2.4GHz/5GHz (802.11n)

802.11n builds upon previous 802.11 standards by adding Multiple-Input Multiple-Output (MIMO), which employs multiple transmitters and receiver antennas to increase data throughput. 802.11n can have up to eight antennas, but most of today’s access points use four. These are sometimes referred to as smart antennas, and if you do have four of them, two would be used for transmitting simultaneously with the other two receiving simultaneously. This setup would allow for much higher data rates than 802.11a/b/g. In fact, the marketing people claim it will provide about 250Mbps, but personally, I’m not buying it. I just don’t believe that’s what our actual throughput levels can be, and even if what they’re saying is true, exactly how would that help if all you’ve got is a 1 or 2Mbps cable or DSL connection to the Internet?

Keep in mind that the 802.11n standard hasn’t yet been ratified and isn’t expected to be until sometime in 2008—maybe later. This means that the products on the shelf today are proprietary,
and they are called “pre-N” products.

With all this in mind, let’s move on and take a look at Cisco’s solution to the growing wireless market.

Comparing 802.11

Before I move on to Cisco-specific products, take at look at Table 1, which lists the pros and cons of 802.11a, b, and g.

Table 1: 802.11 Comparison

Now let’s take a look at Figure 2, which delimits the range comparisons of each 802.11 standard and shows us the different ranges using an indoor open-office environment as a factor. We’ll be using default power settings.

You can see that to get the full 54Mbps benefit of both 802.11a and 802.11g, you need to be between 50 feet and 100 feet (at the farthest) away, and maybe even less if there are any obstructions between the client and the access point.

All good, but there’s one more IEEE 802.11 standard I want to cover that we’ll use to get even higher speeds at greater distances.

Figure 2: Range comparisons of 802.11 standards

5GHz (802.11h)

The FCC added 11 new channels in February 2004, and in 2008, we finally get to begin using these channels based on manufacturers’ releases of more 802.11a 5GHz products. This means that soon, we’ll gain access to up to 23 non-overlapping channels! And there are two new features of the 5GHz radio that are part of the 802.11h specification: Transmit Power Control (TPC) and Dynamic Frequency Selection (DFS).

DFSThis cool feature continuously monitors a device’s operating range for any radar signals that are allowed to operate in portions of the 5GHz band as well as 802.11a before transmitting. If DFS discovers any radar signals, it’ll either abandon the occupied channel or mark it as unavailable to prevent interference from occurring on the WLAN.  

TPCEven though it’s been used by the mobile phone industry for a long time, this technology has some handy new uses. You can set the client machine’s adapter and the access point’s transmit power to cover various size ranges—a feature that’s useful for many reasons. For one, setting the access point’s transmit power to 5mW reduces cell range, which works great if you’ve got a compact area with high-density usage. Further advantages include the fact that TPC enables the client and the access point to communicate.

This means the client machine can fine-tune its transmit power dynamically so it uses just enough energy to preserve its connection to the access point, conserve its battery power, plus reduce interference on the neighboring WLAN cells—sweet!

5GHz (802.11a)

The IEEE ratified the 802.11a standard in 1999, but the first 802.11a products didn’t begin appearing on the market until late 2001—and boy were they pricey! The 802.11a standard delivers a maximum data rate of 54Mbps with 12 non-overlapping frequency channels. Figure 1 shows the UNII bands.

Operating in the 5GHz radio band, 802.11a is also immune to interference from devices that operate in the 2.4GHz band, like microwave ovens, cordless phones, and Bluetooth devices. 802.11a isn’t backward compatible with 802.11b because they are different frequencies, so you don’t get to just “upgrade” part of your network and expect everything to work together in perfect harmony. But no worries—there are plenty of dual-radio devices that will work in both types of networks. A definite plus for 802.11a is that it can work in the same physical environment without interference from 802.11b users.

Similar to the 802.11b radios, all 802.11a products also have the ability to data-rate-shift while moving. The 802.11a products allow the person operating at 54Mbps to shift to 48Mbps, 36Mbps, 24Mbps, 18Mbps, 12Mbps, 9Mbps, and finally still communicate farthest from

the AP at 6Mbps.

There’s also an extension of the 802.11a specifications called 802.11h.

 FIGURE 1: UNII 5GHz band has 12 non-overlapping channels (U.S.).