无线网络权威指南：802.11详解与部署

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"《802.11无线网络：权威指南》是一本由Matthew Gast编写的实用手册，专为网络管理员、架构师和安全专业人员设计。该书于2002年4月由O'Reilly出版社出版，其ISBN为0-596-00183-5，共464页。这本书的核心内容涵盖了802.11无线网络技术的各个方面，从技术家族的历史背景到具体标准的细节，再到实际部署、监控和故障排除的全过程。作为网络专业人士，理解无线局域网（WLAN）的特性、局限性和风险至关重要。作者强调，这本书是分析和部署802.11无线网络的必备工具，它提供了全面的视角，帮助读者从底层规格解析到实际应用的各个环节。第1章介绍了无线网络的基本概念，包括为什么选择无线技术以及无线网络与其他网络类型的区别。第1.1节深入探讨了无线技术的优势，如移动性、灵活性和节省布线成本等，而第1.2节则解释了无线网络如何融入现有的基础设施。第2章是对802.11网络的概述，它首先展示了IEEE 802网络技术的家族树，帮助读者理解不同版本的802.11标准之间的关系和演进。第2.2节专门讨论了802.11名称的命名规则和标准分类，这对于理解每个标准的功能和适用场景极为关键。本书的后续章节将详细讲解802.11的物理层、介质访问控制（MAC）层、网络层和安全性，同时提供如何设计、规划和实施无线网络的实战指导。此外，书中还涉及了无线网络的监控工具和技术，以及如何处理常见的问题和故障。对于那些希望在无线网络领域取得成功并确保网络安全的专业人士来说，《802.11无线网络：权威指南》是一份不可或缺的参考资料，它提供了丰富的理论知识和实践经验，帮助读者在无线网络的世界里游刃有余。"

Internet access past the limitations for DSL into communities where high-speed Internet

access has been only a dream. Community networks have been particularly successful in

out-of-the way places that are too rugged for traditional wireline approaches.

Like all networks, wireless networks transmit data over a network medium. The medium

is a form of electromagnetic radiation.

[3]

To be well-suited for use on mobile networks,

the medium must be able to cover a wide area so clients can move throughout a coverage

area. The two media that have seen the widest use in local-area applications are infrared

light and radio waves. Most portable PCs sold now have infrared ports that can make

quick connections to printers and other peripherals. However, infrared light has

limitations; it is easily blocked by walls, partitions, and other office construction. Radio

waves can penetrate most office obstructions and offer a wider coverage range. It is no

surprise that most, if not all, 802.11 products on the market use the radio wave physical

layer.

[3]

Laser light is also used by some wireless networking applications, but the

extreme focus of a laser beam makes it suited only for applications in which

the ends are stationary. "Fixed wireless" applications, in which lasers replace

other access technology such as leased telephone circuits, are a common

application.

1.1.1 Radio Spectrum: The Key Resource

Wireless devices are constrained to operate in a certain frequency band. Each band has an

associated bandwidth, which is simply the amount of frequency space in the band.

Bandwidth has acquired a connotation of being a measure of the data capacity of a link.

A great deal of mathematics, information theory, and signal processing can be used to

show that higher-bandwidth slices can be used to transmit more information. As an

example, an analog mobile telephony channel requires a 20-kHz bandwidth. TV signals

are vastly more complex and have a correspondingly larger bandwidth of 6 MHz.

The use of a radio spectrum is rigorously controlled by regulatory authorities through

licensing processes. In the U.S., regulation is done by the Federal Communications

Commission (FCC). Many FCC rules are adopted by other countries throughout the

Americas. European allocation is performed by CEPT's European Radiocommunications

Office (ERO). Other allocation work is done by the International Telecommunications

Union (ITU). To prevent overlapping uses of the radio waves, frequency is allocated in

bands, which are simply ranges of frequencies available to specified applications. Table

1-1 lists some common frequency bands used in the U.S.

Table 1-1. Common U.S. frequency bands

Band Frequency range

UHF ISM 902-928 MHz

S-Band 2-4 GHz

S-Band ISM 2.4-2.5 GHz

Table 1-1. Common U.S. frequency bands

Band Frequency range

C-Band 4-8 GHz

C-Band satellite downlink 3.7-4.2 GHz

C-Band Radar (weather) 5.25-5.925 GHz

C-Band ISM 5.725-5.875 GHz

C-Band satellite uplink 5.925-6.425 GHz

X-Band 8-12 GHz

X-Band Radar (police/weather) 8.5-10.55 GHz

Ku-Band 12-18 GHz

Ku-Band Radar (police)

13.4-14 GHz

15.7-17.7 GHz

1.1.1.1 The ISM bands

In Table 1-1, there are three bands labeled ISM, which is an abbreviation for industrial,

scientific, and medical. ISM bands are set aside for equipment that, broadly speaking, is

related to industrial or scientific processes or is used by medical equipment. Perhaps the

most familiar ISM-band device is the microwave oven, which operates in the 2.4-GHz

ISM band because electromagnetic radiation at that frequency is particularly effective for

heating water.

I pay special attention to the ISM bands because that's where 802.11 devices operate. The

more common 802.11b devices operate in S-band ISM. The ISM bands are generally

license-free, provided that devices are low-power. How much sense does it make to

require a license for microwave ovens, after all? Likewise, you don't need a license to set

up and operate a wireless network.

1.1.2 The Limits of Wireless Networking

Wireless networks do not replace fixed networks. The main advantage of mobility is that

the network user is moving. Servers and other data center equipment must access data,

but the physical location of the server is irrelevant. As long as the servers do not move,

they may as well be connected to wires that do not move.

