802.11 WLAN Protocols

wireless space consists of numerous protocols. Specifically in the WLAN area, the Institute of Engineers Electrical and Electronic Engineers (IEEE) has created several protocols within the 802.11 category to facilitate the networking process. These protocols define the data rates, the modulation techniques, and more. An understanding of these protocols is essential for any administrator of wireless networks.

The IEEE helps to standardize wireless protocols. Those that you must be familiar with for the CCNA Wireless Exam are the 802.11 a/b/g and n protocols. These four IEEE standards define the wireless family that is used in almost all wireless LANS today. The standardization of wireless networking started with the original 802.11 protocol in 1997, and each protocol thereafter has simply added to the benefit of wireless technologies. This chapter looks at the 802.11 protocol families, their history, and how they operate. The 802.11 protocols encompass the 2.4-GHz and 5-GHz range.


The Original 802.11 Protocol

The original 802.11 protocol was where wireless LANs find there beginnings. It is rare to find this original protocol in new hardware today, probably because it only operates at 1 and 2 Mbps. The 802.11 standard describes frequency-hopping spread spectrum (FHSS), which operates only at 1 and 2 Mbps. The standard also describes direct sequence spread spectrum (DSSS), which operates only at 1 and 2 Mbps. If a client operates at any other data rate, it is considered non-802.11 compliant, even if it can use the 1- and 2-Mbps rates.


The original 802.11 protocol falls within the industry, scientific, and medical (ISM) bands and operates only in the 2.4-GHz range. The 2.4-GHz range has up to 14 channels depending on the country you are in. In the United States, the FCC allows channels 1 through 11 to be used. This gives you 3 nonoverlapping channels: 1, 6, and 11. This is important because you do not want to have APs and clients operating on the same channel placed near each other for interference reasons.


The 802.11b Protocol

802.11b is a supplement to the 802.11 protocol. To get an better feel for how the 802.11 protocols progressed, understand that technology moves faster than the standards do. 802.11 was quickly outgrown because wired networks offered 10 Mbps versus the 1 and 2 Mbps of 802.11. Vendors developed methods of achieving higher data rates. The danger in vendor-designed protocols, of course, is interoperability. The job of the IEEE was simply to define a standard that all vendors could follow based on the proprietary implementations that they were using.

802.11b offers higher data rates—up to 11 Mbps—with backward compatibility at 1 and 2 Mbps. At 1 and 2 Mbps, the same coding and modulation as 802.11 is used. When operating at the new speeds—5.5 Mbps and 11 Mbps—a different modulation and coding is used. 802.11 uses Barker 11 coding, as covered in Chapter 1, “Introduction to Wireless Networking Concepts,” and 802.11b uses complementary code keying (CCK) for coding. For modulation, 802.11 uses differential binary phase-shift keying (DBPSK), whereas 802.11b uses differential quadrature phase-shift keying (DQPSK). The result is more data sent in the same period.

802.11b was ratified in September 1999. The United States has 11 channels, the same as 802.11. In Europe, the ETSI defines 13 channels, and Japan has 14. 802.11b allows dynamic rate shifting (DRS) to enable clients to shift rates to lower rates as they travel farther away from an AP and higher rates as they get closer to an AP. Today, 802.11b is the most popular and most widely deployed wireless standard. Table 6-3 gives some basic information on the 802.11b standard.


The 802.11g Protocol

The IEEE ratified 802.11g in June 2003. In addition to the four data rates of 802.11b, it added eight more. The maximum data rate of 54 Mbps places 802.11g in the same speed range as 802.11a; however, it remains in the 2.4-Ghz frequency range. On the lower end, 802.11g is still compatible with 802.11b, using the same modulation and coding as 802.11b for the 1-, 2-, 5.5-, and 11-Mbps rates. To achieve the higher data rates, 802.11g uses orthogonal frequency division multiplexing (OFDM) for modulation. OFDM is the same modulation that 802.11a uses.

There are still only three nonoverlapping channels. With OFDM, you must be careful about power outputs; the power needs to be reduced to handle the peaks in the modulation technique and still fall within governmental regulations. Table 6-4 shows some details about 802.11g.


The 802.11a Protocol

802.11a was ratified in 1999 and operates in the 5-GHz frequency range. This makes it incompatible with 802.11, 802.11b, and 802.11g, while avoiding interference from these devices in addition to microwaves, Bluetooth devices, and cordless phones. 802.11a had late-market adoption, so it is not as widely deployed as the 802.11b and g protocols.

Another difference is that 802.11a supports anywhere from 12 to 23 nonoverlapping channels as opposed to the 3 nonoverlapping channels in 802.11b/g. Because OFDM is used, subchannels can overlap. 802.11a requires that the data rates of 6, 12, and 24 Mbps be supported but allows for data rates up to 54 Mbps.

Table 6-5 shows some details on the 802.11a standard.


The rules under ETSI specifications are a little different. ETSI allows 19 channels and requires that dynamic frequency control (DFC) and transmit power control (TPC) be used.

What makes 802.11a unique is the way the 5-GHz frequency band is divided into multiple parts. These parts, the Unlicensed National Information Infrastructure (UNII), were designed for different uses. UNII-1 was designed for indoor use with a permanent antenna. UNII-2 was designed for indoor or outdoor use with an external antenna, and UNII-3 was designed for outdoor bridges and external antennas.

The FCC revised the use of the frequency in 2004 by adding channels and requiring compliance of DFC and TPC to avoid radar. The revision also allows all three parts of the UNII to be used indoors. This is not the case with ETSI, however, because it does not allow unlicensed use of UNII-3.


In the 802.11a spectrum, the higher-band channels are 30 MHz apart. This includes UNII- 2 and above. The lower bands are 20 MHz apart.


The 802.11n Protocol

802.11n is currently a draft standard. Again, technology has progressed more rapidly than the standards, because vendors are already shipping 802.11n APs and clients. What makes 802.11n special is that in a pure 802.11n environment, you can get speeds up to 300 Mbps, but most documentation says it will provide 100 Mbps. This is probably because the expectation is that other 802.11 clients will be present. 802.11n is, in fact, backward compatible with 802.11b/g and a.

The backward compatibility and speed capability of 802.11n come from its use of multiple antennas and a technology called Multiple-Input, Multiple-Output (MIMO). MIMO, pronounced Mee-Moh, uses different antennas to send and receive, thus increasing throughput and accomplishing more of a full duplex operation.

MIMO comes in three types:

• Precoding

• Spatial multiplexing

• Diversity coding

Precoding is a function that takes advantage of multiple antennas and the multipath issue that was discussed in Chapter 3, “WLAN RF Principles.” 802.11n uses transmit beamforming (TxBF), which is a technique that is used when more than one transmit antenna exists where the signal is coordinated and sent from each antenna so that the signal at the receiver is dramatically improved, even if it is far from the sender. This technique is something that you would use when the receiver has only a single antenna and is not moving. If the receiver is moving, then the reflection characteristics change, and the beamforming can no longer be coordinated. This coordination is called channel state information (CSI).

Spatial multiplexing takes a signal, splits it into several lower rate streams, and then sends each one out of different antennas. Each one of the lower rate streams are sent on the same frequency. The number of streams is limited to the lowest number of antennas on either the transmitter or the receiver. If an AP has four antennas and a client has two, you are limited to two.

Currently, the Wi-Fi Alliance is certifying 802.11n devices even though they are still in draft status. The Wi-FI Alliance is doing this using the interim IEEE 802.11n draft 2.0.

Common Antenna Types

The two main types of antennas are directional and omnidirectional. In this section you will learn the difference between the two types and look at some of the antennas that Cisco offers. Both send the same amount of energy; the difference is in how the beam is focused. To understand this, imagine that you have a flashlight. By twisting the head of the light, you can make the beam focus in a specific area. When the beam has a wider fo- cus, it doesn’t appear to be as bright. While you twist the head of the light, you never change its output. The batteries are the same. The power is the same. The light is the same. You simply focus it in different ways. The same goes for wireless antennas. When you look at a directional antenna, it appears to be a stronger signal in one direction, but it’s still emitting the same amount of energy. To increase power in a particular direction, you add gain.

The angles of coverage are fixed with each antenna. When you buy high-gain antennas, it is usually to focus a beam.


Omnidirectional Antennas

There are two ways to determine the coverage area of an antenna. The first is to place the AP in a location and walk around with a client recording the signal-to-noise ratio (SNR) and Received Signal Strength Indicator (RSSI). This could take a really long time. The sec- ond method is a little easier. In fact, the manufacturer does it for you. Figures 5-3 and 5-4 show different views of the wireless signal. Figure 5-3 shows how the wireless signal might propagate if you were standing above it and looking down on the antenna.

This is called the horizontal plane (H-plane) or azimuth. When you look at an omnidi-rectional antenna from the top (H-plane), you should see that it propagates evenly in a 360-degree pattern.

