Networking

Common drags on WLAN performance

How to get peak WLAN performance

Low-budget Wi-Fi networks are extremely popular today, but they are not the only types of wireless networks in existence. There are actually dozens of types of wireless networks ranging in price from under a hundred dollars to millions of dollars. While Wi-Fi problems might not be a big deal to correct, it is a huge problem if you have just spent half a million dollars on a wireless network and the network doesn’t perform as expected. In order to get peak performance out of your wireless network, you need to know some common causes of poor performance on both Wi-Fi and non-Wi-Fi wireless networks.

Too many devices
One of the most common problems with wireless networks is having too many wireless devices within close proximity. This problem can be easily avoided by obtaining a professional site survey prior to installing any wireless equipment. Unfortunately, it seems that a lot of networking people think that if they can install a Wi-Fi network, then the rules are the same for other wireless networks; so, they try to install the devices themselves.

The problem is that certain types of wireless devices are very particular about how many devices can be located within an area. A DS-11, Direct Spread Spectrum network is a good example of this. DS-11 networks have a total of 11 available channels. Because of this, it might stand to reason that you could use 11 different networks within an area without interfering, as long as the networks were on different channels. This isn’t the case, though.

The entire concept behind a spread spectrum network is that multiple channels are used in an effort to boost available bandwidth and to increase security. In a DS-11 network, it is only possible to colocate up to three wireless networks before the frequencies start interfering with each other, because each DS-11 device is using multiple channels.

DS-11 isn’t the only technology that’s subject to this limitation though. You may have heard of a wireless networking technology called FHSS (frequency hopping spread spectrum). FHSS is a type of spread spectrum similar to DS-11 that operates in the 2.4- to 2.483-GHz frequency range. Within this range, there are 79 individual channels and 78 different frequency-hopping sequences that may be used. Even with so many available channels, this type of networking is limited to 15 colocated networks.

The lesson here is that it is extremely important to have a professional site survey conducted prior to installing any wireless hardware. A professional site survey will tell you if other devices already exist within the area that might interfere with the network that you plan to install.

Line-of-sight networks
One commonly used type of wireless networking is line-of-sight. Line-of-sight networks may use either radio signals or lasers to transmit data between two points. As the name implies, line-of-sight technology requires that the transmitter and the receiver have a clear line-of-sight between each other.

There are lots of different problems that can occur with line-of-sight networks. For starters, the antennas used for line-of-sight networks can be notoriously difficult to align. You can easily take care of this problem, though, by having the network professionally installed or by purchasing hardware with a self-aligning mechanism.

I personally like the self-aligning mechanism for more reasons than just that of easy installation. Line-of-sight networks are often used to beam signals between two buildings. The problem is that tall buildings tend to sway a little bit on windy days. Although the swaying may not be more than a few inches in either direction, this is often enough movement to disrupt a wireless network signal. Self-aligning hardware can keep the antennas correctly positioned even if the buildings move.

Keep in mind that although buildings are the most notorious for moving, towers can also move. About nine or ten years ago, I was experimenting with satellite Internet access. I bought a dish in another state. After making the long drive home, I realized that I didn’t have clear line-of-sight with the satellite. To compensate for this, a friend came over and helped me to construct an aluminum tower that was just over 20 feet tall.

For the first couple of days, the signal worked great, but then there was a windy day. Although the naked eye couldn’t pick up on much swaying, the signal faded as the tower moved back and forth in the wind.

Maintaining alignment between antennas on a line-of-sight network is important, but it’s only half of the battle. It is also important to take the fresnel zone into consideration.

Imagine for a moment that you are standing at one end of a field with a large flashlight trying to illuminate a target at the other end of the field. The beam of light at the far end of the field will be much wider than the flashlight. This illustrates a principle of light called divergence, in which light spreads out as it travels.

While laser light does not have nearly as high of a divergence rate as that from a flashlight, it does exist. Radio signals are also subject to this phenomenon and spread out as they travel.

The problem with line-of-sight networks is that a lot of people don’t take divergence into effect. I have seen too many people look out a window, and—if they can see the target—assume that they have a clear line-of-sight to it. However, as the signal spreads out, signal strength is reduced. If you want to receive the signal at full strength, it is important that the receiving antenna have a clear line-of-sight to the entire inbound signal, not just a part of it. The area encompassing the signal is called the fresnel zone.

The fresnel zone identifies the area making up the signal. If there is an object that partially obscures the signal, part of the signal strength would be lost because of the object in the middle. Most line-of-sight networks lack the signal strength to penetrate such obstacles. This shows the importance of a good site survey.

Improper equipment
Another cause of wireless network problems is the use of improper equipment. Earlier, I said that there are dozens of different types of wireless networks. One of the reasons why there are so many different types is because different installations have different requirements. Things like desired bandwidth, climate, distance, and obstacles all play a part in the equipment requirements. If you choose the wrong equipment, your network simply won’t perform adequately.

One of the main pieces of equipment that you need to take into consideration is your antenna. Even if you have purchased the correct radios and have done a good job planning your network, poor antenna choices will undermine all of your efforts. While this article isn’t intended to be a comprehensive guide to choosing an antenna, I want to take a moment to discuss a few of the more common antenna types, just to give you an idea of why antenna choice is so important.

One of the most common types of antennas is the parabolic dish. This antenna looks like a satellite dish and is commonly used for line-of-sight applications. Most of the time, networks using this type of antenna lack the signal strength to penetrate obstacles, but can communicate at great distances. A variation of this type of antenna is the parabolic grid. A parabolic grid works similarly to a parabolic dish, but is better suited to windy environments.

