g., map downloads, emergency calls) 8 4  10/9/2024 GLOBAL CELLULAR NETWORK Current Landscape - No single cellular network; devices support multiple technologies - Generally work within a single operator's network - Need for defined and implemented standards Next-Generation Standards - ITU developing standards for next-gen wireless devices - Higher frequencies to increase capacity - Addressing incompatibilities of 1st and 2nd generation networks d i 1 9 Historical Networks - First-Generation: Advanced Mobile Phone System (AMPS) - Data service using Cellular Digital Packet Data (CDPD) - 19.2-kbps data rate, uses idle voice channel periods - Second-Generation: - GSM (Global System for Mobile Communications) - PCS IS-136 (Personal Communications Service) - Time Division Multiple Access (TDMA) - PCS IS-95 - Code Division Multiple Access (CDMA) - Data services on dedicated channels at 9.6 kbps Future Standards - International Mobile Telecommunications-2000 (IMT-2000) - Seamless global network - 2-GHz frequency band - Data rates up to 2 Mbps 10 5  10/9/2024 broadband Multimedia content is increasingly common on the internet and in business communications. Wireless networks need high data rates comparable to fixed networks to support multimedia. Broadband wireless technologies offer these high data rates. Advantages of broadband wireless include lower deployment costs compared to fixed services, mobility, and versatility in deployment. 11 Various broadband wireless standards are under development for different applications, offering data rates from 2 Mbps to over 100 Mbps. Wireless LANs (WLANs), based on IEEE 802.11 standard with data rates up to 54 Mbps, provide network services where fixed infrastructure is impractical. Compatibility issues between Bluetooth and 802.11 exist due to overlapping frequency bands, potentially causing interference when deployed together. 12 6  10/9/2024 FUTURE TRENDS - Frequency Spectrum and Licensing: Many new wireless technologies utilize unlicensed portions of the spectrum, such as the ISM band near 2.4 GHz and the UNII band at 5 GHz in the US, saving billions in licensing fees. - Development History: Initially overlooked, these bands were later leveraged due to consumer demand and standards development, leading to technologies like Wi-Fi (802.11), which allows wireless LANs and public hotspots. 13 - Wi-Fi and Beyond: Wi-Fi, certified by the Wi-Fi Alliance, covers office LANs, home LANs, and public hotspots. Other technologies like WiMAX (40-50 km range), Mobile-Fi (high-speed mobile internet access), ZigBee (low-cost, low-power sensor networks), and Ultrawideband (high-speed, short-distance data transfer) are also evolving. - Applications: WiMAX serves as a broadband access alternative, while Mobile-Fi aims for high-speed internet on the move. ZigBee supports low-cost sensor networks, and Ultrawideband facilitates high- speed file transfer without cables, catering to diverse communication needs. 14 7  10/9/2024 THE TROUBLE WITH WIRELESS - Standardization Challenges: Wireless technologies face obstacles due to incompatible standards like PCS IS-136 vs. IS-95 in North America and different standards globally. Lack of uniform standards hinders ubiquitous data access, a core ideal of wireless technology. - Device Limitations: Mobile devices often have small displays and limited capabilities, making it difficult to access full web content designed for standard browsers. Wireless Markup Language (WML) is used instead of HTML, further restricting browsing capabilities. - Diverse Needs: No single wireless device can meet all user needs, necessitating integration into various devices tailored to different applications. 15 16 8  10/9/2024 Communication Networking 1  This lecture provides an overview of various approaches to communication networking. The lecture begins with a survey of different type of networks based on geographic extent.  Local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs) are all examples of communications networks. Figure 3.1 illustrates these categories, plus some special cases. By way of contrast, the typical range of parameters for a multiple-processor computer is also depicted. 2 1  10/9/2024 3 Wide Area Networks(WAN)  WANs: Cover large geographical areas. May cross public right-of-ways.  Structure: Consist of interconnected switching nodes. Data is routed through these nodes to reach the destination.  Traditional Capacity: Typically offered modest data rates (e.g., 64,000 bps or less). Business services like T1 provide higher rates (1.544 Mbps).  Advancements: Optical fiber technology has enabled much higher data rates. Modern WANs offer speeds in the 10s and 100s of Mbps. Use asynchronous transfer mode (ATM) for high-speed connections 4 2  10/9/2024 Local Area Networks (LANs)  LANs cover a small geographic area, such as a single building or a cluster of buildings.  