Overview of the IEEE 802.11a Standard

Because IEEE 802.11b operates in the 2.4GHz bandwidth, it is subject to more interference because this bandwidth is basically a "free for all" (see the preceding chapter), meaning that many kinds of devices contend for the frequencies in the 2.4GHz spectrum. Spread spectrum techniques are used in 802.11a and 802.11b wireless radios to help minimize interference, and they are effective to some extent. For more information about spread spectrum techniques, see Chapter 19.

Interference from Consumer Devices

IEEE 802.11a uses frequencies in the 5GHz radio spectrum. This spectrum does not suffer from as much interference from consumer devices, such as microwave ovens, cordless telephones, and other devices that produce radio waves. Additionally, hardware using the 5GHz spectrum does not interfere with the previously mentioned consumer devices. Given the larger bandwidth provided by the 5GHz radio spectrum, there is less interference, and the capability to support more channels than you can get from IEEE 802.11b is a nice plus. Although 5.8GHz cordless telephones use frequencies between 5.725GHz and 5.85GHz, which is a frequency range that overlaps 802.11a channels 149, 153, 157, and 161, it is not a significant issue because only channel 149 is supported by the FCC, and it is only implemented for outdoor use.

Because 802.11a network devices today suffer less interference by other devices, it might be a good choice if you are just starting to use wireless networking. One caveat is that the 802.11g standard, discussed in the next chapter, offers the same data rate but operates in the 2.4GHz spectrum and can easily interoperate with 802.11b.

If you choose a product based on the 5GHz spectrum (which is the subject of this chapter), you need not worry about that microwave oven in the break room or 2.4GHz cordless telephones that can cause problems in an 802.11b or 802.11g network.

Increased Bandwidth in the 5GHz Band

It might not seem like a lotgoing from the 2.4GHz range of radio frequencies to the 5GHz range. However, the larger bandwidth is capable of transmission of data at faster rates.

Note

The 802.11g standard provides the same data rate as 802.11a. With 802.11g using the 2.4GHz radio frequency spectrum, how is this possible? Just as twisted-pair cables in a wired network can be used to transfer data at 10Mbps or 100Mbps, new methods for modulating data on a particular network media make this possible. 802.11g technology can operate at the same data rate as 802.11a because it uses a different method for modulating data. However, that doesn't mean that the 5GHz spectrum has already been pushed to its maximum throughput. Just as newer technology has enabled faster data rates in the 2.4GHz spectrum (802.11g), it is inevitable that advances in technology will enable faster data rates in the 5GHz band sometime in the future.

The 5.4GHz range gives you a maximum data rate of 54Mbps (actual throughput is lower) using the 802.11a standard. And as with other wireless technologies, you do not always achieve the maximum speed defined by the standard. Which technology makes more sense for you when it comes to purchasing equipment should be determined based on budget and need. If you are still operating a network that uses 10BASE-T networking (10Mbps), you won't notice much difference in response time when an 802.11b wireless client exchanges data with a computer on the wired network. This assumes that the Access Points (APs) are placed close to the clients in order to maximize data throughput. If you were to use 802.11a devices that operate at a faster speed than 10Mbps, a 10BASE-T wired network is a bottleneck. A wireless client communicating with a client on the wired network would not utilize the throughput that it is capable of because the wired network cannot operate at that higher speed. Keep in mind, however, that all the wireless technologies discussed in this section of the book probably do not operate at the upper limit that the standards specify, due to environmental factors, signaling, and security overhead.

Note

By adding additional Access Points at strategic points in your network, you can reduce congestion in the wireless portion of your network. And this might just also accomplish diminishing other bottlenecks in your network by moving some departmental clients to wireless, leaving the backbone of the wired network to handle the larger network traffic.

802.11a wireless clients, however, will still be able to transfer data at faster rates among themselves.

The opposite is true when using 802.11a clients with a 100Mbps wired network, because 100Mbps is faster than 54Mbps, and the wireless network then becomes a bottleneck.

