Radio Elements and Frequency Spectrums • Chapter 2 53 The driven element is the only active element on a Yagi antenna and is the only element that connects to the transceiver via a cable.The remaining elements are known as parasitic elements because they feed off of the radiated power from the driven element. As we described ear- lier, if a piece of metal receives a signal and it is not drained from the metal, it will be re-radiated from the metal.This is how the parasitic elements of a Yagi work. Broadcast television antennas are examples of Yagi type antennas. Planar Array Antennas Planar array antennas are similar in concept to Yagi antennas except all elements, both active and parasitic, lie in the same plane.This results in a flat antenna that can be mounted flat on a wall, yet still have the proper- ties and gain of a directional antenna. Figure 2.17 shows a planar array antenna. www.syngress.com Figure 2.17 Planar Array Antenna and Associated Directional Beam Pattern Side View Planar Phased Array Antenna Direction of Beam Antenna G Directional Beam Multiple Antenna Elements Shown with Dipole Antennas Top View Steerable Beam Planar Array Beam Pattern Cross Section View of Steerable Beam 152_wan_02 6/21/01 2:58 PM Page 53 54 Chapter 2 • Radio Elements and Frequency Spectrums Planar arrays can have more than one active element. By changing the phase and power of the signal to hit specific active ele- ments, the beam can be steered without the antenna physically moving. A useful application of this is on military tracking radars. A computer can adjust the input signals to the various elements of a planar array and steer the beam faster than the whole array could be moved physically, thus allowing for tracking of multiple fast-moving targets. Sectorized Array Antennas Sectorized array antennas are a type of phased array antenna designed to split up a circular coverage area into sectors to help in channel allocation and reuse. Most sectorized antennas will have a beam width of about 120° that allow them to divide a circle into three sectors. Sectorized antennas are commonly used in wireless phone applications and can be seen on wireless phone towers all over America. Figure 2.18 illustrates how sectorized antennas work to divide a coverage area. www.syngress.com Figure 2.18 Sectorized Array Antenna and Illustration of Sectorization Cross Section View of Sectorized Beam Pattern Focused Down toward Users Side View Sectorized Array Antenna Top View Sector A Sector CSector B Interference Zone Single Antenna Top View of Common Three Zone Configuration 3 Zone Cluster 152_wan_02 6/21/01 2:58 PM Page 54 Radio Elements and Frequency Spectrums • Chapter 2 55 It is important to plan your zones carefully to minimize interference zones or to make interference zones reside in regions with no users. Parabolic Antennas The most common examples of parabolic antennas are satellite dishes. Parabolic antennas have an emitter that is mounted so that it is aimed into a bowl-shaped reflector. Just as in a common flashlight, the reflector acts to focus the signal from the emitter into a very tight beam. On the receiving end, the dish reflector increases the area of the antenna, col- lecting a lot more of the transmitted signal and focusing that signal back onto the receiver. Figure 2.19 illustrates how parabolic antennas work. Parabolic antennas are used for terrestrial-to-stellar communication (ground-to-satellite) and for terrestrial-to-terrestrial point-to-point com- munication. Microwave long-distance telephone links use parabolic and cone antennas to carry phone conversations from one point to another. The number of microwave telephone links is rapidly diminishing with the advent of fiber optic cables; however, terrestrial point-to-point links using parabolic antennas could see new life in creating cheap alternatives to leased lines for short enterprise network connections. www.syngress.com Figure 2.19 Parabolic Antenna and Focused Beam Pattern Side View Parabolic Antenna Reflector Dish Signal Source Cable Top View Note: Very focused beam yields an antenna with very high gain. 152_wan_02 6/21/01 2:58 PM Page 55 56 Chapter 2 • Radio Elements and Frequency Spectrums Base Stations and Mobile Stations The terms base station and mobile station are very general terms. Base sta- tions are usually fixed locations (such as a cellular telephone tower) that communicate to many mobile stations. In contrast to base stations, and as the name implies, mobile stations usually are not stationary.There are three basic scenarios for communications between mobile stations and base stations: 1. Base Station to Base Station 2. Base Station to Mobile 3. Mobile to Mobile (that is, peer to peer) Base stations are usually an aggregation point for many mobile sta- tions. Also, the base station is usually a gateway for