LANs are localized, leading to different technical solutions compared to WANs.  Internal Data Rates: LANs typically offer much higher data rates than WANs. Traditional LANs provided rates from 1 to 20 Mbps. Modern LAN (from 100 Mbps to 10 Gbps.)  Structure: Devices are attached to a shared transmission medium. All devices on the network can receive transmissions from any other device. 5 Metropolitan Area Networks (MANs)  Position Between LANs and WANs: - MANs serve as an intermediary between LANs and WANs.  Purpose and Need: Emerging due to the inadequacies of traditional WAN techniques for the growing needs of organizations. Aimed to provide high capacity at lower costs over large areas.  Benefits and Features: Combines the high-speed, shared-medium approach of LAN standards with metropolitan scale coverage. Covers greater distances and higher data rates than LANs, with some geographical overlap.  Primary Market: Targeted at customers with high-capacity needs within metropolitan areas. Offers a more cost-effective and efficient alternative to local telephone company services. 6 3  10/9/2024 Transmission Medium  Definition: Transmission medium is the physical path between transmitter and receiver.  Classification: Guided Media: Waves are guided along a solid medium. Examples: Copper twisted pair, copper coaxial cable, optical fiber. Unguided Media: Waves are not guided; transmission occurs through the atmosphere or outer space. Also known as wireless transmission.  Form of Communication: In both guided and unguided media, communication is in the form of electromagnetic waves. 7  Data Transmission Quality: - Determined by both the medium and the signal characteristics.  Guided Media: - Medium is more important in determining transmission limitations.  Unguided Media: - Signal bandwidth from the transmitting antenna is more important. - Signal directionality is key. - Lower frequencies: Omnidirectional (signal propagates in all directions). - Higher frequencies: Can focus signal into a directional beam.  Electromagnetic Spectrum: - Figure 2.10 shows frequencies for guided and unguided media. 8 4  10/9/2024 9  Unguided Media: Transmission and reception use an antenna. Directional Configuration: Focused beam, requires careful alignment. Omnidirectional Configuration: Signal spreads in all directions, can be received by multiple antennas.  Frequency Ranges: Microwave Frequencies: 1 GHz to 100 GHz, suitable for directional, point-to-point, and satellite communications. Radio Range: 30 MHz to 1 GHz, suitable for omnidirectional applications. Infrared Range: 3 x 10¹¹ to 2 x 10¹⁴ Hz, useful for local point-to-point and multipoint applications within confined areas. 10 5  10/9/2024 Terrestrial Microwave  Physical Description: Common antenna: Parabolic "dish," ~3m in diameter. Fixed rigidly, focuses a narrow beam for line-of-sight transmission. Antennas usually elevated to extend range and clear obstacles. Long-distance transmission uses series of microwave relay towers.  Applications:  Long-haul telecommunications: Alternative to coaxial cable or optical fiber, requires fewer amplifiers/repeaters.  Voice and television transmission.  Short point-to-point links: Between buildings for closed-circuit TV or data links between LANs. 11  Transmission Characteristics: - Frequencies: 2 to 40 GHz. - Higher frequencies offer higher bandwidth and data rates.  As with any transmission system, a main source of loss is attenuation. For microwave (and radio frequencies), the loss can be expressed as where d is the distance and is the wavelength, in the same units. Thus, loss varies as the square of the distance.  Another source of impairment is Interference: - Overlapping transmission areas can cause interference. - Frequency bands are strictly regulated. 12 6  10/9/2024  Common Bands: 4-GHz to 6-GHz: Long-haul telecommunications. 11-GHz: Increasingly used due to congestion in lower bands. 12-GHz: Component of cable TV systems. 22-GHz: Short point-to-point links between buildings. Higher frequencies are less useful for long distances due to increased attenuation but are adequate for short distances. Higher frequency antennas are smaller and cheaper. 13 Satellite Microwave  Physical Description: Function as microwave relay stations. Link ground-based microwave transmitter/receivers (earth stations). Receive signals on one frequency band (uplink), amplify or repeat the signal, and transmit on another frequency (downlink).  Applications: 1.Television distribution: Well-suited due to broadcast nature. Direct Broadcast Satellite (DBS) transmits signals directly to home users. 