CWNA Certified Wireless Network Administrator Certification Exam Preparation Course in a Book for Passing the CWNA Certified Wireless Network Administrator ... on Your First Try Certification Study Guide

1 Foreword This Exam Preparation book is intended for those preparing for the Certified Wireless Network Administrator ...

Author: William Manning

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1 Foreword This Exam Preparation book is intended for those preparing for the Certified Wireless Network Administrator Exam. The Art of Service is an Accredited Training Organization and has been training IT professionals since 1998. The strategies and content in this book are a result of experience and understanding of the Certified Wireless Network Administrator methods, and the exam requirements. This book is not a replacement for completing the course. This is a study aid to assist those who have completed an accredited course and are preparing for the exam. Do not underestimate the value of your own notes and study aids. The more you have, the more prepared you will be. While it is not possible to pre-empt every question that may be asked in the exam, this book covers the main concepts covered within the Certified Wireless Network Administrator discipline. The Certified Wireless Network Administrator (CWNA) is a foundation level certification that measures the ability to administer any wireless LAN. The exam covers a wide range of wireless LAN topics and is directed toward professionals who want to work with 802.11 wireless technology, rather than vendor-specific products. The CWNA certification demonstrates a candidate’s ability to successfully administer enterprise-class wireless LANS. Due to licensing rights, we are unable to provide actual CWNA Exams. However, the study notes and sample exam questions in this book will allow you to more easily prepare for a CWNA exam. Ivanka Menken Executive Director The Art of Service

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Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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2 Table of Contents 1 2 3 4 5 6

Foreword ............................................................................ 1 Table of Contents............................................................... 3 Certified Wireless Network Administrator........................... 7 Exam Specifics .................................................................. 8 Exam Prerequisites ............................................................ 8 Understanding Wireless LANs ........................................... 9 6.1 The Evolution of Wireless LANs................................... 9 7 Fundamentals of Radio Frequency (RF) .......................... 12 7.1 How Radio Frequencies Work ................................... 12 7.2 VSWR – Voltage Standing Wave Ratio ..................... 14 7.3 Understanding Antennas ........................................... 15 7.4 Measuring Radio Frequencies ................................... 16 8 Spread Spectrum Technology ......................................... 18 8.1 Communication Types ............................................... 18 8.2 Wireless LANs ........................................................... 19 8.3 Frequency Hopping Spread Spectrum (FHSS) .......... 21 8.4 Direct Sequence Spread Spectrum (DSSS) .............. 23 9 Wireless LAN Organizations and Standards .................... 27 9.1 Federal Communications Commission (FCC) ............ 27 9.2 Institute of Electrical and Electronics Engineers (IEEE) 31 9.3 Major Organizations ................................................... 34 9.4 Competing Technologies ........................................... 36 10 Infrastructure Devices for Wireless LANs ........................ 39 10.1 Access Points ......................................................... 39 10.2 Wireless Bridges ..................................................... 42 10.3 Wireless Workgroup Bridges .................................. 43 10.4 Wireless LAN Client Devices .................................. 44 10.5 Wireless Residential Gateways .............................. 47 10.6 Enterprise Wireless Gateways ................................ 48 11 Antennas and Accessories .............................................. 51 11.1 Radio Frequency Antennas .................................... 51 11.2 Concepts of RF Antennas....................................... 53 11.3 Antenna Installation ................................................ 56 Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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11.4 Power over Ethernet (PoE) Devices ....................... 59 11.5 Accessories for Wireless LAN ................................ 62 12 802.11 Network Architecture ............................................ 70 12.1 Identifying Wireless LANs ....................................... 70 12.2 Authentication and Association ............................... 74 12.3 Authentication Methods .......................................... 75 12.4 Emerging Authentication Protocols ......................... 77 12.5 Service Sets ........................................................... 80 12.6 Roaming ................................................................. 82 12.7 Power Management Features ................................ 85 13 MAC and Physical Layers ................................................ 87 13.1 Communicating with Wireless LANs ....................... 87 13.2 Interframe Spacing ................................................. 92 13.3 RTS/CTS ................................................................ 95 13.4 Modulation .............................................................. 96 14 Troubleshooting Wireless Installations............................. 98 14.1 Multipath ................................................................. 98 14.2 Hidden Node ......................................................... 102 14.3 Near/Far ............................................................... 103 14.4 System Throughput .............................................. 104 14.5 Co-location Throughput ........................................ 105 14.6 Types of Interference ............................................ 106 14.7 Range Considerations .......................................... 109 15 Wireless LAN Security ................................................... 111 15.1 Wired Equivalent Privacy ...................................... 111 15.2 Wireless LAN Attacks ........................................... 114 15.3 Securing Wireless LANs ....................................... 115 16 Fundamentals of Site Surveying .................................... 117 16.1 Understanding Site Surveys ................................. 117 16.2 Site Survey Preparation ........................................ 118 16.3 Site Survey Equipment ......................................... 120 16.4 Conducting Site Surveys ...................................... 120 17 Practice Exam ................................................................ 123 17.1 Questions ............................................................. 123 18 Answer Guide ................................................................ 134 18.1 Answers ................................................................ 134 19 References .................................................................... 140 Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Notice of Rights All rights reserved. No part of this book may be reproduced or transmitted in any form by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Notice of Liability The information in this book is distributed on an “As Is” basis without warranty. While every precaution has been taken in the preparation of the book, neither the author nor the publisher shall have any liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the instructions contained in this book or by the products described in it. Trademarks Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations appear as requested by the owner of the trademark. All other product names and services identified throughout this book are used in editorial fashion only and for the benefit of such companies with no intention of infringement of the trademark. No such use, or the use of any trade name, is intended to convey endorsement or other affiliation with this book.


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3 Certified Wireless Network Administrator Certified Wireless Network Administrator is the foundation level certification for CWNP. It is actually the second exam in the four-level certification process. The main subject areas dealt with by the CWNA exam are:  Wireless Standards and Organization  Radio Technologies  Antennas  Wireless LAN Hardware and Software  Network Design  802.11 Network Architecture  Wireless LAN security  Troubleshooting  Site Surveys. The CWNA certification is valid for three years, and can be renewed by retaking the test or moving to the next level in the certification path.


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4 Exam Specifics Exams cost $175 at time of printing. Duration – 90 minutes 60 questions Question Type – Multiple-choice / Multiple answer Passing Score – 70% Registration of the exam must be made with Pearson VUE.

5 Exam Prerequisites There are no prerequisites for the Certified Wireless Network Administrator exam.


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6 Understanding Wireless LANs 6.1

The Evolution of Wireless LANs

Wireless LANs began as a military technology, providing a simple method of exchanging data that could be implemented easily in a combat environment. At first, the cost of the technology was enormous. As the cost decreased, enterprises began to integrate wireless capabilities into their network architecture. The driving force for most enterprises was providing an inexpensive way to connect campus environments. As the cost continued to decrease and quality increased, the mobility aspects of wireless computing became advantageous for businesses. In addition, households started to adopt wireless technologies in home offices and wireless gaming stations. While this growth continued, the need for standards between different manufacturers and installed LANS also grew. Stress on compatibility of systems and interoperability between networks became key issues. 6.1.1

Wireless LAN Standards

Wireless LANs transmit radio frequencies and are regulated by the same type of laws governing AM/FM radios. In the United States, the Federal Communications Commission (FCC) regulates the use of wireless LAN devices. The Institute of Electrical and Electronic Engineers (IEEE) have created and currently maintain several operational standards. The current IEEE standards:  802.11 is the original LAN standard and specifies the lowest data transfer rates for both radio frequencies and light-based transmissions.  802.11b has faster data transfer rates and has been promoted to great extent as Wi-Fi by the Wireless Ethernet Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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  

Compatibility Alliance (WECA). 802.11a uses 5GHz UNII frequency bands. It is much faster than 802.11b but is not compatible with that standard. 802.11g has data transfer rates as fast as 802.11a and is backwards compatible to 802.11. 802.11-2007, also referred to as 802.11REVma, was an effort to create a single document out of the original 1999 standard and all 8 of its amendments.