the mobile stations to reach a traditional wire-based network. Examples of base stations include: ■ Cellular phone tower Connects many mobile phones to the Public Switched Telephone Network (PSTN). ■ Police dispatcher station Coordinates and communicates with many mobile units. ■ Access point for a wireless LAN Connects various com- puters to a traditional wired Ethernet network. Some mobile stations rely on a base station for all connectivity, such as mobile phones. Mobile phones cannot talk peer-to-peer.This is due partly to technology and partly to business. If mobile phones could recognize when another mobile phone was in range to engage in peer- to-peer communication, then the service provider could not bill for the air-time of that connection. Other mobile stations are designed to operate completely independent of any base station, such as the Motorola Talkabout Family Radio Service (FRS) handheld units.These units are designed for peer-to-peer communication. Still other mobile units, such as the IEEE 802.11 wireless LAN mobile units, can talk to a base station or can talk peer-to-peer. www.syngress.com 152_wan_02 6/21/01 2:58 PM Page 56 Radio Elements and Frequency Spectrums • Chapter 2 57 Access Points Access point is another term that can be used very generally in reference to a point of access to a network. However, in the context of this book, access point most often refers to a base station for the IEEE 802.11 wireless LAN protocol. Access points provide computers that are equipped with a mobile radio card to access a LAN, usually via an Ethernet connection. Channelizing the Frequency Spectrum Radio waves are just one of many electromagnetic bands that exist along the frequency spectrum.The range of frequency of EM waves goes from a few Hz, which is just above DC (Direct Current) all the way to cosmic radiation, which is above 10 22 Hz. Figure 2.20 illustrates the entire EM spectrum.The spectrum is divided into the following broad categories: 1. Radio Spectrum 3 KHz–300 GHz 2. Microwave Spectrum 100 MHz–500 GHz 3. Infrared Light 500 GHz–400 THz (Terahertz, or 10 12 Hz) 4. Visible Light 400 THz–750 THz 5. Ultra Violet Light 750 THz–30 PHz (Petahertz, or 10 15 Hz) 6. X-Ray ~30 PHz – ~10 EHz (Exahertz, or 10 18 Hz) 7. Gamma ~10 EHz – ~10 ZHz (Zettahertz, or 10 21 Hz) 8. Cosmic Rays >10 22 Hz www.syngress.com 152_wan_02 6/21/01 2:58 PM Page 57 58 Chapter 2 • Radio Elements and Frequency Spectrums www.syngress.com Figure 2.20 Entire EM Spectrum on Logarithmic Scale 0 10 100 1k 10k 100k 1M 10M 100M 1G 10G 100G 1T 10T 100T 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 1P 10P 100P 1E 10 16 10 17 10 18 10E 100E 10 19 10 20 1Z 10 21 10Z 10 22 100Z 10 23 1Y 10 24 Audible Range Hz Hz The Radio Frequency Spectrum VLF LF MF HF VHF UHF SHF EHF Microwave Spectrum Infrared Light Visible Light Ultraviolet Light X-Ray Gamma Cosmic Rays 152_wan_02 6/21/01 2:58 PM Page 58 Radio Elements and Frequency Spectrums • Chapter 2 59 The most important part of the entire spectrum for wireless com- munication is the RF (Radio Frequency) spectrum, which includes a large chunk of the microwave spectrum.To prevent chaos and interrup- tions in wireless communications, government agencies regulate the use of frequencies. In the United States of America the Federal Communi- cations Commission (FCC) and the National Telecommunications and Information Administration (NTIA) regulate the use of the RF spec- trum and divide the RF spectrum into bands of frequencies that they assign for specific types of services, such as broadcast television or cel- lular phone use.These bands are then divided into blocks or individual channels that are allocated or sold to individual customers. The other piece of the spectrum that is often used in wireless com- munication and worth briefly mentioning is the infrared band of fre- quencies. Infrared devices have a very short line-of-sight range. Channelizing All RF communications require a little segment of the total RF spec- trum to transmit their signal.These individual segments of the spectrum are called channels. In order to allow multiple simultaneous signals, dif- ferent channels are assigned to different frequencies.This type of mul- tiple access is called Frequency Division Multiple Accessing (FDMA). FDMA is the most common type of multiple accessing used in RF communications. Channel Bandwidth Channels are named by their center frequency but they contain a range of frequencies both above and below the center frequency.This is referred to as channel bandwidth.The bandwidth of a channel depends on several factors, such as frequency and modulation technology, but in general, the more information you are trying to send over the channel, the wider the channel bandwidth. By looking back at the FM example in Figure 2.3, you can see why a channel has a width.The carrier wave is the main determining factor for the frequency of the channel, yet clearly, the instantaneous frequency www.syngress.com 152_wan_02 6/21/01 2:58 PM Page 59 60 Chapter 2 • Radio Elements and Frequency Spectrums of the resulting signal varies from one point in time to the next based on the modulating wave frequency.Therefore the channel bandwidth is the maximum range that the signal fluctuates from the center frequency. An application of this can be seen on a digital display FM radio receiver. Most receivers jump in increments of 0.2 MHz, like from 94.1 MHz to 94.3 MHz, which includes the channel bandwidth plus a buffer zone between the channels called channel spacing. Channel Spacing and Buffer Zones Theoretically, you could put the channels right next to each other to get the most number of channels in a certain band; to receive a channel, you would tune your receiver to that channel, and it would allow just that channel through and completely block all the others. However, in the real world, filters are not that precise.They require a buffer zone to attenuate other signals sufficiently to keep them from interfering with the desired channel. Figure 2.21 illustrates how a perfect passband filter would work, and how a real filter works. The requirements of channel spacing are part of the regulations that come out of the FCC and NTIA concerning the allocation of frequencies. www.syngress.com Figure 2.21 Channels and Channel Spacing Signal Power Center Frequency Ideal Channel with Filters with Instant Cut Off Channel Bandwidth Realistic Channel with Filters with Roll Off Channel Spacing Noise Floor Center Frequency Center Frequency Frequency Note: Channel spacing must be great enough to allow filters to attenuate the signal near noise floor. Center Frequency AB A B 152_wan_02 6/21/01 2:58 PM Page 60 Radio Elements and Frequency Spectrums • Chapter 2 61 Multichannel Systems and Channel Offsets In a Citizen’s Band (CB) radio, your radio is in listen mode or receive mode most of the time.When you want to talk, you push the transmit button and talk over the same channel to which you were previously lis- tening.This type of communication is called half-duplex because you cannot send and transmit at the same time. In this example only one stream of information can utilize any given channel at one time. There are many forms of communication in which you want mul- tiple sources of information to be transmitted simultaneously. An example is wireless phone communications where you want a full-duplex connection.To accomplish this, there is a secondary channel assigned to the main channel, which allows two streams of information to propagate simultaneously (full duplex), or two receive channels such as the left and right audio channels for FM broadcast radio. Extending the Number of Channels (Frequency Reuse) The RF spectrum is becoming a very valuable commodity.With more and more users demanding more and more different services, we are rapidly using all of the available channels. Frequency utilization is of growing concern for everyone in the wireless communications industry. One of the simplest ways to extend the number of channels is by reusing frequencies. As a signal propagates away from the transmitter, it is constantly losing power. At a great enough distance, the signal is little more than background noise. At this point, the frequency used to transmit that signal can be reused by a different transmitter. A simple example of this is broadcast radio between distant cities. For example, Kansas City, MO has a country music station at 104.3 MHz.That same channel is used for a different station in Denver, CO. Since the stations are so distant from each other, their signals do not interfere.This type of frequency reuse is not designed specifically into the broadcast radio system. It happens almost naturally and therefore has been around almost as long as radios. However, the demand for wireless www.syngress.com 152_wan_02 6/21/01 2:58 PM Page 61 62 Chapter 2 • Radio Elements and Frequency Spectrums phones has led to systems whose design is based on the reuse of fre- quencies. One of the first services to design a frequency reuse plan was the Advanced Mobile Phone Service (AMPS), also known as a cellular plan. Seven Cell Frequency Reuse Before AMPS there were a limited number of channels available for wireless phones; the towers to transmit and receive the signals from the mobile units were designed to cover large geographic areas, so as to limit the number of towers and lower costs. However, this resulted in very low user density and crippled the systems in highly populated areas such as New York City. AMPS was designed to allow frequencies to be reused in a smaller geographical area, thereby increasing the available user den- sity. Figure 2.22 illustrates how the AMPS or cellular telephones reuse plan works. www.syngress.com Figure 2.22 AMPS and Cellular Telephone Seven Cell Reuse Plan Seven Cell Cluster A Seven Cell Cluster B 7A 2A 7B 6B 2B 1B 5B 6A 1A 3A 3B 4B 5A 4A 7 Cell Reuse Cell Pattern 152_wan_02 6/21/01 2:58 PM Page 62 [...]