We have recently come across few production networks, where distance between two APs or APs and a client stations was much closer than I was comfortable with.
Natural reaction was to suggest moving radios at least few metres apart. But why exactly? What happens when two or more transmitting radios are too close to one another? How would placing a laptop next to the AP affect wireless network quality?
Intrigued and eager to answer questions that, interestingly, no one was asking, I have decided to lab it up, research it more deeply and document my findings. I quickly realised that there is not one, but two issues potentially affecting wireless transmission, where distances between radios were too short. Let’s discuss Channel Leakage and Near-Field Interference.
- Channel Leakage
- Near-Field Interference
Few months after a successful design and refresh of a WiFi estate for a financial institution in London, I came back to work on a periodic wireless survey and to assess if there is anything that would be potentially stopping them from introducing more agile working heavily relying on the new WiFi deployment.
I was told that there is a separate company that is working closely with my client in the same offices and that they use their own, separate WiFi network. They have decided to put their own APs next (few inches away) to our APs convinced that since it worked perfectly for my client, it would continue working when two overlaid networks operate simultaneously. This is how the new original vs new deployment looked like:
Having completed the survey, we have concluded that city centre location inside a multi-tenant building with multiple WiFi networks leaking from the outside and adjacent floors combined with two separate, overlaid WiFi networks contributed to very high CCI averaging at 10 or sometimes more. RF tuning across both networks has helped a lot with the contention but the overall quality of the WiFi was still not great. It felt slow despite having strong underlying infrastructure, little CCI and fast Internet pipe. Quick wireless capture has revealed excessive retransmissions rates peaking at 30% in some areas even when there were not too many clients (maybe 10 per radio, often less) competing for the airtime, channel utilisation peaking at 10% and with no obvious faults with the configuration.
Next step was to reproduce the issue in the lab, where I wanted to show how channel separation and AP-to-AP distance impacts percentage of retries in the wireless transmission.
- 2x Cisco 3702i APs registered to C9800-CL WLC broadcasting separate BSSIDs on 5GHz only
- 20MHz static non-overlapping, clean channels (100 & 104 and 100 & 140) and max Tx power (1) set
- One mobile station associated to each BSSID, running external speed test in the loop
- Ekahau Sidekick used for wireless captures
4 different tests were performed, each test involved wireless capture over 30 seconds and was repeated 5 times to minimise measurement errors:
- Channels 100 & 104 (no channel spacing)
- Test 1: APs 0 metres away from one another
- Test 2: APs 3 metres away from one another
- Channels 100 & 140 (200MHz channel
- Test 3: APs 0 metres away from one another
- Test 4: APs 3 metres away from one another
We can clearly see in Test 1, that close physical distance between radios combined with no channel separation resulted in 15.4% retries.
Introducing 200MHz channel separation (Test 3) without extending distance between radios has reduced retries ratio by about half, to 8.2%.
Moving APs 3 metres away from one another has further reduced the retries to 3.9% (Test2; no channel separation) and 2.3% (Test 4; 200MHz channel separation).
Unplanned channel leakage really is an adjacent-channel interference, even though our channels theoretically don’t overlap. ACI will naturally cause increased number of retries, and therefore decrease the throughput and contribute to the slow WiFi perception.
Note: we used flagship Cisco APs with great quality of components providing great RF accuracy. Using cheaper APs that pack cheaper antennas and radios would result in less stellar performance of spectral masks (algorithms applied to the levels of radio transmissions used to reduce main channel leaking to adjacent channels) and amplify the effects of intermodulation (signal modulation on two or more different, non-harmonic, frequencies), increasing effects of ACI and percentage of retries when reasonable channel separation and physical distance between AP radios are not maintained.
Another distance issue that I have frequently seen in an enterprise environment is placing receivers too close to transmitters.
Let’s start with a quick definition of what near-field is in wireless.
Near-field is a 1 wavelength region, where electromagnetic field EM charges and electric charge effects are extensively produced, potentially negatively affecting quality of the received transmission within this region.
Near-field interference decreases drastically, in a logarithmic fashion, when the receiver is moved farther away from the transmitter. It is normally considered enough to be 1 wavelength away from the transmitter to negate the impact of near-field interference. Near-field also affects reflected signal for Rx antennas.
Based on the above, we can conclude that:
- Radios should never be positioned closer than 1 wavelength apart from one another
- Receive antennas should not be positioned closer than one wavelength to any reflecting obstacle due to reflection indicted multi-path phase cancellation
How far is 1 wavelength? It depends on the frequency. The higher the frequency the shorter the wavelength. Here are the wavelength calculations for our beloved 802.11 bands:
2.4GHz frequency wavelength = 12.5cm
5GHz frequency wavelength = 6cm
Now we know, that when we position 2.4GHz radios closer than 12.5cm away from one another (or 6cm in 5GHz) we would suffer from extensive near-field interference and that the situation would improve greatly when we start increasing this distance. Bear in mind, that unwanted channel leakage might contribute to packets corruption here too, so it typically is recommended to keep at least few metres distance between the radios!
Couple of examples, where near-field interference can affect quality of WiFi transmission:
- Enterprise-class AP placed on a small desk with few people sitting around it and their laptops virtually adjacent to the AP; note, that external antennas can increase the negative effect of a near-field interference
- Capturing wireless packets with a capturing device positioned to close to a client or an AP
We must remember that WiFi is not NFC! ????
In this test I targeted impact of near-field interference on wireless receive quality, where receiver (AP in a sniffing mode) was within 1 wavelength of a transmitter (AP serving clients).
- 2x Cisco 3702i APs registered to Cisco 3504 WLC and one test client associated to client serving AP
- AP with blue LED is serving clients
- AP with green LED is running as a sniffer and capturing packets on the same channel as the AP serving clients
- Below tests are based on 5GHz, but in my tests I could have easily reproduced them across both 5GHz and 2.4GHz bands
It was enough to perform a very simple test here – check captured packets integrity in two scenarios:
- Test 1: AP sniffer placed 60 centimetres away from a client-serving AP (more than 1 wavelength distance between radios)
- Test 2: AP sniffer placed on top of a client-serving AP (less than 1 wavelength distance between radios)
Here are the results:
Test 1 captures, where APs are fairly far away from one another, are clean. Putting transmitting and receiving radios very close together (Test 2) results in corruption of almost all packets transmitted by client-serving AP.
When receiver is placed too close to transmitter (especially
true when the distance is less than 1 wavelength apart), near field
interference will cause packets to lose integrity and result in failed Frames
Check Sequence (FCS).
As shown in the above tests, unwanted channel leakage can seriously affect the transmission quality over distances less than few metres, especially without proper channel separation on the neighbouring BSSIDs. Near-field interference can cause corruption of most of the transmitted frames, where distance between transmitting and receiving radios is less than 1 wavelength apart. Finally, after having a chat with my friend Nigel (Twitter @WiFiNigel), we have concluded that the Rx overdrive might also play a role in the frames corruption. Unfortunately, it is difficult to ascertain which phenomena impacts the frames corruption most using the home lab, so I’ll just conclude with this: don’t stack APs! 🙂 And put them away from strong reflectors. All above issues can be easily avoided by using good quality enterprise equipment, solid RF design and having a high-level understanding of how the distance induced interference can affect the quality of the wireless transmission.