New standards are always being proposed and take years to adopt. Some standards, such as 802.11g, are in such high demand that they are adopted before the standard is finalized. Another standard, called OpenAir, was created by the Wireless LAN Interoperability Forum (WLIF) to assist manufacturers in interoperability testing. 6.1.2

Uses of Wireless LANs

Wireless LANs provide another point of access to a wired network. Like other access methods, wireless LANs are data-link layer networks and implemented in the access layer role. Unlike other access methods, wireless solutions increase the mobility of an organization. Due to problems with speed and resiliency, wireless networks are not usually implemented in distribution or core roles in an organization. Wireless allows the extension of a wired network, specifically when using wired solutions would be cost-prohibitive. Seamless connectivity to remote areas within a building can allow employees to gain and maintain access, even when moving. Two wired networks can be connected using cost-effective wireless connections. Point-to-point (PTP) connectivity speaks to the wireless connection between two buildings and will use semi-directional or highly directional antennas at both ends. Point-to-multipoint (PTMP) solutions typically connect three or more buildings in a hub/spoke fashion. Omni-directional antennas are installed at the hub, with Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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semi-directional antennas at each of the spokes of the network. All residential businesses and families rely on services provided by a telecommunications or cable company. Wireless solutions have increased the availability of these services to customers by providing low-cost “last mile” connectivity. The term, “last mile,” refers to the connectivity between the customer and the central office of the service provider. The connection can be wired or wireless. For a few customers such as rural or hard to reach locations, a wireless solution is the only option available. Wireless LANS provide the best option for users and organizations interested in maintaining little or almost no infrastructure, and to raise the mobility capabilities of the user.


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7 Fundamentals of Radio Frequency (RF) 7.1

How Radio Frequencies Work

Radio frequencies are high frequency alternating current (AC) signals. They are passed through a copper conductor then radiated into the air through an antenna. The antenna will take a wireless signal and convert it into a wired signal, as well as reversing the process. The signal moves (propagates) from the antenna, or source, in a straight line is all directions. 7.1.1

Behaviors of Radio Waves

The basic behaviors of radio frequencies of importance include:  Gain  Loss  Reflection  Refraction  Diffraction  Scattering. 7.1.2

Gain

Gain refers to an increase in a radio frequency's signal amplitude. Typically, the amplitude changes from an external power source, such as an RF amplifier, making frequency gain an active process. However, there is a passive process of reflecting, or bouncing, a signal to increase the signal's strength. 7.1.3

Loss

Loss is a decrease in signal strength. Loss of a signal is possible within a wire and as a radio signal. Some of the reasons for signal loss include:  Resistance of cables and connectors Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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  

Mismatch or impedance which can reflect signals back Obstacles which can absorb, reflect, or destroy signals An RF attenuator is intentionally used to convert high frequency AC to heat.

Wireless devices which read signals have a sensitivity threshold. This threshold is the point where the device can distinguish a signal from mere background noise. 7.1.4

Reflection

Reflection happens when a propagated electromagnetic wave bounces off a large object of high density, such as the earth, buildings, walls, etc. The smoother the surface of the object, the more intact the signal remains, except for some absorption that may occur. The rougher the surface, the more likely the reflected signal becomes multipathed. This multipath effect can severely degrade the signal and requires antenna diversity to compensate for it. 7.1.5

Refraction

Where reflection bounces off of high density objects, refraction, or bending of the signal, happens when the signal hits an object of medium density. In truth, the signal is typically reflected and refracted. The refraction will go through the object, but its direction has changed. Refraction is a consideration for long distance RF links. Atmospheric conditions are primary causes for refraction. 7.1.6

Defraction

When a radio wave between a transmitter receiver is obstructed, how the signal reacts is dependent on the geometry of the object and the amplitude, phase, and polarization of the wave at the point of impact. Besides refection and refraction, the signal may be diffracted, or bend around the object. Strictly speaking, diffraction is the slowing of the wave at the point of impact while the rest of the wave maintains the same speed. The higher the frequency, the greater the defraction. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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7.1.7

Scattering

Signals can also be scattered and this is the result of obstructions with a rough surface or irregularities in the signal path, such as foliage, street signs, or furniture. Scattering happens when a wave hits an uneven surface and reflects in multiple directions simultaneously. Dust clouds or rainstorms can be enough to reflect and refract a signal to cause scattering to the point where transmissions are severely degraded or lost.

7.2

VSWR – Voltage Standing Wave Ratio

Impedance is the resistance to current flow measured in Ohms. When the signal is reflected, the point of impedance is mismatched in the signal path creating a VSWR (Voltage Standing Wave Ratio). Some power is reflected back towards the transmitter resulting in loss of forward energy, called the return loss. VSWR shows then the impedance of the ends of a connection vary and the maximum amount of the transmitted power is not received at the antenna. VSWR is expressed as a relationship between two numbers, shown as x:1. The second number will always be one and represents a perfect impedance match. The first number represents an imperfect impedance match and the closer it is to one, the better the impedance matching available to the system. 7.2.1

Impact of VSWR

The more mismatched the VSWR is, the more problems with the RF circuit. In most cases, the amplitude of the RF signal is decreased. However, when transmitters are not protected against returned power, the electronics of the transmitter can burn out. Preventative measures against VSWR include:  Tight connections between cables and connectors  Use of impedance matched hardware  Use of high-quality equipment capable of being calibrated. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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7.3

Understanding Antennas

Transmitting antennas convert electrical energy into radio frequency waves. Receiving antennas convert radio frequency waves into electrical energy. The frequency of the signal that an antenna can transmit or receive propagated waves is dependent on the physical dimensions of the antenna. Some concepts about antenna pertinent to wireless LANs include:  Line of Sight (LOS)  The Fresnel Zone  Antenna Gain. 7.3.1

Line of Sight

A visual line of sight is the straight line between an object in view, or transmitter, and the observer, or receiver. With radio waves, the effects of reflection, refraction, and diffraction have some impact on how straight the line of sight remains. 7.3.2

Frensel Zone

The RF line of sight can be changed by the diffraction and reflection of the signal by objects blocking or degrading the signal. The objects of concern are present in the Frensel Zone, an area around the line of sight which can introduce RF signal interference when blocked. The Fresel Zone can be viewed as a series of concentric ellipsoid-shaped areas around the LOS path. Some blockage within the Fresnel Zone can occur without any disruption to the link. Specifically, 20%-40% blockage in the zone will introduce little or no interference at all.


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7.3.3

Antenna Gain

A signal can be conditioned, amplified, and manipulated by an antenna. When none of this exists, the antenna element is considered passive. Amplification of the signal can be obtained by the physical characteristics of the antenna. An omni-directional antenna has a 360-degree horizontal beamwidth. A beamwidth is a measuring of the focused radiation in horizontal and vertical degrees. By limiting the beamwidth, the radio waves can be radiated further. Intentional radiators and Equivalent Isotropically Radiated Power (EIRP) are key considerations in understanding the power considerations of antennas. The FCC defines an intentional radiator as an RF device that is designed to generate and radiate RF signals. Each component of the hardware used by and connected to the intentional radiator has an impact on the power used by the radiator. The power output is actually regulated by the FCC, so it is important to understand how power is measured, how much power is allowed, and how to calculate these values. Equivalent Isotropically Radiated Power (EIRP)is the term for the power radiated by the antenna element. EIRP is regulated by the FCC and is used to calculate the viability of a wireless link.

7.4

Measuring Radio Frequencies

In calculating power for wireless LANs, the areas of concern are:  Power at the transmitting device  The loss and gain of connectivity devices, such as cables, connectors, amplifiers, attenuators, and splitters, between the transmitting device and the antenna  Power at the intentional radiator  Power (EIRP) at the antenna element.


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7.4.1

Units of Measure

The basic unit of power is watt (W). Defined as one ampere (A) of electrical current at one volt (V). A typical 120-volt night light is about 7 watts and can be seen 50 miles away on a clear night. The FCC requires that only 4 watts of power be radiated from an antenna in point-to-multipoint wireless LANs. Power levels for most wireless LANs are measured in milliwatts, or 1/1000 watt. Milliwatts are abbreviated as “mW.” A single LAN segment is typically no more than 100mW and will communicate up to half a mile in the best conditions. 7.4.2

Relative Measurements

A receiver can pick up radio signals as small as 0.000000001 W. This number is so small, that a logarithmic relationship is often used to communicate power of a signal, referred to as decibel (dB). Power loss and gain is measure in decibels. Some important numbers used by an administrator include: -3 dB = half the power in mW 3 dB = double the power in mW -10 dB = one tenth the power in mW 10 dB = ten times the power in mW The mathematical notation, dBm, refers to the reference point between the logarithmic dB scale and the linear watt scale, represented by: 1 mW = 0 dBm The mathematical notation, dBi, refers to the gain of an antenna. The “i” stands for “isotropic” which is a change in power referenced against an isotropic radiator. An isotropic radiator refers to a theoretical ideal transmitter which produces useful electromagnetic field output in all directions with equal intensity and used to measure output power, or EIRP. 1 W + 10 dBi = 10 W


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8 Spread Spectrum Technology Spread spectrum is a communications technique with a wide bandwidth and low peak power. It is hard to detect, which prevents it from being intercepted and demodulated without the right equipment.

