Lesson 4:Wireless Networking

When speaking of data networks, it's most common to think of a cable as the network medium. However, there have been wireless data networking technologies available for several years. Until recently, a wireless LAN was usually synonymous with slow transmission speeds and unreliable service, but there are now wireless LAN technologies available that provide reasonably reliable service at speeds that are at least acceptable to the average user accustomed to a cable network.

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After this lesson, you will be able to

• Describe the two basic wireless topologies
• List the IEEE 802.11 physical layer options
• Describe the CSMA/CA MAC mechanism

Estimated lesson time: 20 minutes

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Although wireless LANs can now reach respectable speeds (up to 11 Mbps), they are not likely to ever be as fast or reliable as a cabled LAN. For that reason, wireless LANs are typically used only in specific situations that require them, such as when users must be able to roam around a site with a portable computer while remaining connected to the LAN or when network access is required in places where it is impractical or impossible to install LAN cables. For example, many organizations use wireless LAN technologies to keep employees with portable computers or handheld devices in constant touch with the company LAN. Hotel and airport kiosks, rental car agents performing curbside check-ins, and retail workers scanning products on display for inventory control are just some of the many applications for this technology. There are also wireless LAN products designed for home use, where a cable installation would be inconvenient, expensive, or unsightly.

The dominant wireless LAN standard today is IEEE 802.11, developed by the same organization responsible for the current Ethernet and Token Ring stan-dards. The IEEE 802.11 working group was convened in 1990 for the purpose of developing a global wireless networking standard with a transmission rate of 1 to 2 Mbps. This standard has come to be known as IEEE 802.11a. The later IEEE 802.11b standard provides transmission speeds of 5.5 and 11 Mbps.

The IEEE 802.11 Physical Layer

As mentioned in Chapter 2, "Network Hardware," wireless LANs support two topologies, an ad hoc topology and an infrastructure topology. The ad hoc or independent topology is one in which computers equipped with wireless network interface adapters communicate directly with each other on a peer-to-peer basis; there is no cabled network involved. This type of network is designed to support only a limited number of computers, such as those in a home or small business. The infrastructure topology is designed to extend the range and flexibility of a normal cabled network by enabling wireless-equipped computers to connect to it using a specialized module called an access point.

In some cases, an access point is a computer with a wireless network interface adapter as well as a standard adapter connecting it to a standard cabled LAN, or it can be a dedicated device. The wireless clients communicate with the cabled network using the access point as an intermediary. The access point is essentially a translation bridge because it converts between the wireless network signals and those of the cabled network, preserving the single broadcast domain. As with all wireless communication technologies, distance and environmental conditions can have great effects on the performance realized by the mobile workstations. A single access point can typically support 10 to 20 clients, depending on how heavily they use the LAN, as long as they remain within an approximately 100- to 200-foot radius of the access point. Intervening walls and interference can diminish this performance substantially.

To extend the range of the wireless part of the network and provide support for more clients, you can use multiple access points in different locations, or you can use an extension point. An extension point is essentially a wireless signal repeater that functions as a way station between wireless clients and an access point. An IEEE 802.11 LAN is divided into cells, each of which is controlled by a base station. The 802.11 standard refers to each cell as a basic service set (BSS) and to each base station as an access point. If the network uses multiple access points, they are connected by a backbone, which the standard calls a distribution system (DS). The DS is usually a cabled network, but it can conceivably be wireless as well.

Run the WirelessLANs video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of the ad hoc and infrastructure topologies.

The IEEE 802.11 standard supports three different types of signals at the physical layer, which are as follows:

• Direct Sequence Spread Spectrum (DSSS).  A radio transmission method in which the outgoing signals are modulated using a digital code (called a chipping code) that uses a redundant bit pattern. The end result is that each bit of data is converted into multiple bits, enabling the signal to be spread out over a wider frequency band. The use of DSSS in combination with a technique called complimentary code keying (CKK) enables IEEE 802.11b systems to achieve their 11-Mbps transmission rates.
• Frequency Hopping Spread Spectrum (FHSS).  A radio transmission method in which the transmitter continuously performs rapid frequency shifts according to a preset algorithm. The receiver performs the exact same shifts to read the incoming signals. IEEE 802.11a systems can use FHSS, but IEEE 802.11b doesn't support it.
• Infrared.  Infrared communications use high frequencies, just below the visible light spectrum. Infrared is a "line of sight" technology, meaning that the signals cannot penetrate through opaque walls and objects. This severely limits the utility of infrared technology, which is why it is rarely used for LAN communications, outside of simple links between computers and peripherals, such as printers and handheld devices.

The IEEE 802.11 MAC Layer

Like all of the protocols developed by the IEEE 802 working groups, IEEE 802.11 splits the data-link layer into two sublayers, LLC and MAC. The LLC sublayer used to package the network layer data to be transmitted is the same for all of the IEEE 802 protocols. The IEEE 802.11 protocol's MAC sublayer defines the data, control, and management frames used by the protocol, as well as its MAC mechanism. IEEE 802.11 uses a variation on the CSMA/CD MAC mechanism used by Ethernet, called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).

CSMA/CA is similar to CSMA/CD in that computers listen to the network to see if it is in use before they send their data, and if the network is free, the transmission proceeds. Also like CSMA/CD, two computers can transmit at the same time on a CSMA/CA network, causing a collision. The difference between the two MAC mechanisms is that in a wireless environment, the CSMA/CD collision detection mechanism would be impractical, because it would require full-duplex communications. A computer on a twisted-pair Ethernet network assumes that a collision has occurred when an incoming signal arrives over its receive wire pair while it's sending data over the transmit wire pair. Making wireless LAN devices that can transmit and receive signals simultaneously is far more difficult.

Instead of detecting collisions as they occur, the receiving computer on a CSMA/CA network performs a CRC check on the incoming packets and, if no errors are detected, transmits an acknowledgment message to the sender. This acknowledgment serves as an indication that no collision has occurred. If the sender does not receive an acknowledgment for a particular packet, it automatically retransmits it until it either receives an acknowledgment or times out. If the sender still doesn't receive an acknowledgment after a specific number of retransmissions, it abandons the effort and leaves the error correction process to the protocols at the upper layers of the networking stack.

Exercise 1: IEEE 802.11 Concepts

Match the concepts in the left column with the correct definitions in the right column.

1. Extension point

2. BSS

3. Base station

4. DS

a. An access point

b. A backbone connecting access points

c. Another term for a cell

d. A repeater for wireless signals

Lesson Review

1. What MAC mechanism does an IEEE 802.11 network use?
   1. DSSS
   2. FHSS
   3. CSMA/CA
   4. CSMA/CD
2. Which of the following terms describes a wireless LAN that does not use access points?
   1. Infrastructure
   2. Distribution
   3. Ad hoc
   4. Basic
3. Which of the following is not a physical layer option supported by IEEE 802.11?
   1. DSSS
   2. Infrared
   3. BSS
   4. FHSS

Lesson Summary

• Wireless LAN technologies enable wireless computers to communicate among themselves or with a standard cabled network.
• The IEEE 802.11 protocol supports three physical layer options and provides transmission speeds up to 11 Mbps.
• IEEE 802.11 splits the data link layer into LLC and MAC sublayers and uses CSMA/CA as its MAC mechanism.