Wireless LANs (Networking)

A wireless local-area network (LAN) is a data communications system implemented as an extension—or as an alternative—to a wired LAN. Using a variety of technologies including narrowband radio, spread spectrum, and infrared.6 Wireless LANs transmit and receive data through the air, minimizing the need for wired connections.


Wireless LANs have gained strong popularity in a number of vertical markets, including health care, retail, manufacturing, warehousing, and academia. These industries have profited from the productivity gains of using hand-held terminals and notebook computers to transmit real-time information to centralized hosts for processing. The following are among the many applications of wireless LANs:

■ Hospital staff are more productive because hand-held or notebook computers with wireless LAN capability deliver patient information, regardless of their location.

■ Consulting or accounting audit teams, small workgroups, or temporary office staff can increase productivity with quick network setup.

■ Network managers in dynamic environments can minimize the overhead caused by moves, extensions to networks, and other changes with wireless LANs.

■ Network managers can quickly install networked computers in older buildings with wireless technology, without having to upgrade existing wiring or install new wiring.

■ Warehouse workers can use wireless LANs to exchange information with central databases, thereby increasing productivity.

■ Branch office workers can minimize setup requirements by installing preconfigured wireless LANs.


With wireless LANs, users can access shared information without looking for a place to plug in, and corporate managers can set up or augment networks without installing or moving wires. Wireless LANs offer the following advantages:

MOBILITY Wireless LANs can provide users with access to real-time information anywhere in an organization. This mobility improves productivity.

INSTALLATION SPEED AND SIMPLICITY Installing a wireless LAN can be fast and easy, since it eliminates the need to pull cable through walls and ceilings.

INSTALLATION FLEXIBILITY Wireless technology allows users to go where wires cannot go.

REDUCED COST-OF-OWNERSHIP While the initial investment required for wireless LAN hardware can be higher than the cost of wired LAN hardware, overall installation expenses and life-cycle costs can be significantly lower. Long-term cost benefits are greatest in dynamic environments requiring frequent moves, adds, and changes.

SCALABILITY Wireless LANs can be configured in a variety of topologies to meet the needs of specific applications and installations. They can be expanded with the addition of access points and extension points to accommodate virtually any number of users.


There are several technologies to choose from when selecting a wireless LAN solution, each with its own advantages and limitations.

NARROWBAND A narrowband radio system transmits and receives user information on a specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as possible to pass the information. Undesirable cross-talk between communications channels is avoided by carefully coordinating different users on different channel frequencies.

A radio frequency is much like a private telephone line in that people on one line cannot listen to the calls made on other lines. In a radio system, privacy and noninterference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio signals except the ones on its designated frequency.

SPREAD SPECTRUM Most wireless LAN systems use spread-spectrum technology, a wideband radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband transmission, but the trade off produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to the right frequency, a spread-spectrum signal looks like background noise. There are two types of spread-spectrum radio: frequency hopping and direct sequence.

FREQUENCY HOPPING Frequency-hopping spread spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.

DIRECT SEQUENCE Direct-sequence spread spectrum (DSSS) generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip (or chipping code). The longer the chip, the greater the probability that the original data can be recovered. Of course, this method requires more bandwidth. If one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low-power wideband noise and is ignored.

INFRARED Infrared (IR) systems use very high frequencies just below visible light in the electromagnetic spectrum. Like light, IR cannot penetrate opaque objects; to reach the target system, the waves carrying data are sent in either directed (line-of-sight) or diffuse (reflected) fashion. Inexpensive directed systems provide a very limited range of not more than three feet. They are typically used for personal-area networks, but are occasionally used in specific wireless LAN applications. High-performance, directed IR is impractical for mobile users and is therefore used only to implement fixed sub-networks. Diffuse IR wireless LAN systems do not require line-of-sight, but cells are limited to individual rooms.


As noted, wireless LANs use electromagnetic waves (radio or infrared) to communicate information from one point to another without relying on a wired connection. Radio waves are often referred to as radio carriers because they simply perform the function of delivering energy to a remote receiver. The data being transmitted is superimposed on the radio carrier so it can be accurately extracted at the receiving end. This process is generally referred to as carrier modulation. Once data is modulated onto the radio carrier, the radio signal occupies more than a single frequency since the frequency or bit rate of the modulating information adds to the carrier.

Multiple radio carriers can exist in the same space at the same time without interfering with each other if the radio waves are transmitted on different radio frequencies. To extract data, a radio receiver tunes into one radio frequency while rejecting all other frequencies.

In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure. A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point (or the antenna attached to the access point) is usually mounted high but may be mounted essentially anywhere that is practical as long as the desired radio coverage is obtained.

Users access the wireless LAN through wireless LAN adapters, which provide an interface between the client network operating system (NOS) and the airwaves via an antenna. The nature of the wireless connection is transparent to the NOS.


Wireless LANs can be simple or complex. The simplest configuration consists of two PCs equipped with wireless adapter cards that form a network whenever they are within range of one another (Figure 136). This peer-to-peer network requires no administration. In this case, each client would have access to only the resources of the other client and not to a central server.

Installing an access point can extend the operating range of a wireless network, effectively doubling the range at which the devices can communicate. Since the access point is connected to the wired network, each client could access the server’s resources as well as to other clients (Figure 137). Each access point can support many clients—the specific number depends on the nature of the transmissions involved. In some cases, a single access point can support up to 50 clients.

Figure 136

A wireless peer-to-peer network.

A wireless peer-to-peer network.

Figure 137

A wireless client connected to the wired LAN via an access point.

A wireless client connected to the wired LAN via an access point.

Access points have an operating range of about 500 feet indoors and 1000 feet outdoors. In a very large facility such as a warehouse or on a college campus, it will probably be necessary to install more than one access point (Figure 138). Access point positioning is determined by a site survey. The goal is to blanket the coverage area with overlapping coverage cells so clients can roam throughout the area without ever losing network contact. Access points hand the client from one to another in a way that is invisible to the client, ensuring uninterrupted connectivity.

To solve particular problems of topology, network designers might choose to use extension points (EPs) to augment the network of access points (Figure 139). Extension points look and function like access points (APs), but they are not tethered to the wired network as are APs. EPs function just as their name implies; they extend the range of the network by relaying signals from a client to an AP or another EP.

Another component of wireless LANs is a directional antenna. If a wireless LAN in one building must be connected to a wireless LAN in another building a mile away, one solution would be to install a directional antenna on the two buildings—each antenna targeting the other and connected to its own wired network via an access point (Figure 140).

Figure 138

Multiple access points extend coverage and enable roaming.

Multiple access points extend coverage and enable roaming.

Figure 139

Use of an extension point in a wireless network.

Use of an extension point in a wireless network.

Figure 140

A directional antenna can be used to interconnect wireless LANs in different buildings.

A directional antenna can be used to interconnect wireless LANs in different buildings.

Network Management

Wireless LANs are typically set up and managed with Windows-based tools. Lucent Technologies, for example, offers a Windows-based site survey tool to facilitate remote management, configuration, and diagnosis of its spread-spectrum WaveLAN wireless LANs, which include access points and adapters that are available in 900-MHz and 2.4-GHz versions.

WaveManager makes it easy for system administrators to monitor the quality of communications at multiple WaveLAN stations in a wireless network (Figure 141). It can also be used to verify building coverage, identify coverage patterns, select alternate frequencies, locate and tune around RF interference, and customize network access security. Five basic functions are offered by WaveManager:

COMMUNICATIONS INDICATOR This is located on the Windows 95 taskbar and provides mobile users with graphical, real-time information on the level of communication quality between a WaveLAN station and the nearest WavePoint access point.

LINK TEST DIAGNOSTICS Verifies the communications path between neighboring WaveLAN stations, as well as between a WaveLAN station and WavePoint access points within one wireless cell. Link Test Diagnostics measure signal quality, signal-to-noise ratio, and the number of successfully received packets.

Figure 141

Lucent’s WaveManager provides an administrative graphical user interface through which WaveLAN wireless LANs can be configured, managed, and maintained.

