General Wireless Topologies

When you’re talking about wireless topologies, there are a number of ways it could go. If you are talking about how your wireless network looks next to your wired network, you are most likely talking about a wireless local-area network (WLAN). The goal of a WLAN versus a wireless personal-area network (WPAN) is quite different. The following sections discuss the purpose of each network type, what they try to accomplish, and what types of wireless technologies you might encounter there. Figure 4-1 shows the various wireless topologies.

WPAN

If you were to consider all the options, a WPAN would be the solution to choose if you wanted to wirelessly connect to something that is very close to you. It seems funny to put it that way, because if something close to you needs to be networked, you might as well just walk over and grab it, right? Wrong. Even though this is called a network, its form can mislead you into thinking that it’s not a networking technology. What forms are we talking about? Headsets, headphones—even a mouse.
A WPAN has the following characteristics:

• The range is short—about 20 feet.
• Eight active devices
• Unlicensed 2.4-GHz spectrum
• Called a piconet

A WPAN is a network that is designed to operate within a 20-foot range. The most common WPAN is Bluetooth. In a Bluetooth network, you communicate on the 2.4-GHz spec- trum. Thinking about how many people have Bluetooth headsets and mice and such, you would expect a lot of interference, but that’s not the case. Bluetooth uses Frequency Hop- ping Spread Spectrum (FHSS). Although this book doesn’t discuss FHSS, it’s good to un- derstand that even though Bluetooth operates on the same frequency as 802.11b and 802.11g, they don’t interfere as much as another AP in the same frequency spectrum would, but they do interfere. The fact that Bluetooth communicates with a shared hopping sequence in a local area is what makes it a piconet.

Bluetooth piconets consist of up to eight active devices but can have many inactive de- vices. WPANs usually fall into the unlicensed 2.4-GHz spectrum and are standardized by the 802.15 IEEE workgroup. A WPAN study group was formed in 1998, and two months later a Bluetooth Special Interest Group (SIG) was formed. Shortly thereafter the study group became the IEEE 802.15 group. The Bluetooth SIG has more than 9000 members and continues to further the technology.


WLAN

WLANs are designed for a larger area than that of a WPAN. These can scale from very small home offices to large enterprise networks. The fact that they are local-area means that the organization where the WLAN exists also manages and probably owns the equip- ment. WLANs have the following characteristics:

• 2.4-GHz or 5-GHz spectrum.
• A larger range than a WPAN—close to 100 meters from AP to client.
• To achieve further distance, more power output is required.
• It’s not personal; rather, more clients are expected.
• WLANs are very flexible, so more than eight active devices/clients are expected, un- like a WPAN.

Normally you find a mix of dual-band wireless access points, laptops, and desktops in a WLAN. A WLAN operates in either the 2.4-GHz spectrum for 802.11b/g or the 5-GHz spectrum for 802.11a. Of the protocols seen in WLANs, 802.11b was the first to really get market penetration. Others, such as the 802.11a, have followed. Now the 802.11a, b, g, and n WLAN standards are commonly found in networks around the world. The frequency spectrums used by 802.11a/b, g, and n are all unlicensed.

Because WLANs cover larger areas, they require more power output than a WPAN. The issue to watch in WLANs is that you don’t exceed the power rules that the government sets forth. For example, in the U.S., the Federal Communications Commission (FCC) man- dates radiated power levels.

WLANs are designed to give mobile clients access to network resources. For this reason, a WLAN expects to see multiple users. In addition to wireless users, there are wireless print servers, presentation servers, and storage devices. You end up with many devices connecting to each other or sharing information with each other, usually over a common distribution system such as the local-area network. This makes WLANs much more com- plex than WPANs.

What makes WLANs flexible is the fact that the APs and clients are dual-band. This makes it easy to deploy different transmission methods in different areas, and most clients can still operate.


WMAN

A wireless metropolitan-area network (WMAN) covers a large geographic area and has the following characteristics:

• Speeds decrease as the distance increases.
• Close to broadband speeds versus Ethernet speeds.
• Used as a backbone, point-to-point, or point-to-multipoint.
• Most well-known is WiMax.

WMANs are used as backbone services, point-to-point, or even point-to-multipoint links that can be a replacement for technologies such as T1 and T3. Sometimes, a WMAN can use unlicensed frequencies. However, this isn’t always a preferred solution, because others could use the same frequency, thus causing interference. Instead, many prefer to use a li- censed frequency range; however, this requires payment for exclusive rights.

It’s normal for the speeds in a WMAN to decrease with distance. This places them in a closer category to broadband than to Ethernet. The most widely known WMAN is WiMax (802.16b). WiMax can be used to offer last-mile access as an alternative to broad- band services such as DSL or cable connections. WiMax is an excellent solution where fa- cilities or distance are a limitation. With WiMax, you pay a service provider for access, because the cost of deployment is normally very high.


WWAN

A wireless wide-area network (WWAN) covers a large geographic area. WWANs have the following characteristics:

• Low data rates
• Pay-for-use
• High cost of deployment

Because they cover a large geographic area, WWANs usually are very expensive to deploy. To better understand what a WWAN is, consider your cellular service. Your cell serv- ice is a WWAN and probably offers data access as well as voice access. The data rates are probably around 115 kbps, although some providers offer higher data rates. The most widely deployed WWAN technologies are Global System for Mobile Communication (GSM) and Code Division Multiple Access (CDMA). Payment for data access or even voice access is typically based on usage.