To put it another way, if you choose 802.11a hardware, you will benefit from this increased bandwidth only if your other network components can work at this speed (or faster). Because most enterprise networks, as well as SOHO networks, now operate using Fast Ethernet (100Mbps), IEEE 802.11a is a better fit than 802.11b. Though not as fast as Fast Ethernet, 802.11a does offer more than a fivefold increase in potential bandwidth over 802.11b, just more than one-half of the bandwidth that Fast Ethernet gives you. In comparison, 802.11b, the first widespread wireless standard, is rated at only 11Mbps, which is almost a tenth of what Fast Ethernet can attain. These are performance statistics under perfect conditions, however. Have you ever been driving late at night and that great radio station just fades away, and then you have to look for another one? With wireless, including 802.11a, other factors, such as distance, buildings, and electrical devices, can also limit the actual bandwidth you achieve.

802.11. a Signal Modulation

One of the advantages of 802.11a over 802.11b is the method of signal modulation it uses. 802.11a uses a signaling method called orthogonal frequency-division multiplexing (OFDM) for almost all data rates.

OFDM transmits multiple narrowband data streams at different frequencies selected to avoid crosstalk (interference). This method is much different than the DSSS (spread-spectrum) method that 802.11b wireless networks use. Because most 802.11a networks are indoors, OFDM is a perfect choice because it provides higher data rates than DSSS and minimizes the effects of multi-path propagation on signal quality and throughput.

Multi-path propagation takes place when radio signals are reflected on their way between sender and receiver. Metal office furniture, structural elements, and other features common in office buildings can all reflect radio waves. This causes errors in received data and requires that it be retransmitted. Multi-path propagation has a big impact on the performance of 802.11b because DSSS is very susceptible to this type of interference. However, OFDM is not affected very much by multi-path propagation.

Because OFDM signal modulation provides better data rates, signal quality, and throughput, both 802.11a and 802.11g use it. Note that 802.11g hardware is also compatible with DSSS, and switches to DSSS (and thus 11Mbps or lower data rates) when connecting with 802.11b hardware.

802.11. a Channels

Given the fact that 802.11a has the same 54Mbps maximum data rate as 802.11g, but cannot interoperate with 802.11g unless dual-mode network adapters or APs are used, what is the most compelling reason to use 802.11a network hardware? In a word, channels. As you learned in Chapter 19, 802.11b (and 802.11g) networks offer 11 channels, but only three channels (1, 6, and 11) do not overlap with each other. In a large-scale building or campus-wide installation, the ability to use only three channels can reduce real-world throughput and make avoiding interference from other 802.11b or 802.11g-based wireless networks difficult.

Unlike 802.11b/g wireless networks, 802.11a wireless networks have eight non-overlapping channels. In other words, you can choose any combination of channels for a multi-AP environment without interference with each other. And, even if your installation is next to another 802.11a installation, it's going to be relatively easy to choose channels that are not in use by other nearby networks to avoid interference.

Table 20.1 lists the channels supported by 802.11a wireless networks in North America. Note that Asian locations support fewer (and different) channel combinations, and that Europe has been slow to standardize support for 802.11a hardware.

Table 20.1. 802.11a Channels in North America

Channel	Frequency (MHz)	Category	Maximum Power Level	Usage
36	5180	U-NII Low Band	40mW	Indoor
40	5200	U-NII Low Band	40mW	Indoor
44	5220	U-NII Low Band	40mW	Indoor
48	5240	U-NII Low Band	40mW	Indoor
52	5260	U-NII Medium Band	200mW	Indoor
56	5280	U-NII Medium Band	200mW	Indoor
60	5300	U-NII Medium Band	200mW	Indoor
64	5320	U-NII Medium Band	200mW	Indoor
149	5745	U-NII High Band	800mW	Outdoor

Proprietary Extensions to 802.11a

Just as some vendors created proprietary extensions to 802.11b to improve performance, some vendors of 802.11a hardware have developed clients and APs claiming throughput of up to 108Mbps. Most of these products use Atheros Super AG chipsets (www.atheros.com), which use techniques such as channel bonding (turbo mode) and special bursting techniques to achieve faster speeds with both 802.11a and 802.11g clients. As with proprietary extensions to 802.11b standards, the biggest performance boost is seen when all network clients support the same proprietary extensions.