Lucent's WaveManager provides an administrative graphical user interface through which WaveLAN wireless LANs can be configured, managed, and maintained

SITE MONITOR Ensures optimal placement of WavePoint access points. While carrying a WaveLAN-equipped computer through the facility, Site Monitor graphically displays changing communication quality levels with the various access points installed in the building. This tool makes it easy to locate radio dead spots or sources of interference.

FREQUENCY SELECT Manages RF Channel selection. It enables users to choose from up to eight different channels (in the 2.4-GHz frequency band).

ACCESS CONTROL TABLE MANAGER Enables system administrators to provide extra levels of security by restricting access to individual computers in a facility.

Wireless LAN Standard

A wireless LAN standard has been in the making since 1990. In mid-1997, the IEEE 802.11 committee defined a transmission rate of up to 2 Mbps over infrared or radio frequency bands. The standard also includes the media access control protocol Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which allows devices implementing the standard to interoperate with wired Ethernet LANs.

The standard provides for an optical-based, physical-layer implementation that uses infrared light to transmit data. It also provides two physical-layer choices based on the radio frequency (RF): Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Both operate in the 2.4-GHz ISM band.

In the standard, a 2-Mbps peak data rate is specified for DSSS with optional fallback to 1 Mbps in very noisy environments. The standard defines the FHSS implementation to operate at 1 Mbps and allows for optional 2-Mbps operation in very clean environments.

The 802.11 media access control (MAC) can work seamlessly with standard Ethernet via a bridge to ensure that wireless and wired nodes on an enterprise LAN are logistically indistinguishable and can interoperate. The 802.11 MAC is necessarily different from the wired Ethernet MAC— the wireless LAN standard uses a carrier sense multiple access with collision-avoidance scheme, whereas standard Ethernet uses a collision-detection scheme—but the difference is masked by an access point that connects a wireless LAN channel to a wired LAN backbone.

The roaming provisions built into 802.11 also provide several advantages. It includes mechanisms to allow a client to roam among multiple access points that can operate on the same or separate channels. For example, an access point transmits a beacon signal at regular intervals. Roaming clients use the beacon to gauge the strength of their existing connection to an access point. If the connection is weak, the roaming station can attempt to associate itself with a new access point.

Although the 802.11 standard addresses roaming, it is with the understanding that all the access points in an installation are manufactured by the same vendor. The standard does not ensure that clients can roam among access points from different vendors. With the advent of 802.11 products, however, users may want to mix and match access points. For example, some customers might need standard commercial-grade bridges in the office but want ruggedized bridges for the factory floor.

To address multivendor roaming, Aironet Corp., Digital Ocean, Inc. and Lucent have collaborated to develop the Inter Access Point Protocol (IAPP) specification. That will extend the 802.11 multivendor interoperability benefits with comprehensive roaming protocols. Several others, including IBM, have voiced support for IAPP as a necessary step toward true multivendor interoperability.

The 802.11 specification adds features to the MAC that can maximize battery life in portable clients via power-management schemes. Power management causes problems with wireless LAN systems because typical power-management schemes place a system in sleep mode (low or no power) when no activity occurs for a user-definable time period. This can cause a sleeping system to miss critical data transmissions. To support clients that periodically enter sleep mode, the 802.11 standard specifies that access points include buffers to queue messages.

The 802.11 standard also addresses data security. The standard defines a mechanism through which the wireless LANs can achieve Wired Equivalent Privacy (WEP). The optional WEP mechanism is especially important because RF transmissions—even spread-spectrum transmissions—can be intercepted more easily than wired transmissions.

The next step in the evolution of the 802.11 standard will likely be the inclusion of higher data rates, expected in the 10-Mbps and above range in the 5.2-GHz band. The higher speed could encourage development of such applications as streaming video, telephony, and multimedia for wireless networks.

Last Word

Wireless LANs provide all the functionality of wired LANs, without the physical constraints of the wire itself. Wireless LAN configurations range from simple peer-to-peer topologies to complex networks offering distributed data connectivity and roaming.

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