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.
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.
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.
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).
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
If this page has moved, simply Google for “FCC ID Search Form”.
Now enter your Grantee Code and Product Code in the Search Form.
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.
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)
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.
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:
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.
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.
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.
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
- List of WLAN channels (Wikipedia)
- IEEE 802.11 (Wikipedia)
- U-NII (Wikipedia)
- Why 802.11ac will kill the 5 GHz WiFi band (SmallNetBuilder)
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