Wireles Networks The Definitive Guide.pdf - Csbdu.in

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15.2.1.4 Coverage and physical installation restrictions Part of the end user requirement is a desired coverage area, and possibly some physical restrictions to go along with it. Physical restrictions, such as a lack of available electrical power and network connections, can be mundane. Some institutions may also require that access points and antennas are hidden; this may be to maintain the physical security of the network infrastructure, or it may be simply to preserve the aesthetic appeal of the building. Some organizations may want to provide coverage outdoors as well, though this is confined to mild climates. Any equipment placed outdoors should be sturdy enough to work there, which is largely a matter of waterproofing and weather resistance. One solution is to install access points inside and run antennas to outdoor locations, but external antenna cables that are long enough are not always available. Outdoor network extensions can be difficult because most 802.11 equipment is not suited to outdoor use, and even if it was, power and Ethernet connections are not readily available outdoors. The best approach to providing outdoor coverage is to keep the access points inside and use external weatherproof antennas on the roof. 15.2.2 The Building It is a great help to get blueprints or floor plans and take a tour of the installation site as early as possible in the process. Based on a walk-through with the floor plans, you can note where coverage must be provided, nearby network and power drops, and any relevant environmental factors. Most importantly, you can correct the blueprints based on any changes made to the structure since the blueprints were drawn. Many minor changes will not be reflected on the blueprints. Different materials have different effects on the radio link. Signal power is most affected by metal, so elevator shafts and air ducts cause significant disruption of communications. Tinted or coated windows frequently cause severe disruption of radio signals. Some buildings may have metal-coated ceilings or significant amounts of metal in the floor. Wood and most glass panes have only small effects, though bulletproof glass can be quite bad. Brick and concrete have effects somewhere between metal and plain untreated glass. To a large extent, though, the expected drop in signal quality due to building construction is a judgment call that improves with experience. During a pre-survey walk-though, also note any potential sources of interference. The 2.4-GHz ISM band is unlicensed, so many types of devices using the band can be deployed without central coordination. Newer cordless phones operate in the 2.4-GHz band, as well as Bluetooth-based devices and a number of other unlicensed radio devices. Depending on the quality and amount of shielding, microwave ovens may also emit enough radiation to disrupt 802.11 communications. If you anticipate a large amount of interference, testing tools called spectrum analyzers can identify the amount of radiation in the wireless LAN frequency band. If your organization does RF testing, it may be necessary to shield any labs where testing is done to avoid interference with the wireless LAN. As a rule of thumb, keep access points at least 25 feet away from any strong interference sources. End user devices also suffer if they are located too close to sources of interference, but only end user communications are interrupted in that case. 15.2.3 The Network There are two components to network planning. The first, physical planning, is largely legwork. In addition to the building map, it helps to obtain a physical network map, if one exists. It is much easier to install wireless LAN hardware when no expensive and time-consuming wiring needs to be done. Knowing the location and contents of all the wiring closets is an important first step. The second component of network planning is the plan for changes to the logical network. How will mobile stations be addressed How will access points be reconnected to their firewall or router 254
15.2.3.1 Network addressing 802.11 provides mobility between access points as long as both access points are part of the same ESS. Roaming works only when mobile stations can transfer from one access point to another and keep the same IP address. All access points of the same ESS must therefore be connected to the same IP subnet so that wireless stations can keep addresses as they associate with different access points. To get the IP space allocated, you will probably need to work with a network administrator. The administrator will want to know how many addresses you need and why. In addition to the planned number of wireless stations, be sure to include an address for each access point and any servers and security devices on the wireless subnet. After tentatively locating access points on a blueprint or sketch of the area, work with the physical network map to plug the access points into the nearest wiring closet. If the access device is a switch with VLAN capability, the access point can probably be placed on the access point backbone VLAN. If not, it may be necessary to patch the access point back to a switch capable of VLAN connections or replace the access device with a small multi-VLAN switch. 15.2.4 Preliminary Plan Based on the floor plans, use the map to come up with a preliminary plan. The preliminary plan is based on the coverage area required and the typical coverage radius from an access point. At this point, detailed radio channel use planning is not yet necessary. The main use of the preliminary plan is to come up with trial access point locations to begin signal quality measurements as part of the detailed site survey. Table 15-2 is based on the best-case coverage radius from a typical omnidirectional antenna. Table 15-2. "Rule-of-thumb" coverage radius for different types of space Type of space Maximum coverage radius Closed office up to 50-60 feet Open office (cubicles) up to 90 feet Hallways and other large rooms up to 150 feet Outdoors up to 300 feet 15.3 The Site Survey After coming up with the preliminary plan, it is time to move on to the heart of the deployment routine: the site survey. Several options exist for performing a site survey. Vendors may provide site surveys to early adopters who agree to be reference accounts. Value-added resellers may also have the skills to perform detailed site surveys; resellers may sell site survey consulting services or use site surveys as a way of coming up with a wireless LAN deployment bid. Some companies that specialize in technical education also offer classes on performing site surveys. Refining the preliminary design is the purpose of the site survey. Radio transmission is complicated, and some things must be done by experiment. All sites will require adjustments to the preliminary design as part of the site survey. In many cases, use of site survey tools can help eliminate access points from a network design and result in substantial cost savings. The major goal of a site survey is to discover any unforeseen interference and redesign the network accordingly. In most cases, interference problems can be repaired by relocating an access point or using a different antenna. The site survey should assess the following: 255
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802.11® Wireless Networks: The Def
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8. Contention-Free Service with the
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ERRATA: Confirmed errors: {47} Figu
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network can often be reduced to a m
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suffer from a number of propagation
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Chapter 2. Overview of 802.11 Netwo
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Figure 2-3. Components of 802.11 LA
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the number of mobile stations that
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system. After all, they are connect
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The services are described in the f
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2.3.1.1 Station services Station se
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Chapter 3. The 802.11 MAC This chap
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3.1.2 The Hidden Node Problem In Et
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sensing hardware for RF-based media
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grab the medium before another type
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3.3.2 Backoff with the DCF After fr
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802.11 MAC frames do not include so
