Wi-Fi

Issues to consider when incorporating WLAN

What a WLAN is and how it works

I must admit that I was totally wrong back in 1994 when I said that dial-up Internet service providers would never survive. Eight years later, not only are more people demanding Internet access, but they also expect to be able to take it on the road. A number of wireless networking products have been around for a few years, and new ones are springing up every day.

In this Daily Drill Down, I’ll discuss wireless local area network (WLAN) technology, focusing on the IEEE 802.11 standard. I’ll explain some of the issues you should consider when incorporating WLAN technologies into your network. Finally, I’ll examine several WLAN products and vendors.

What is WLAN?
A WLAN is an on-site data network that uses radio technology to allow computers to communicate without the restrictions of wiring. Because the range of the devices is limited to less than 1,000 ft, a WLAN typically exists within a building or surrounding areas. The obvious benefits of such a network include:
  • Easy setup: There are no wires to run and no holes to drill.
  • Easy reconfiguration: Just move the terminal to the new location—you don't have to change addresses.
  • Extreme portability: The WLAN’s portability makes it perfect for inventory management applications or other applications where workers must be mobile.

WLANs use several types of communication standards. The most popular WLAN standard is IEEE 802.11, which has been around for several years. There are also the Bluetooth and infrared technologies.

What about Bluetooth?
There’s a new Bluetooth standard in the works, and you can read about it at Bluetooth’s Web site. For the purposes of this Daily Drill Down, I've chosen to ignore Bluetooth because Bluetooth is still relatively new and unproven, and it’s not widely deployed. In addition, Bluetooth’s limited range (30 ft) makes it impractical for most wireless needs. It’s also not widely available yet. You have to be a member of the Bluetooth consortium in order to read most parts of the site. However, I did find a Toshiba advertisement at the Toshiba Web site that claims a data rate of up to 720 Kbps at up to 100 ft! That’s slower than IEEE 802.11, and far more range restricted. If this is the best that Bluetooth has to offer, I think customers will favor IEEE 802.11.

For several reasons, I believe IEEE 802.11 will drive competing technologies like Bluetooth out of the marketplace. First, Bluetooth is way behind in terms of deployment. A number of companies, such as Wal-Mart, Ford, and Home Depot, have been using IEEE 802.11 WLAN equipment for inventory management and point-of-sale applications for years. So IEEE 802.11 enjoys an established base of existing installations and a proven track record.

In addition, costs are coming down on IEEE 802.11, especially the 2-Mbps units. All the R&D costs have long since been recovered, so vendors can offer these devices at a far lower cost than Bluetooth devices. Early Bluetooth products will include the R&D costs until many units have been sold.

IEEE 802.11 products have been around for several years. Many products have been designed around this standard, and vendors are still aggressively producing new products that maintain compatibility with 802.11. If you take a look at the Wireless Ethernet Compatibility Alliance (Wi-Fi Alliance) Web site, you’ll see that many vendors are conforming to the IEEE standard. While Motorola, Nokia, and Ericsson are working on Bluetooth products, more than 50 vendors are working together to conform to the IEEE standard.

Another wireless technology, infrared (IR) wireless, is restricted to line-of-sight applications. When used outdoors, it’s extremely sensitive to obstructions, such as rain or fog. IR bridges can achieve very high data rates, however, so they have legitimate applications.

Based on these facts, I recommend sticking with IEEE 802.11-compliant WLAN equipment. The early IEEE 802.11 devices reached data rates of up to 2 Mbps using spread spectrum radio frequency (RF) transmissions. Today, 11-Mbps products are available from a number of vendors.

802.11a vs. 802.11b
Currently, the 802.11 standard is broken down into two separate substandards: 802.11a and 802.11b. Both of these standards accomplish the same thing: They allow you to fully access your network without being attached to a cable. However, they go about it in different ways.

Even though the “b” in 802.11b suggests that 802.11b came second, it was actually the first widely available and affordable wireless standard. The 802.11b standard runs on the 2.4-GHz wireless band and at a maximum rated speed of 11 Mbps. As distance from the central wireless access point increases, 802.11b’s speed decreases. In an indoor setting, 802.11b’s speed and distance measurements are as follows:
  • Up to 150 ft: 11 Mbps
  • 151-240 ft: 5.5 Mbps
  • 241-360 ft: 2 Mbps
  • 360-450 ft: 1 Mbps

The 802.11a standard is a newer implementation of 802.11. It runs faster and at greater distances than 802.11b. Unfortunately, it also costs more, and it’s incompatible with 802.11b equipment.

The 802.11a standard runs in the 5-GHz wireless band. This is a mixed blessing. Because it runs in this band, 802.11a equipment can’t communicate with 802.11b equipment. However, this band is also immune to the crosstalk that plagues 802.11b due to things like cordless phones that also share the 2.4-GHz band.

Equipment that meets the 802.11a standard can communicate at a maximum speed of 54 Mbps. The maximum distance of 802.11a is shorter than 802.11b: only 300 ft. Like 802.11b, 802.11a steps down in speed as distance from the wireless access point increases in the following increments:
  • Up to 20 ft: 54 Mbps
  • 21-40 ft: 48 Mbps
  • 41-75 ft: 36 Mbps
  • 76-85 ft: 24 Mbps
  • 86-135 ft: 18 Mbps
  • 136-175 ft: 12 Mbps
  • 176+ ft: 6 Mbps

Additional 802.11 standards are under development. For example, 802.11g runs at speeds faster than 802.11b but maintains compatibility with 802.11b networks. In addition, the emerging 802.11e standard adds error correction and other features to the 802.11 standard. However, there are few products available that take advantage of these new technologies.

