Wireless Networks dor Dummies phần 10 pot

802.11 802.11 is the granddaddy of wireless networking standards. The IEEE 802.11 specifications specify an “over-the-air” interface between a wireless client and a base station or access point, as well as at the peer-to-peer level among wireless clients. These standards can be compared to the IEEE 802.3 standard for Ethernet for wired LANs. They address two of the OSI layers, the Physical (PHY) layer and Media Access Control (MAC) sub-layer of the Data Link layer. The wireless standard is designed to resolve compatibility issues between manufacturers of Wireless LAN equipment. This standard provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), or infrared. 802.11a After the finalization of the 802.11 standard, it became apparent that the 2 Mbps bit rate wouldn’t cut it, especially compared to 10 Mbps Ethernet. Soon after, the IEEE started two taskgroups to work on this problem: 802.11a and 802.11b. The goal of the two groups was to define higher bit rate refinements to the 802.11 standard. Hence, 802.11a is an extension to 802.11 that provides up to 54 Mbps speed in the 5GHz band. At the Physical layer, 802.11a uses an orthogonal frequency division multi- plexing encoding scheme rather than FHSS or DSSS. It offers less potential for radio frequency (RF) interference because it operates in the 5GHz band. This standard is quickly gaining ground on 802.11b. Even though approval for the 802.11a and 802.11b standards came at roughly the same time, it took a year longer for 802.11a equipment to hit the market due to the complexity of the standard. In fact, it took until late 2002 before 802.11a equipment hit critical mass. 802.11b This extension provides 11 Mbps transmission (with a fallback to 5.5, 2, and 1 Mbps) in the 2.4 GHz band. You may hear people refer to 802.11b as Wi-Fi 348 Part VI: Appendixes 29_575252 appb.qxd 9/2/04 4:15 PM Page 348 or Wireless-Fidelity: a throwback to Hi-Fi. One of the original wireless specifi- cations in use, 802.11b uses only DSSS. It was first implemented in a 1999 rati- fication to the original 802.11 standard. 802.11b equipment is backward compatible to 802.11 equipment using DSSS. It was at the time considered a stopgap until the adoption of the 802.11a standard, but it soon became the dominant standard with the largest installed base. Most wireless solutions today either use or support this standard. It is also probably the least expen- sive to implement, although 802.11a is quickly catching up. 802.11c You don’t see or hear about this one much. 802.11c provides required infor- mation to ensure proper bridge operations. Product developers use this stan- dard when developing access points, so most users will not notice it. 802.11d 802.11d is a little-known standard. The intent of 802.11d was to harmonize fre- quency and bandwidth around the world so that wireless equipment can interoperate. Enough said. 802.11e 802.11e is being defined to provide support for Quality of Service (QoS) traf- fic and thereby improve support of audio and video (such as MPEG-2) appli- cations. Because 802.11e falls within the MAC layer, it will be common to all 802.11 PHYs and be backward-compatible with existing 802.11 wireless LANs. 802.11f You don’t see or hear much about 802.11f. 802.11f provides the necessary information that access points need to exchange in order to support the dis- tribution system functions, like roaming. Without this standard, the IEEE rec- ommends using similar vendors to support interoperability. 349 Appendix B: Wireless Standards 29_575252 appb.qxd 9/2/04 4:15 PM Page 349 802.11g The 802.11g standard defines the way wireless communicates at higher bit rates of up to 54 megabits per second while remaining backward-compatible with the 11 Mbps 802.11b standard. This bit rate enables streaming media, video downloads, and more users. 802.11g is gaining ground on the earlier 802.11a/b standards in industry and quickly becoming the more prevalent standard for wireless access. 802.11h 802.11h is a new specification that addresses the requirements of the European regulatory bodies. It provides for dynamic channel selection (DCS) as well as transmit power control (TPC) for devices operating in the 5GHz band, like the 802.11a specification does. In Europe, there is a greater need to avoid interference with satellite communications, which have “primary use” designations and can be interfered with by the 5 GHz band. This standard helps eliminate any potential for that interference. 