Chapter 7: A WLAN Primer

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Overview

Take a moment to look around you. Unless you are outside, you should see walls and ceilings decked out in technological artifacts: lighting fixtures, jacks (telephone and data), power outlets, and so forth. They are so common they've become a part of the landscape. Now an invisible artifact is joining this landscape. It provides high-speed, low-cost, low-power, wireless access to the Internet and other networks over a short range. It is Wi-Fi, which is a marketing term that has grown to encompass technology built upon the three latest 802.11 Physical Layer specifications: 802.11a, 802.11b and 802.11g.

In its base set of applications, Wi-Fi provides wireless connectivity via portable devices, including laptops, tablet PCs, handheld computers, personal digital assistants (PDAs), digital cameras, audio and video players, cell phones, headsets, and even portable devices that people can wear on their clothing. Wi-Fi also provides end-users wireless access to a host of new services through a topology referred to as a "wireless local area network" (WLAN), which usually consists of a wired Internet connection and more traditional wired voice and data connections.

Wireless LAN technologies (i.e. the 802.11 series of specifications) emphasize a high data speed and a range that makes using these technologies in a networked environment feasible. Typically, WLANs provide wireless links from portable computing devices to a wired LAN via access points (essentially a wireless hub), but there are also a growing number of stand-alone WLANs.

Now, what many people don't realize is that the wireless LAN's foundation is built upon wired technology. Let me explain.

Computers have shared information across a network via a wired connection since the 1980's. Initially, the physical path over which computers shared information was a wire, usually a coaxial cable or a Category 3 (and later Category 5 and 5e) cable. As users began to connect computers together to share information and common resources, it became essential that each computer agree on how to send that information across the networked computers. This need for different computers, built by different manufacturers, to share information eventually led to the development of a standard set of rules that each computer obeyed in order to communicate with one another.

The Institute of Electrical and Electronics Engineers (IEEE) formed the 802 Executive Committee to design specifications to standardize the physical path over which computers communicated. The result was an Ethernet-like standard published in 1985 by the 802.3 Working Group. This first publication was named "IEEE 802.3 Carrier Sense Multiple Access with Collision Detection Access Method and Physical Layer Specifications." The local area network (LAN) standard defined in that document is more commonly known as "Ethernet," although IEEE doesn't refer to 802.3 as "Ethernet," because Ethernet is a specific product trademarked by Xerox, whereas 802.3 is a set of standards. The 802.3 standard, albeit with many extensions, still governs how computers communicate today.

As the need for user mobility increased along with the cost of installing a network's cables, the business community demanded alternative methods to network its computers. The result was the development of wireless connections that use radio frequencies to transmit information.

In 1990, the IEEE 802 Executive Committee established the 802.11 Working Group to create a wireless local area network standard to govern how computers communicate over a wireless connection. The goal of that Working Group was to describe a wireless LAN that delivers services commonly found in wired networks, i.e. high-speed throughput, reliable data delivery, and continuous network connections.

The resulting 802.11 series of standards (which are in many ways extensions of the 802.3 standard) define a Medium Access Control (MAC) sublayer (of the Data Link Layer) and three Physical (PHY) Layers. The MAC sublayer is mostly made up of software-based protocols that enable devices to talk to each other and to wired local area networks. The PHY Layer defines the physical characteristics of the radio signal, i.e. the frequency, power levels, and type of modulation.

All Wi-Fi networks have an architecture that is specifically designed to support a network where all decision-making is distributed across the network's stations (i.e. the components, which may be mobile, portable, or stationary, that connects to the wireless medium). The building blocks of all Wi-Fi networks include:

• Support of all station (computer, printers, scanners, etc.) services including authentication, de-authentication, privacy, and delivery of the data (MAC service data unit).

• Basic Service Set (BSS), a set of stations (e.g. computing devices) that communicate with one another. When all the stations in the BSS communicate directly with each other and there is no connection to a wired network, the BSS is called an "Independent BSS" (IBSS), although it is more commonly known as an "ad hoc network" (i.e. it is typically a short-lived network with a small number of stations in direct communication range). When a BSS includes an access point (AP), the BSS is no longer independent and is called an "infrastructure BSS" or simply "BSS." In an infrastructure BSS, all computing devices (stations) communicate with the AP. The AP provides both the connection to the wired LAN (if there is one) and the local relay function within the BSS.

• Extended Service Set (ESS), a set of Infrastructure BSSs, where the APs communicate among themselves to forward traffic from one BSS to another. The APs perform this communication via what is referred to as a Distribution System (DS). The DS is the backbone of the WLAN and may be constructed of either wired or wireless networks.

(See Fig. 7.7 for a visual explanation of each of the above-referenced Basic Service Sets.)

As explained in previous chapters, the original 802.11 standard defines the OSI's (Open System Interconnection) Data Link Layer's MAC (Medium Access Control) sublayer and PHY (Physical) Layer. And, to date, all of the ensuing amendments and supplemental standards either enhance the original MAC for QoS (Quality of Service) and security, or extends the original PHY for high-speed data transmission.

The design of 802.11's MAC interface means it is compatible with 802.3 networks. But the 802.11 MAC offers many other functions as well, including:

• Providing a reliable delivery mechanism for user data over wireless media, which is achieved through a frame exchange protocol at the MAC level.

• Controlling access of the shared wireless media via two different access mechanisms: the contention-based mechanism, called the distributed coordination function (DCF), and a centrally controlled access mechanism, called the point coordination function (PCF).

• Protecting the data it delivers, which is done through a privacy service, called Wired Equivalent Privacy (WEP) that encrypts the data sent over the wireless medium.

Furthermore, Physical Layer dependent parameters (like timing intervals and backoff slots) are also modeled in the MAC.

ACCOMMODATING DIFFERENT APPLICATIONS

As applications using the 802.11 specifications broaden, e.g. HotSpots, integration into 2.5G and 3G cellular phone devices, support for streaming media, and more, a different implementation focus will be needed. All 802.11 implementations are comprised of four key components: the PHY, the MAC (both are normally delivered via a silicon chip), the Distribution Services, and the Management Services (the latter two are software components that enable 802.11 Access Points and Gateways to be created). And while all implementations of 802.11 require a MAC and PHY to operate, Management Services and Distribution Services are only required in certain applications. For instance:

The MAC and PHY components implement the 802.11 standards for MAC and PHY.

The Distribution Services provide the functionality to enable communication with a wired and wireless infrastructure via routing of frames that provide scalable, reliable and secure networks.

The Management Services provide the ability to manage a large-scale wireless infrastructure, to monitor performance, to determine the best method to tune network performance, and to modify operational parameters such as security and Quality of Service.

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