The speed of wireless networks is constrained by the available bandwidth. Information

theory can be used to deduce the upper limit on the speed of a network. Unless the

regulatory authorities are willing to make the unlicensed spectrum bands bigger, there is

an upper limit on the speed of wireless networks. Wireless-network hardware tends to be

slower than wired hardware. Unlike the 10-GB Ethernet standard, wireless-network

standards must carefully validate received frames to guard against loss due to the

unreliability of the wireless medium.

Using radio waves as the network medium poses several challenges. Specifications for

wired networks are designed so that a network will work as long as it respects the

specifications. Radio waves can suffer from a number of propagation problems that may

interrupt the radio link, such as multipath interference and shadows.

Security on any network is a prime concern. On wireless networks, it is often a critical

concern because the network transmissions are available to anyone within range of the

transmitter with the appropriate antenna. On a wired network, the signals stay in the wires

and can be protected by strong physical-access control (locks on the doors of wiring

closets, and so on). On a wireless network, sniffing is much easier because the radio

transmissions are designed to be processed by any receiver within range. Furthermore,

wireless networks tend to have fuzzy boundaries. A corporate wireless network may

extend outside the building. It is quite possible that a parked car across the street could be

receiving the signals from your network. As an experiment on one of my trips to San

Francisco, I turned on my laptop to count the number of wireless networks near a major

highway outside the city. I found eight without expending any significant effort. A

significantly more motivated investigator would undoubtedly have discovered many

more networks by using a much more sensitive antenna mounted outside the steel shell of

the car.

1.2 A Network by Any Other Name...

Wireless networking is a hot industry segment. Several wireless technologies have been

targeted primarily for data transmission. Bluetooth is a standard used to build small

networks between peripherals: a form of "wireless wires," if you will. Most people in the

industry are familiar with the hype surrounding Bluetooth. I haven't met many people

who have used devices based on the Bluetooth specification.

Third-generation (3G) mobile telephony networks are also a familiar source of hype.

They promise data rates of megabits per cell, as well as the "always on" connections that

have proven to be quite valuable to DSL and cable modem customers. In spite of the

hype and press from 3G equipment vendors, the rollout of commercial 3G services has

been continually pushed back.

In contrast to Bluetooth and 3G, equipment based on the IEEE 802.11 standard has been

an astounding success. While Bluetooth and 3G may be successful in the future, 802.11 is

a success now. Apple initiated the pricing moves that caused the market for 802.11

equipment to explode in 1999. Price erosion made the equipment affordable and started

the growth that continues today.

This is a book about 802.11 networks. 802.11 goes by a variety of names, depending on

who is talking about it. Some people call 802.11 wireless Ethernet, to emphasize its

shared lineage with the traditional wired Ethernet (802.3). More recently, the Wireless

Ethernet Compatibility Alliance (WECA) has been pushing its Wi-Fi ("wireless fidelity")

certification program.

[4]

Any 802.11 vendor can have its products tested for

interoperability. Equipment that passes the test suite can use the Wi-Fi mark. For newer

Chapter 2. Overview of 802.11 Networks

Before studying the details of anything, it often helps to get a general "lay of the land." A

basic introduction is often necessary when studying networking topics because the

number of acronyms can be overwhelming. Unfortunately, 802.11 takes acronyms to new

heights, which makes the introduction that much more important. To understand 802.11

on anything more than a superficial basis, you must get comfortable with some esoteric

terminology and a herd of three-letter acronyms. This chapter is the glue that binds the

entire book together. Read it for a basic understanding of 802.11, the concepts that will

likely be important to users, and how the protocol is designed to provide an experience as

much like Ethernet as possible. After that, move on to the low-level protocol details or

deployment, depending on your interests and needs.

Part of the reason why this introduction is important is because it introduces the

acronyms used throughout the book. With 802.11, the introduction serves another

important purpose. 802.11 is superficially similar to Ethernet. Understanding the

background of Ethernet helps slightly with 802.11, but there is a host of additional

background needed to appreciate how 802.11 adapts traditional Ethernet technology to a

wireless world. To account for the differences between wired networks and the wireless

media used by 802.11, a number of additional management features were added. At the

heart of 802.11 is a white lie about the meaning of media access control (MAC). Wireless

network interface cards are assigned 48-bit MAC addresses, and, for all practical

purposes, they look like Ethernet network interface cards. In fact, the MAC address

assignment is done from the same address pool so that 802.11 cards have unique

addresses even when deployed into a network with wired Ethernet stations.

To outside network devices, these MAC addresses appear to be fixed, just as in other

IEEE 802 networks; 802.11 MAC addresses go into ARP tables alongside Ethernet

addresses, use the same set of vendor prefixes, and are otherwise indistinguishable from

Ethernet addresses. The devices that comprise an 802.11 network (access points and other

802.11 devices) know better. There are many differences between an 802.11 device and

an Ethernet device, but the most obvious is that 802.11 devices are mobile; they can

easily move from one part of the network to another. The 802.11 devices on your

network understand this and deliver frames to the current location of the mobile station.

2.1 IEEE 802 Network Technology Family Tree

802.11 is a member of the IEEE 802 family, which is a series of specifications for local

area network (LAN) technologies. Figure 2-1 shows the relationship between the various

components of the 802 family and their place in the OSI model.

Figure 2-1. The IEEE 802 family and its relation to the OSI model

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