802.11i 802.11i is the security panacea for wireless LANs. 802.11i incorporates stronger encryption techniques, such as AES (Advanced Encryption Standard). It is designed to improve on the weaknesses found in the existing WEP standards used by the other wireless standards. 802.11i includes strong encryption and a robust key management scheme. On the flipside, 802.11i will require new hardware chipsets, so it will not be compatible with existing hardware. 802.11j The 802.11j task group has the mandate to refine some physical and data link issues for 5 GHz wireless networking with the view to the coexistence and eventual convergence of the IEEE 802.11a and European/Japanese HIPERLAN/2 standards. 350 Part VI: Appendixes 29_575252 appb.qxd 9/2/04 4:15 PM Page 350 802.11k 802.11k is another Quality of Service standard, but this one is for the Radio layer (physical). The mandate is to ensure the quality of service over an 802.11 link. 802.11n 802.11n is an effort to provide user throughput speeds of 100M bits/sec or more, with vendors like Agere pushing for 500M bits/sec. Current speeds in 802.11g for example, have data rates of 54M bits/sec which usually results in user throughput of considerably less, arguably around only 18 to 22M bits/sec. Originally anticipated January of 2004 it is still waiting approval. 802.15 In March 1998, the IEEE formed the WPAN Study Group. The study group’s goal was to investigate the need for a wireless network standard for devices within a personal operating space (POS). In May of 1998, the Bluetooth Special Interest Group (SIG) formed. In March of 1999, the WPAN study group became IEEE 802.15, the WPAN Working Group. The 802.15 WPAN (Wireless Personal Area Network) is an effort to develop standards for Personal Area Networks or short distance wireless networks. These WPANs address wire- less networking of portable and mobile computing devices, such as PCs, Personal Digital Assistants (PDAs), peripherals, cell phones, and pagers, let- ting these devices easily communicate with one another. Since the formation of 802.15, three projects have started. The first (TG1) was the Bluetooth project that released the Bluetooth 1.0 Specification in July of 1999. The project will produce an approved IEEE standard derived from the Bluetooth standard. The second, or TG2, will address the issue of co-existence of 802.11 and 802.15 networks. Currently, Bluetooth networks create havoc with 802.11 networks. And the third, or TG3, will work on deliv- ering a standard for high bit rate (20 Mbps or higher) WPANs. 351 Appendix B: Wireless Standards 29_575252 appb.qxd 9/2/04 4:15 PM Page 351 802.16 The 802.16 standard is a broadband wireless standard for Wireless Metropolitan Area Networks (WirelessMAN or WMAN). This standard, also known as Broadband Wireless Access (BBWA), addresses the “first-mile/l ast-mile” connection in wireless metropolitan area networks, focusing on the efficient use of bandwidth between 10 and 66 GHz. Unless you are a large business, it is unlikely you’ll deal with this standard. 352 Part VI: Appendixes 29_575252 appb.qxd 9/2/04 4:15 PM Page 352 Appendix C The Fundamentals of Radio Frequency In This Chapter ᮣ Radio frequency (RF) ᮣ Behavior of radio waves ᮣ RF units of measure ᮣ RF mathematics W e cannot do justice to the discussion of radio frequency in one Appendix. It is the stuff of many books. Having said that, you must understand some concepts in order to set up and administer your WLAN. This Appendix provides a glimpse into the fascinating world of radio frequen- cies. You may want to peruse this appendix before calculating your link budget in Chapter 2. Radio Frequency When teaching networking, we often use the example of throwing a rock into a river to teach the concept of attenuation. Think of going down to the water and throwing in a rock. You see an epicenter where the rock went in and waves rippling out from that epicenter. The farther you get from the center, the weaker the waves get. The concentric circles that you see are similar to the radio waves as they propagate away from the antenna. Radio frequencies are high-frequency (and in our, case ultra- and super-high frequency, as shown in Table C-1) alternating current (AC) signals passed along copper wire or some other conductor until an antenna radiates them into the air. The antenna transforms the wireless signal into a wired signal and vice versa. When the antenna propagates the high-frequency AC signal into the air, it forms radio waves. These radio waves propagate, or move away, from the source in a straight line in all directions. Just imagine the rock going into the water. 