8.1

Communication Types

8.1.1

Narrow Band Transmission

The precursor to spread spectrum communication is narrowband transmissions. Narrowband refers to a communication technology that uses only enough of the frequency spectrum to support the data signal. To send a transmission over such a small frequency range, a great deal of power is required. This is required to overcome the noise floor, or the general level of noise inherent in all transmissions. Narrowband transmissions can be intentionally overpowering a transmission using unwanted signals on the same band, or jammed. Additionally, noise on the same frequency or another signal can interrupt the transmission. 8.1.2

Spread Spectrum Transmission

Requirements required to be considered spread spectrum:  The bandwidth of the signal must be wider than needed to send information  The peak power needed to send the signal must be low. It has noise-like characteristics which prevents transmissions from being jammed or corrupted.


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8.2

Wireless LANs

8.2.1

Uses of Spread Spectrum

Spread spectrum communication has been used for:  Military  Cordless telephony  Digital cellular telephony (CDMA)  Personal communications system (PCS)  Global positioning (GPS)  Wireless LAN  Bluetooth (WPAN). Wireless networks are available in several architectures:  Wireless local area network (WLAN)  Wireless personal area network (WPAN)  Wireless metropolitan area network (WMAN)  Wireless wide area network (WWAN). Spread spectrum is used differently in each of the architectures. 8.2.2

Same Rules, Different Technologies

Wireless LANs are used:  To provide connectivity for mobile users within a building  As a bridge between buildings across a campus setting. The most common use of spread spectrum technologies is the combination of WLANs and Bluetooth devices. WLANS are IEEE 802.11 compliant. Bluetooth devices are IEEE 802.15 compliant. Bluetooth and WLANs function differently, work within the same FCC rules, and interfere with each other. But much effort has been put towards research, time, and resources to have the two technologies coexist.


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8.2.3

Frequency Hops

WPANs, or its common name Bluetooth, is specified by the 802.15 standard. The regulation is broad, allowing for different types of spread spectrum uses. Frequency hopping is the act of jumping from one frequency to another within a frequency band while transmitting data.  Bluetooth devices hop approximately 1600 times per second.  HomeRF technology hop approximately 50 times per second.  802.11 WLANs hop 5-10 times per second. 8.2.4

Licensed Frequencies

WMANs are networks that consist of several high-power point-topoint wireless links across a city. WMANS use licensed frequencies. WLANs use unlicensed frequencies. The purpose of licensing a frequency is to ensure that the wireless network can be implemented without concern of a conflicting implementation elsewhere. Wireless Wide Area Networks also utilize licensed frequencies. 8.2.5

Specifications from the FCC

Two types of spread spectrum technologies are specified by the FCC:  Direct sequence spread spectrum (DSSS)  Frequency hopping spread spectrum (FHSS). The specific regulation is a collection of laws found in the Codes of Federal Regulation (CFR), volume 47 “Telegraphs, Telephones, and Radiotelegraphs,” part 15. Wireless LAN devices are sometimes called “part 15 devices” for this reason. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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8.3

Frequency Hopping Spread Spectrum (FHSS)

Frequency agility refers to the ability to abruptly change transmission frequency within a usable RF frequency band. FHSS uses frequency agility to spread data over 83 MHz of the usable 2.4 Ghz ISM. 8.3.1

How It Works

Hops, or frequency changes, in FHSS systems occur based on a pseudorandom sequence. The sequence is actually a list of several frequencies which will be hopped at specific times before the pattern is repeated. The time that a carrier sits at a specific frequency is called dwell time. The time it takes a carrier to complete a single hop is called hop time. The receiving device is synchronized to the hop sequence of the transmitting device to receive the signal properly. Narrow band interference can still interfere with the signal of frequency hopping systems. However, the interference only affects the frequency that is shared between the narrow band device and the frequency hopping device. The rest of the signal will remain intact and the lost data would be transmitted again. 8.3.2

Systems Using Frequency Hopping

The regulations of the FCC provide confines to use technologies such as frequency hopping. The IEEE creates operating standards within those confines. In regards to FHSS, IEEE standards describe FHSS systems based on:  What frequency bands are allowed  The hop sequences  Dwell times  Data rates. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Frequency hopping systems must operate in the 2.4 Ghz ISM band. IEEE 802.11 specifies data rates of 1 Mbps and 2 Mbps. The 2.4 Ghz ISM band is defined by the FCC to be between 2.4000 Ghz and 2.5000 Ghz. Specific hop patterns are called channels. Typical frequency hopping systems use the 26 standard hop patterns as defined by the FCC or subsets of those patterns. Some systems will allow the creation of custom hop patterns. Some systems will allow synchronization between systems in a colocated environment to eliminate any collisions. Up to 79 synchronized, co-located access points is possible, though rather expensive. Recommendations generally prescribe a maximum of 12 co-located systems, though the use of non-synchronized devices can raise the number to 26 for medium-traffic networks. More traffic on the network will reduce the limit. The dwell time is the time a system transmits on a specific frequency. When that time expires, the system transmits on a different frequency. Times are typically measured in milliseconds (ms). The maximum dwell time defined by the FCC is 400 ms in any 30 second time period. When frequencies change within a system, it does so by:  Switching to a different circuit tuned to a new frequency  Using a different element in the current circuit. The change must complete before transmissions continue. Electrical latencies in the circuit will delay the time required to make the change. Hop times are measured in microseconds. The typical hop time for 802.11 FNSS systems is 200-300 microseconds. Data throughput has a correlation with the length of the dwell and hop times, namely, the longer dwell time will result in greater throughput. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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8.4

Direct Sequence Spread Spectrum (DSSS)

DSSS is the most commonly known and used spread spectrum type with most wireless LAN equipment using the technology. The type sends data where the transmission and receipt of the data are both using a 22 Mhz-wide set of frequencies. 8.4.1

How It Works

The data signal at a transmitting station is combined with a higher data rate bit sequence. The data rate bit sequence is referred to as a chipping code or processing gain. The signal's resistance to interference is increased when the processing gain increases. Most commercial products operate under 20. The FCC allows a minimum processing gain of 10 and the IEEE 802.11 has set the minimum at 11. The direct sequence starts with a carrier being modulated with a code sequence. The code has a specified number of “chips” that aid in determining the data rate, specifically:  Number of chips per bit  The speed of the code in chips/sec. 8.4.2

Direct Sequence Systems

IEEE specifications for the 2.4 Ghz ISM band include:  For 802.11 devices – 1 or 2 Mbps data rate  For 802.11b devices – 5.5 or 11 Mbps data rates  For 802.11g devices – 54 Mbps data rate. Devices adhering to 802.11a operate on 5 Ghz UNII bands. DSSS systems use channels, but unlike FHSS systems are not defined using hop sequences. Each channel is a contiguous band of frequencies 22 Mhz wide with a 1 Mhz carrier frequency similar to FHSS. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Channels overlap one another. The expression of a channel is by its band mean. Channel one is 2.412 Ghz represents a band from 2.401 GHz to 2.423 Ghz, or 2.412 Ghz ± 11 Mhz. Only 11 non-licensed channels are specified by the FCC. Their European counterpart, ETSI, specifies 9 channels. A complete list includes: Channe l ID

FCC ETSI Channel Channel Frequencie Frequencie s GHz s GHz

1

2.412

n/a

2

2.417

n/a

3

2.422

2.422

4

2.427

2.427

5

2.432

2.432

6

2.437

2.437

7

2.442

2.442

8

2.447

2.447

9

2.452

2.452

10

2.457

2.457

11

2.462

2.462

Using DSSS systems with overlapping channels in the same physical space will cause interference. Co-location of channels should happen if the channels are at least five apart, i.e. 1 and 6, 2 and 7, and so on. A maximum of three co-located channels are available, namely using 1, 6, and 11. Since the DSSS band is smaller than FHSS systems, they are more sensitive to narrow band interference. Additionally, since no frequency hopping is present with DSSS, the information is transmitted across the entire band. Still, DSSS uses a wide band and are highly resistant to any interference from narrow band transmissions. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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8.4.3

Comparing FHSS and DSSS

The factors used to compare FHSS and DSSS are:  Narrow band interference  Co-location  Cost  Equipment compatibility and availability  Data rate and throughput  Security  Standards support. FHSS utilizes 79 Mhz-wide bands while DSSS uses 22 Mhz. This allows FHSS to be more resistant to narrow band interference. DSSS costs less than FHSS systems. Frequency hopping allows FHSS to use 79 discrete channels over the 3 channels available for DSSS. This provides an advantage to FHSS for co-located access points. Unfortunately in order to get the same throughput from both configuration, a FHSS solution would require 16 access points over the DSSS maximum 3 access points. Compatibility for DSSS equipment is widespread; being the focus on of the interoperability standard called Wireless Fidelity, or Wi-Fi, by the Wireless Ethernet Compatibility Alliance (WECA). Numerous tests are available to ensure devices are “Wi-Fi” compliant. Though FHSS adheres to the standards of 802.11, there are no compatibility tests available. FHSS systems which are compliant to 802.11 or OpenAir will have a data rate of no greater than 2 Mbps. The data throughput for both FHSS and DSSS systems is typically half the data rate. However, determining the data throughput has to take into consideration the power output: HomeRF devices utilize 125 mW of power compared to the single watt of 802.11 systems. Despite the ability to hop frequencies, FHSS systems are not secure for a number of reasons:  To promote products effectively, manufacturers have to adhere to 802.11 or OpenAir standards. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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 

A standard set of hop sequences come from a predetermined list. Each beacon used will broadcast the channel number and the MAC address which can be easily determined with a spectrum analyzer.