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Retry bit From time to time, frames
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destined to a node on an Ethernet c
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Both RFC 1042 and 802.1h are deriva
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to boost speed over a single hop wi
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stations receiving a PS-Poll to upd
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Chapter 4. 802.11 Framing in Detail
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4.1.3 Addressing and DS Bits The nu
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Figure 4-6. Wireless distribution s
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Figure 4-9. Data frames from the AP
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Control frames arbitrate access to
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Duration The sender of a CTS frame
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the association. Including this ID
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This section presents the fixed fie
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Table 4-4. Interpretation of pollin
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6 Incorrect frame type or subtype r
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Figure 4-33. Supported Rates inform
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DTIM Period This one-byte field ind
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sending Probe Response frames for t
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Figure 4-47. Authentication frames
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Chapter 5. Wired Equivalent Privacy
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WEP on the card, and patents preven
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stations in a service set. Once a s
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2. In spite of vendor claims to the
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Furthermore, 802.11 networks announ
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Figure 6-1. EAP architecture 6.1.1
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6.1.2.4 Type code 4: MD-5 Challenge
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8. Once again, the user types a res
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0011 0000 0100 EAPOL- Encapsulated-
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6.3 802.1x on Wireless LANs 802.1x
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Chapter 7. Management Operations Wh
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Figure 7-2. Passive scanning 7.2.2
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These three facets of the network h
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to connect. While address filtering
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Figure 7-6b shows what happens when
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2. The access point processes the R
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Figure 7-10. PS-Poll frame retrieva
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interval, a frame has also been buf
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Figure 7-15. ATIM effects on power-
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7.6.1 Infrastructure Timing Synchro
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Chapter 8. Contention-Free Service
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the result is a bit strange. A sing
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Figure 8-3. Data+CF-Ack usage This
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8.2.1 Contention-Free End (CF-End)
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CFP DurRemaining This value is the
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The IR PHY 802.11 also includes a s
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• Bluetooth, a short-range wirele
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9.2.2.1 Types of spread spectrum Th
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usually buy some cable to separate
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This figure shows three paths from
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Figure 10-1. Frequency hopping Freq
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Table 10-2. Size of hop sets in eac
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Figure 10-5. 2-level GFSK The rate
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with ISO reference model terminolog
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transmitted using a different encod
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used in the encoding process. There
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power at the channel center. Figure
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Figure 10-19. DBPSK encoding Table
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10.2.3 DS Physical-Layer Convergenc
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10.2.4.3 CS/CCA for the DS PHY 802.
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Barker spreading, as used in the lo
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Figure 10-27. Service field in the
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Figure 10-30. 802.11b transmission
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area. The total aggregate throughpu
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OFDM devices use one wide frequency
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11.1.3 Guard Time With the physical
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11.1.5 Convolution Coding Strictly
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11.2.3 Operating Channels In the U.
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Figure 11-12. Preamble and frame st
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keying (QPSK) to encode 2 bits per
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Chapter 12. Using 802.11 on Windows
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Figure 12-3. Driver installation pr
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Figure 12-7. Addressing and login o
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When troubleshooting connectivity p
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12.1.4 Configuring WEP Despite its
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Long retry limit 4 attempts Passive
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Figure 12-21. Smart-card management
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12.2.2 The Client Manager and Netwo
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Figure 12-30. Power management conf
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Chapter 13. Using 802.11 on Linux O
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Figure 13-1. Linux PCMCIA configura
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Module install directory [/lib/modu
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Table 13-3. Common I/O ports Device
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• Building with debugging informa
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mibattribute=dot11WEPDefaultKey3=01
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17.4 Physical Operations Most wirel
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Chapter 18. The Future, at Least fo
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However, 802.11 offers only link-la
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network provider that leases space
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A.2 Station Management Station mana
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dot11DesiredBSSType (enumerated) Li
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dot11WEPDefaultKeyValue (WEPKeytype
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dot11TransmittedFragmentCount (Coun
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A.4 Physical-Layer Management Physi
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A.4.3 DSSS Table The direct-sequenc
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choosing how to configure TCP/IP. F
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Figure B-4. AirPort status icon In
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Figure B-8. AirPort preferences tab
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Figure B-10. Network name and passw
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B.2.2.2 Configuration of the WAN in
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http://www.msrl.com/airport-gold/ O
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