30_575252 appc.qxd 9/3/04 8:40 AM Page 353 354 Part VI: Appendixes Table C-1 Radio Frequency Spectrum Frequency Description Up to 300 Hz Extremely Low Frequency (ELF) 300 Hz–3 kHz Voice frequency 3 kHz–30 kHz Very Low Frequency (VLF) 30 kHz–300 kHz Low Frequency (LF) 300 kHz–3 MHz Medium Frequency (MF) 3 MHz–30 MHz High Frequency (HF) 30 MHz–300 MHz Very High Frequency (VHF) 300 MHz–3 GHz Ultra-High Frequency (UHF) 3 GHz–30 GHz Super High Frequency (SHF) 30 GHz–300 GHz Extremely High Frequency (EHF) In the table, Hz denotes hertz. We use the term hertz to represent the unit for frequency. One hertz simply means one cycle (event) per second; 10 Hz means ten cycles (events) per second; and so on. You can apply hertz to any periodic event. For example, the clock speed of your Pentium might be said to tick at 2.2 GHz. The reciprocal of frequency is time (period): a frequency of 1 Hz is equivalent to a period of 1 second, and a frequency of 1 MHz is equal to a period of 1 microsecond. You should know some multiples, as follows: Term Symbol Equivalence 1 kilohertz kHz 10 3 Hz or 1,000 Hz 1 megahertz MHz 10 6 Hz or 1,000,000 Hz 1 gigahertz GHz 10 9 Hz or 1,000,000,000 Hz This may or may not make sense, but either way, some examples cannot hurt. For example, standard domestic AC electric power (220v [volt] or 110v volt- age) is 50–60 Hz. If you play music on the side, middle C is 261.625 Hz. If you don’t play music but listen to it, FM radio broadcasts are 88–108 MHz. The clock speed of the Intel 4004, the world’s first commercial microprocessor, was 104 kHz. Today’s Pentium 4 has a clock speed of around 3 GHz. The Federal Communications Commission (FCC) in the United States as well as other government agencies around the world have made parts of the spectrum available for unlicensed radio networks as long as they meet local regulations. Currently, these bands are the Industrial, Scientific, and Medical (ISM) band (operating at 2.4 GHz) and the U-NII (Unlicensed National Information Infrastructure) band (operating at 5.8 GHz). 30_575252 appc.qxd 9/3/04 8:40 AM Page 354 355 Appendix C: The Fundamentals of Radio Frequency In Appendix B, you can read about the various wireless standards. The 802.11b standard supports data rates of up to 11 Mbps, whereas 802.11a and 802.11g support data rates of up to 54 Mbps. The difference in data rates is caused by one of two things: more bandwidth or better encoding. The 802.11g standard works in the same 2.4 GHz band used by 802.11b but uses OFDM rather than DSSS and uses 64QAM rather than DQPSK. The 802.11a standard has more bandwidth. 64QAM and DQPSK? That’s got to be Greek! No, it’s Geek for modulation tech- nology. Because we have digital bits to transmit over the air, our radio trans- ceiver must convert the digital data to an analog signal. Converting digital data to analog signals is modulation. You can modulate data by using the amplitude, the frequency, or the phase of the signal (all three components of the signal). The term shift keying is sometimes substituted for the term modulation, and we use that term interchangeably here. Tables C-2 and C-3 provide the modulation techniques for 802.11, 802.11a, 802.11b, and 802.11g. Table C-2 Modulation Techniques: 802.11, 802.11b, and 802.11g Spreading Code Modulation Technology Data Rate 2.4 GHz DSSS Barker Code DBPSK 1 Mbps DQPSK 2 Mbps CCK DQPSK 5.5 Mbps DQPSK 11 Mbps 2.4 GHz FHSS Barker Code 2GFSK 1 Mbps 4GFSK 2 Mbps CCK: Complimentary Code Keying DBPSK: Differential Binary Phase Shifting Key DQPSK: Differential Quadrature Phase Shifting Key GFSK: Gaussian Phase Shifting Key Table C-3 Modulation Techniques: 802.11a Modulation Technology Data Rate BPSK 6 Mbps 9 Mbps QPSK 12 Mbps 18 Mbps (continued) 30_575252 appc.qxd 9/3/04 8:40 AM Page 355 356 Part VI: Appendixes Table C-3 (continued) Modulation Technology Data Rate 16QAM 24 Mbps 