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Wireless LANs: Extending the Reach of a LAN

Chapter Description

This chapter describes the reasons for extending the reach of a LAN and the methods that can be used to do so, with a focus on RF wireless access.

Exploring Wireless Networking

Wireless networking technology has developed like most new technologies; business needs drive technology developments, which in turn drive new business needs, which in turn drive new technology developments. To keep this cycle from spinning out of control, several organizations have stepped forward to establish WLAN standards and certifications. This lesson describes the trends and standards that impact WLAN development.

The Business Case for WLAN Service

Productivity is no longer restricted to a fixed work location or a defined time period. People now expect to be connected at any time and place, from the office to the airport or even the home. Traveling employees used to be restricted to pay phones for checking messages and returning a few phone calls between flights. Now employees can check e-mail, voice mail, and the web status of products on personal digital assistants (PDA) while walking to a flight. Figure 3-1 shows the trends involved with wireless networking and mobility.

Figure 3-1

Figure 3-1 Wireless Market Trends

Even at home, people have changed the way they live and learn. The Internet has become a standard in homes, right along with TV and phone service. Even the method of accessing the Internet has quickly moved from temporary modem dialup service to dedicated digital subscriber line (DSL) or cable service, which is always connected and is faster than dialup. In 2005, users of PCs purchased more Wi-Fi–enabled mobile laptops (i.e., products that are based on the IEEE 802.11 standards) than fixed-location desktops.

The most tangible benefit of wireless is the cost reduction. Two situations illustrate cost savings. First, with a wireless infrastructure already in place, savings are realized when moving a person from one location in an office to another, when reorganizing a lab, or when moving from temporary locations or project sites. On average, the IT cost of moving an employee from one location to another where wiring changes are required is $375. For the business case, assume that 15 percent of the staff is moved every year. With a staff of 800, the savings represented by using wireless would be $45,000. The second situation to consider is when a company moves into a new building that does not have a wired infrastructure. In this case, the savings from wireless is even more noticeable because running cables through walls, ceilings, and floors is a labor-intensive process.

Finally, another advantage of using a WLAN is the increase in employee satisfaction brought on by having mobility in their working environment, leading to less turnover and the cost savings of not hiring as many new employees. Employee satisfaction also results in better customer support, which can't be easily quantified, but is a major benefit.

Besides the increase in productivity, WLAN also means better quality in daily work (better responsiveness to customers, a better can-do attitude from employees, and so on) and other benefits that cannot be easily measured.

Differences Between WLANs and LANs

Although WLANs and LANs both provide connectivity between the end users, they have some key differences that include both physical and logical differences between the topologies. In WLANs, radio frequencies are used as the physical layer of the network. Differences also exist in the way the frame is formatted and in the transmission methods, detailed as follows:

  • WLANs use carrier sense multiple access with collision avoidance (CSMA/CA) instead of carrier sense multiple access collision detect (CSMA/CD), which is used by Ethernet LANs. Collision detection is not possible in WLANs, because a sending station cannot receive at the same time that it transmits and, therefore, cannot detect a collision. Instead, WLANs use the Ready To Send (RTS) and Clear To Send (CTS) protocols to avoid collisions.
  • WLANs use a different frame format than wired Ethernet LANs use. WLANs require additional information in the Layer 2 header of the frame.

Radio waves cause problems not found in LANs, such as the following:

  • Connectivity issues occur because of coverage problems, RF transmission, multipath distortion, and interference from other wireless services or other WLANs.
  • Privacy issues occur because radio frequencies can reach outside the facility.

In WLANs, mobile clients connect to the network through an access point, which is the equivalent of a wired Ethernet hub. These connections are characterized as follows:

  • There is no physical connection to the network.
  • The mobile devices are often battery-powered, as opposed to plugged-in LAN devices.

WLANs must meet country-specific RF regulations. The aim of standardization is to make WLANs available worldwide. Because WLANs use radio frequencies, they must follow country-specific regulations of RF power and frequencies. This requirement does not apply to wired LANs.

Radio Frequency Transmission

Radio frequencies range from the AM radio band to frequencies used by cell phones. This section identifies the characteristics of the radio frequency transmissions used by WLANs.

Radio frequencies are radiated into the air by antennas that create radio waves. When radio waves are propagated through objects, they might be absorbed, scattered, or reflected. This absorption, scattering, and reflection can cause areas of low signal strength or low signal quality. Understanding this phenomena and the causes is important when you are building and designing WLAN networks.

