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.).

2.4GHz (802.11g)

The 802.11g standard was ratified in June 2003 and is backward compatible with 802.11b. The 802.11g standard delivers the same 54Mbps maximum data rate as 802.11a but runs in the 2.4GHz range—the same as 802.11b.

Because 802.11b/g operates in the same 2.4GHz unlicensed band, migrating to 802.11g is an affordable choice for organizations with existing 802.11b wireless infrastructures. Just keep in mind that 802.11b products can’t be “software upgraded” to 802.11g. This limitation is because 802.11g radios use a different chipset in order to deliver the higher data rate.

But still, much like Ethernet and Fast Ethernet, 802.11g products can be commingled with 802.11b products in the same network. Yet, for example, completely unlike Ethernet, if you have four users running 802.11g cards and one user starts using an 802.11b card, everyone connected to the same access point is then forced to run the 802.11b CSMA/CA method—an ugly fact that really makes throughput suffer. So to optimize performance, it’s recommended that you disable the 802.11b-only modes on all your access points.

To explain this further, 802.11b uses a modulation technique called Direct Sequence Spread Spectrum (DSSS) that’s just not as robust as the Orthogonal Frequency Division Multiplexing (OFDM) modulation used by both 802.11g and 802.11a. 802.11g clients using OFDM enjoy much better performance at the same ranges as 802.11b clients do, but—and remember this—when 802.11g clients are operating at the 802.11b rates (11, 5.5, 2, and 1Mbps), they’re actually using the same modulation 802.11b does.

Figure 1 shows the 14 different channels (each 22Mhz wide) that the FCC released in the 2.4GHz range.

In the U.S., only 11 channels are configurable, with channels 1, 6, and 11 being nonoverlapping. This allows you to have three access points in the same area without experiencing
interference.

Figure 1:ISM 2.4GHz channels

2.4GHz (802.11b)

First on the menu is the 802.11b standard. It was the most widely deployed wireless standard, and it operates in the 2.4GHz unlicensed radio band that delivers a maximum data rate of 11Mbps.

The 802.11b standard has been widely adopted by both vendors and customers who found that its 11Mbps data rate worked pretty well for most applications. But now that 802.11b has a big brother (802.11g), no one goes out and just buys an 802.11b card or access point anymore because why would you buy a 10Mbps Ethernet card when you can score a 10/100 Ethernet card for the same price?

An interesting thing about all Cisco 802.11 WLAN products is that they have the ability to data-rate-shift while moving. This allows the person operating at 11Mbps to shift to 5.5Mbps, 2Mbps, and finally still communicate farthest from the access point at 1Mbps. And furthermore, this rate shifting happens without losing connection and with no interaction from the user. Rate shifting also occurs on a transmission-by-transmission basis. This is important because it means that the access point can support multiple clients at varying speeds depending upon the location of each client.

The problem with 802.11b lies in how the Data Link layer is dealt with. In order to solve problems in the RF spectrum, a type of Ethernet collision detection was created called CSMA/CA, or Carrier Sense Multiple Access with Collision Avoidance. Check this out in Figure 1.

CSMA/CA is also called a Request To Send, Clear To Send (RTS/CTS) because of the way that hosts must communicate to the access point (AP). For every packet sent, an RTS/CTS and acknowledgment must be received, and because of this rather cumbersome process, it’s kind of hard to believe it all actually works!

Figure 1:802.11b CSMA/CA

The 802.11 Standards

Taking off from what you learned in Chapter 1, “Internet working,” wireless networking has its
own 802 standards group—remember, Ethernet’s committee is 802.3. Wireless starts with 802.11,
and there are various other up-and-coming standard groups as well, like 802.16 and 802.20. And
there’s no doubt that cellular networks will become huge players in our wireless future. But for
now, we’re going to concentrate on the 802.11 standards committee and subcommittees.

IEEE 802.11 was the first, original standardized WLAN at 1 and 2Mbps. It runs in the
2.4GHz radio frequency and was ratified in 1997 even though we didn’t see many products pop
up until around 1999 when 802.11b was introduced. All the committees listed in Table 1 are
amendments to the original 802.11 standard except for 802.11F and 802.11T, which are both
stand-alone documents.

Table 1:802.11 Committees and Subcommittees