The vertical pattern does not propagate evenly, though. Figure 5-4 shows the elevation plane (E-plane). This is how the signal might propagate in a vertical pattern, or from top to bottom. As you can see, it’s not a perfect 360 degrees. This is actually by design. It’s what is known as the “one floor” concept. The idea is that the signal propagates wider from side to side than it does from top to bottom so that it can offer coverage to the floor it is placed on rather than to the floor above or below the AP.


Another way to look at this is to imagine an AP, as shown in Figure 5-5. If you draw in the H-plane and E-plane, you can relate the signal to each plane.

Now that you have a better understanding of how to determine the propagation patterns of an antenna, let’s look at some antennas.


2.2-dBi Dipole

The 2.2-dBi dipole,orrubberduck,showninFigure5-6,ismostoftenseenindoorsbe- cause it is a very weak antenna. In fact, it’s actually designed for a client or AP that doesn’t cover a large area. Its radiation pattern resembles a doughnut, because vertically it doesn’t propagate much. Instead, it’s designed to propagate on the H-plane. The term dipole may be new to you. The dipole antenna was developed by Heinrich Rudolph Hertz and is con- sidered the simplest type of antenna. Dipoles have a doughnut-shaped radiation pattern. Many times, an antenna is compared to an isotropic radiator. An isotropic radiator assumes that the signal is propagated evenly in all directions. This would be a perfect 360- degree sphere in all directions, on the H and E planes. The 2.2-dBi dipole antenna doesn’t work this way; rather, it has a doughnut shape.


IR-ANT1728

The AIR-ANT1728, shown in Figure 5-7, is a ceiling-mounted omnidirectional antenna op- erating at 5.2 dBi.

You would use this when a 2.14-dBi dipole doesn’t provide adequate coverage for an area. This antenna has more gain, thus increasing the H-plane, as shown in Figure 5-8.

The easiest way to express the effect of adding gain—in this case, 5.2 dBi versus 2.2 dBi— is to imagine squeezing a balloon from the top and the bottom, as shown in Figure 5-9.

The squeezing represents the addition of gain. The H-plane widens and the E-plane short- ens, as shown in Figure 5-10.

Table 5-2 details the statistics of the AIR-ANT1728.

Antenna Communications

Principles of Antennas

If someone asked you what the most important part of a wireless network is, what would you say? I’d have to say the antenna. Why? Without it, you have a nice little AP that can offer network services for anyone within about 3 feet. But that’s not what you want. You want to make sure that your space is properly covered. You need antennas to do this. In fact, you need the right antennas to do this. In this section you will learn about the factors involved in dealing with antennas, which include polarity and diversity.


Polarization

The goal of an antenna is to emit electromagnetic waves. The electro portion of the term electromagnetic describes the wave and that it can move in different ways. The way that it moves is its polarization. There are three types of polarization:

• Vertical
• Horizontal
• Circular

As shown in Figure 5-1, vertical polarization means that the wave moves up and down in a linear way. Horizontal polarization means that the wave moves left and right in a linear way.

The third type of polarization, circular polarization, indicates that the wave circles as it moves forward, as illustrated in Figure 5-2.

The electric field is generated by stationary charges, or current. There is also a magnetic field hence the term electromagnetic. The magnetic field is found perpendicular (at a 90- degree angle) to the electric field. This magnetic field is generated at the same time as the electric field; however, the magnetic field is generated by moving charges. Cisco antennas are always vertically polarized in wireless networks. This makes the electric field vertical. Why is this important? The importance is that the antenna is designed to propagate signals in a certain direction. Here is where installation errors can hurt you. For example, if you have a long tube-like antenna, it would face up/down. If you placed it flat instead, the signal would propagate in a different direction and would end up degraded.

Although this is not a huge factor in indoor deployments, it can be in outdoor deployments. Usually other factors degrade your wireless signal propagation on indoor deployments.


Diversity

By now you should understand what the multipath issue is. Traffic takes different paths because of the obstacles in the wireless path. One way to deal with multipath issues is to use two antennas on one AP. Diversity is the use of two antennas for each radio to increase the odds that you receive a better signal on either of the antennas.



Here is how it works: The two antennas are placed one wavelength apart. When the AP hears a preamble of a frame, it switches between the two antennas and uses an algorithm to determine which antenna has the better signal. After an antenna is chosen, it is used for the rest of that frame. You can switch antennas and listen to the preamble because it has no real data. As soon as the real data gets there, it uses only one of the antennas.

Most of the time this happens with a single radio in the AP and two antennas connected to it. This is important because the two antennas cover the same area. You wouldn’t try to cover two different areas with the same radio. Additionally, the antennas need to be the same. If you used a weaker antenna on one side versus the other, the coverage area would not be the same.