Back in the day, the two competing LAN technologies were Ethernet and Token Ring. Many argued that Token Ring was the better technology because the users would never experience a collision of data on the network. This is because only one user could transmit data at a time due to the token-passing mechanism. Ethernet ended up winning the competition because its method of collision detection (CSMA/CD) allowed for higher transmission speeds.

When we move into the WLAN environment, we move our transmission methods to a collision avoidance (CSMA/CA) strategy because wireless devices have no collision detection mechanism. Likewise, because a wireless device that is sending data cannot receive data at the same time, WLAN devices run at half duplex as a rule. When you combine this limitation with signal degradation due to wireless range and interference, you’ll find that wireless devices rarely run at the advertised speed (11Mbps for 802.11b, 54Mbps for 802.11b and 802.11a). Most of the time, the actual speed you are receiving is less than half of the maximum data rate!

Much of the wireless signal degradation is due to the nature of RF. Most of us are used to the land of Ethernet, where data signals travel cozily through a padded copper wire, free from most interference. Wireless RF signals are sent through the air using an antenna, and unfortunately, the air is an ugly place. The first challenge that the wireless signal faces is physical objects. Every object the signal must pass through can degrade the signal in some way. Reflective surfaces, such as metal or glass, cause RF waves to bounce off, shooting in a different direction. Uneven surfaces, such as a gravel road, piles of merchandise in a warehouse, a desk, or a cubicle can cause the signal to reflect and scatter in many directions. Finally, as wireless signals pass through physical objects, they are absorbed. This absorption rate differs depending on the type of material the signal passes through.

As if physical objects weren’t enough, the wireless signal is also subject to interference. All 802.11 technology uses unlicensed wireless bands (discussed further in a moment). This means that they are not sanctioned by the United States Federal Communications Commission (FCC), and any wireless device can use them. In many ways, this is fantastic. Without unlicensed wireless bands, every new wireless network in the world would need approval from the FCC, along with all the licensing fees associated with the process! However, unlicensed RF bands also present the challenge of conflict. Cordless phones, Bluetooth, microwave ovens, and WLAN technology all share the same unlicensed RF band. Never before have network administrators faced the challenge of a Stouffer’s Microwavable Macaroni Dish being responsible for a 1-minute network outage. It’s a good thing this is reduced to 30 seconds in high-powered microwaves!

The FCC maintains three unlicensed RF bands called the Industrial, Scientific, and Medical (ISM) bands. Figure 15.1 shows where these RF bands fit in the entire wireless spectrum.

Figure 15.1. FCC unlicensed bands in view of the entire wireless spectrum.

The three specific bands are as follows:

Keep in mind that these ranges of frequency are for the U.S. Other countries may support more frequencies, and others support fewer. Some countries may not allow unlicensed wireless transmission at all!

Every company that manufactures equipment that transmits wirelessly and that does not want to license frequencies from the FCC is forced to share these unlicensed bands. Combining different devices in the same area that are sharing the same frequencies results in interference. For our WLAN, that means impaired performance or potentially a completely unusable network, depending on the severity of the interference.

By far, more equipment uses the 2.4GHz band, making it more difficult to keep the airwaves free from interference. The more popular 802.11b, 802.11g, and 802.11n standards fit into this RF band. The 5GHz band is home to the 802.11a standard. On that note, let’s also discuss some key facts about RF:

So, building a 500-foot-high tower on top of your company building powered by a small nuclear generator most likely is not an option for achieving campus-wide network coverage. The alternative solution requires you to purchase many small transmitters and strategically place them throughout your network.

Because government restrictions keep us from saturating our campus network with an ultrapowerful wireless signal, we are forced to deploy many transmitters (WAPs) that have a much smaller coverage radius. The problem with this solution is that these transmitters all use the same RF band. Because a good wireless design requires overlapping signals, we are stuck with a problem: the adjacent transmitters will interfere with each other.