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9 Wireless LAN Organizations and Standards Understanding the basis of the standards developed for wireless LANs ensures that the wireless solutions remain compatible within the network and for use with our wireless devices.

9.1

Federal Communications Commission (FCC)

The FCC is an independent government agency of the United States. Established by Congress through the Communications Act of 1934, the FCC is accountable for regulating interstate and international communications using radio, television, wire, satellite, and cable technologies. Their jurisdiction covers the 50 states, the district of Columbia and all U.S. possessions. Wireless LAN solutions must operate by laws made by the FCC. Specifically, the FCC regulates:  Where on the radio frequency spectrum wireless LAN devices must operate  The power utilization of wireless devices  The transmission technologies used  How and where various wireless LAN equipment can be used. 9.1.1

License-Free Bands

The limits on the frequency spectrum and power used by wireless LANs is one of the most significant regulations by the FCC. Wireless LANS can use the Industrial, Scientific, and Medical (ISM) bands. These bands are license free. Their locations start at 902 Mhz, 2.4 Ghz, and 5.8 Ghz. The widths of the bands vary from 26 Mhz to 150 Mhz. Three Unlicensed National Information Infrastructure (UNII) bands are specified by the FCC. These UNII bands are in the 5 Ghz range and 100 Mhz wide. These bands are also license free. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Implementing wireless systems on license-free bands holds no requirement to petition the FCC for bandwidth or power needs. Though limits do exist, no permission is needed to transmit at these limits. Additionally, a no license, no cost policy is associated with licensing. It is within the license-free environment of ISM and UNII bands that allow small businesses and households to utilize wireless capabilities and encourages wireless LAN market growth. The freedom provided by license-free bands comes with drawbacks. Since multiple users can utilize the same license-free band, it can cause possible interference when two LAN segments are installed near each other. This is particularly present in residential implementations. Additionally, if one installation has a higher-power system, it could “white out” the wireless traffic of the other wireless LAN installation. This is a simple disabling of wireless LAN traffic, and the two systems do not have to be on the same channel or same spread spectrum technology. 9.1.2

Industrial Scientific Medical (ISM) Bands

The three license-free ISM bands specified by the FCC include:  900 MHz  2.4 Ghz  5.8 Ghz The 900 MHz ISM band represents the range of frequencies between 903 MHz to 928 MHz, or 915 MHz ± 13 MHz. Though widely used for wireless LAN solutions, this band has been abandoned in favor of higher frequency bands. Some wireless home phones and wireless camera systems still use the band. Replacement of obsolete equipment supporting the 900 MHz is quite expensive. Support of older 900 MHz units is difficult to obtain. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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The most populated space of the three ISM bands is the 2.4 GHz ISM band. The range of frequencies for this band is 2.4000 GHz - 2.5000 GHz, or 2.45000 GHz ± 50 MHz. Wireless LANs utilize only 100 MHz of the band, particularly 2.4000 – 2.4835 GHz. This applies to all devices complaint to 802.11, 802.11b, and 802.11g. The limitation is in place because the FCC has only specified power output for this range of frequencies. Growing in popularity is the 5 GHz ISM band, specifically 150 MHz bandwidth ranging from 5.725 GHz - 5.875 GHz. This ISM band is confusing because it is not specified for use by wireless LAN devices, but overlaps with the 5 GHz Upper UNII band which is specified for use by wireless LANs. 9.1.3

Unlicensed National Information Infrastructure (UNII) Bands

The 5 GHX UNII bands are comprised of thee 100 MHz-wide bands used by 802.11a-compliant devices. The three bands are known as lower, middle, and upper bands. The lower band is specified for indoor use. The upper band is specified for outdoor use. The middle band can be used indoors or outdoors. Each of these three bands has four non-overlapping DSSS channels separated by 5 MHz. Since most access points are mounted indoors, these bands allow for 8 non-overlapping access points by using both the lower and middle UNII bands. The lower band range is 5.15 GHz - 5.25 GHz and has a maximum output power of 50 mW specified by FCC. The IEEE has specified 40 mW as the maximum output power for 802.11a-compliant radios. These reserve the lower band for indoor operation only. The middle UNII band range is 5.25 - 5.35 GHz. The specified power output by the FCC is 250 mW: by the IEEE, 200 mW. This power limit allows operation of devices indoors and outdoors. The band is often used to handle short outdoor hops between closely spaced buildings. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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The upper band is reserved for outdoor links and has a FCC power output of 1 Watt (1000 mW). The IEEE limits the power output to 800 mW. The frequency range for the upper band is from 5.725 GHz 5.825 GHz. These limitations are still excellent for installing wireless LANs for large campuses and long-distance RF links. 9.1.4

Power Output

FCC regulations regarding power radiated from antenna elements is dependent on the use of point-to-multipoint (PtMP) or point-to-point (PtP) implementations. Power radiated by the antenna is called Equivalent Isotropically Radiated Power (EIRP). PtMP links are typically configured as a hub-n-spoke topology. In this topology, a central point of connection is typically in place and may or may not be supported by an omni-directional antenna. If the network is, the FCC automatically considers the link a PtMP link. The EIRP of a PtMP is limited by the FCC to 4 Watts in either the 2.4 GHz ISM band and 5 GHz UNII band. The intentional radiator which transmits the RF signal has a power limit of 1 Watt (+30 dBm). The FCC regulates that for every 3 dBi above the antenna's initial dBi of gain, the power of the intentional radiator must be reduced by the appropriate dB below the initial dBi. Simply, the EIRP of a PtMP is set. The power and gain of the antenna and intentional radiator can be used in combination to meet the limit. A PtMP Power Compensation Table shows most common combinations: Power at Antenna (dBm)

Antenna EIRP (dBm) Gain (dBi)

EIRP (watts)

30

6

36

4

27

9

36

4

24

12

36

4

21

15

36

4


30

18

18

36

4

15

21

36

4

12

24

36

4

Point-to-Point (PtP) links have a single directional transmitting antenna and a single directional receiving antenna. TMP implementation, PtP have a sliding power limit. The FCC mandates that for every 3 dBi above the initial 6 dBi of antenna gain, the power at the intentional radiator must be reduced by 1 dB below the+30 dBm. A similar compensation table for PtP shows common combinations: Power at Antenna (dBm)

9.2

Antenna EIRP (dBm) Gain (dBi)

EIRP (watts)

30

6

36

4

29

9

38

6.3

28

12

40

10

27

15

42

16

26

18

44

25

25

21

46

39.8

24

24

48

63

23

27

50

100

22

30

52

158

Institute of Electrical and Electronics Engineers (IEEE)

When it comes to information technology in the United States, the IEEE is the primary standards creator for most subjects. They utilize the laws created by the FCC as guidelines to create standards. IEEE 802.11 is the recognized technology standard for wireless LANs. The standard consists of a main body and several amendments. The most referenced parts of the standard include:  802.11 Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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    9.2.1

802.11a 802.11b 802.11g 802.11n 802.11

The 802.11 is the main body of the standard describing the operation of wireless LANs. The standard contains the transmission technology descriptions for Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS), and infrared. The 802.11 standard describes the operation of DSSS system at 1 Mbps and 2 Mbps data rate transfer. For systems that can operate at other data rates including 1 Mbps and 2 Mbps, the device is 802.11compliant. However, the system must be operating at 1 or 2 Mbps (802.11 compliant mode) to be expected to communicate with other 802.11 compliant devices. IEEE provides one of two standards for the operations of FHSS systems. The IEEE standard describes FHSS systems at 1 and 2 Mbps. The same compatibility issues are in place as with DSSS systems. 802.11 focuses on products that operate within the 2.4 GHz ISM band. The exception is infrared, a light-based technology which is covered by the standard but does not fall into the 2.4 GHz ISM band. 9.2.2