The transmission of radio waves is influenced by the following factors:

  • Reflection: Occurs when RF waves bounce off objects (for example, metal or glass surfaces)
  • Scattering: Occurs when RF waves strike an uneven surface (for example, a rough surface) and are reflected in many directions
  • Absorption: Occurs when RF waves are absorbed by objects (for example, walls)

The following rules apply for data transmission over radio waves:

  1. Higher data rates have a shorter range because the receiver requires a stronger signal with a better signal-to-noise ratio (SNR) to retrieve the information.
  2. Higher transmit power results in a greater range. To double the range, the power has to be increased by a factor of four.
  3. Higher data rates require more bandwidth. Increased bandwidth is possible with higher frequencies or more complex modulation.
  4. Higher frequencies have a shorter transmission range because they have higher degradation and absorption. This problem can be addressed by more efficient antennas.

Organizations That Standardize WLANs

Several organizations have stepped forward to establish WLAN standards and certifications. This topic identifies the organizations that define WLAN standards.

Regulatory agencies control the use of the RF bands. With the opening of the 900-MHz Industrial, Scientific, and Medical (ISM) band in 1985, the development of WLANs started. New transmissions, modulations, and frequencies must be approved by regulatory agencies. A worldwide consensus is required. Regulatory agencies include the Federal Communications Commission (FCC) for the United States (http://www.fcc.gov) and the European Telecommunications Standards Institute (ETSI) for Europe (http://www.etsi.org).

The Institute of Electrical and Electronic Engineers (IEEE) defines standards. IEEE 802.11 is part of the 802 networking standardization process. You can download ratified standards from the IEEE website (http://standards.ieee.org/getieee802).

The Wi-Fi Alliance offers certification for interoperability between vendors of 802.11 products. Certification provides a comfort zone for purchasers of these products. It also helps market WLAN technology by promoting interoperability between vendors. Certification includes all three 802.11 RF technologies and Wi-Fi Protected Access (WPA), a security model released in 2003 and ratified in 2004, based on the new security standard IEEE 802.11i, which was ratified in 2004. The Wi-Fi Alliance promotes and influences WLAN standards. A list of ratified products can be found on the Wi-Fi website (http://www.wi-fi.org).

ITU-R Local FCC Wireless

Several unlicensed RF bands exist. Figure 3-2 shows an overview of the FCC bands and where the wireless bands are located.

Figure 3-2

Figure 3-2 Wireless Bands

Three unlicensed bands exist: 900 MHz, 2.4 GHz, and 5.7 GHz. The 900-MHz and 2.4-GHz bands are referred to as the ISM bands, and the 5-GHz band is commonly referred to as the Unlicensed National Information Infrastructure (UNII) band.

Frequencies for these bands are as follows:

  • 900-MHz band: 902 MHz to 928 MHz.
  • 2.4-GHz band: 2.400 GHz to 2.483 GHz (in Japan, this band extends to 2.495 GHz.)
  • 5-GHz band: 5.150 GHz to 5.350 GHz, 5.725 GHz to 5.825 GHz, with some countries supporting middle bands between 5.350 GHz and 5.725 GHz. Not all countries permit IEEE 802.11a, and the available spectrum varies widely. The list of countries that permit 802.11a is changing.

Figure 3-2 shows WLAN frequencies. Next to the WLAN frequencies in the spectrum are other wireless services such as cellular phones and Narrowband Personal Communication Services (NPCS). The frequencies used for WLAN are ISM bands.

A license is not required to operate wireless equipment on unlicensed frequency bands. However, no user has exclusive use of any frequency. For example, the 2.4-GHz band is used for WLANs, video transmitters, Bluetooth, microwave ovens, and portable phones. Unlicensed frequency bands offer best-effort use, and interference and degradation are possible.

Even though these frequency bands do not require a license to operate equipment, they still are subject to the local country's code regulations inside the frequencies to regulate areas such as transmitter power, antenna gain (which increases the effective power), and the sum of transmitter loss, cable loss, and antenna gain.

Effective Isotropic Radiated Power (EIRP) is the final unit of measurement monitored by local regulatory agencies. Therefore, caution should be used when attempting to replace a component of a WLAN, for example, when adding or upgrading an antenna to increase range. The possible result could be a WLAN that is illegal under local codes. The equation for calculating EIRP is as follows:

EIRP = transmitter power + antenna gain - cable loss

802.11 Standards Comparison

IEEE standards define the physical layer and the Media Access Control (MAC) sublayer of the data link layer of the OSI model. The original 802.11 wireless standard was completed in June, 1997. It was revised in 1999 to create IEEE 802.11a/b and then reaffirmed in 2003 as IEEE 802.11g.

By design, the standard does not address the upper layers of the OSI model. IEEE 802.11b was defined using Direct Sequence Spread Spectrum (DSSS). DSSS uses just one channel that spreads the data across all frequencies defined by that channel. Table 3-1 shows the different standards and how they compare.