2. Long-distance telephone transmission: Optimal for high-usage international trunks and competitive for long-distance intranational links. 3.Private business networks: Capacity divided into channels and leased to businesses. Used for private networks by organizations with high-volume requirements. 14 7  10/9/2024  Transmission Characteristics: Frequency Range: Optimum between 1 to 10 GHz.  Propagation delay: ~0.25 seconds due to long distances, noticeable in telephone conversations.  Many stations can transmit to the satellite.  Satellite transmissions can be received by many stations. 15 Broadcast Radio:  Physical Description: Omnidirectional, unlike directional microwave. Does not require dish-shaped antennas or precise alignment.  Applications: Encompasses frequencies from 3 kHz to 300 GHz. Informally covers VHF (30 MHz to 300 MHz) and part of UHF (300 MHz to 1 GHz). Used for FM radio, VHF/UHF television, and various data networking applications.  Transmission Characteristics: Effective range: 30 MHz to 1 GHz. Less sensitive to attenuation from rainfall compared to microwave. Multipath interference: Reflections from various surfaces can create multiple signal paths, causing issues like multiple TV images when airplanes pass by. 16 8  10/9/2024 Infrared Communications:  Physical Description: Uses transceivers to modulate noncoherent infrared light. Requires line of sight between transceivers, either directly or via reflection from light-colored surfaces like ceilings.  Advantages: Security and Interference: Does not penetrate walls, eliminating security and interference issues common in microwave systems. Frequency Allocation: No licensing required, avoiding frequency allocation issues. 17 18 9  10/9/2024 Wireless LAN Technology 1 Wireless LAN Technology  Overview  Wireless LAN Applications  Wireless LAN Requirements  Wireless LAN Technology  Infrared LANs  Strengths and Weaknesses  Transmission Techniques  Spread Spectrum LANs  Configuration  Transmission Issues  Narrowband Microwave LANs  Licensed Narrowband RF  Unlicensed Narrowband RF 2 1  10/9/2024 Wireless LAN Applications  Application areas for wireless LANs:  LAN Extension  Cross-Building Interconnect  Nomadic Access  Ad Hoc Networks. 3 LAN Extension  Wireless LANs save on installation costs and simplify relocation and network modifications.  Wireless LANs are suitable in certain environments:  Large open areas (e.g., manufacturing plants, stock exchange trading floors, warehouses)  Historical buildings where drilling for new wiring is prohibited.  Small offices where wired LAN installation and maintenance are not economical.  In these cases, wireless LANs provide an effective and attractive alternative. 4 2  10/9/2024  Organizations often use a combination of wired and wireless LANs: - Wired LANs support servers and stationary workstations. - Wireless LANs link different areas within the same premises  Figure 13.2, is a multiple-cell wireless LAN. In this case, there are multiple control modules interconnected by a wired LAN. Each control module supports a number of wireless end systems within its transmission range. For example, with an infrared LAN, transmission is limited to a single room; therefore, one cell is needed for each room in an office building that requires wireless support. 5 Multiple-cell Wireless LAN 6 3  10/9/2024 Cross-Building Interconnect  Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs.  In this case, a point-to-point wireless link is used between two buildings.  The devices so connected are typically bridges or routers.  This single point-to-point link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN. 7 Nomadic Access  Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop computer or notepad computer.  One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a personal portable computer to a server in the office.  Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move around with their portable computers and may wish access to the servers on a wired LAN from various locations. 8 4  10/9/2024 Ad Hoc Networking  An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need.  For example, a group of employees, each with a laptop or computer, may convene in a conference room for a business or classroom meeting.  The employees link their computers in a temporary network just for the duration of the meeting. 