... isotropic antenna All other things being equal, a high-gain antenna will transmit and receive weaker signals farther than a low-gain antenna Omnidirectional antennas, such as dipole, will have lower gain than directional antennas because they distribute their power over a wider area Parabolic antennas usually have the highest gain of any type of antenna A half-wave dipole antenna will have a gain of near... application and terrain Base stations have two primary functions: serving as aggregation points for multiple mobile stations and as gateways for mobile stations to access a wire-line network or backbone Access points and PC wireless cards are used in wireless local area networks (WLANs) Access points function similar to base stations in a mobile wireless network The RF range is optimized for wireless. .. have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the “Ask the Author” form Q: What is the gain of an antenna and how is it different from one type to another? A: Antenna gain is the measure of the amount of signal the antenna radiates or receives It is given as a decibel ratio, compared to a theoretical omnidirectional antenna called an... Application Presentation Applications Session Transport Host-to-Host Network Internet Data-Link Network Access Physical Understanding the Network Access Layer As shown in Figure 3. 1, the network access layer of the DoD model is a combination of the physical and data-link layers.The functionality of the www.syngress.com 81 152_wan_ 03 82 6/21/01 3: 18 PM Page 82 Chapter 3 • TCP/IP and the OSI Model layer... transmitted is accurate.The frames are translated into bits and eventually radio waves that are sent by the physical layer for transmission The data-link layer is actually divided into two sublayers: the logical link control (LLC) and the MAC.The LLC is the liaison between the protocols within the network layer and the media access control www.syngress.com 152_wan_ 03 6/21/01 3: 18 PM Page 77 TCP/IP and... There are basically two types of design for wireless LANs (WLANs), referred to as infrastructure and ad hoc Infrastructure is similar to a traditional LAN, with APs bridging traffic Ad hoc is typically a temporary www.syngress.com 87 152_wan_ 03 88 6/21/01 3: 18 PM Page 88 Chapter 3 • TCP/IP and the OSI Model network with limited users that does not use APs.Within the group of users, a master is dynamically... wider area of coverage (IR requires direct line of sight, meaning that it cannot penetrate solid surfaces such as walls.) Other Network Access Protocols Some additional network access protocols are Frame Relay, Asynchronous Transfer Mode (ATM), and Synchronous Optical Networking (SONET) Frame Relay is a wide area network (WAN) technology.This allows communication between LANs that interconnect ATM, on... proposed by federal, state, and local agencies in an evolving business and regulatory climate, so it is recommended to reference the regulations committees before designing a wireless network Regulatory Agencies The International Telecommunication Union (ITU) (www.itu.int/ITU-R), headquartered in Geneva, Switzerland, is an international organization that coordinates standards and regulations, and promotes... Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA), and internationally, the International Telecommunication Union (ITU) Solutions Fast Track Transmitting Radio Signals Over Electromagnetic Waves Electromagnetic waves are the result of applying alternating current (AC) to an electric field, which in turn, produces a magnetic field Oscillating between... short of accommodating the demand AMPS and its use of the Seven Cell Reuse Plan was a giant step forward for wireless communication, because it broke apart coverage areas into small cells that had fewer users in each cell However, even the Seven Cell Reuse Plan is being saturated in dense population areas.This has lead to some other types of multiple access techniques that are being used, in addition . antennas. Planar Array Antennas Planar array antennas are similar in concept to Yagi antennas except all elements, both active and parasitic, lie in the same plane.This results in a flat antenna that can be mounted. mounted flat on a wall, yet still have the proper- ties and gain of a directional antenna. Figure 2.17 shows a planar array antenna. www.syngress.com Figure 2.17 Planar Array Antenna and Associated. power over a wider area. Parabolic antennas usually have the highest gain of any type of antenna. A half-wave dipole antenna will have a gain of near 1, or nearly equal the isotropic antenna. Q: Why