802.11b

802.11b is often referred to as “high-rate” and Wi-Fi. The original standard provided a basic description for operating wireless LANs based on the technology at the time of adoption. Technology grew and eventually outgrew the standard. 802.11b was a successful attempt to update the standard. It's considered high-rate because it describes the operation of DSSS systems at 1, 2, 5.5 and 11 Mbps. By default, 802.11b-compliant Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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devices are 802.11-compliant. This allows backward compatibility and allows upgrades to existing networks without replacing core hardware, providing a low-cost solution to remain competitive. The high data rate provided by 802.11b-compliant devices is possible by the use of a different coding technique. The system is still a direct sequencing system, but the chips are coded using CCK rather than Barker Code. The information is modulated differently as well allowing a greater amount of data to be transferred in the same time frame. 802.11b products will only operate in the 2.4 GHz ISM band between 2.4000 and 2.4835 GHz. 9.2.3

802.11a

The 802.11a describes wireless operations in the 5 GHz UNII bands. Operating in the UNII bands make 802.11a devices incompatible with all other devices compliant to other 802.11 series of standards because 5 GHz frequencies do not communicate with systems using 2.4 GHz frequencies. With the UNII bands, devices can achieve data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Proprietary technology can raise the data rate as high as 108 Mbps using rate doubling. However, 802.11a specifies data rates of only 6, 12, and 24 Mbps. To be 802.11a-compliant, devices must support these data rates at the very least. The maximum data rate specified by the standard is 54 Mbps. 9.2.4

802.11g

802.11g standard provides data speeds similar to 802.11a but backwards compatible to 802.11 devices. The standard still operates in the 2.4 Ghz ISM band, but utilizes Orthogonal Frequency Division Multiplexing (OFDM) modulation Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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which allows higher data rates. Backwards compatibility allows QSPK modulation to communicate with slower 802.11 and 802.11b compliant devices. 9.2.5

802.11n

The primary difference with 802.11n is the inclusion of option modes and configurations. This allows manufacturers to maintain baseline performance parameters while accommodating different customer demands. The options are not required to be supported to be 802.11n compliant. OFDM modulation has been improved to allow an attainable raw data rate of 65 Mbps, up from 54 Mbps in other standards. One of the biggest changes is the adoption of Multiple Input Multiple Output (MIMO), which exploits multipathing by splitting the data stream into multiple parts, called spatial streams, and transmitting each stream from different antennas. 4 spatial streams are supported by the standard. Using 2 spatial streams instead of one essentially doubles the data rate, though power consumption and cost also increases. The technique used to split the data stream is called space-division multiplexing. 802.11n is backwards compatible with 802.11b and 802.11g.

9.3

Major Organizations

In the United States, the FCC provides the laws and regulations for the proper use of wireless networking. The IEEE provides the operational standards used to design and manufacture wireless devices. The results of these two organizations have an impact on the global adoption of wireless standards. However, there are several other organizations that contribute to the growth, education, and adoption of wireless technology in the marketplace. The most recognized of these organizations are: Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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   9.3.1

Wireless Ethernet Compatibility Alliance (WECA) European Telecommunications Standards Institute (ETSI) Wireless LAN Association (WLANA) Wireless Ethernet Compatibility Alliance

Wireless Ethernet Compatibility Alliance (WECA) promotes and tests for wireless LAN interoperability. The term, Wi-Fi, is often attributed to WECA as it is their mission “to certify interoperability of Wi-Fi products and to promote Wi-Fi as the global wireless LAN standard across all market segments. The term was so powerful, the WECA is now known as the Wi-Fi Alliance. Interoperability requires resolving conflicts from interference, incompatibility, and other problems. When a product meets the requirements described in Wi-Fi Alliance's test matrix, the product is granted a certification of interoperability. This certification can be advertised by the vendors and provides end-users confidence in building a wireless network with devices bearing the Wi-Fi logo. One of the primary checks for interoperability is the use of the 40-bit WEP keys. A 40-bit “secret” key assists in securing the network. The key is actually concatenated with a 24-bit Initialization Vector (IV) to reach 64-bits. Therefore, 40-bit and 64-bit keys are the same thing. In like manner, 104-bit and 128-bit keys are the same; however, WECA does not specify interoperability for 128-bit keys though general practice has shown it exists. Other factors sought after for interoperability for Wi-Fi Alliance include:  Fragmentation  PSP mode  ESSIDs  SSID probe requests. 9.3.2

European Telecommunication Standards Institute

The ETSI has the same charter for creating standard in Europe as the Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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IEEE has for the United States. Often, the standards of the two organizations compete against each other and some discussion has made to unify the standard on certain wireless technologies. IEEE attempted to reach interoperability with 802.11h. The original ETSI standard for wireless networking is called HiperLAN/1. It supported rates up to 24 Mbps using DSSS with a range of 150 feet. The lower and middle UNII bands were used. The HiperLAN/2 standard provides support for rates up to 54 Mbps across all three UNII bands. HiperLAN/2 supports interchangeable convergence layers, including ATM, Ethernet, PPP, FireWire, and 3G. 9.3.3

Wireless LAN Association

The purpose of WLANA is to educate and raise consumer awareness about the use and availability of wireless LANs. It is an educational resource to learn about the concepts of wireless LANs and specific products and services. WLANA also promotes the cooperation of several partners to generate content to the directory of information such as white papers and case studies.

9.4

Competing Technologies

Several technologies compete with the 802.11 family of standards. This is natural as business needs change and technologies improve. Other wireless LAN technologies being used include:  HomeRF  Bluetooth  Infrared  OpenAir. 9.4.1

HomeRF


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HomeRF operates within the 2.4 GHz band and utilizes frequency hopping technology. The devices hop approximately 50 hops per second, which is 5-20 times faster than 802.11-compliant FHSS devices. The newest versions of HomeRF take advantage of “wide band” frequencies approved by the FCC, which entail:  Maximum of 5 MHz wide carrier frequencies  Minimum of 15 hops in a sequence  Maximum of 125 mW of output power. The SWAP protocol is used which is a combination of CSMA used in local area networks and TDMA used in cellular phones. SWAP is actually a hybrid of 802.11 and DECT standards. HomeRF is considered to be more secure than 802.11 products using WEP. This is the result of the 32-bit initialization vector (IV) used over 802.11's 24-bit IV. Additionally, HomeRF specifies how the IV is chosen during encryption which is not done by 802.11. 9.4.2

Bluetooth

Bluetooth also operates in the 2.4 GHz ISM band as a frequency hopping technology. The hop rate for Bluetooth is 1600 hops per second. Bluetooth devices are designed for:  Low throughput  Simple use  Low power  Short range. One disadvantage of Bluetooth is its persistent interruptions of other 2.4 GHz networks and FHSS systems due to the faster hop rate. The Bluetooth signal appears to other systems as all-band noise or allband interference. Bluetooth operate in three power classes: 1 mW, 2.5 mW and 100 mW. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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9.4.3

Infrared Data Association (IrDA)

IrDA is not a standard, but an organization. Founded in 1993 and funded by members, IrDA's mission is “to create an interoperable, low cost, low power, half duplex, serial data interconnection standard that supports a walk-up point-to-point user model that is adaptable to a wide range of computer devices”. Infrared (IR) is a light-based transmission technology. At close range, the maximum data rate of 4 Mbps is possible, though the typical rate found is 111 kps. Unfortunately, being light-based, other sources of IR can interfere with the transmission. Security for IR devices have two major advantages:  IR cannot travel through walls  The light beam must be directly intercepted to gain access to the information. A popular use for infrared transmissions is between laptops and handheld devices because of their easy point-to-point connectivity. The maximum range of a point-to-point connection is 1 kilometer, or 3280 feet. 9.4.4

Wireless LAN Interoperability Forum (WLIF)

The WLIF is a defunct organization responsible for the creation of the OpenAir standard. OpenAir was meant to be an alternative to 802.11. Two speeds were specified: 800 kbps and 1.6 Mbps. OpenAir and 802.11 are not compatible. Several products still exist that comply to OpenAir standards.


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10 Infrastructure Devices for Wireless LANs 10.1 Access Points Access points provide an entry point into the wireless network. The hardware is half duplex with an intelligence equivalent to a sophisticated Ethernet switch. Access points have the ability to communicate with the client, the network and other access points. 10.1.1 Modes An access point can be configured in three modes:  Root mode  Repeater mode  Bridge mode. Root mode is typically the default configuration. It is used when connecting the access point to a wired network backbone through a wired interface. Root-based access points are typically Ethernet driven. When multiple access points are connected to the same wired network distribution, they are in communication with each other coordinating roaming functions Bridge mode connects two or more wired networks using wireless access points. Repeater mode provides a wireless upstream link to a wired link. A client will connect to an access point in repeater mode. A wireless connection from the repeater is made with a root access point upstream.