Another type of antenna is a panel or sector antenna. This antenna functions like a parabolic dish, but looks more like a pizza box. These antennas can accept signals varying from 60 to 180 degrees, and are suitable for wide area broadcasts.

Still another common type of antenna is the omni. An omni looks like a CB antenna or like a radio antenna that would be used on a boat. An omni has a 360-degree coverage area, but only along a flat horizon. This means that the signal will travel out in all directions, but won’t really travel up or down.

An alternative to an omni is a patch antenna. A patch antenna is a small circular antenna that also has a 360-degree coverage area. Unlike the omni though, a patch antenna does not have a completely flat horizon. Patch antennas are used primarily for indoor networks.

Poor antenna connection
Yet another common problem is poor antenna connections. A wireless signal is at its strongest when it leaves the receiver. However, there is usually a barrel connector that connects the antenna cable to the receiver, and another barrel connector that connects the antenna cable to the antenna. Barrel connectors diminish the signal strength greatly, as does the length of the antenna cable and even the antenna itself.

Your goal should be to minimize signal loss. To do so, don’t use any more barrel connectors than are absolutely necessary, and use the minimum practical cable length. I should also point out that using amplifiers is usually a bad idea. Amplifiers not only amplify the signal, they also amplify noise. More importantly, though, they generally require you to use more cable and a couple more barrel connectors than would be required to connect the radio directly to an antenna, thus diminishing the signal quality. Even if signal distortion were not an issue, an amplified signal often exceeds FCC-mandated signal strengths.

Recently, a friend told me about a network in which the owner was having problems with poor signal strength. The radio was linked to an amplifier. On the other side of the amplifier was a splitter and two antenna cables, which fed two large antennas. Because of the resistance of the three cables, six barrel connectors, two antennas, and the splitter, virtually no signal was being produced. In this situation, my friend simply connected the radio directly to an antenna via a single cable and two barrel connectors and the radio began to perform as it was designed to.

Wi-Fi performance problems
Since Wi-Fi networks are so popular, I wanted to take a moment to discuss some of the problems that are common to Wi-Fi network performance. Although there are some long-distance Wi-Fi implementations in existence, Wi-Fi is designed primarily to be an indoor networking solution. Therefore, this section will address performance problems in an indoor environment.

Just as the most common performance-related problem on big, expensive, outdoor networks is poor signal strength, Wi-Fi networks tend to suffer from poor signal strengths as well. Access point type and position is very important. Some Wi-Fi implementations simply work better than others around obstacles.

For example, several years ago, I bought an 802.11B wireless access point. After installing this access point, I was able to get a wireless signal anywhere in my entire house or yard. Although I liked the convenience, I was always frustrated by the slow speed.

When 802.11A became available, I installed an 802.11A network in my home. This network operates on a frequency of 5.8 GHz as opposed to the 2.4-GHz frequency used by 802.11B. This means that data rates are much higher. However, a 5.8-GHz signal has much more trouble penetrating obstacles than a 2.4-GHz signal does. Consequently, I now have a very fast wireless network, but there are places in my home where I simply can’t get a signal.

Another common problem with Wi-Fi networks is that an access point may become oversubscribed. For example, the access point that I’m using in my home supports up to 256 simultaneous Wi-Fi connections. Even the first access point that I ever bought back in 1999 could accept up to 64 connections. The problem is that these high numbers of wireless connections tend to be impractical. It has been my experience that performance starts dropping off once more than about 10 clients are using a single access point.

It may seem that the solution is to add more access points so as to reduce the workload on existing access points. While this is a solution in some situations, having too many access points can cause problems because of interference. If you do have more than one access point, it’s a good idea to lock each NIC to a specific access point.

The reason for this is that wireless NICs are designed to roam from one access point to another and to latch onto the access point that has the strongest signal strength. If multiple access points exist in an environment, and two or more have comparable signal strengths, then a NIC may constantly switch back and forth between access points. This greatly reduces network performance.

Locking a NIC to a specific access point accomplishes three things. First, it eliminates the constant switching between access points. Second, it increases security because no NIC may use an access point unless you have specifically authorized it to do so. Finally, it prevents any of your access points from being oversubscribed.

One last problem that tends to occur with wireless NICs is something called multipath. The best way to describe multipath is by comparing it to a television. You have probably seen a television that used rabbit ears or another type of air antenna. With air antennas, it’s common to have some channels in which the main image on the screen is superimposed with a ghosted image. The ghosted image is caused by multipath.

Multipath is caused when a signal bounces off of nearby objects and arrives at the receiver at different times. For example, suppose that you place a wireless NIC into a room that also contained an access point. As the access point transmits a signal, the signal spreads out in all directions and finds its way to the NIC. However, the signal may also bounce off of another object in the room and find its way to the wireless NIC by this diverted path. This means that the wireless NIC is actually receiving the signal twice.

In the real world, it’s almost impossible to get rid of multipath signals. The most that you can do is to try to reduce it by not having any large metal objects in the proximity of the access point or of the wireless NIC.

No wires doesn't mean no problems
As you can see, there are many different types of wireless networking problems. Over the last year, I have seen a lot of networking professionals make a trip to the computer store and spend a couple hundred dollars on a wireless access point and a few NICs. Often, when this equipment gets installed the results can be disappointing. Once you know what can cause problems with wireless networks, you can figure out how to solve those problems and get the most out of your investment.
0 comments

Editor's Picks