9 Figure 13.3 suggests the differences between a wireless LAN that supports LAN extension and nomadic access requirements and an ad hoc wireless LAN. 10 5  10/9/2024 Wireless LAN Requirements A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs:  Throughput  Number of nodes  Connection to backbone LAN  Service area  Battery power consumption  Transmission robustness and security  Collocated network operation  License-free operation  Handoff/roaming  Dynamic configuration 11 Wireless LAN Categories Wireless LANs are generally categorized according to the transmission technique that is used. All current wireless LAN products fall into one of the following categories:  Infrared (lR) LANs: An individual cell of an IR LAN is limited to a single room, because infrared light does not penetrate opaque walls.  Spread spectrum LANs: This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands so that no FCC licensing is required for their use in the United States.  Narrowband microwave: These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands. 12 6  10/9/2024 Infrared (lR) LANs Optical wireless communication in the infrared portion of the spectrum is commonplace in most homes, where it is used for a variety of remote-control devices. More recently, attention has turned to the use of infrared technology to construct wireless LANs. Strengths of Infrared Over Microwave Radio:  Spectrum for infrared virtually unlimited  Possibility of high data rates  Infrared spectrum unregulated  Equipment inexpensive and simple  Doesn’t penetrate walls  More easily secured against eavesdropping  Less interference between different rooms 13 Drawbacks of Infrared Medium:  Indoor environments experience infrared background radiation  Sunlight and indoor lighting  Ambient radiation appears as noise in an infrared receiver  Transmitters of higher power required  Limited by concerns of eye safety and excessive power consumption  Limits range 14 7  10/9/2024 IR Data Transmission Techniques There are three alternative transmission techniques commonly used for IR data transmission: 1. Directed Beam IR 2. Ominidirectional 3. Diffused 15 Directed Beam Infrared  Used to create point-to-point links  Range depends on emitted power and degree of focusing  Focused IR data link can have range of kilometers  Cross-building interconnect between bridges or routers 16 8  10/9/2024 Ominidirectional IR  Single base station within line of sight of all other stations on LAN  Station typically mounted on ceiling  Base station acts as a multiport repeater  Ceiling transmitter broadcasts signal received by IR transceivers  IR transceivers transmit with directional beam aimed at ceiling base unit 17 Diffused IR  All IR transmitters focused and aimed at a point on diffusely reflecting ceiling  IR radiation strikes ceiling  Reradiated omnidirectionally  Picked up by all receivers 18 9  10/9/2024 Spread Spectrum LAN The most popular type of wireless LAN uses spread spectrum techniques. Configuration:  Multiple-cell arrangement (Figure 13.2)  Within a cell, either peer-to-peer or hub  Peer-to-peer topology  No hub  Access controlled with MAC algorithm  Appropriate for ad hoc LANs 19  Hub topology  Mounted on the ceiling and connected to backbone  May control access  May act as multiport repeater  Automatic handoff of mobile stations  Stations in cell either:  Transmit to / receive from hub only  Broadcast using omnidirectional antenna 20 10  10/9/2024 Transmission Issues Licensing: Wireless LANs ideally usable without licensing. Licensing regulations vary by country, complicating the objective. In the U.S., FCC authorizes two unlicensed ISM band applications: spread spectrum systems (up to 1 watt) and very low power systems (up to 0.5 watts). Frequency Bands in the U.S.: Three unlicensed spread spectrum bands: 902-928 MHz (915 MHz band) 2.4-2.4835 GHz (2.4 GHz band) 5.725-5.825 GHz (5.8 GHz band) 2.4 GHz band also used in Europe and Japan. Higher frequency bands offer higher potential bandwidth. 21 Interference: 900 MHz band: Interference from devices like cordless phones, wireless microphones, amateur radio. 2.4 GHz band: Fewer competing devices; notable interference from microwave ovens. 5.8 GHz band: Minimal competition, but more expensive equipment. Data Rates: Typical spread spectrum wireless LANs were limited to 1-3 Mbps. Newer standards provide up to 54 Mbps. 22 11  10/9/2024 Narrowband Microwave LANs  The term narrowband microwave refers to the use of a microwave radio frequency band for signal transmission, with a relatively narrow bandwidth-just wide enough to accommodate the signal.  Narrowband Scheme: Uses a cell configuration with nonoverlapping frequency bands. Adjacent cells operate on different frequencies within the 18- GHz band. 23 Licensed Narrowband RF  Licensing and Coordination: - Used for voice, data, and video transmission. - Licensed and coordinated to avoid interference. - Each geographic area has a radius of 28 km and can contain five licenses. - Each license covers two frequencies.  Security and Advantages: All transmissions are encrypted for security. Licensed narrowband LANs guarantee interference-free communication. 24 12  10/9/2024 Unlicensed Narrowband RF  RadioLAN introduced narrowband wireless LAN in 1995  Uses unlicensed ISM spectrum  Used at low power (0.5 watts or less)  Operates at 10 Mbps in the 5.8-GHz band  Range:  50 meters in a semi-open office.  100 meters in an open office.  Configuration:  Peer-to-peer setup.  Automatically elects a Dynamic Master node based on location, interference, and signal strength.  Master node identity changes automatically with changing conditions.  Features: Dynamic relay function: Each station can act as a repeater to move data between stations that are out of range of each other. 25 26 13  10/9/2024 27 14  10/9/2024 Antennas and Propagation 1 Antennas  Antenna Definition: An antenna is an electrical conductor or system of conductors used to radiate or collect electromagnetic energy.  Signal Transmission: In transmission, the antenna converts radiofrequency electrical energy from a transmitter into electromagnetic energy, which is then radiated into the environment.  Signal Reception: In reception, the antenna converts electromagnetic energy from the environment into radiofrequency electrical energy, which is fed into the receiver.  TwoWay Communication: The same antenna can be used for both transmission and reception if the same frequency is used. The efficiency of energy transfer is the same in both directions. 2 1  10/9/2024 Radiation Patterns  Radiation Directionality: Antennas radiate power in all directions but not equally in all directions.  Radiation Pattern: A graphical representation of an antenna's radiation properties as a function of space coordinates.  Beam Width: The angle within which the radiated power is at least half of its maximum value. A measure of the antenna's directivity.  Reception Pattern: When used for reception, the radiation pattern indicates the best direction for receiving signals. The longest section of the pattern denotes the optimal reception direction. 3  Isotropic Antenna: An idealized point in space radiating power equally in all directions. Its radiation pattern is spherical, depicted in 2D as a circle. Figure 5.1a: Shows the isotropic antenna’s radiation pattern as a circle, indicating equal power in all directions.  Directional Antenna: Radiates more power in a preferred direction along one axis. Figure 5.1b: Shows a directional antenna’s pattern with varying power in different directions. Relative Power Measurement: Relative power in a direction is determined by the distance from the antenna position to the point on the radiation pattern. 4 2  10/9/2024 Types of Antennas 1 Dipole Antennas  HalfWave Dipole (Hertz Antenna): Consists of two straight collinear conductors of equal length, separated by a small gap. Length: Half the wavelength of the signal it transmits most efficiently (Figure 5.2a). HalfWave Dipole: Suitable for broad, uniform coverage.  QuarterWave Vertical (Marconi Antenna): Commonly used for automobile radios and portable radios. Length: Quarter of the signal's wavelength (Figure 5.2b). QuarterWave Vertical: Ideal for mobile and portable radio communications. 5 2. Parabolic Reflective Antenna  The parabolic reflective antenna is commonly used in terrestrial microwave and satellite communications.  A parabola, when revolved around its axis, forms a paraboloid, which is key to the antenna's design.  Paraboloids are utilized in applications like headlights, telescopes, and antennas because they reflect waves into a parallel beam when the source is at the focus.  While some beam dispersion occurs in practice, the parabolic antenna still creates a highly directional beam, with larger antennas producing more focused beams. 6 3  10/9/2024  Radiation Patterns:  HalfWave Dipole (Figure 5.3a)  Uniform/omnidirectional in one plane.  Figureeight pattern in perpendicular planes.  Directional Antenna (Figure 5.3b): Main strength focused in a specific direction (xdirection in the example). 7 Antenna Gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Effective area Related to physical size and shape of antenna Relationship between antenna gain and effective area 4πAe 4πf 2 Ae G  λ2 c2 G = antenna gain Ae = effective area f = carrier frequency  = carrier wavelength c = speed of light (» 3 x 108 m/s) 8 4  10/9/2024 9 Propagation Modes Groundwave propagation Skywave propagation Lineofsight propagation 10 5  10/9/2024 Ground Wave Propagation Follows the earth's contour and can travel beyond the visual horizon at frequencies up to 2 MHz. Electromagnetic waves in this band tend to follow the earth's curvature due to induced currents in the earth's surface, which slow the wavefront and cause it to tilt downward. These waves are scattered by the atmosphere and do not penetrate the upper atmosphere. AM radio is the most wellknown example of ground wave communication. 