Having a 10 to 15% overlap in signals, as shown in Figure 15.2, is a good design. With the right configuration, this overlap can allow clients to roam seamlessly between transmitters. However, the client will experience heavy interference and performance degradation within the overlapping coverage areas.

Figure 15.2. The problem with overlapping wireless signals.

The solution? Wireless channels. Just because a wireless access point uses the 2.4GHz range of RF doesn’t mean it consumes the entire range. The 2.4GHz RF band is divided into different channels, as shown in Figure 15.3.

Figure 15.3. 2.4GHz band wireless channels.

The 2.4GHz band supports up to 14 different channels (the FCC allows only channels 1 to 11 in the U.S.). Each channel consumes 22MHz of frequency bandwidth. When you walk through the setup wizard of a wireless access point, one of your options is a channel selector, typically populated with channels 1 to 11. Your goal is to put adjacent access points on separate channels. Be careful when doing this—it’s not as simple as it looks! First, most of the wireless channels the FCC has sanctioned are overlapping. Figure 15.3 shows that Channel 1 (2401 to 2423MHz) overlaps with Channel 2 (2406 to 2428MHz), Channel 3 (2411 to 2433MHz), Channel 4 (2416 to 2438MHz), and Channel 5 (2421 to 2443MHz). Placing one access point on Channel 1 and an adjacent access point on Channel 3 would cause interference problems.

For this reason, it is commonly said that the 2.4GHz band supports only three channels: 1, 6, and 11. These three channels do not overlap with each other. With this in mind, we need to arrange our wireless access deployment into logical, nonoverlapping “cells,” as shown in Figure 15.4.

Figure 15.4. Arranging 2.4GHz channels into logical, adjacent cells.

Looking at Figure 15.4 should make you realize that, indeed, deploying an organization-wide wireless network does take planning.

One of the big advantages that the 5GHz band has over the 2.4GHz band is the number of channels supported. Depending on your locale and the channels you choose, you could have up to 23 nonoverlapping channels in the 5GHz range! This makes it much more viable to find a clear channel without interference. Remember our Stouffer’s Macaroni Dish microwave issue? Using different channels can be one solution to help alleviate some of the microwave interference.

In 1999, two competing standards were released: 802.11a and 802.11b. As with most brand-new standards, vendors did not really begin implementing and releasing equipment that used 802.11a or 802.11b until about 2002. The two standards offered the following features:

It’s an interesting story how these two standards competed. 802.11a was clearly the better standard. It offered faster speeds (54Mbps versus 11Mbps), more channels (23 versus 3), and an overall cleaner RF band (nowhere near as many devices use the 5GHz band as the 2.4GHz band). Alas, the 802.11b standard won the competition (for now) just because it was more available. The silicon used to make the 802.11a chips was in short supply, and the industry was hungry for wireless, so it gobbled up 802.11b instead.

After the newness of the 802.11b standard wore off, users and administrators alike demanded more speed. The IEEE answered with the 802.11g standard. This standard borrowed much of the 802.11a technology and implemented it in the 2.4GHz RF band. One of the major hurdles that was overcome was to achieve backward compatibility with 802.11b, thus allowing 802.11g access points to also support 802.11b clients. For this reason, most access points are labeled as 802.11b/g. Let’s pull the same stats on 802.11g as we did for 802.11a and 802.11b:

At the time of this writing, 802.11n is currently in prestandard state, set to become a standard in September 2008. Therefore, I will write about the basics of what we currently think the 802.11n standard will become. 802.11n adds multiple input, multiple output (MIMO) technology to wireless cards and access points. Simply put, you now have multiple antennas that can send or receive between devices. This can bring about increased range and throughput. The irony of the situation is that even though the 802.11n standard is not yet complete, vendors are releasing prestandard devices, placing their bets on what they believe the standard will be. This is quite a risk, because it has yet to be decided which RF band 802.11n will use; it could use the 2.4GHz and/or 5GHz bands. Perhaps by the time you read this, the standard will have been decided. Here is what we currently know about 802.11n: