2.1 802.11 Wireless Standards

The 802.11 wireless standard is a family of specifications for wireless technology. It was (and is still) developed by the Institute of Electrical and Electronics Engineers (IEEE). 802.11 specifies a client communicating over the air with another client (or through a base station). It comprises the following specifications:


This is the original specification for wireless networks. The 802.11 standard specifies a transmission rate of 1 or 2 Mbps, and it operates over the 2.4 GHz spectrum. It uses either a Frequency Hopping Spread Spectrum (FHSS) or a Direct Sequence Spread Spectrum (DSSS) modulation scheme.


802.11b is more popularly known as Wi-Fi (Wireless Fidelity). This is an extension of the original 802.11 specification. More significantly, 802.11b operates at a much higher data rate: 11 Mbps. However, it can also fall back to a slower rate of 5.5, 2, or 1 Mbps. 802.11b uses only DSSS. Like the original 802.11 standard, 802.11b operates in the 2.4 GHz spectrum. Most wireless networks deployed at the time of this writing are 802.11b networks.

The term Wi-Fi has now been extended to cover not only 802.11b, but the entire family of 802.11 specifications, such as 802.11a and 802.11g. The Wi-Fi Alliance (http://www.wi-fi.org/), a nonprofit organization formed to promote 802.11 wireless technologies, uses the term Wi-Fi to refer to 802.11a, 802.11b, and 802.11g wireless networks.


The 802.11a is a relatively new extension to the 802.11 specification. It addresses the slower data rate of the 802.11 and 802.11b specifications by allowing data rates of up to 54 Mbps. It does not use the FHSS or DSSS encoding scheme, but instead uses the Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme. The most significant point about 802.11a is that it is not compatible with the existing 802.11b networks because it operates in the 5 GHz spectrum.

Understanding DSSS and OFDM

DSSS is a transmission technology that transmits data in small pieces over a number of discrete frequencies. Essentially it splits each byte into several bits and sends them on different frequencies. (802.11 uses FHSS, which is similar, but hops across frequencies many times a second.)

OFDM, on the other hand, uses several overlapping frequencies to send packets of data simultaneously. What's more, it splits the overlapping frequencies into smaller frequencies for its own data transmission. Hence, 802.11a and 802.11g can transmit at a higher data rate, since these two standards use OFDM.


The latest addition to the 802.11 family is the 802.11g specification. 802.11g is a competing standard to 802.11a. Similar to 802.11a, 802.11g uses OFDM, but it operates in the 2.4 Ghz spectrum, which is compatible with 802.11b. 802.11g supports up to a maximum of 54 Mbps.

The data rates specified for 802.11b (11 Mbps) and 802.11a/g (54 Mbps) are raw, and don't reflect what you'll actually experience in everyday use. Because of network overhead, 802.11b users will get between 4 and 5.5 Mbps; 802.11a/g users can expect about 25 Mbps. Further, this is shared between all users of a given base station.

The predominant 802.11 specification in use today is 802.11b. With the explosion of network-intensive applications such as streaming video between a computer and Personal Video Recorder (PVR), 802.11b networks are increasingly unable to meet the speed demands of users. Hence network infrastructure designers are looking for faster standards available today and tomorrow. They have to weigh the different criteria when deciding on the suitable specifications to follow. In the next section, you will see a more detailed comparison of the various 802.11 specifications, which will help you make an informed decision.

2.1.1 Comparing 802.11a, 802.11b, and 802.11g

To start off, let's compare 802.11a and the current 802.11b standard by looking at a few parameters. Table 2-1 lists the comparisons sorted according to basic characteristics.

Table 2-1. Comparing 802.11a and 802.11b



Modulation and power consumption

802.11b uses the DSSS modulation scheme.

In terms of power efficiency, DSSS is more efficient than OFDM.

802.11a uses OFDM.

802.11a devices consume more power than 802.11b devices.


802.11b uses the 2.4 GHz spectrum, which is overcrowded with devices such as cordless phones and microwave ovens. Even Bluetooth devices use the 2.4 Ghz spectrum.

802.11a uses the 5 GHz spectrum. Though the 5 GHz spectrum is less crowded, the signals have a higher absorption rate and are easily blocked by walls and objects.


802.11b has a range of 300 feet. But this is dependent on the environment. Obstacles such as concrete walls and metal cabinets can reduce the effective range of 802.11b networks.

Due to the higher absorption rate at the 5 GHz spectrum, 802.11a devices have shorter operating range of about 150 feet, compared to the 300 feet achievable by 802.11b (optimally under ideal conditions). As a result, more transmitters are required for 802.11a networks.

Data rate

802.11b supports raw speeds of up to 11 Mbps with a peak speed of 4 to 5.5 Mbps after accounting for network overhead.

802.11a supports raw speeds up to 54 Mbps with a peak speed of approximately 25 Mbps after accounting for network overhead.


Cost of 802.11b devices have gone down due to the maturity of the 802.11b technologies.

Components for 802.11a devices are more expensive to produce and hence their price tags are higher than 802.11b devices. Also, the increased number of transmitters required for the 802.11a network will drive up the cost of implementing an 802.11a network.


802.11b wireless networks are prevalent, and most current wireless networks use 802.11b devices.

802.11a is not compatible with the 802.11b protocol. Hence 802.11a devices cannot work with existing 802.11b wireless access points (but see Dual Band and Compatibility later in this chapter). Note that if you plan on migrating to the newer 802.11g standard, your 802.11b cards can still access the "g" network. This will not be the case for 802.11a radio cards.


Supports between 30 and 60 simultaneous wireless users per base station, depending on the device specifications and kind of user activities.

The 802.11a network can accommodate more users (compared to 802.11b) due to the increase in radio frequency channels and increased operating bandwidth.

The main draw of migrating to an 802.11a network is no doubt increased bandwidth. With the near five-fold increase in data rate (54 Mbps), applications like streaming audio and video and networked games would now be possible (or at least more responsive).

Dual Band and Compatibility

The main drawback in adopting 802.11a networks is compatibility. Businesses and institutions that have invested in 802.11b networks are reluctant to migrate to a faster but incompatible 802.11a network. For these reasons, vendors are coming out with dual-band wireless access points and network adapters. These dual-band access points and network adapters contain two sets of hardware, one for 802.11a and one for 802.11b, which let you deploy both 802.11a and 802.11b devices in the same environment. Best of all, since these two protocols operate in different frequencies, interference is minimized.

How about the newer 802.11g standard? How does it compare to 802.11a? Table 2-2 is a comparison of 802.11a and 802.11g standards.

Table 2-2. Comparing 802.11a and 802.11g



Modulation and power consumption

Both 802.11a and 802.11g utilize OFDM.

Both 802.11a and 802.11g utilize OFDM, which leads 802.11a and 802.11g to have similar power consumption needs.


Uses the 5 GHz spectrum.

The number of nonoverlapping channels in 802.11a is eight. The eight nonoverlapping channels in 802.11a make for an increase in throughput (see Section 2.2.3 later in this chapter).

Unlike 802.11a, 802.11g uses the same 2.4 GHz spectrum as 802.11b. The number of nonoverlapping channels in 802.11g is three (see Section 2.2.3 later in this chapter), compared to eight in 802.11a networks. This makes channel assignment more difficult for 802.11g networks and reduces the effective throughput in a given area.


Both 802.11a and 802.11g have a shorter operating range than 802.11b, due to the higher absorption rate.

See 802.11a column.

Data rate

Both 802.11a and 802.11g support a maximum raw data rate of 54 Mbps, with a peak speed of approximately 25 Mbps after accounting for network overhead.

See 802.11a column.


802.11a and 802.11g networking equipment cost about the same.

802.11g can preserve the current investment in the 802.11b network, allowing you to gradually phase in 802.11g, replacing equipment at whatever pace suits your budget.


802.11a is neither compatible with 802.11g nor 802.11b (see Dual Band and Compatibility for information about devices that can operate on 802.11b and 802.11a networks).

802.11g allows both 802.11b and 802.11g devices to coexist in the same network.

The question remains: which standard should you go for, 802.11a or 802.11g? The answer depends very much on your environment. Since each offers similar performance in terms of transfer rate, the other criteria that you should consider is your investment in current 802.11b technologies. If preserving compatibility with older equipment is your priority, then 802.11g should be the clear choice. If you are more concerned about pure performance and need to avoid interference with the already crowded 2.4 GHz spectrum, 802.11a would be the recommended route. Another concern is interoperability with equipment from other vendors. For example, Apple has decided not to invest in 802.11a and instead supports 802.11g in its Airport Extreme line of wireless networking equipment. So, an Apple computer or access point would work with 802.11b or 802.11g equipment, but not 802.11a.

At the time of this writing, D-Link has launched a tri-mode wireless access point (the DI-774) that supports 802.11a, 802.11b, and 802.11g wireless networks.

2.1.2 Wireless Cards and Adapters

Now that we have seen the various wireless standards available in the market, let's turn our attention to the client side of things. Wireless cards and adapters come in the following flavors:

  • PCI adapters

  • USB adapters

  • PCMCIA cards

A PCI wireless adapter is useful for a desktop with an empty PCI slot. Figure 2-1 shows the D-Link DWL-AB520 Multimode Wireless PCI Adapter. It supports both 802.11a and 802.11b wireless standards.

Figure 2-1. The D-Link AirPro DWL-AB520 Multimode Wireless PCI Adapter (802.11a and 802.11b)

If you do not want to open up your computer casing or you simply want to share a wireless adapter among many computers, the Linksys WUSB11 (as shown in Figure 2-2) is a good choice. Simply connect the USB Wireless Adapter to the USB port on your computer and you can get on the wireless network. It supports the 802.11b standard.

Figure 2-2. The Linksys WUSB11 USB Wireless Adapter (802.11b)

Finally, for notebook users, the most popular choice is a PCMCIA card. Figure 2-3 shows the D-Link DWL-G650 wireless card, which supports the 802.11b and 802.11g standards.

Figure 2-3. The D-Link DWL-G650 AirPlus wireless card (802.11b and 802.11g)

2.1.3 Wireless Networking Modes

There are two modes in which your computer can participate in a wireless network. The first, ad-hoc mode , is a wireless network where two or more computers communicate with one another directly. It is known as a Basic Service Set (BSS). An ad-hoc network is also known as an Independent Basic Service Set (IBSS). An ad-hoc network does not involve the use of a wireless access point. Each computer on the network communicates with each other in a peer-to-peer fashion. When two or more BSSs operate within the same network, it is then called an Extended Service Set (ESS).

If you want your computer to participate in an ad-hoc network, each computer on the network must have a unique IP address. You can either assign a fixed IP address to your computer or rely on link-local addressing to have your computer automatically assign itself an IP address. See Chapter 5 for information on how to set up an ad-hoc wireless network.

The second mode is infrastructure mode, in which a wireless access point is used. A wireless access point routes the network traffic from one computer to another. It also moves data to the wired network.

Windows XP Unwired(c) A Guide for Home, Office, and the Road
Windows XP Unwired(c) A Guide for Home, Office, and the Road
Year: 2005
Pages: 92

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