11 Sky Wave Propagation Sky wave propagation is used for amateur radio, CB radio, and international broadcasts like BBC and Voice of America. In sky wave propagation, a signal from an earthbased antenna is refracted by the ionized layer of the upper atmosphere (ionosphere) back down to earth. The signal can travel through multiple hops, bouncing between the ionosphere and the earth's surface, allowing it to cover thousands of kilometers. This mode of propagation enables longdistance communication far from the transmitter. 12 6  10/9/2024 LineofSight Propagation Above 30 MHz, communication relies on line of sight as ground wave and sky wave propagation modes are ineffective. For satellite communication, signals above 30 MHz are not reflected by the ionosphere, allowing transmission between an earth station and a satellite beyond the horizon. In groundbased communication, transmitting and receiving antennas must be within an effective line of sight, as microwaves are bent or refracted by the atmosphere. Microwaves typically bend with the curvature of the earth, allowing them to propagate farther than the optical line of sight. 13 14 7  10/9/2024 LOS Wireless Transmission Impairments  In any communication system, the received signal will differ from the transmitted signal due to transmission impairments.  For analog signals, these impairments cause random modifications that degrade signal quality.  For digital data, impairments can introduce bit errors, where binary 1s are transformed into 0s, and vice versa.  This section examines the different transmission impairments and their impact on the informationcarrying capacity of a communication link, with a focus on lineofsight (LOS) wireless transmission. Attenuation and attenuation distortion Free space loss: signal disperses with distance Noise Atmospheric absorption Multipath Refraction Thermal noise 15 1Attenuation Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than noise to be received without error Attenuation is greater at higher frequencies, causing distortion Amplifiers are introduced to amplify high frequences 16 8  10/9/2024 2Free Space Loss  In wireless communication, signal strength decreases with distance due to dispersion.  An antenna with a fixed area receives less signal power as it moves farther from the transmitting antenna.  In satellite communication, this distancerelated signal reduction is the primary mode of signal loss.  The attenuation of a signal over distance, known as free space loss, occurs because the signal spreads over a larger area.  Free space loss can be expressed as the ratio of transmitted power to received power, or in decibels by taking 10 times the logarithm of that ratio. 17 3Noise  In data transmission, the received signal includes the transmitted signal modified by distortions from the transmission system.  Additional unwanted signals, known as noise, are also introduced during transmission.  Noise is the primary factor limiting the performance of communication systems.  Noise may be divided into four categories:  Thermal Noise  Intermodulation noise  Crosstalk  Impulse Noise 18 9  10/9/2024 Thermal Noise Thermal noise results from the thermal agitation of electrons and is present in all electronic devices and transmission media. It is uniformly distributed across the frequency spectrum, earning the name "white noise." Thermal noise cannot be eliminated and sets an upper limit on communication system performance. This type of noise is especially significant in satellite communication due to the weak signals received by satellite earth stations. 19  Intermodulation noise occurs when signals at different frequencies share the same medium, creating new frequencies that are sums or differences of the originals. This is due to nonlinearity in the transmission system, caused by issues like component malfunction or excessive signal strength.  Crosstalk is an unwanted signal coupling between communication paths, often experienced as hearing another conversation on a telephone. It can result from electrical coupling in cables or from microwave antennas picking up stray signals and is more significant in unlicensed ISM bands.  Impulse noise is irregular, highamplitude noise caused by factors like electromagnetic disturbances or system faults. While it minimally impacts analog signals, it is a major source of errors in digital data transmission, potentially corrupting hundreds of bits in a brief spike. 20 10  10/9/2024 Other Impairments 4 Atmospheric Absorption: Signal loss due to atmospheric absorption is mainly caused by water vapor and oxygen, with significant attenuation at 22 GHz (water vapor) and 60 GHz (oxygen). Lower frequencies (