As mentioned in Chapter 1, “The Basics of Wi-Fi Hotspots,” Wi-Fi networks use a technology that is specified within a standard called 802.11. The standard is basically written documents formed by members of the IEEE. These documents help manufacturers develop wireless products so that they will work together with wireless devices from every vendor that follows the standard.

802.11 comes in several versions that have different characteristics, as shown in Table A-1.

Note

It is a frequently misunderstood belief that the actual traffic on a wireless network travels at the maximum or “raw” data rate, such as shown in Table A-1.

The actual data rates that users will experience will be much less, up to 50 percent less, due to the normal overhead of the network protocol.

The most widely used standard today is 802.11g, which is similar to 802.11b. However, 802.11g can handle a much greater data rate, up to 54 Mbps. The 802.11g standard is an improved version of 802.11b, in which the main upgrade is the higher data rates, or speeds. The 802.11g standard was created to be backward-compatible with 802.11b. Therefore, it is possible to mix 802.11b and 802.11g devices within the same wireless network. The 802.11a standard, though, is not compatible with these two standards and does not even use the same frequency band.

A drawback to using the 802.11b and 802.11g standards is that they both use the 2.4-GHz frequency band, which has more potential congestion. Other devices such as cordless phones, wireless speakers and headphones, and baby monitors use this band. Microwave ovens also emit radio waves in this range and can cause interference. However, these two standards are most likely the best choice for use in public hotspots because nearly all client devices today implement 802.11g. The 802.11a standard is not widely used throughout the consumer market.

802.11n is an evolving wireless networking standard used for Wi-Fi devices. This upcoming standard plans to use the 2.4-GHz frequency band, as do 802.11b and g, with data rates up to 540 Mbps, which is 10 times faster than existing standards. In addition, these products will have a much longer range by using a powerful smart-antenna technology, called Multiple Input Multiple Output (MIMO), which is already available in some wireless networking products.

Wireless networking vendors have already started selling products based on this technology. The products are referred to as pre-n or 802.11n draft products. With these prestandard components, the user adapter and your AP (or router) must be manufactured by the same vendor.

In addition, even with finalized 802.11n products, both the AP, or wireless router, and the user’s wireless adapter have to be 802.11n to achieve all the benefits. Even though it might take a few years for the majority of Wi-Fi users to upgrade, implementing 802.11n (when finalized) for your Wi-Fi hotspot might not be a bad idea, because 802.11b/g users might still receive benefits such as a slight increase in range.

Note

Keep in mind that even though 802.11n might increase tenfold the data rate or speed of a wireless network, typical Internet connection speeds are much lower, such as 3 or 4 Mbps or lower. Therefore, even if the network operates at 540 Mbps, users who are just browsing the web and sending e-mail, such as at hotspots, do not see much of a speed advantage when using 802.11n. However, file sharing between computers on the same private network is much faster for 802.11n users.

All 802.11 technologies operate in the same manner. The operational functions, such as connecting, scanning, and roaming between APs, are controlled by the MAC layer, which is specified in the 802.11 standard. This section covers how the main MAC operations of wireless networks work.

Beacons

Access points (APs) and wireless routers broadcast beacons via the airwaves, as shown in Figure A-1. The beacons provide a means of identifying APs and wireless routers to wireless clients. These beacons also contain pertinent information about the wireless network to inform nearby wireless clients of the network and to synchronize the wireless clients that are already part of the network.

Figure A-1. Example of Beacon Broadcasts

Beacons are sent periodically, starting as soon as you plug in the AP or wireless router, at an interval of 1 to 65,535 milliseconds. The default interval that most vendors use is 100 milliseconds, which equates to 10 beacons being sent every second. For hotspot installations, leave the beacon interval set to 100 milliseconds.

Note

Most APs and wireless routers allow you to disable service set identifier (SSID) broadcasting, which removes the SSID from beacons. This helps hide the network from others. Obviously, you would not want to disable SSID broadcasting on your hotspot network; however, this feature might be useful if you have a separate private network.