Table 3-1. 802.11 Standards





Frequency band

2.4 GHz

5 GHz

2.4 GHz

Number of channels


Up to 23



Direct Sequence Spread Spectrum (DSSS)

Orthogonal Frequency Division Multiplexing (OFDM)

Direct Sequence Spread Spectrum (DSSS)

Orthogonal Frequency Division Multiplexing (OFDM)

Data Rates in Mbps

1, 2, 5.5, 11

6, 9, 12, 18, 24, 36, 48, 54

1, 2, 5.5, 11

6, 9, 12, 18, 24, 36, 48, 54

IEEE 802.11 divided the 2.4-GHz ISM band into 14 channels, but local regulatory agencies such as the FCC designate which channels are allowed, such as channels 1 through 11 in the United States. Each channel in the 2.4 GHz ISM band is 22 MHz wide with 5 MHz separation, resulting in overlap with channels before or after a defined channel. Therefore, a separation of 5 channels is needed to ensure unique nonoverlapping channels. Given the FCC example of 11 channels, the maximum of nonoverlapping frequencies are channels 1, 6, and 11.

Recall that wireless uses half-duplex communication, so the basic throughput is only about half of the data rate. Because of this, the IEEE 802.11b main development goal is to achieve higher data rates within the 2.4-GHz ISM band to continue to increase the Wi-Fi consumer market and encourage consumer acceptance of Wi-Fi.

802.11b defined the usage of DSSS with newer encoding or modulation of Complementary Code Keying (CCK) for higher data rates of 5.5 and 11 Mbps (Barker Coding of 1 and 2 Mbps). 802.11b still uses the same 2.4-GHz ISM band and is backward compatible with prior 802.11 and its associated data rates of 1 and 2 Mbps.

The year that the 802.11b standard was adopted, IEEE developed another standard known as 802.11a. This standard was motivated by the goal of increasing data rates by using a different OFDM spread spectrum and modulation technology and using the less crowded frequency of 5 GHz UNII. The 2.4-GHz ISM band was widely used for all WLAN devices, such as Bluetooth, cordless phones, monitors, video, and home gaming consoles, and it also happens to be the same frequency used by microwave ovens. 802.11a was not as widely known because materials for chip manufacturing were less readily available and initially resulted in higher cost. Most applications satisfied the requirements following the cheaper and more accessible standards of 802.11b.

A more recent development by IEEE maintains usage of the 802.11 MAC and obtains higher data rates in the 2.4-GHz ISM band. The IEEE 802.11g amendment uses the newer OFDM from 802.11a for higher speeds, yet is backward compatible with 802.11b using DSSS, which was already using the same ISM frequency band. DSSS data rates of 1, 2, 5.5, and 11 Mbps are supported, as are OFDM data rates of 6, 9, 12, 18, 24, 48, and 54 Mbps. IEEE requires only mandatory data rates of OFDM using 6, 12, and 24 Mbps, regardless whether it is 802.11a or 802.11g OFDM.

Wi-Fi Certification

Even after the 802.11 standards were established, a need to ensure interoperability among 802.11 products still existed. The Wi-Fi Alliance is a global, nonprofit industry trade association devoted to promoting the growth and acceptance of wireless LANs. One of the primary benefits of the Wi-Fi Alliance is to ensure interoperability among 802.11 products offered by various vendors by providing certification. Figure 3-3 shows an example of the Wi-Fi Alliance certification logo.

Figure 3-3

Figure 3-3 Wi-Fi Alliance Certification Logo

Certified vendor interoperability provides a comfort zone for purchasers. Certification includes all three IEEE 802.11 RF technologies, as well as an early adoption of pending IEEE drafts, such as one that addresses security. The Wi-Fi Alliance adapted the IEEE 802.11i draft security as WPA and then revised it to WPA2 after final release of IEEE 802.11i.

Summary of Exploring Wireless Networking

The following summarizes the key points that were discussed in this section:

  • People now expect to be connected at any time at any place. However, the most tangible benefit of wireless is cost reduction.
  • Both WLANs and LANS use CSMA. However, WLANs use CA, whereas LANs used CD.
  • Radio frequencies are radiated into the air by antennas where they are affected by:
    • Reflection
    • Scattering
    • Absorption
  • The IEEE defines the 802.11 standards.
  • The ITU-R local FCC wireless bands are unlicensed.
  • The 802.11 standards are a set of standards that define the frequencies and radio bands for WLANs.
  • One of the primary benefits of the Wi-Fi Alliance is to ensure interoperability among 802.11 products.
3. Understanding WLAN Security | Next Section Previous Section

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