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Use of repeater access points is not recommended because:  Cells around each access point must overlap by more than 50%, reducing the range available to clients.  Throughput is reduced since the repeater is communicating with the upstream access points and all clients.  Users attached to repeaters typically experience low throughput and high latency. 10.1.2 Access Point Options Access points are portals allowing client connectivity from wireless 802.11 networks to wired 812.3 or 802.5 networks. Several hardware and software options are available, including:  Fixed or detachable antennas  Advanced filtering capabilities  Removable (modular) radio cards  Variable output power  Varied types of wired connectivity. Devices with detachable antennas allow connection to any antenna with any length of cable required. Some access points are shipped with diversity antennas which allow the use of multiple antennas with multiple inputs on a single receiver. An access point may include MAC or protocol filtering capabilities which is used to prevent intruders accessing the wireless LAN. Access points can be configured to filter devices that are not listed in the MAC filter list in the administrative controls of the access point device. Protocol filtering controls what protocols can be used on a wireless link. Some access points have the ability to add special functionality by providing PCMCIA slots. This allows additional radios to be added or removed from the device. With two PCMCIA slots, a device can have one radio card to become an access point while another radio card can act as a bridge or as independent access points increasing the number of users that can connect. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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The administrator can control the power used by the access point to send data through variable power output functions. Controlling powers allow the range of the access point to be controlled. The more power used, the greater the distance available to access the wireless network. Fixed output access points are alternatives. Changes to power can be made using:  Amplifiers  Attenuators  Long cables  High-gain antennas. Access points can link to most network types. Understanding the limitations of wired connections to the access point from the core network follows because of network restrictions. 10.1.3 Managing Configurations Configuring and managing access points are dependent on the features set by the manufacturer. Most devices include at least a console, telnet, USB, or built-in web server. Some models may include custom configuration management software. An access point is typically preconfigured with an IP address. A hardware reset button is available to reset the device to factory defaults. Several additional features are available. The more features available, the greater the expense for the device. Some of the features available on Small Office, Home Office (SOHO) devices and Enterprise devices include:     

SOHO devices MAC filtering WEP (64-bit or 128-bit) USB or console configuration interfacing Built-in web server configuration interface (simple) Custom configuration applications (simple). Enterprise


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         

Custom configuration applications (advanced) Built-in web server configuration interface (advanced) Telnet access SNMP management 802.1x/EAP RADIUS client VPN client and server Static or dynamic routing Repeater functions Bridging functions.

Functionality support can vary drastically within the same feature; some devices partially support the feature while others fully support.

10.2 Wireless Bridges A wireless bridge will connect two wired LAN segments. Typical attributes of bridges include:  Half-duplex devices  Capable of layer 2 wireless connectivity only  Used in point-to-point and point-to-multipoint configurations. 10.2.1 Bridge Modes Wireless bridges communicate to other wireless bridges in one of four modes:  Root mode  Non-root mode  Access point mode  Repeater mode. In a group of bridges, one bridge will always be configured as the root bridge. A root bridge will never be associated with another root bridge. The communication it has will always be with non-root bridges and client devices. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Non-root mode allows wireless bridges to attach to other wireless bridges which are not in root mode. Some manufacturers support the ability for clients to connect to non-root bridges while in bridging mode; allowing the device to be an access point and a bridge at the same time. Access point mode provides the bridge access point functionality. In repeater mode, the wireless bridge is placed between two other bridges in order to extend the length of the bridged segment. Unfortunately, use of repeaters typically will reduce the throughput available. 10.2.2 Bridge Options Options available for bridges are the same for wireless access points, including:  Fixed or detachable antennas  Advanced filtering capabilities  Removable (modular) radio cards  Variable output power  Varied types of wired connectivity.

10.3 Wireless Workgroup Bridges Different from wireless bridges are wireless workgroup bridges, or WGB. WGBs are client devices, allowing multiple wired LAN clients to be aggregated into a single collective wireless LAN client. Workgroup bridges are useful in:  Mobile classrooms  Mobile offices  Remote campus kiosks. With WGBs, the workgroup bridge appears as a single client device in the association table. The MAC addresses of the devices supported by the workgroup bridge are not seen by the access point. A standard bridge can be used, by if an access point is used at the central site, Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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an extra bridge device is required. Purchasing a workgroup bridge can alleviate this problem. 10.3.1 Options and Configuration Since wireless workgroup bridges are a type of bridges, they have some of the same options that standard bridges have. Because the WGB supports multiple clients, some thought should be given to how many clients should be supported. Depending on the manufacturer, the range can be from 8 to 128 clients; though more than 30 will cause significant degradation to the throughput.

10.4 Wireless LAN Client Devices A client is any wireless device that uses the access point to enter the network. The devices can include:  PCMCIA cards  Compact flash cards  Ethernet connectors  Serial connectors  USB adapters  PCA adapters  ISA adapters. 10.4.1 PCMCIA and Compact Flash Cards PCMCIA cards, or PC cards, are used in notebook computers and PDAs. They provide a connection between the client and the network. The main difference in most PC cards is the antenna. Some antennas are flat and small, while others are detachable and connected using a cable. Compact Flash Cards, or CF cards, are similar to PC cards in Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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functionality. They are typically used in PDAs and are the size of a matchbook. 10.4.2 Wireless Ethernet and Serial Converters Converters are for any device having Ethernet or legacy 9-pin serial ports. They convert network connections into wireless LAN connections. Ethernet converters are external devices that connect using the category 5 (Cat5) cable. They have the ability to convert a large number of wired nodes to wireless in a matter of minutes. Serial devices are used on older equipment such as terminals, telemetry equipment and serial printers. 10.4.3 USB Adapters USB Adapters are plug-n-play devices and uses power delivered through the connection from the computer. Some USB clients utilize modular radio cards while others have a fixed internal card. When a PC card is used, it is recommended to use an adapter and card from the same manufacturer. 10.4.4 PCA & ISA Adapters PCI and ISA adapters are installed inside desktop or server equipment. Wireless PCI devices are plug-n-play and may come with an “empty” PCI card which requires the insertion of a separate PCMCIA card to function. Most ISA cards are not plug-n-play and require some configuration through a software utility and the operating system. Typically, the administrator has to change the settings of the adapter to match the settings of the operating system. Drivers for PCI and ISA adapters are usually different and provided Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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for by the manufacturer. When a PC card is used, it is recommended to use an adapter and card from the same manufacturer. 10.4.5 Wireless Fixed Adapters The increased demand of wireless capability has caused most computer manufacturers to include a fixed adapter in new equipment, particularly notebook computers and personal devices. 10.4.6 Configuring Adapters Wireless adapters are installed using:  drivers  manufacturer's wireless utilities Drivers are typically included with the software. Most adapters are plug-and-play, so a prompt for the software is made when the client device is installed. Some devices, such as Ethernet and Serial converters, do not use any special drivers. Utilities provided by manufacturers can range in functionality from simple connectivity to a full suite of utilities, including:  Site Survey Tools  Spectrum analyzer  Power and speed monitoring  profile configuration  Link status monitor  Link testing Site survey tools can be used to:  find networks  identify MAC address or access points  quantify signal strength Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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 

quantify signal-to-noise ratios identify interfering access points

Spectrum analyzer software will find interfering sources and overlapping wireless LAN channels. Power and speed configuration utilities monitor the behavior of the wireless link at any given time. Profile configuration utilities assist in changing from one wireless network to another by creating a profile for each network. Link status monitor utilities allow users to view a variety of components including:  Packet errors  Successful transmissions  Link viability  Connection speed  Configurable parameters. Though the functionality of the utilities can vary greatly, the configurable parameters are typically the same. They include:  Infrastructure mode /Ad Hoc mode  SSID  Channel  WEP Keys  Authentication type.

10.5 Wireless Residential Gateways A wireless residential gateway is designed to connect a small number of wireless nodes to a single device. Connectivity to the Internet or another network is capable through the Layer 2 or Layer 3. Most gateways include a built-in hub or switch and an access point which is fully configurable and Wi-Fi compliant. Most residential users are familiar with the WAN port: an Internetfacing Ethernet port that allows connection through: Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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   

Cable modems xDSL modems Analog modems Satellite modems.

Common options found on wireless residential gateways include:  Point-to-point protocol over Ethernet (PPPoE)  Network address translations (NAT)  Port address translation (PAT)  Ethernet switching  Virtual servers  Print services  Fail-over routing  Virtual private networks (VPN)  Dynamic host configuration protocol (DHCP)  Configurable firewall. Configuring gateways typically require browsing to a built-in Ethernet ports in order to change the settings for the particular users. Some of the options that are configurable if available are:  ISP settings  LAN settings  VPN settings  Console connectivity  Telnet connectivity  USB connectivity.