See "Proprietary Extensions to 802.11b," p. 293, for details on proprietary extensions.

Using Wireless Networking in Public Places

Wireless can be used for so many situations in which wired-network components would not be a good fit. As mentioned in previous chapters, just being able to sit in an airport or a shopping mall (waiting on that other shopper), and connect directly to the Internet while you are waiting is going to be where wireless networking succeeds with the ordinary consumer, as well as computer enthusiasts. Today several large telecommunications companies are laying groundwork for this capability by creating a large network of 802.11b/g APs in public places where computers are likely to be used.

For example, in many airports there are rooms set aside for business travelers who need access to computer services, the Internet, faxing, and so on. These services aren't necessarily cheap. Yet if an Internet provider can offer its services over a network that spans most of the country, the price for an Internet connection will continue to drop, and you will be able to use your laptop computer pretty much anywhere in a public place. The main drawback to this for the next few years is that it will take time to create a large network, and builders will concentrate on the larger metropolitan areas first because that's where revenues from the service will be larger. Such a network will have to be built-out over the long run, just as the telephone network was when that technology was first introduced more than a hundred years ago.

This type of network will not be entirely wireless. Instead, the wireless APs will be connected to backbone cabling in a similar way that wireless APs are used in a corporate LAN. For the long haul, this backbone cabling will be joined to the WAN using existing high-speed technologies such as ATM and Frame Relay. This is also the way that the Internet operates. Your connection to the local ISP can be accomplished using a telephone line (DSL service) or a cable modem. Whichever method is used, your line terminates back at the ISP's central office, and from there it is connected to the Internet using high-speed connections.

The only problem with this technology is that because it has already been adopted by so many current users, virtually all installations support only 802.11b or 802.11g technology. Although 802.11a is technologically superior in terms of avoiding interference, it is not directly compatible with 802.11b or the newer 802.11g installations seen in public hot spots. Although Internet communications are becoming more bandwidth-intensive, and a mere 11Mbps (if you can get that maximum speed) won't suffice for many users, 802.11g is the logical successor because it's backward-compatible and faster.

If you can afford it, 802.11a is a good start for an enterprise network because it offers the speed of 802.11g and avoids most sources of interference. For those who do not require multiple Access Points, 802.11g is idealfor example, in a SOHO or home network.

Note

If you use a portable computer, normally use 802.11a in your corporate network, and travel domestically or internationally, I recommend you use a dual-mode (802.11a/802.11g) network adapter. This is rarely a problem because most integrated wireless network adapters support dual-mode operation (if your system uses a mini-PCI card, ask your vendor about an upgrade), and you can also use CardBus or USB dual-mode network adapters if you don't have onboard dualmode wireless network support. The reason is that most North American public hot spots support 802.11b or 802.11g hardware, and most European localities don't support 802.11a wireless hardware.

If you need to buy an 802.11a network adapter, about all that's on the market these days are dual-mode 802.11a/g network adapters. Virtually all vendors have discontinued their 802.11a-only network adapters.

Security Concerns

As first shipped, the 802.11b standard suffers from a weak security link: the Wireless Equivalent Privacy (WEP) security standard. The first version of WEP used a 40-bit key size that was too easy for a dedicated hacker to exploit and penetrate a network. IEEE 802.11a also uses WEP, with much larger keys, ranging from 64 to 256 bits. However, note that encryption sizes above 128-bit are usually vendor-specific. In other words, if you have some wireless hardware that supports 256-bit encryption, but others that support only 128-bit encryption, you must use the lower standard across the network.

Although some 802.11b hardware has been updated to support superior security technologies such as Wi-Fi Protected Access (WPA), many products cannot be upgraded.

Current generations of 802.11a and 802.11g network adapters and APs also support superior security technologies such as WPA and WPA2 and others in addition to WEP. You can read more about wireless security mechanisms in Chapter 23, "Security and Other Wireless Technologies."