Spectrum Analysis II - #CWAP #Revision

April 29, 2020

Spectrum Analysis II – #CWAP #Revision

Exam Specific only as I’ve outlined another post on this topic here

1. Spectrum Analysis – 15%
1.1 Capture RF spectrum data and understand the common views available in spectrum analyzers
1.1.1 Install, configure and use spectrum analysis software and hardware
 Configure Wi-Fi integration
 Save and export capture data
1.1.2 Capture RF spectrum data using handheld, laptop-based and infrastructure spectrum capture solutions
1.1.3 Understand and use spectrum analyzer views
 Real-time FFT
 Waterfall, swept spectrogram, density and historic views
 Utilization and duty cycle
 Detected devices
 WLAN integration views

Introduction

Since the nature of 802.11 communication involves both layer 1 and 2, most of the times we have to inspect the environment at layer 1(PHY) as well.
The nature of Wi-Fi using unbounded medium often requires to identify potential interference sources.
The non 802.11 device signals do not comply with the medium contention standards and often cause intentional or accidental disruptions.
Adding spectrum/packet analysis to the investigation process can allow to better understand what is going on in the WLAN environment.

Overcoming non-Wi-Fi interference summrised

Locate for offending devices and remove them if possible.
Change channels, frequency bands.
Shield the noise, move the Wi-Fi devices away from noise.
Increase the Tx Power, Use directional antenna if needed.

Capturing Data and using Spectrum Analysis

Standard WLAN adapters in client devices are not capable to capture layer 1 RF energy.
Specialised radios might need additional software to interpret the capture.
AP doing spectrum analysis cannot perform normal AP functions during the use.

Install, Configure and Use Spectrum Analysis Tools

Used for capturing RF signals from 802.11 and non 802.11 sources/devices.
There are some dedicated devices for undertaking this and also some which only work on laptops, tablets etc..
Some devices used PCMCIA cards back in the day now replaced with USB connectors.
Most of these devices use omni directional antenna, one may need to turn about to find the direction in which the noise source is located.
It is suggested to use spectrum analysis adapters which scan both the 2.4GHz and 5GHz spectrum.
Some of the spectrum analysers are able to sweep the bands faster than others to offer the ability to scan for user defined duration in their sweeps. Reading the energy on the frequencies as it passes across them. It is sometimes called the “sweep cycle”.
Once the noise source is determined it is advisable to lock the capture on that channel and sweeping the single channel.
Resolution Bandwidth (KHz)> It is the reference to the smallest frequency that can be resolved by the receiver. RBW should be low enough to resolve spectral components of the transmissions being measured. The lower the RBW wider the frequency inspection.
The newer radios are able to sweep the spectrum much faster allowing to save battery life, improving the graphical representation and reducing the need for multiple radios.

Capturing RF Spectrum Data

Determine the tools for capture. For remote and immediate diagnosis, the infrastructure spectrum analysis is the best choice. For onsite, a portable spectrum analysis is a better choice.
Determine the type of antenna used for capture (omni directional or patch). Just FYI, water absorbs Wifi signals, humans are 70% water approx. so would see some absorbance at 2.4GHz.
Once the tool is determined it will be easier to scan and detect the source of potential interference. Steps can be taken based on the noise source, if it is a mission critical source or not.

Using Spectrum Analyzer Views

There are some complex and simple views. The hand-held all-in-one devices tend to have smaller screens and simpler interfaces.
Laptop or infrastructure based tools tend to have richer and complex interfaces.

Real Time FFT (Fast Fourier Transform)

Algorithm which samples over a period of time (or space) and divides into frequency components. These components are single sine wave oscillations at distinct frequencies.
Simply put, FFT plot shows amplitude on y axis plotted over frequency on x axis.

Swept Spectrogram

Shows RF energy present at a particular frequency over the course of time. Essential same info as Real Time FFT, but it is presented in the different format and tracked over time.
It will display the frequency and signal strength (amplitude) of the detected RF signals.

Waterfall View

Displays the same information shown in swept spectrogram horizontally.
Some tools use one view whereas others can use multiple/toggle views.

Power Spectral Density

This view displays the strength of the detected energy as a function of frequency. In other words, it shows at which frequencies any detected RF energy variations are strong and which frequencies they are weaker.

Historical Views

Can be useful in diagnosing problems when you have to compare the patterns. This can help identify any new sources of interferences which were not visible on a particular day/time and then troubleshoot accordingly.

Spectrum Utilization & Duty Cycle

The spectrum utilization reporting allows you to determine how much airtime is being used on a given frequency. This can be from both 802.11/non 802.11 devices.
Low-density deployments generating small amounts of retransmissions can have very high spectrum utilization due to noise.
Spectrum utilization views can help us see the entire picture not just what WLAN devices are doing.
Duty cycle indicates the fraction of time a resource is busy. When a single device transmits on a channel for 2 times every 10 time units, this device has a duty cycle of 20%.
The duty cycle is traditionally calculated for a specific signal and is defined as the pulse duration divided by the pulse period.
Many spectrum analysers calculate duty cycle as the amount of time the measured amplitude is above the noise floor or another arbitrary threshold.
Device Possible Duty Cycle Microwave Oven – 50%, Jammer – 100% &Wireless Video Camera -100%

WLAN Integration Views

Allow us to see both the RF energy visualised in the spectrum analysis and the SSID and in some cases the MAC Address of the detected AP.

Analysing Spectrum Captures

RF Noise Floor

Determine the signal strength of the noise floor
Many applications require a signal strength that is based upon a measure above the noise floor to work optimally.
Adjustments may be required on the power levels of AP to compensate for high noise floor.
In some cases adjustments may be required by adding additional AP or moving existing AP location (changing antenna direction) away from potential noise floor interference such as Microwaves etc..

SNR (Signal to Noise Ratio)

Comparison of level of signal power to the level of noise power. Often expressed in decibels dB. It is the difference in dB between received signal and background noise level.
SNR value of 25 dB to 30 dB is recommended for networks.

Interference

Identifying the sources of interference is the key to fixing underlying WLAN problems.
Spectrum Analysers can help track the interference and we can remediate accordingly.
A common RF interferer is microwave oven, they operate in 2.4GHz and can have duty cycles of up to 50%.

Channel Utilization

Spectrum Analyzers can help identify the sources of 802.11/non 802.11 RF signals. We can determine which channels being used and find the width depending on the way the channel bonding is displayed.