10.6 Enterprise Wireless Gateways Enterprise wireless gateways are appropriate for large-scale wireless LAN environments to provide a range of manageable wireless LAN services, such as:  Rate limiting  Quality of Service(QoS)  Profile management. Particularly, the gateway provides specialized authentication and connectivity to wireless clients. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Enterprise Wireless Gateways differ from residential devices with the existence of more powerful CPUs and faster Ethernet interfaces. This is required to support multiple access points sending traffic to and through the gateway. 10.6.1 Option Support Additionally, most enterprise gateways will support a variety of WLAN and WPAN devices. SNMP is supported and user profiles can be upgraded simultaneously. Other common options include:  Hot fail-over  RADIUS support  LDAP support  Windows NT authentication databases  Data encryption  VPN connectivity  802.1x/EAP connectivity  Role-Based Access Control (RBAC)  Class of Service support  Mobile IP  MAC spoofing prevention  Complete session logging. 10.6.2 Configuring Enterprise Gateways Enterprise gateways are configured through:  Console ports  Telnet  Internal HTTP or HTTPS servers. Typically, they are installed in the data path of a wired LAN just past the access point. The gateway provides centralized management of multiple devices. Enterprise wireless gateways are upgraded similar to wired switches Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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and routers through TFTP. Configuration backups can be automated. Additionally, most gateways are built as rack-mountable 1U and 2U devices to accommodate existing data center designs.


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11 Antennas and Accessories Antennas are a basic component to allow wireless devices to communicate and extend the range of wireless LAN systems. Antennas can enhance the security of the wireless network by reducing signal leaking and preventing signal interception. The right antenna in the right position can provide a proper and secure network environment. Wireless LAN antennas are categorized as:  Omni-directional  Semi-directional  Highly-directional.

11.1 Radio Frequency Antennas Antennas convert high frequency (RF) signals from a cable or waveguide into propagated waves in the air. The electrical fields emitted from an antenna are called beams or lobes. Different categories of antennas offer different RF characteristics. A similar characteristic of all antennas is the effect of controlling gain: namely, as the gain is increased the coverage area narrows. 11.1.1 Omni-Directional Antennas Omni-directional antennas are the most commonly used wireless LAN antenna. Also called Dipole antenna, the omni-directional antenna is standard equipment for most access point. The primary characteristic of a dipole antenna is that the energy emissions radiate in all directions around the antenna's axis. The radiating element of a dipole is one inch long. They support wireless LAN frequencies in the 2.4 Ghz microwave spectrum. One characteristic of an antenna is their size: the smaller they are, the Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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higher the frequency supported. An antenna that is capable of radiating in all directions equally, in the shape of a sphere, is called an isotropic radiator. The possibility of this type of antenna is theoretical, the actual radiation of dipole is more closely shaped like a doughnut with the antenna at the center. This is because the dipole will radiate equally in all directions by not along the length of the wire itself As the gain increases, the more horizontal the doughnut is shaped. High gain antennas have a complete horizontal beam with little or no change in the vertical emissions. A number of reasons exist for using omni-directional antennas:  Coverage in all directions  Large areas of coverage around a central point  Point-to-multipoint designs in a hub-n-spoke topology. The positioning of the antenna is important:  For outdoors, the antenna should be placed at the top of a structure  For indoors, the antenna should be placed near the ceiling at the center of the building. They are most suitable in warehouse or convention type environments. 11.1.2 Semi-Directional Antennas Supported types of semi-directional antennas include:  Patch  Panel  Yagi. These types of antennas can come in different styles and shapes. All are generally flat and can be mounted to the wall. The differences are in the coverage characteristics. Unlike omni-directional antennas, the semi-directional antennae direct the energy in one particular directional, often in a hemispherical Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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(Patch) or cylindrical (Yagi) coverage pattern. Semi-directional antennas are typically used for bridging at short or medium ranges. A couple of properly placed semi-directional antennas can provide better coverage than several omni-directional antennas. 11.1.3 Highly-Directional Antennas The signal beam from a highly directional antenna is the narrowest signal beam of any type, as well as the greatest gain. Most highly-directional antennas are dish-shaped devices and are either parabolic dishes or grid antennas. These antennas are ideal for sending signals over distances up to 25 miles. They do not have the coverage usable for client devices, but rather are used for point-to-point communications.

11.2 Concepts of RF Antennas When using RF antennas, several concepts are important to understand. The most important concepts are:  Polarization  Gain  Beamwidth  Free space path loss. These concepts are important for determining:  The placement of the antenna  The positioning of the antenna  The power being radiated  The distance traveled by the beam  The power being received.


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11.2.1 Polarization A radio wave is comprised of two fields: electric and magnetic. Together, they create an electro-magnetic field. Energy is transferred between the two fields in a process called oscillation. The fields are on planes which are perpendicular to each other. The plane that is parallel to the antenna element is referred to as the “Eplane.” The plane perpendicular to the antenna element is referred to as the “H-plane.” Polarization refers to the physical orientation of the antenna. The antenna element is the metal part of the antenna which performs the radiating of the electrical field. The electrical field is always parallel to the radiating element:  Horizontal polarization is the electrical field parallel to the ground  Vertical polarization is the electrical field perpendicular to the ground. Wireless LANs typically use vertical polarization. Most access points have dual antennas that are vertically polarized. Antennas that are not polarized in the same way are not able to communicate with each other. The topic of polarization explains the reasons why PCMCIA cards do not have adequate coverage since it is difficult to build appropriate antennas into devices that normally sit horizontally. 11.2.2 Gain Gain has the ability to increase the distance that a propagated wave will travel. Antennas that feature passive gain do not increase the power that is inputted, but will change the radiation field to lengthen or shorten the propagated field. 11.2.3 Beamwidth Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Vertical and horizontal considerations are in place when discussing beamwidths. A vertical beamwidth is measured in degrees and perpendicular to the surface of the Earth. A horizontal beamwidth is also measured in degrees and parallel to the surface of the Earth. For different antenna types, here are the documented specifications: Antenna type

Horizontal Beamwidth

Vertical Beamwidth

Omnidirectional

360

Ranges from 7-80

Patch/Panel

Ranges from 30180

Ranges from 6-90

Yagi

Ranges from 3078

Ranges from 1464

Parabolic Dish

Ranges from 4-25 Ranges from 4-21

11.2.4 Free Space Path Loss Sometimes simply referred to as Path Loss, Free Space Path Loss is the measurable loss incurred by an RF signal due to “signal dispersion.” Signal dispersion is the natural broadening of the wave front. The wider the front is, the less power which is induced into the receiving antenna. Power is important factor in link viability. When a signal travels through the atmosphere, its power decreases at a rate inversely proportional to the distance traveled and proportional to the wavelength of the signal. Path Loss is the single greatest source of loss in a wireless system and one of the foundations of calculating link budgets using the formula: Path Loss = 2LOG10[4Πd/λ]{dB} Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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The equation above identifies a relationship between the loss in decibels and the distance traveled by the signal. As the distance doubles, an increase of 6dB in the EIRP occurs. Below is a sample of calculations: Distance

