ISP Wireless - Link with Poor Wireless Performance (Signal, Throughput, Latency/Jitter)
This article aims to assist the user in obtaining the proper CINR/SNR for every deployment. If the link faces low throughput or instability, please follow these guidelines to improve your overall wireless link health.
Introduction
The CINR is Carrier to Interference+Noise Ratio which is basically "signal to noise ratio" (SNR) in dB. Having the proper CINR means having a much stronger signal over the noise in an environment. In most cases, wireless links that suffer from poor throughput or instability do not have the proper CINR to maintain a stable link. Having a greater CINR is paramount for a proper link stability. This article mentions seven factors that will affect stability.
Increasing SNR and CINR values by following the steps to assess the factors below should result in improved capacity and throughput on the dashboard. Using iPerf3 across the link before and after these steps will help track the results of any improvements. See our Related Article below for more.
Factors to Assess
Line of Sight & Fresnel Zone
Path obstructions between devices at link ends greatly affect the signal strength of the link. While some frequencies can penetrate some obstructions, all signals will suffer when line of sight is blocked in any way.
Achieving clear line of sight does not merely refer to a straight line between link ends, but rather, ensuring Fresnel Zones remain unobstructed as well. Fresnel Zones represent an elliptical shapes around the beam in which radio waves propagate. At least 60% of the first Fresnel Zone should be clear of all obstructions in order to ensure strong and stable signal levels sufficient for the link to connect. The clearer this first Fresnel Zone remains, the better the signal can become.
- If the line of sight between the two devices is obstructed in any way, signal levels will be weak and unstable.
- The first Fresnel Zone requires at least 60% clearance for stable signal levels.
- Mounting devices higher can improve line of sight and Fresnel Zone clearance.
- Lower frequency radio signals have greater Fresnel Zones; higher frequency signals, narrower Fresnel Zones.
Mounting & Alignment
Physical alignment of devices is a crucial determining factor of signal strength. It is important to ensure that the center of the radiation patterns meet as directly as possible. How To Align airMAX Antennas
- Ensure that the devices are correctly aligned according to the radiation pattern found in their respective data sheets. Data Sheets and Quick Start Guides can be found in our Downloads page by navigating to the right product line on the top bar, and selecting the corresponding product from the left hand menu, and then find the right document in the Documentation subsection along with User Guides and other helpful docs.
- Ensure that the devices are tightly secured after verifying alignment so they do not twist on the mount.
- Once aligned, tightly secure the mount to ensure antenna remains aligned (despite winds, heat/cold, etc.).
- Ensure that no objects (ex. walls, roofing, poles) are causing reflection issues. If the antennas are installed in a roof with metal or any reflective surfaces, install the antennas high up as possible, at least 3meters (10ft) above any reflective surface.
- Align until signals across MIMO chains are equal (ex. -40 dBm/-40 dBm). Maximum deviation should be less than 4dB (ex. -45 dBm/-46dBm) to avoid degraded performance. Greater than 3dB may indicate either improper alignment, or obstructions causing multipath/reflections on a given chain.
- Receive signal should match expected signal as reported by ISP Design Center simulation and/or airOS GUI. Align until the reported signal reaches within 1-2dB of expected signal. If it does not, you have a line of sight issue, or your alignment is off.
Interference
Competing signals from other devices can introduce noise on the spectrum that will disrupt your link's ability to capture and filter its signal in isolation. Use airView to scan the spectrum and select the clearest channel that meets your needs with respect to output power regulations. In addition, reducing your channel width will help to limit interference. Selecting the Best Channel Width
- Use airMagic at the AP to find the optimized channel based on reported noise levels by all CPEs.
- Use airView from both sides of the link to find a clear channel.
- Use a narrower Channel Width to reduce the effect of interference in noisy environments and improve link stability.
- Avoid Auto Channel selection without creating a Frequency List based on airView results.
Beamwidth
A narrower beamwidth device generally encounters less noise from competing, in-band radios on same & neighboring channels.
Output Power
Some channels have different EIRP limits. If there are more available channels clear of interference, higher output power may become available.
Antenna Gain
Higher gain antennas provide better signal strength by increasing efficiency of conversion between electrical power and radio waves.
Atmospheric Inversion & Thermal Ducting
On long links, generally greater than 15 kilometers, it's possible to experience an effect known as atmospheric inversion or temperature inversion. This is caused by the RF passing through a layer of air that is warmer or cooler than the air above and below, causing the air to have a different density than the surrounding air, which causes the RF path to bend and no longer be a straight line. Since the RF path is no longer straight, your signal will become unstable until this atmospheric event passes.
In most cases, this will be noticeable during specific time frames each day and will always go back to normal after some time. Generally, you will see this effect during the late evening or early morning hours when the sun is rising or setting. But can happen at any point during the day or night when the ground temperature is vastly different than the air temperature. This occurrence is exacerbated in links that pass over bodies of water due to the massive temperature difference between the water and the air directly above it.
There are only a few ways to overcome this issue.
- Set up a failover link that follows a new path.
- Shorten your link by adding a hop in the middle. Shorter links do not have to cover as much distance, so there is a greater chance of avoiding inversion events by using multiple hops.
- Raising the antennas can get your RF path above the inversion event. The higher the RF path, the less noticeable the inversion is.
Speed Tests
airOS Metrics & Built-in Speed Tests
Use the airOS metrics to quickly measure performance, including the built-in Speed Test tool to quickly measure speed from one radio device to another. However, for true speed test performance measuring across wireless links, it's recommended to use a dedicated laptop/desktop device with iPerf3, since built-in speed tests can saturate/limit CPU performance on the device itself.
Internet Speed Tests
Internet Speed Tests (at client premises) can fail for a variety of reasons including, but not limited to the following:
- Server location
- Server load
- Server connection speed
- Server networking issues
- Simultaneous tests being run to the same server
This is why it is useful to rule out connection issues that are out of your control by testing from one end of a link to another.
iPerf3 Speed Tests
Run iPerf3 between two PCs connected directly/locally to each side of the wireless link.
- This is the best way to test your throughput. It will rule out any other networking equipment on your network and allow you to test only that wireless link.
- This test will also be able to show any limiting factors on your LAN connections because it will test through the LAN interface on one radio, through the LAN interface of the other radio, without causing too much overhead on the radios themselves.
Note: These instructions assume you are running Windows on both PCs.
1. Visit https://iperf.fr/iperf-download.php#windows to download the current version of iPerf3 for your Windows PCs.
2. Once you download the file, you will need to extract it. You may put the extracted contents on C:\.
3. Once you have downloaded iPerf3 and extracted the files to C:\, you can open the command prompt and navigate to the directory that you saved iPerf3 to.
cd C:\
4. Now run iperf3.exe as a server on one PC.
iperf3.exe -s
5. Run iperf3.exe as the client on the other PC.
iperf3.exe -c 203.0.113.26 -t 100 -P 5
Note: 203.0.113.26 is the IP address of the example iPerf3g on the opposite PC. You will need to change the IP address to match that of the PC that you started the server on.
This will start an iPerf3 TCP throughput test and run for 100 seconds while showing the results every one second.