Factors Affecting Wireless Signals

Because wireless signals travel through the atmosphere, they are susceptible to different types of interference than with standard wires networks. Interference weakens wireless signals and is therefore an important consideration when working with wireless networking.

Interference Types

Wireless interference is an important consideration when planning a wireless network. Interference is unfortunately inevitable, but the trick is to minimize the levels of interference. Wireless LAN communications are typically based on radio frequency signals that require a clear and unobstructed transmission path.

What are some of the factors that cause interference?

• Physical objects Trees, masonry, buildings, and other physical structures are some of the most common sources of interference. The density of the materials used in a building's construction determines the number of walls the RF signal can pass through and still maintain adequate coverage. Concrete and steel walls are particularly difficult for a signal to pass through. These structures will weaken or, at times, completely prevent wireless signals.

• Radio frequency interference Wireless technologies such as 802.11b/g use RF range of 2.4GHz, and so do many other devices such as cordless phones, microwaves, and so on. Devices that share the channel can cause noise and weaken the signals.

• Electrical interference Electrical interference comes from devices such as computers, fridges, fans, lighting fixtures, or any other motorized devices. The impact that electrical interference has on the signal depends on the proximity of the electrical device to the wireless access point. Advances in wireless technologies and in electrical devices have reduced the impact these types of devices have on wireless transmissions.

• Environmental factors Weather conditions can have a huge impact on wireless signal integrity. Lighting, for instance, can cause electrical interference, and fog can weaken signals as they pass through.

Some of the equipment and materials that can interfere with wireless LAN transmissions include

Equipment such as cordless phones or microwaves that produce radio waves in the 2.4 or 5.2GHz range

RF noise caused by two wireless LANs operating in close proximity

Outdoor broadcast television used by mobile television cameras

Uninterruptible power supply (UPS) devices

Large objects such as pine trees

Fluorescent lights

Heavy machinery

Heavy-duty motors found in elevators or other large devices

Plants and trees

Close proximity to smaller electric devices such as computers or air conditioners

Transformers

This is not an exhaustive list, but it shows how wireless signals can be influenced by many different factors.

Wireless Antennas

A wireless antenna is an integral part of the overall wireless communication. Antennas come in many different shapes and sizes with each one designed for a specific purpose. Selecting the right antenna for a particular network implementation is a critical consideration and one that could ultimately decide how successful a wireless implementation will be. In addition, using the right antennas can save money as networking costs because you'll need fewer antennas and access points.

When a wireless signal is low and being influenced by heavy interference, it might be possible to upgrade the antennas to create a more solid wireless connection. To determine the strength of an antenna, we refer to its gain value.

An antenna's gain is a measure of how well the antenna will send or pick up a radio signal. The gain value is measured in decibels-isotropic, or dBi. The gain value of an antenna is a unit of comparison to a referencethat reference being an isotropic antenna. An isotropic antenna is an antenna that sends signals equally in all directions (including up and down). An antenna that does this has a 0dBi gain.

An antenna's rating (gain) is the difference between the 0db isotropic antenna and the actual antenna rating. As an example, a wireless antenna advertised as a 15-dBi antenna is 15 times stronger than the hypothetical isotropic antenna.

The initials "dB" reference decibels, and the "i" references the isotropic antenna. dBi is a unit measuring how much better the antenna is compared to isotropic signals.

When looking at wireless antennas, remember that a higher gain rating means stronger sent and received signals. In terms of performance, the rule of thumb is that every 3dBi of gain added doubles the effective power output of an antenna.

Antenna Coverage

When selecting an antenna for a particular wireless implementation, it is necessary to determine the type of coverage used by an antenna. In a typical configuration, a wireless antenna can be either omni directional or directional.

An omni directional antenna is designed to provide a 360-degree dispersed wave pattern. This type of antenna is used when coverage in all directions from the antenna is required. Omni directional antennas are good to use when a broad-based signal is required. Because of the dispersed nature of omni directional antennas, the signal is weaker overall and therefore accommodates shorter signal distances. Omni directional antennas are great in an environment in which there is a clear line of path between the senders and receivers. The power is evenly spread to all points, making omni directional antennas well suited for linking several home and small office users.

Omni directional antennas provide a wide coverage but weaker signal strength in any one direction than a directional antenna.

Directional antennas are designed to focus the signal in a particular direction. This focused signal allows for greater distances and a stronger signal between two points. The greater distances enabled by directional antennas allow a viable alternative for connecting locations, such as two offices, in a point-to-point configuration.

Directional antennas are also used when you need to tunnel or thread a signal through a series of obstacles. This concentrates the signal power in a specific direction and allows you to use less power for a greater distance than an omni directional antenna.

In the wireless world, polarization refers to the direction that the antenna radiates wavelengths. This direction can either be vertical, horizontal, or circular. Today, vertical antennas are perhaps the most common type. As far as configuration is concerned, both the sending and receiving antennas should be set to the same polarization.