The beacons contain information, such as the SSID (name) of the network, supported data rates, timestamp, and more about the wireless network. The wireless client uses this information to determine which AP or wireless router to connect to.

Wireless Client Scanning

Wireless clients (or radio cards) are constantly listening for activity in their specific frequency band. Wireless clients have two main methods to scan for wireless networks (APs and wireless routers):

• Active scanning

• Passive scanning

Active scanning is used when a wireless client wants to search for a specific wireless network. The wireless client is depicted in Figure A-2. It sends a packet, called a probe request, on each channel, asking if a network, or an AP/wireless router, is nearby. The wireless client can ask either for a specific network or for any wireless network.

Figure A-2. Example of a Wireless Client Actively Scanning

If the wireless client is probing for a specific network, only networks that match the SSID in the request respond with a probe response. However, if the wireless client is probing for any wireless networks nearby, all the APs or wireless routers that “hear” the request respond with a probe response.

Passive scanning, by contrast, takes a hands-off approach. In this case, the wireless client just listens for beacons on each channel. Most wireless clients use a combination of active and passive scanning using proprietary mechanisms. Even when a wireless client is connected to a wireless network, it still periodically scans other channels for nearby wireless networks. This is necessary if the user is roaming through a facility to determine whether to connect to a different AP or wireless router. With both scanning methods, the wireless client stores the information from the wireless networks it discovers. Client software, such as Windows XP, displays a list of applicable wireless networks to the user.

Sending and Receiving

In infrastructure wireless networks, such as hotspots, the APs or wireless routers are the coordinators that regulate the traffic from the wireless clients. A common misconception is that the wireless clients send and receive traffic directly to and from each other. However, as shown in Figure A-3, all the traffic goes through the AP, even if a wireless client is accessing or transferring shared files from another wireless client on the network. Figure A-4 depicts the traffic flow when a wireless client is accessing the Internet.

Figure A-3. Example of Transferring a File Within a Wireless Network

Figure A-4. Example of a Wireless Client Communicating with the Internet

The two widely used wireless networking standards (802.11b and g) operate in the 2.4-GHz frequency band. The total RF allocated space for transmissions is approximately 72 MHz. In this total allocated RF space, 14 defined channels exist.

Note

Although 14 channels exist overall, only 11 of them are legal for use in the United States. However, in Japan, all 14 channels are available. Countries in Europe and most of the rest of the world can use channels 1 through 13.

When a wireless client or AP sends something, it transmits signals on a channel (1 through 14) that occupies about 22 MHz out of the 72 MHz total allocated space. Simple math (14 channels × 22 MHz) shows that at least 308 MHz of RF space is needed to accommodate 14 channels. However, only 72 MHz of space exists for all the channels, and the frequencies that the channels use overlap. Theoretically, not enough RF space is available for each channel to be active at the same time, at least without the potential for problems.

Therefore, as illustrated in Figure A-5, all but three channels overlap each other. Keep in mind that the figure shows only channels 1 through 11, which are legal for use in the United States.

Figure A-5. DSSS Channels

This is why you should use only the nonoverlapping channels (1, 6, and 11) if you are implementing more than one AP or wireless router in your hotspot. The use of a different nonoverlapping channel for adjacent APs and routers improves performance by reducing interference. Figure A-6 illustrates an example of how you can assign the channels of your APs. Even though the wireless coverage of the APs (called radio cells) might overlap, you should not have a problem if the nonoverlapping channels are utilized.

Figure A-6. Example of Using Nonoverlapping Channels

Keep in mind, when selecting the channels for your APs, that other wireless networks might be nearby. You also should plan your channel assignments based on any existing Wi-Fi signals that are in or around your designated coverage area.

Note

NetStumbler is free software that you can download from http://www.netstumbler.com to identify the channel settings of nearby APs and wireless routers.

If you are implementing a hotspot that needs to support many users in a dense area, such as in a huge auditorium filled with students who are accessing the web from their laptops, the overlapping channel situation might cause some difficulties. This is because you likely need to designate many APs in the dense area to support all the users. Even though a single AP might be able to handle more than 200 users connected to it at once, the throughput (speed) that each user experiences would be extremely low. When designating more than three APs in a single space, even though it might be in a huge auditorium, the cells of the APs will probably overlap with other cells. It is a problem only when cells on the same channel overlap.

Tip

When many APs are colocated or when it is necessary to decrease their coverage or their cell size, you can turn down their transmitter power. In addition, you might want to make sure you are not using a high-gain antenna.

You should be familiar with two different antenna types:

Omnidirectional antennas propagate the Wi-Fi signals the same in all directions. Most manufacturers provide this antenna type on their APs, wireless routers, and client devices. Omnidirectional antennas do well in most facilities.

Figure A-7 shows a pair of omnidirectional antennas.

Figure A-7. Example of Omnidirectional Antennas

Wireless networking vendors usually sell high-gain omnidirectional antennas (see Figure A-8 for an example) for use with their products. This allows a larger Wi-Fi coverage area in all directions.

Figure A-8. Example of a High-Gain Omnidirectional Antenna

Directional antennas, however, focus the Wi-Fi signals and provide much greater wireless coverage in one direction than in others. Unlike omnidirectional antennas, most directional antennas are external, meaning you need to mount the antenna somewhere and connect the antenna cable to the AP or wireless router. Figure A-9 shows an example of a directional antenna, called a patch antenna. Figure A-10 depicts how this antenna type might propagate the Wi-Fi signals.

Figure A-9. Example of a Patch Antenna

Figure A-10. Coverage Area of a Patch Antenna

A directional antenna is useful when you want more horizontal than vertical coverage, such as when providing coverage for a long set of offices.

One of the biggest challenges when working with wireless networks is the RF interference and attenuation. It takes practice to develop the skills necessary to predict the coverage and performance of wireless networks to aid in a proper installation. Keep in mind that 802.11 wireless devices share the airwaves with other wireless technologies that might cause significant interference with your wireless network or hotspot. The most common of these devices are as follows:

Using these devices in or around your wireless coverage area will likely degrade the performance of your network, or hotspot. In some cases, you might not even be able to keep a connection with the wireless network such as when using a wireless client next to an operating microwave oven. You can take some simple steps to help prevent these issues. Try to use cordless phones and other wireless devices in other frequency bands, such as 900 MHz or 5 GHz. Try not to use microwave ovens around APs or wireless clients. You might also avoid RF interference from microwave ovens by avoiding channel 11, because the top one-third of a band is typically affected the worst. You can also refrain from installing APs close to microwave ovens.

Attenuation, as discussed in this book, means the loss of radio signal strength. Free space loss is a type of attenuation that is the natural loss of the radio signal when propagating through the air without obstructions. As Figure A-11 illustrates, the signal gets weaker and weaker when traveling away from the AP.

Figure A-11. Depiction of Free Space Loss

If buildings did not have walls, desks, filing cabinets, or elevators, it would be much easier to install wireless networks. This is because pretty much all you would have to think about would be free space loss when determining how many APs a facility needs, where to put them, and what antennas to use. However, this is not the case; buildings are usually filled with things that cause significant attenuation to your Wi-Fi signals. For instance, Figure A-12 shows how a wall might block RF signals.

Figure A-12. Attenuation from an Obstruction

Suppose that you need good coverage on the other side of the brick wall, as shown in Figure A-12. You could replace the standard antennas on the AP with high-gain antennas. Then you would likely increase the signal quality enough to better penetrate the brick wall, and wireless clients on the other side would have a good connection. Alternatively, you could move the AP closer to the wall.

This appendix covered many different factors relating to Wi-Fi that can help during the installation and administration of your hotspot.

The following are a few items you need to remember: