It should come as no surprise to anyone that there are several issues faced with a WLAN deployment. These issues can be very complex, so a careful understanding of RF technology is needed to successfully deploy a WLAN. There are several factors that can affect the radio waves being sent out by WLAN devices, causing corruption, loss, gain, and many other issues. I will discuss these issues now.
First I would like to touch upon loss. Loss occurs when there is a decrease in signal strength. Loss can occur both before and after the signal hits the air. It can happen for a variety of reasons, such as resistance in electrical cables and obstacles. Like all electrical signals, RF signals weaken as they get further away from the source. Obstacles, such as brick walls, will decrease the strength as well. As an example, the apartment in which I live is in a brick building. Forty years ago, when the building was converted from a single-family home into a multi-unit dwelling, there was an extension built onto the back of this brick building to provide a kitchen area as well as a fire escape. I have most of my computers in the front half of this building, but we often take my wife’s laptop into the back part of the building, as it is handy to look up recipes and perform other tasks while in the kitchen or dining room. Something I noticed right away was that as soon as I left the room containing our wireless router the signal strength went from five bars down to two bars. By the time I reached the back of the house I was no longer able to access the wireless network, as the signal strength had degraded so much in that short distance. When the signal passed through the brick wall, I experienced obstruction loss. As I moved further away from the access point I experienced path loss.
Gain is another factor affecting RF behaviors. Gain is almost the opposite of loss, but not quite. Gain is an increase in the amplitude of the RF signal. It can be caused by the use of an amplifier or a high antenna. An antenna can not amplify a signal, but it can focus it, thereby increasing the amplitude. The ability of an antenna to focus a signal is called gain. It is possible that a device may be able to receive a signal that has significant gain but it will not be able to establish two-way communication. If you are implementing a situation where gain is required, it is optimal to implement gain on both ends.
Lobe refers to the amount of coverage area of an RF signal sent from an antenna. An antenna with gain can focus the area of coverage and narrow (or focus) the lobe so that it has less coverage vertically, but more coverage horizontally. This will increase the range of the signal while decreasing the amount of up-and-down coverage area. This is ideal for long-range wireless communications from one point to another, as the signal can be focused directly on the receiving antenna.
Refraction occurs when an RF signal passes through an obstacle and bends. Refraction can cause the RF signal to be diverted from its intended destination. This is particularly a problem on long-distance point-to-point links.
Diffraction occurs when an RF signal bends as it passes around an obstacle. Diffraction can result in the distortion of an RF signal.
Scattering occurs when an RF signal bounces off a rough surface, or off of particles in the air. Both situations cause the RF signal to scatter in many directions.
Absorption occurs when an RF signal is absorbed by an object and is not allowed to pass through at all. The problem here should be obvious!
Finally, reflection can occur when an RF signal bounces off a large obstacle, causing degradation of signal strength. The receiving end receives the data through both the direct wave and the reflected wave and attempts to process both signals simultaneously. This can cause a multi-path signal, which can become a serious problem. If the multi-path signal is 180 degrees out of phase with the original signal, the two signals can cancel each other out! Mathematically speaking, the degradation of signal strength occurs at a fourth power of distance, or Strength = -1/d4.
There are also some delay issues that are expected with a WLAN. As mentioned earlier, WLANs use ACK packets to signify that the data has been correctly received. If a packet is not received, an automatic repeat request (ARQ) is used by the WLAN to request the retransmission of the packet. In a VoIP environment delays can be experienced while the data is being retransmitted. Delays can also be experienced because forward and reverse direction MAC flows that the WLAN uses to maintain link reliability and control use bandwidth as well. Since stations are always contending for access to the media we can experience further delays. Typical WLAN delays include:
1. Interframe Space (50 μs)
2. Back off (average of 80 μs)
3. Synchronization Preamble (192 μs)
4. Data (approximately 171 μs)
5. Gap between Data and ACK (10 μs)
6. Next Preamble (192 μs)
7. MAC ACR (11 μs)
As we can see, we can expect an average delay of 706 μs. In addition we can expect to see further delays on our WLAN as we add more MUs to the network, each trying to compete with each other for access to the medium.
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