CompTIA Network+ Training Kit: Wireless Networking
Wireless networking technologies have existed for decades, but it is only in recent years that wireless local area networks (LANs) have been available in the consumer networking market. For many home and small business users, until recently the primary factor preventing them from installing a network was the cabling.
Given a choice between an unsightly external cable installation and a costly and elaborate internal installation, many chose no network at all, or relied instead on stopgap technologies, such as dial-up connections or the “sneakernet.” The advent of inexpensive wireless LANs offers a compromise between cost and performance that is attractive to many people, and wireless LANs are now all but ubiquitous features of homes, offices, restaurants, and coffee shops.
In Chapter 2, you learned about the various types of cables used in data networking, including the properties of those cables and how to install them. In Chapter 3, you learned about the various components that administrators use to build wired networks. In this chapter, you will learn about the wireless equivalents of both of these subjects, including the radio signals that replace the cables and the devices used by wireless computers to communicate with a wired network.
Exam objectives in this chapter:
Objective 2.2: Given a scenario, install and configure a wireless network.
Compatibility (802.11 a/b/g/n)
Objective 3.3: Compare and contrast different wireless standards.
802.11 a/b/g/n standards
Wireless LAN Standards
The wireless LAN equipment on the market today is based on the 802.11 standards published by the Institute of Electrical and Electronics Engineers (IEEE), from the same LAN/MAN Standards Committee—IEEE 802—that publishes the 802.3 Ethernet standards. Because its standards are produced by the same standards body, the 802.11 wireless technology fits neatly into the same layered structure as the Ethernet specifications.
As discussed in Chapter 4, the 802.3 standards divide the data-link layer of the Open Systems Interconnection (OSI) model in two, with the Logical Link Control (LLC) layer on top, and the media access control (MAC) layer on the bottom, as shown in Figure 5-1. The LLC layer is defined in a separate standard: IEEE 802.2. A wireless LAN uses the same LLC layer as an 802.3 Ethernet network, with the 802.11 documents defining the physical layer and MAC layer specifications.
Figure 5-1. IEEE standards in the OSI model.
Building a Wireless Standard
Despite the inclusion of the 802.11 standards in the same company as 802.3, the use of wireless media calls for administrators to make certain fundamental changes in the way they think about a local area network and its use. Some of the most significant differences are as follows:
Unbounded media A wireless network does not have readily observable connections to the network or boundaries beyond which network communication ceases.
Dynamic topology Unlike cabled networks, in which the LAN topology is meticulously planned out before the installation and remains static until the administrator makes deliberate changes, the topology of a wireless LAN changes frequently, if not continuously.
Unprotected media The stations on a wireless network are not protected from outside signals as those on cabled networks are. On a cabled network, outside interference can affect signal quality, but there is no way for the signals from two separate but adjacent networks to be confused. On a wireless network, roving stations can conceivably wander into a different network’s operational perimeter, compromising performance and security.
Unreliable media Unlike with a cabled network, on a wireless network, a protocol cannot work under the assumption that every station on the network receives every packet and can communicate with every other station.
Asymmetric media The propagation of data to all of the stations on a wireless network does not necessarily occur at the same rate. There can be differences in the transmission rates of individual stations that change as one of the devices moves or the environment in which it is operating changes.
Because of these differences, the traditional elements of a data-link layer LAN protocol—the MAC mechanism, the frame format, and the physical layer specifications—have to be designed with different operational criteria in mind. Therefore, in addition to the material required in any physical/data-link layer standard, such as media specifications, signaling techniques, and frame formats, the 802.11 standard includes a list of wireless-specific functions that the document is intended to provide, including the following:
The means by which devices compliant with the standard can participate in a network with other wireless equipment or with wired devices
Procedures for the operation of a compliant device in an environment with multiple overlapping wireless networks
Procedures to support applications with quality of service (QoS) requirements, such as streaming video
Procedures to support asynchronous delivery services
Requirements for authenticating devices and ensuring data confidentiality
Mechanisms by which the wireless networking equipment can satisfy government regulatory standards
IEEE 802.11 Standards
As with the 802.3 Ethernet standard, the IEEE has updated and expanded on the 802.11 specification several times over the years, increasing the maximum transmission speed of the network and altering the frequencies and modulation techniques. The standard publications and their basic specifications are listed in Table 5-1.
Table 5-1. IEEE 802.11 Standards
Transmission Rate (Mbps)
Range (Indoor/Outdoor) (meters)
802.11 – 1997
802.11a – 1999
6 to 54
802.11b – 1999
5.5 to 11
802.11g – 2003
6 to 54
802.11 – 2007
|Republication of the base standard with eight amendments
2.4 and 5
7.2 to 288 (at 20 MHz) 15 to 600 (at 40 MHz)
6 to 54
433 to 867 (at 80 MHz) 867 Mbps to 6.93 Gbps (at 160 MHz)
These standards are described in greater detail in the following sections.
