Troubleshooting Wireless LAN Installations

Just as traditional wired networks have challenges during implementation, wireless LANs have their own set of challenges, mainly dealing with the behavior of RF signals. In this chapter, we will discuss the more common obstacles to successful implementation of a wireless LAN, and how to troubleshoot them. There are different methods of discovering when these challenges exist, and each of the challenges discussed has its remedies and workarounds.

The challenges to implementing any wireless LAN discussed herein are considered by many to be “textbook” problems that can occur within any wireless LAN installation, and, therefore, can be avoided by careful planning and simply being aware that these problems can and will occur.


Multipath

If you will recall from Chapter 2, RF Fundamentals, there are two types of line of sight (LOS). First, there is visual LOS, which is what the human eye sees. Visual LOS is your first and most basic LOS test. If you can see the RF receiver from the installation point of the RF transmitter, then you have visual line of sight. Second, and different from visual LOS, is RF line of sight. RF LOS is what your RF device can “see”.

The general behavior of an RF signal is to grow wider as it is transmitted farther. Because of this type of behavior, the RF signal will encounter objects in its path that will reflect, diffract, or otherwise interfere with the signal. When an RF wave is reflected off an object (water, tin roof, other metal object, etc.) while moving towards its receiver, multiple wave fronts are created (one for each reflection point). There are now waves moving in many directions, and many of these reflected waves are still headed toward the receiver. This behavior is where we get the term multipath, as shown in Figure 9.1. Multipath is defined as the composition of a primary signal plus duplicate or echoed wave fronts caused by reflections of waves off objects between the transmitter and receiver. The delay between the instant that the main signal arrives and the instant that the last reflected signal arrives is known as delay spread.


Effects of Multipath

Multipath can cause several different conditions, all of which can affect the transmission of the RF signal differently. These conditions include:

• Decreased Signal Amplitude (downfade)

• Corruption

• Nulling

• Increased Signal Amplitude (upfade)

Decreased Signal Amplitude

When an RF wave arrives at the receiver, many reflected waves may arrive at the same time from different directions. The combination of these waves' amplitudes is additive to the main RF wave. Reflected waves, if out-of-phase with the main wave, can cause decreased signal amplitude at the receiver, as illustrated in Figure 9.2. This occurrence is commonly referred to as downfade and should be taken into consideration when conducting a sight survey and selecting appropriate antennas.


Corruption

Corrupted signals (waves) due to multipath can occur as a result of the same phenomena that cause decreased amplitude, but to a greater degree. When reflected waves arrive at the receiver out-of-phase with the main wave, as illustrated in Figure 9.3, they can cause the wave to be greatly reduced in amplitude instead of only slightly reduced. The amplitude reduction is such that the receiver is sensitive enough to detect most of the information being carried on the wave, but not all.


In such cases, the signal to noise ratio (SNR) is generally very low, where the signal itself is very close to the noise floor. The receiver is unable to clearly decipher between the information signal and noise, causing the data that is received to be only part (if any) of the transmitted data. This corruption of data will require the transmitter to resend the data, increasing overhead and decreasing throughput in the wireless LAN.