36 Mbps 64QAM 48 Mbps 54 Mbps BPSK: Binary Phase Shifting Key QPSK: Quadrature Phase Shifting Key QAM: Quadrature Amplitude Modulation The 802.11g standard achieves its high data rates through the use of the Quadrature Amplitude Modulation (QAM) technique. With QAM, there are 12 phase angles with 2 different amplitudes. Eight phase angles have a single amplitude, and four have two amplitudes, resulting in 16 different combina- tions. QAM uses each signal change to represent 4 bits. Consequently, the data rate is four times the baud rate. You may find vendors who support 802.11a, b, and g in a single device. We cover that in Chapter 3. Early in this Appendix, we talk about the pebble creating a splash in the water and waves rippling out until they dissipate. We call this phenomenon attenuation. Figures C-1 and C-2 demonstrate attenuation for outdoors and indoors by using the 802.11b standard as an example. You can see that the signal travels farther outdoors because no walls, floors, or any other obstruc- tions absorb, reflect, refract, or diffract the signal. Figure C-1 shows what you would expect in theory. The access point does not radiate perfect circles at precisely these distances in the real world. In reality, the radiation patterns are more oblong and flatter than a circle. Popular wisdom holds that 802.11b technology, which is generally held to be effec- tive up to about 300 feet, offers better coverage than 802.11a equipment. Theoretical calculations put 802.11a coverage at roughly one-fourth of that range. However, tests show that 802.11a operates with acceptable reliability to well over 200 feet. Moreover, throughout most of its range, including the maximum, it offers a throughput advantage over 802.11b at the same dis- tances. Products based on 802.11a use the 5.8 GHz band. Physics dictate that higher frequencies have a larger path loss (greater spa- tial attenuation) — and therefore, shorter range than lower frequencies — when all other variables are the same. Thus, the 802.11g products have a greater range than 5 GHz products for the same data rate. But the current higher susceptibility to interference in the 2.4 GHz band might affect the range of 802.11g products more in noisy and congested environments than products in the 5 GHz band. 30_575252 appc.qxd 9/3/04 8:40 AM Page 356 357 Appendix C: The Fundamentals of Radio Frequency Currently the 5 GHz band is cleaner from interference than the 2.4 GHz band. However, both bands are unlicensed. With the emergence of products creat- ing interference in the 5 GHz band (at least three cordless telephone prod- ucts workin the 5.8 GHz band, representing the top four 802.11a channels), interference may eventually affect 802.11a products much like 802.11b and g products. At the same time, the 5 GHz band has more bandwidth than the 2.4 GHz band for unlicensed devices, and thus there is more room to avoid such interference. Specific implementation details of different vendors, such as power output, receiver sensitivity, antenna design, and other factors will also affect the range. Another consideration is the number of usable channels. 802.11b (or Wi-Fi) is limited to three clear channels. When you deploy more than three contiguous cells, you likely will find some performance degradation (up to as much as 50 percent) because of co-channel interference (CCI) between cells operating on a given channel. With Wi-Fi, there’s no way to avoid duplication of channel usage more than one cell diameter away. And the closer together the cells, the more interference. Table C-4 shows the various channels in use for 802.11b and g and their frequencies. Channels 1, 6, and 11 are the non- overlapping channels. 1 Mbps: 1800 ft. 2 Mbps: 1476 ft. 5.5 Mbps: 984 ft. 11 Mbps: 590 ft. Note: typical coverage. Your mileage may vary. Figure C-1: Attenu- ation — 802.11b outdoors. 30_575252 appc.qxd 9/3/04 8:40 AM Page 357 [...]... dBm gain (+33 dBm) You can break 33 down into 10 + 10 + 10 + 3 Remember the handy rules that state that +10 is equivalent to 10 times and that +3 is equivalent to 2 times You know that 1 mW is equal to 0 dBm, so this is where you start You can calculate that +33 dBm equals 2 watts, as follows: 1 mW × 10 = 10 mW 10 mW × 10 = 100 mW 100 mW × 10 = 100 0 mW 100 0 mW × 2 = 2000 mW or 2 watts Consider a negative... proved): dB = 10 log10 (P2 / P1) –3 dB = 10 log10 (P2 / 100 ) –0.3 = log10 (P2 / 100 ) 10 – 0.3 = P2 / 100 0.50 = P2 / 100 P2 = 50% Now, you may not feel like using a slide rule (or Google) to calculate dB, but you can use the handy following rules We have the 10s and 3s of RF math as follows: 365 366 Part VI: Appendixes 1 –3 dB = 12 the power in mW ⁄ 2 +3 dB = 2 times the power in mW 3 10 dB = 110 the power... 