Avoid interference between 5 GHz WiFi and 5.8 GHz cordless phones

interference wifi cordless phones

Are you running 5 GHz WiFi (a.k.a. 802.11a, 802.11n or 802.11ac)?  Are you also using a 5.8 GHz cordless phone system?  If so, you may be experiencing interference resulting in performance problems with both systems – problems like:

  • strong WiFi signal strength “bars” yet slow network speeds; or
  • phone calls that sound choppy or “noisy”.

You may be able to avoid interference between 5 GHz WiFi and 5.8 GHz cordless phones by simply by selecting channels 36 through 48 in your WiFi router.  If you want further details, just read on.

Summary

This article explains the frequencies used by 5 GHz WiFi and 5.8 GHz cordless phones.  If you have these devices, are experiencing performance problems caused by interference and just want to fix the problems, try selecting channels 36, 40, 44 or 48 in your WiFi router.

Caveats

This article will specifically address 5 GHz WiFi (aka 802.11 a/n/ac) and not 2.4 GHz WiFi (aka 802.11 b/g/n).  It also specifically addresses 5.8 GHz cordless phones and not 900 MHz, 2.4 GHZ, or 1.9 GHz (DECT 6.0) cordless phones.  However, many of the principles in this article could be applied to these other systems when diagnosing frequency contention.

In addition, this article will focus on diagnosing these problems in the United States of America.  Different countries have different ways of allocating the frequency spectrum and the manner in which they regulate devices.  My experience is with systems in the USA.

Finally, I am primarily focused on addressing this problem in a home environment where you can control the devices involved.

5 GHz WiFi frequencies

How do you know if you are using 5 GHz WiFi?  The easiest way to look at the documentation that came with your wireless router or access point.  Usually it will obviously identify itself as 2.4 GHz or 5 GHz.  It may also list specifications such as 802.11a, 802.11n or 802.11ac.  All of these specifications operate in the 5 GHz WiFi spectrum.  802.11n can operate in both the 5 GHZ WiFi spectrum and the 2.4 GHz WiFi spectrum, so double check your frequency settings if using 802.11n.

In the USA, 5 GHz WiFi uses 24 non-overlapping frequencies – each 20 MHz wide.  These frequencies are not created equal.  Because of restrictions, most 5 GHz consumer routers and access points use only eight frequencies.  These eight frequencies are labeled channels 36, 40, 44, 48, 149, 153, 157, and 161.

The following table contains details about the various 5 GHz WiFi frequencies, but all you really need to know is that most consumer routers use eight frequencies – four in the 5.2 GHz range and four in the 5.8 GHz range.

5 GHz WiFi frequency table
5 GHz WiFi frequency table

Note that channels 149, 153, 157, and 161 are allowed to transmit with much higher power (1000 mW) than channels 36, 40, 44, and 48 (50 mW).  More power means that the signal travels farther before decaying into uselessness.  In consumer applications, some routers will boost the power in the upper channels – but only slightly to perhaps 100 mW – 150 mW.  Remember that WiFi requires two way communication, so even if your WiFi router can transmit at 1000 mW, your WiFi device (laptop, tablet, etc.) can’t “shout back” at the same power and thus cannot communicate with the WiFi router.  These higher powers can be used in point-to-point applications.

 5.8 GHz cordless phones frequencies

How do you know if you are using a 5.8 GHz cordless phone?  The easiest way to look at the documentation that came with your cordless phone.  Usually it will obviously identify itself as 5.8 GHz as opposed to 900 MHz, 2.4 GHZ, or 1.9 GHz (DECT 6.0).

Unlike WiFi where systems use standard frequencies in order to ensure interoperability, 5.8 GHz cordless phones are not made to work with other systems.  A handset from one system will not work with the base station from another system.  There is no standard frequency that they use nor standard way of passing information between the handsets and the base station.  Many 5.8 GHz cordless phone use multiple frequencies and can even change their frequency within a certain range if they so desire.  These frequencies will be somewhere around 5.8 GHz, but finding the exact frequencies that they use can be tricky.

So, how do you determine the exact frequency used by your cordless phone?  You could try looking in the manual, but you most likely won’t find it there.  You could try to use a special tool called a “frequency spectrum analyzer” that would show you the frequencies in use, but such tools are expensive and can be difficult to operate.

In the United States, all wireless systems must be certified by the Federal Communications Commission (“FCC”) to ensure that the design is operating within the regulations of a given frequency range.  This helps to ensure that our consumer electronics operate as advertised and don’t end up fighting with each other.  On every wireless device, you will find an FCC ID which is a special number that is awarded when that particular device was tested by the FCC.  One of the FCC test results lists the exact frequencies and their associated power levels for the particular device.  Best of all, this information is publicly available on the web.

How to use the FCC database to find the frequencies used by your cordless phone

Step 1. Determine your FCC ID

The FCC ID will probably be listed underneath your cordless phone base station on the back of your cordless phone base station or in the battery compartment of your handset (look under the battery).

Finding your FCC ID
Finding your FCC ID

My FCCID is AMWUP758

Step 2. Determine your “Grantee Code” and “Product Code”

The Grantee Code is given to a specific vendor (e.g. Uniden).  The product code identifies the model of the device (e.g. model series TRU-9460).

The first three characters of the FCC ID is the Grantee Code and the remaining characters are the Product Code.

For my FCC ID: AMWUP758

Grantee Code = AMW (Uniden)

Product Code = UP758 (series TRU-9460)

Step 3. Use the FCC ID Search page

Visit http://transition.fcc.gov/oet/ea/fccid/

If this page has moved, simply Google for “FCC ID Search Form”.

Now enter your Grantee Code and Product Code in the Search Form.

FCC ID search screen
FCC ID search screen

Step 4. Find the right document

Pull up the detailed list of documents.  Here you will find all kinds of great information about your cordless phone.  Not only will you find the results of the FCC tests, but you could also possibly find photos of your phone (inside and out) and even a copy of an early version of your owner’s manual.

Look for the SAR Test Report.  This will provide a summary of the frequency tests that were conducted.

List of FCC documents
List of FCC documents

Step 5. Find the frequency range

The second page of the SAR Test Report will likely be the declaration of compliance.  On this page you are looking for two lines:

Tx Frequency Range(s):  (Transmit Frequency Range)

Max RF Output Power Tested: (Maximum Radio Frequency Output Power Tested)

SAR Compliance Summary
SAR Compliance Summary

For my phone, the results are:

Tx Frequency Range(s):  5.741 – 5.828 GHz

Max RF Output Power Tested: 152 mW

 

Is there frequency contention?

Channels 149 – 161 on 5 GHz WiFi use the same frequency with similar power as my 5.8 GHz cordless phone.

WiFi and phone frequencies compared
WiFi and phone frequencies compared

This explains why I was experiencing strong WiFi signal bars yet up to 30% packet loss resulting in what appeared to be a slow WiFi connection.  It also explains why my cordless phone sounded choppy and noisy at times.

If we combine WiFi frequencies with the cordless phone frequencies on a chart, it is easy to see the potential for interference:

5 GHz frequency chart
5 GHz frequency chart

Spectrum Analyzer

A quick look at the 5 GHz frequencies using a Wi-Spy spectrum analyzer confirms our theory.  The graph below shows the interference generated when a phone is in use.  The x-axis shows the Wi-Fi channels.  The upper graph shows the real time usage and the lower graph shows the usage over time.

Spectrum Analyzer Graph showing 5.8 GHz cordless phone interference
Wi-Fi channels

Most of my wireless network was turned off during this test; however, you can see a small amount of traffic on the lower Wi-Fi channels 36 – 48 which is generated by my wireless network.  Most of the interference from the cordless phone was on Wi-Fi channels 149 – 165.

The center frequency used by the cordless phone changed over time.  In the graph below, the x-axis is the frequency.

Spectrum Analyzer Graph showing 5.8 GHz cordless phone interference
5 GHz frequencies

The center frequency started centered around 5.820 GHz  and then it shifted to 5.798 GHz.  I tried using multiple handsets at the same time and the center frequency didn’t change based on the number of handsets used – even when one handset was using telephone line 1 and another was using telephone line 2.