The first version of the IEEE 802.11 standard, published in 1997, defined the specifications for a wireless networking protocol that would meet the following requirements:
The protocol would provide wireless connectivity to automatic machinery, equipment, or stations that require rapid deployment—that is, rapid establishment of communications.
The protocol would be deployable on a global basis.
The protocol would support stations that are fixed, portable, or mobile, within a local area.
This document, in its original form, was known as IEEE 802.11, 1999 Edition, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” This original standard, which was not widely implemented, defined three physical layer specifications: an infrared medium running at 1 Mbps and two types of radio signal modulation using the 2.4-GHz band: Direct-Sequence Spread Spectrum (DSSS) and Frequency-Hopping Spread Spectrum (FHSS), both running at 1 or 2 Mbps.
Despite its continued inclusion in the standard, no one has ever marketed an implementation of the infrared option, and the slow transmission speeds of this early standard made it an unattractive solution, even for users of the original 10-Mbps Ethernet network.
Despite its later designation, the IEEE 802.11b standard represents the next step in the evolution of the original document. All of the lettered publications are amendments to the original 802.11 standard, containing only revisions and additions.
The 802.11b document retains the 2.4-GHz frequency and the DSSS modulation from the original standard, but increases the transmission speed to as much as 11 Mbps. This was, for the first time, a wireless LAN that could run at an acceptable speed for the typical network user. For users of 10-Mbps Ethernet, it theoretically represented an improvement, and even Fast Ethernet users could accept that level of wireless performance. Adoption of the 802.11b standard was quick; many manufacturers released products, and prices fell rapidly.
Although it might appear to be an interim step in the development of 802.11 networking, the IEEE 802.11a amendment actually represented a fundamental change in the technology. One of the ongoing problems with 802.11 networks is the heavy use of the 2.4-GHz band by a variety of consumer products, including cordless telephones and Bluetooth devices. Wireless LAN performance can degrade in such a crowded environment, causing speed reductions or even service interruptions.
The 802.11a amendment calls for the use of the relatively vacant 5-GHz band and a different form of modulation called Orthogonal Frequency-Division Multiplexing (OFDM). The data transfer rate can be as high as 54 Mbps, with fallbacks to 48, 36, 24, 18, 12, 9, and 6 Mbps.
Because of complications in manufacturing, 802.11a products arrived on the market after 802.11b devices had achieved a considerable popularity, and they cost significantly more. The 802.11a technology also developed a reputation—perhaps unfounded—for having a shorter range than 802.11b and for being more susceptible to signal loss from attenuation. Whatever the reason, dedicated 802.11a equipment did not sell well. Later devices supporting both the 802.11a and 802.11b standards eventually came to market, and cross-compatibility between standards soon became a major selling point.
The IEEE 802.11g amendment built on the 802.11b technology by adopting the OFDM modulation from 802.11a while retaining the 2.4-GHz frequency from 802.11b. The result was a new standard that was fully backward compatible with 802.11b equipment but that pushed the maximum transfer rate up to 54 Mbps.
Even before the 802.11g amendment was officially ratified by the IEEE, the public began buying new wireless LAN products at an unprecedented rate. Most of the new products on the market supported both the “draft” 802.11g and 802.11b standards, and some added support for the 802.11a standard as well.
IEEE 802.11 – 2007
As mentioned earlier, the lettered documents published by the 802.11 working group are amendments to the previously released standard. These amendments consist only of the new material and the sections that have been changed, and they frequently contain extensive strikeouts and rewrites. In addition, some of the amendments contain revisions to previous amendments. Understanding the changes made to the original standard requires a careful study or an intimate knowledge of the original document and all of the previously released amendments.
To simplify understanding and further development, the working group published a composite standard in 2007 that incorporated all of the amendments released up to that time. This document incorporates all the modifications and additions from the 802.11a, b, d, e, g, h, i, and j amendments into the original base standard and is published as “IEEE Standard 802.11 – 2007: IEEE Standard for Information technology – Telecommunications and information exchange between systems – Local and metropolitan area networks – Specific requirements – Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.”
As with Ethernet, administrators and users of wireless LANs are constantly asking for faster networks. However, the development of 802.11b/802.11g technology had reached a point at which a fundamental change was needed to improve performance. The IEEE 802.11n amendment introduces several modifications to the technology, including the potential doubling of channel widths and the addition of the 5-GHz frequency band from 802.11a to the standard 2.4-GHz band from 802.11b/g. 802.11n also includes two innovations that, combined with these other improvements, can push network transmission speeds well beyond the 54 Mbps realized by 802.11g to levels as high as 600 Mbps. These two innovations—MIMO and frame aggregation—are described in the following sections.
Multiple-Input Multiple-Output (MIMO) is a physical layer enhancement that enables wireless devices to multiplex signals over a single channel simultaneously, by using a technique called Spatial Division Multiplexing (SDM). Each 802.11n device has an array of transmit and receive antennae and a transceiver capable of sending and receiving separate signals by using separate frequencies.
802.11n designations describing the MIMO capabilities of each device use the format axb:c, where a is the number of transmit antennae in the device, b is the number of receive antennae, and c is the number of data streams that the radio in the device supports. The maximum configuration defined by the standard is 4x4:4, indicating that a device has four transmit and four receive antennae and can send or receive on four channels at once.
802.11n devices are available in various configurations from 2x2:2 up to the maximum. The more antennae and the larger the number of simultaneous signals, the greater the throughput the device can achieve, and of course, the higher its price. As with any networking technology, though, both of the devices involved in a transaction must have the same capabilities to achieve the best possible performance. MIMO therefore adds another compatibility factor to the process of implementing a wireless network. It makes no sense to purchase a 4x4:4 access point if all of your workstations are equipped with 2x2:2 network interface adapters.
In addition to MIMO, the 802.11n standard also enables devices to double their channel widths from 20 megahertz (MHz) to 40 MHz, nearly doubling the data transfer rate in the process. However, in the 2.4-GHz band, this practice only exacerbates the existing signal crowding problem. For more information, see Frequencies and Channels, later in this chapter.
The second innovation defined in the 802.11n document is a MAC layer technique called frame aggregation. In wireless networking, physical layer improvements can only do so much, because the control overhead is so high. In addition to the data-link layer frame, there are acknowledgment messages, spaces between frames, and radio communication transmissions, which in some circumstances can add up to more data than is carried in the payload.
Frame aggregation is a technique that combines the payload data from several frames into one large frame, thus reducing the amount of overhead and increasing the information throughput of the network.
The 802.11n standard retains the fallback capabilities of the previous documents. Depending on a multitude of conditions, devices operating at peak speeds of 600 Mbps (using 4x4:4 MIMO and a 40-MHz channel) can drop down to successively lower rates and then speed up again when environmental conditions change. In addition, most 802.11n devices also support 802.11g and 802.11b, with some adding 802.11a as well.
Transfer rates for 802.11n networks are dependent on the capabilities of the equipment, the configuration settings selected by the administrator, and the usual environmental conditions that affect all wireless LANs. To calculate transfer rates, the 802.11n standard defines lists containing dozens of Modulation and Coding Schemes (MCSs), which are indexed combinations of factors including the following:
Modulation type The modulation scheme used for the subcarriers created by OFDM
Spatial streams The number of MIMO streams (1 to 4)
Channel width 20 MHz or 40 MHz
Guard interval The space inserted between transmissions to prevent interference (400 or 800 nanoseconds)
Coding rate The proportion of the data stream that is useful
For example, to achieve the maximum possible transmission rate of 600 Mbps (MCS index 31), an 802.11 network must have all of the following attributes:
Modulation type 64-Quadrature Amplitude Modulation (64-QAM)
Spatial streams 4
Channel width 40 MHz
Guard interval 400 nanoseconds
Coding rate 5/6
Modifying any of these factors will reduce the data transfer rate to some degree. For example, each spatial stream accounts for 25 percent of the data transfer rate, and dropping to a 20 MHz channel width will reduce the rate by a little more than half.
As with 802.11g, equipment manufacturers released “draft” 802.11n devices to market long before the standard was officially ratified. In 2009, though, the standard was officially ratified, and products conforming to its specifications should be interoperable. However, because many of the attributes defined in the standard are not required, administrators must carefully examine product specifications to ensure interoperability.
The IEEE 802.11y amendment defines a modification to the 802.11a standard that would enable licensed operators to construct high-powered wireless networks by using the 3.7-GHz band. By transmitting with up to 20 watts of power, 802.11y networks are expected to achieve ranges of 5 kilometers or more.
IEEE 802.11ac is a draft of a standard currently under development that is expected to represent the next iteration in wireless LAN technology. Expanding on the technology of 802.11n, 802.11ac devices will be able to support up to eight MIMO streams (instead of four) and channels 80 or 160 MHz wide, for theoretical transfer rates as high as 6.93 gigabytes per second (Gbps). The standard also calls for a new type of modulation technology called 256-Quadrature Amplitude Modulation (256-QAM).
The term Wi-Fi has entered the daily lexicon of mobile computer users, as increasing numbers of businesses and public places provide wireless LANs for the use of their customers. Despite its interchangeability with the term “802.11” in common parlance, “Wi-Fi” is a privately owned trademark; it is not a name sanctioned by the IEEE for 802.11 networks or equipment.
The name “Wi-Fi” is owned by an organization of hardware and software manufacturers called the Wi-Fi Alliance. The group operates an interoperability certification program for wireless LAN equipment and allows certified products to carry a special logo indicating their participation. Not all manufacturers submit their products for testing, however, which does not necessarily mean that they are not compatible.