326 Firewalls For Dummies (Wiley), 167 First Mile Wireless Web site, 258 5ivenetworks Web site, 370 flashing, firmware, 110 Flexible Authentication via Secure Tunneling (FAST), 207 377 378 Wireless Networks For Dummies Fluhrer, Scott (“Weaknesses in the Key Scheduling Algorithm of RC4”), 309 Fluke Networks software OptiView Series II Network Analyzer, 267 WaveRunner, 286 1400 Series Wireless Bridge,... Password Authentication Protocol (PAP), 219 Patras Wireless Network Web site, 242 payload, 215 PCF (Point Coordination Function), 240 PCI (Peripheral Component Interconnect), 105 106 , 107 381 382 Wireless Networks For Dummies PCMCIA (Personal Computer Memory Card International Association), 105 , 346 PCTEL SoftAP software, 302 PCU (Proxim Client Utility), 110, 114–115 pdaconsulting Web site, 120, 298 PEAP... mW × 2 = 2000 mW or 2 watts Consider a negative example Suppose that you have –23 dBm You can break 23 down into 10 + 10 + –3 You know that 1 mW is equal to 0 dBm, so this is where you start You can calculate that –23 dBm equals 5 microwatts as follows: 1 mW /10 = 100 µW 100 µW /10 = 10 µW 10 µW/2 = 5 µW We both bought +16 dBi antennas from Hugh Pepper (http://mywebpages comcast.net/hughpep) If you apply... example, to calculate the loss over 10 feet, the calculation is 10/ 100 × 12.7 = 1.27 dB Over 15 feet, it is 15 /100 × 12.7 = 1.91 dB You can calculate the power ratio associated with the dB value by dividing the dB value by 10 and raising 1 10 to that power For example 1 /100 .127 = 0.746 This means that you’ll have only 75 percent of your input power at the end of the 10- foot run of cable It goes without... DP-311P print server, 108 DWL-G 810 Wireless Bridge, 251 key length, 164 DMZ (Demilitarized Zone), 99, 180, 322–324 DoS (denial of service) attack, 291, 300 DP-311P print server, 108 DQPSK (Differential Quadrature Phase Shifting Key), 355 Drizzle Web site, 309 DSSS (Direct Sequence Spread Spectrum), 49, 238, 355 dstumbler software, 286 Dummies Web site, 6 Dummynet software, 267 DWL-G 810 Wireless Bridge,... + (20 × 0) = 104 .4 (miles) Using Channel 6 across a one-mile path, the loss is 104 .4 dB That’s quite a lot! That formula is pretty intimidating, so we provide Table C-8 as an estimate of path loss for 2.4 GHz networks Table C-8 Distance (In Meters) Free Space Loss Distance (In Feet) Loss (In dB) 100 328.08 80.23 200 656.17 86.25 500 1,640.42 94.21 1,000 3,280.84 100 .23 2,000 6,561.68 106 .25 5,000 16,404.20... 95–96, 100 101 , 129 subnet, 87, 321–325 time zone, configuring, 89 transmission rate, specifying, 93 virtual server setup, 98–99 WEP, 96–98, 164 Wi-Fi compliance, 27 APRS (Automatic Position Reporting System), 153 Arpwatch software, 298–299 arstechnica Web site, 154, 313 Aruba Wireless Networks switch product line, 262 WIDS software, 298 Asanté FriendlyNet AeroLAN AL1511 PCMCIA adapter card, 105 Ash-Tec... www.glenbrook.k12.il.us/gbssci/phys/Class/waves/u10l3b.html ߜ http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html ߜ www.5ivenetworks.com/index2.asp?act=tool Who knows, may be you’ll go on to become a Certified Wireless Network Administrator (www.cwne.com) Index • Numbers & Symbols • @stake (at stake) Web site, 75, 151 3Com Network Director software, 287 3G Americas (organization), 342 3GPP (3rd Generation Partnership Project), 343 5ivenetworks . mW × 10 = 10 mW 10 mW × 10 = 100 mW 100 mW × 10 = 100 0 mW 100 0 mW × 2 = 2000 mW or 2 watts Consider a negative example. Suppose that you have –23 dBm. You can break 23 down into 10 + 10 + –3 demonstrandum: that is, that which was to be proved): dB = 10 log 10 (P2 / P1) –3 dB = 10 log 10 (P2 / 100 ) –0.3 = log 10 (P2 / 100 ) 10 – 0.3 = P2 / 100 0.50 = P2 / 100 P2 = 50% Now, you may not feel like using. you start. You can calculate that –23 dBm equals 5 microwatts as follows: 1 mW /10 = 100 µW 100 µW /10 = 10 µW 10 µW/2 = 5 µW We both bought +16 dBi antennas from Hugh Pepper ( http://mywebpages. comcast.net/hughpep ).