Loss

100 meters

80.23

200 meters

86.25

500 meters

94.21

1,000 meters 100.23 2,000 meters 106.25 5,000 meters 114.21 10,000 meters

120.23

11.3 Antenna Installation Installation of antennas is one of the most important considerations after determining the type of antenna. The factors associated with installing antennas include:  Placement  Mounting  Proper use  Orientation  Alignment  Safety  Maintenance. 11.3.1 Placement Omni-directional antennas should be located as close to center of the desired coverage area as possible. The antenna should be placed as high as possible while considering that the high-gain antennas have a narrower vertical axis to the transmission field. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Outside antennas should be mounted above obstructions such as tree and buildings that could encroach on the Fresnel Zone. Planning for future development of growth should be considered, i.e. trees that currently do not block the antenna may grow to significantly block the signal in 10-20 years. 11.3.2 Mounting Several options are available to mount antenna:  Ceiling mount – hung from crossbars of drop ceilings  Wall mount – forces the signal away from a perpendicular surface  Pillar mount – mounts flush to a perpendicular surface  Ground plane – sits flat on the ground  Mast mount – mounted to a pole  Articulating mount – mast mount that can be moved  Chimney mount – mounted to structure's chimney  Tripod-mast – mounted to a tripod. 11.3.3 Proper Use The most important factor in proper use of antennas is whether it is designed for inside or outside use. Outside antennas are designed with seals and plastics to protect the element from water, heat, and cold. Inside antennas are not. 11.3.4 Orientation The orientation of the antenna determines polarization. Two antennas must have the same orientation in order to communicate between each other. Therefore, if the access point is parallel to the Earth's surface, the antennas on the clients must also be parallel. The same is true for antennas perpendicular to the surface. 11.3.5 Alignment Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Alignment becomes increasingly important the more highly directional the antenna. Some antennae have very wide beamwidths so that two antennas can be aimed in the general direction of each other. Wireless bridges come with alignment software to optimize antenna reception. The concerns using omni-directional or semi-directional are usually focused on getting the greatest coverage of the appropriate area. 11.3.6 Safety RF antennas are electrical devices and as such can be dangerous to implement and operate. Some basic guidelines include:  Use professional installers, especially for elevated installations  If not using professional, follow the manual  High-gain antennas utilize high levels of power: touching or directing an antenna while it is transmitting is very dangerous  Antennas should be installed away from any metal obstructions to reduce the amount of multipathing as well as reflection of dangerous levels of high power  Recommendations related to power lines is keep antennas at least 2 times the height of the antennas from all overhead power lines  Grounding must be appropriate and follow the National Electric Code and local electrical codes. 11.3.7 Maintenance Outdoor antennas require the greatest amount of maintenance because of their exposure to heat, cold, and other weather conditions. Water is the biggest problem to antennas requiring sufficient sealing of connectors.


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11.4 Power over Ethernet (PoE) Devices When AC power receptacles are not available to power a unit, wireless devices can receive DC power over a Cat5 Ethernet cable. The method is called Power of Ethernet (PoE). The cable used will carry both power and data to the unit. This is done with a POE device on the cable line. Cable requirements related to data transfer still apply, namely that Ethernet cables can only carry data reliably for 100 meters. PoE has such great advantages that some manufacturers build devices that will only use PoE. 11.4.1 PoE Options Several types of PoE devices exist, including:  Single-port DC voltage injectors  Multi-port Voltage injectors  Ethernet switches designed to inject DC voltage. Configuration and management is not necessary for a PoE device, though some considerations must be taken into account. No industry standard exists for the implementation of PoE to describe how PoE equipment interfaces with other devices, therefore, if you are using a PoE device to power an access point, it is best to use devices from the same manufacturer. The output voltage required to power a wireless LAN will be different form one manufacturer to the next. Using devices from the same manufacturer will mitigate any compatibility issues. The current that is carried over Ethernet utilizes unused pins that are not standardized. One manufacturer may use pins 7 and 8 while another uses 4 and 5. If a wireless LAN device does not accept power on those pins, the device will not power up. Single-port DC voltage injectors are used to power wireless LAN devices. They are specifically used when the use of PoE is Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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mandatory for the access point or bridge. Single-port injectors should only be used in small networks to prevent clutter. For medium-sized networks, a multi-port injector will support the power requirements for several devices, typically manufactured to have 4, 6, or 12 output ports. The PoE device is typically sitting within a wiring closet and connected to a switch or hub on the input port and the wireless devices on the output ports. Up to 50 access points can be supported. Large sized networks would benefit from the installation of active Ethernet switches. The device incorporates DC voltage injections into the Ethernet switch. The benefit is the support of large numbers of PoE devices without additional hardware. In most cases, an active Ethernet switch can auto-detect any PoE requirements on the network. If PoE is not required, than the DC voltage is switched off on the connecting port. 11.4.2 Compatibility for PoE Devices that are not designed to use PoE can be converted using a DC “picker” or “tap.” Also called active Ethernet “splitters,” these devices siphon the DC voltage that was injected in the Cat5 cable by the injector and makes it available to the non-PoE device through a regular DC power jack. Using Power-over-Ethernet requires the use of an injector. If the wireless device is PoE compatible, no other equipment is required. If the device is not PoE compatible, a picker is required. 11.4.3 Injector Types Injectors come in two types:  Passive  Fault protected. A passive injector simply places a DC voltage onto a Cat5 cable. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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A fault protected injector also places a DC voltage on a Cat5 cable, but also provides continuous fault monitoring and protection to detect short circuits and over-current conditions. 11.4.4 Picker Types Pickers come in two types:  Passive  Regulated. A passive picker will take the voltage from the Cat5 cable and direct it to the equipment. Therefore, the equipment will receive whatever VDS (Volts of Direct Current) were originally injected. A regulated tap will take the voltage from the Cat5 cable and convert it to another voltage, typically standard regulated voltages of 5 VDC, 6 VDC, and 12 VDC. 11.4.5 Standards for Voltage and Pinouts The injected PoE voltage has been standardized at 48 VDC by the IEEE. However higher voltage will reduce the current flowing through the Cat5 cable. This will cause the load to increase and the limitations on the length of the Cat5 cable. Some manufacturers have decided to utilize 12 or 24 VDC. 11.4.6 Fault Protection Fault protection is meant to protect the cable, equipment and power supply from short-circuit or fault. Under normal operations, faults are highly unexpected; though several causes may contribute:  Incompatibility with PoE  Non-standard or defective connectors  Incorrectly wired Cat5 cabling  Cut or crushed Cat5 cabling. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Fault protection will shut DC voltage off whenever a fault condition exists. Circuit operation varies from one model to the next. Restoring power is dependent on the device model. Some models will continuously monitor the cable and automatically restore power when the fault has been resolved. Other models must be manually reset.

11.5 Accessories for Wireless LAN Several accessories can be added to a wireless network to:  Maximize throughput  Minimize signal loss  Ensure proper connections. Types of accessories possible are:  RF Amplifiers  RF Attenuators  Lightning Arrestors  RF Connectors  RF Cables  RF Splitters. 11.5.1 RF Amplifiers Devices designed to “amplify;” to increase the amplitude of the RF signal. The measure of an amplified signal is noted as +dB. Typically amplifiers are used to compensate for the expected loss of the signal due to distance. Most amplifiers are powered using DC voltage and located near the access point or bridge. The DC voltage is sometimes called phantom voltage because the RF amplifier seems to power up without cause. Two types of amplifiers exist:  Unidirectional  Bi-directional. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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Unidirectional amplifiers will compensate for signal loss by increasing the signal level before being injected into the transmitting antenna. Bi-directional amplifiers will amplify the received signal before it is fed into the access point, bridge, or client device. Each type of amplifier can have one of two additional options:  Fixed gain  Variable gain. Fixed gain amplifiers apply a fixed amount of gain to the RF signal, while variable gain provides a manual means of configuring the desired amount of gain required from the equipment. To determine the right amplifier, certain specifications should be taken into consideration, including:  Frequency response (range in Ghz)  Impedance (ohms)  Gain (dB)  VSWR  Input (mW or dBm)  Output (mW or dBm). Frequency response is a big determiner for selecting the right amplifier. If a wireless LAN uses a 5 Ghz frequency spectrum, an amplifier that works within the 2.4 Ghz spectrum will not work. The impedance of the amplifier should be the same as all the other wireless LAN hardware between the transmitter and the antenna, though typically they will be about 50 ohms. Since the amplifier will be connected to the network, consideration must be made around the kinds of connectors used. Most amplifiers use either SMA or N-type connectors. Amplifiers are generally mounted to a solid surface. Unless the amplifier allows variable gains, there are no configurations required. Variable amplifiers have to be adjusted to the proper amount of amplification. Copyright The Art of Service Brisbane, Australia │ Email: [email protected] │Web: http://theartofservice.com │eLearning: http://theartofservice.org Phone: +61 (0)7 3252 2055

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11.5.2 RF Attenuators A device that causes precise measured loss in an RF signal. The common reason for decreasing the RF signal is to meet FCC rules when the equipment available for the network will provide a much greater signal strength than allowed by the regulation. Like amplifiers, attenuators can be either fixed or variable. The same considers for selecting amplifiers and attenuators are the same. Fixed attenuators typically have BNC or SMA connectors. The placement of an attenuator is directly between the transmitter and the antenna. Configuration is not required unless the attenuator allows variable loss. 11.5.3 Lightning Arrestors These devices provide protection to access points, bridges, and workgroup bridges attached to coaxial transmission lines. Coaxial transmission lines are susceptible to lightning strikes. A lightning arrestor will redirect the transient current caused by lightning into the ground. Generally, the lightning arrestor will redirect up to 5000 amperes at up to 50 volts. When lightning strikes a nearby object, transient currents from the strike are induced into the antenna or transmission line. The lightning arrestor senses these currents and ionizes the gases to cause a short directly to the earth ground. The cost of a lightning arrestor is between $50 and $150. When purchasing an arrestor, some considerations include:  Must meet IEEE standard of

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