It was very curious to see significant usage at 5.354 GHz when the phone was in use.  The spike at this frequency disappears when the phone is not in use.  Note that this frequency is not part of the Wi-Fi bands.

Solution

The solution is to simply choose WiFi frequencies that don’t conflict with the cordless phone.  In my case, these channels are 36, 40, 44 and 48.

I may lose a little bit of distance since these lower channels transmit at lower power; however, the alternative is to purchase a new phone system (DECT 6.0 instead of 5.8 GHz).  If distance is a problem, a second WiFi access point can be installed.

Further reading and reference

If you found this article helpful, please leave a reply below.  Thanks!

5 thoughts on “Avoid interference between 5 GHz WiFi and 5.8 GHz cordless phones

  1. I bought a Digitech Audio/Video Sender with 5.8GHz frequency from Australia. I’m trying to use in Philippines and found out it’s not working. Are frequency standard globally. I was assuming it was and that my AV sender will work here in Philippines.

    Thanks for any info you can impart.

  2. Hello,

    Your advice was right on target…!

    Using the WiFi Analyzer app on my smartphone, I could see that the shape of the router’s 5 Ghz wifi signal was often triangular with sharp peaks and steep dropoffs. I thought that it was caused by interference of some sort but lacked the know-how to pursue it.

    Based on your guidance, I learned that our Linksys E4200 v1 wifi router was operating on Channel 149 and our 5.8 Ghz cordless phone operates between 5.725 and 5.850 Ghz, basically blanketing the 4 upper wifi channels. Initially I changed the 5.8 Ghz Channel to 48 butsoon noticed a dramatic drop in our wifi connect speed (from 16 Mbps to 9 Mbps). I assume this was caused by the reduced power allowed on the 4 lower channels. When I reset the channel to 161, the wifi connect speed has been restored and the 5 Ghz signal is stable.

    Thank you for taking the time and trouble to write this article. I really appreciated your systematic and thorough explanation.

    Regards,
    Dave C.

  3. Hello there. I am not a subject matter expect on wireless interference, but as you clearly are I was hoping you could help me with a problem:

    I experience a lot of wireless interference (cracking and popping) in my wireless headphones for my xbox. They operate at 2.4ghz along with a lot of other products in my house.

    I have two dual band 2.4 – 5ghz Triton CGN3 modems in the same room as my xbox which provide internet to my home and also my tenants in the basement.

    I am looking to buy a new pair of wireless headphones to avoid the problem and have found one that advertises it operates at 5ghz and the other at 5.8ghz. They 5ghz headphone are much cheaper and i would like to save money if possible, but not if the problem persists.

    My questions:
    1) will the 5ghz headphones experience the wireless interference in this scenario that I experienced with the older set due to the two modems that operate at both 2.4-5?
    2) Will the headphones at 5.8 ghz avoid this problem entirely as they are “above” the range of the modems?

    Thank you in advance

    1. Duncan,

      Most likely the new headphones won’t fix the problem if it is due to interference. My best advice is to figure out exactly which frequencies you want to use for each device and assign them specifically if you can.

      The WiFi modems should have a way for you to configure the channels. Be sure that both modems are not configured to use the same channels. For the 2.4 GHz bands, only select 1, 6, or 11 since these are the only 3 non-overlapping channels. On the 5Ghz bands, you can select any available channel since none overlap. However, if your modem supports “channel bonding” or “40 MHz” wide channels, then remember that this consumes 2 channels so keep your assignments 2 channels apart.

      Try to figure out exactly which frequencies your headphones use. Use the steps in the article under the heading “How to use the FCC database to find the frequencies used by your cordless phone”. You probably can’t change these frequencies, but once you know which ones are being used, you can select your Wifi frequencies so that they don’t conflict.

      Best of luck.

      – Brad

  4. Just wanted to say I was having the exact problem you described and your solution above worked perfectly for me. Thanks very much!

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