How WLAN works
Wireless radio transmissions shift frequencies to avoid interference. There are two methodologies for changing frequencies in WLAN transceivers: frequency hopping and direct sequencing. Frequency-hopping spread spectrum (FHSS) follows a seemingly random pattern of frequency shifts. Both the transmitter and the receiver know the frequency pattern. Direct sequencing spread spectrum (DSSS) moves sequentially through a range of frequencies. Typically, the direct sequencing radios should offer a higher data rate, while frequency hopping tends to be less sensitive to noise.

The spread spectrum concept is critical to the operation of WLAN equipment. Since you have a number of radios all trying to talk on the same frequencies, you need to ensure a communication path for each transmitter/receiver pair. You could configure each WLAN device pair to communicate on a specific frequency, but then you’d have to configure each device specifically for the given network. This would complicate mobility and create configuration problems far worse than wired networks.

Spread spectrum allows all of the radios to share the same frequency ranges. If two radios try to communicate at the same time, one of them will fail and retry after a time interval. In this manner, you can communicate with a large number of radios from a single access point. Additionally, the spread spectrum concept allows the radios to function in a region in which a particular kind of RF noise is present. If one portion of the spectrum is too noisy, the radios are still able to communicate over the remaining portions of the spectrum. Some vendors may even have intelligent algorithms for skipping over noisy portions of the spectrum so that radios don’t waste time retransmitting lost data on those frequencies.

The actress who discovered spread spectrum
Just after World War I, the “most beautiful woman in the world” was singing with her accompanist when she came up with the concept of spread spectrum communication. Believe it or not, the actress Hedy Lamarr realized that listeners have no trouble comprehending the lyrics of a song despite all the changes in pitch that occur in the song (Click here for more on this). Before she came to the United States, Lamarr was married to an Austrian arms dealer. She understood the many problems that plagued RF transmissions, and she realized that a radio transmitter/receiver pair could change frequencies through a wide range of the spectrum in order to avoid interference. This fact would be especially useful in war, since the enemy couldn't jam the entire spectrum without disabling their own communications. This technology also provided a fair amount of security at the time, since only the transmitter and the receiver would know the pattern of frequency changes. Every other receiver would just hear noise.

WLAN components
The simplest WLAN network consists of two computers with WLAN interface adapter cards. These cards behave just like Ethernet cards, except they don’t require any wires. The configuration is peer-to-peer, and the devices are limited to a distance of approximately 500 feet, depending on such things as the wireless cards, networking environment, and any interference. If you add a repeater device, you can effectively double the range. These repeaters, often called extension points (EP), simply echo the signals they receive. This, of course, adds latency, and the devices potentially reduce your data rate since you introduce an extra transmitter to the network.

Like the EP, the wireless access point (WAP) sends and receives WLAN transmissions. The WAP connects the WLAN to conventional wired networks. It also performs all the same functions as the EP. When setting up a WLAN network, you must analyze the region to be covered and arrange the access points to make sure each region has adequate coverage.

What happens when a WLAN card partially overlaps into the coverage of two different access points depends on the manufacturer. For example, Symbol Technologies uses a strategy called "preemptive roaming." In this strategy, each WLAN client card retains a list of all WAPs that it can hear. It sorts this list according to signal strength. Normally, each WLAN client card chooses the WAP with the best signal strength or the least load imbalance. Basically, the card is always aware of all the WAPs it can talk with. It favors the one with the best signal strength, unless that WAP is heavily loaded with other WLAN clients. If the traffic is too high on that WAP, it favors another one, unless its signal strength is too weak. Also, there is a heavy amount of hysteresis that prevents it from flipping back and forth between WAPs. Other vendors have similar solutions.

Network design considerations
You must consider a number of factors before deciding to use WLAN. In the event that WLAN makes sense, choosing a vendor is the next step.

First, figure out how much money you have to spend. Prices have really come down on WLAN interface cards. Some vendors are offering cards below $100. For example, you can find wireless PC cards for laptops from SMC in the $75 range. These days, 802.11a wireless access points and cards normally run at least twice the cost of 802.11b wireless access points and cards. You must balance your budget with your other needs.

Next, you must evaluate your mobility needs. If all hardware will be located within a 500-1,000-feet radius, you can get away with one WAP. If not, you must determine how many WAPs you’ll need. These WAPs should have sufficient overlap so you don’t have dead zones between them.

Also, you need to identify potential sources of RF noise and find ways to correct these problems. Metal blocks RF energy, so you can use metallic foil to prevent RF energy from passing from one side of the foil to the other. You can shield noisy equipment with metal shrouds. You might want to shield power cords all around the coverage region to prevent RF noise from propagating through the power cords. Finally, you might need to improve the shielding in your facility. A layer of aluminum foil between the insulation and the wall would certainly do the trick.

Next, you must determine what throughput you’ll require. If you want 11-Mbps transmission speeds, you’ll be paying more, and your coverage areas will be smaller. If you can get by with 2-Mbps radios, you’ll save a great deal on costs. Of course, if you wait a year or two, 11-Mbps radios will probably be as cheap as 2-Mbps radios are now. Also, you need to review the differences between frequency hopping and direct sequencing to see if one appeals to you more than the other. Frequency hopping uses less power and costs less. However, frequency hopping is limited to a maximum practical speed of 2 Mbps, while direct sequencing can be much faster.

The final step is choosing a vendor. I select exclusively from the list of companies that support the Wi-Fi standard. Every network grows beyond its original specification, and you need to make sure the equipment you buy will be compatible with as many vendors as possible. Wireless is a very competitive field, and the players change pretty rapidly. The last thing you want is to build half a network out of obsolete and unsupported equipment!
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