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Your Wi-Fi Design Questions Answered, Part 2

Expert insights into designing a reliable Wi-Fi network.

Key takeaways

  • When designing a Wi-Fi network, the placement and number of access points will depend on the design requirements.
  • Even Wi-Fi experts come to 7SIGNAL to have questions answered.
  • Before placing an AP in a stairwell, passage, corridor, or elevator, it's important to learn the local fire code and talk to inspectors to ensure it's allowed.
  • Instead of increasing the 2.4 GHz power to compensate for insufficient coverage in critical areas, it's best to add another AP and design to the requirements to avoid flooding out other areas.

When designing your Wi-Fi network, sometimes you need a quick answer from an expert who has done it all. In part two of Wi-Fi Design Questions Answered, 7SIGNAL answered more design questions. The 7SIGNAL team previously covered several design questions in Your Wi-Fi Design Questions Answered, Part 1. This article provides valuable insights into placing access points, using outdoor APs in open spaces, increasing power on 2.4 GHz bands, and more. 

1. What are the recommended placement and number APs in floorplans? Is there any automated tool or a rule of thumb?

The answer to this is subject to the design’s requirements. What are you designing for? Is it a location? Should it be a voice design? Do you just need coverage? What are you trying to accomplish from this? That will dictate cell size, overlap, and AP placement.

For placement specifically, are you trying to keep cell sizes down? If so, keep them out of hallways and put them in rooms. You must understand the building’s architecture, including where the stairwells, elevators, and conference rooms are. Design requirements will give you the size and placement of where the APs should go.

Plenty of programs will put dots on a map for you, but you need to take the proper measurements, consider what walls are made of and when the structure was built, and look at AP readings. If you have good information coming in, you can have good information going out.

2. Is placing an AP in a stairwell/passage/corridor/elevator recommended for coverage and roaming?

First, learn the local fire code. Talk to whoever is doing inspections, like fire inspectors, architects, or building designers. Are you allowed to put anything in those areas? In some states, you can’t put anything in those locations unless necessary, such as an exit sign.

If you’re designing for that situation, you can put an AP outside the stairwell to get some bleed-through, but factor in how it’s insulated there. It’s the same thing for elevator shafts. Inspectors may give out permits to mount APs on the cars.

3. Do outdoor APs cause CCI to indoor APs in campus buildings with open spaces/courtyards?

Yes. There will be co-channel interference (CCI). The APs will bleed back through the walls, and cause CCI. This goes back to design and architecture. What are you trying to cover? What are the walls made of?

With outdoor APs, you can get away with them as long as you stay within power levels and that it is allowable in the first place. Check with your jurisdiction. Outdoor APs are rated for the outdoors by the FCC. If you’re running APs too hot, you can cause CCI. Keep an eye on it and do the same with channel planning and power levels. Make sure to keep everything in check.

4. Dual-band SSID: Is increasing 2.4 GHz power to compensate for insufficient coverage in critical areas recommended?

If you have a critical area, adding another AP is best. Design to the requirements and don’t try to cover the area with power. You can introduce too many other issues if you try to resolve the problem with power. You end up flooding out other areas that aren’t being considered, so we don’t recommend doing that. You should use another AP instead.

If you crank up 2.4 GHz power, specifically on dual-band SSID, all those clients will roam to 2.4 because the signal is so loud, and you don’t want that. So, you need to fix the design.

5. Where is the list of data an AP puts out when it transmits beacons while just sitting up there advertising itself? Is any of that transmission user selectable other than beacon frequency?

The best way to see it is to do a packet capture (PCAP) on the channel. Look at that beacon frame; you’ll see all the information elements. Especially with 802.11ax APs, there’s so much complexity, which isn’t exposed anywhere in the GUI or even the CLI for an AP’s configuration. A PCAP will show you everything in the beacon.

Note that you can do a packet capture like this from the Sapphire Eye® with 7SIGNAL.

6. When a device hears the beacon transmission, is it correct when it reports the RSSI in that it’s actually determining the RMS value of that complex waveform? Or, when an AP is transmitting, would it be measuring RSSI?

In Wi-Fi, RSSI is not measured in any standardized way. If you read the standard, you won’t find a clear definition– no specific way that hardware should be calibrated. It just says it’s a measure of the received power of an 802.11 signal and that it should be used relatively to compare previous signals to future signals.

The vendors that make Wi-Fi chipsets have to fill in the rest. They do it their own way, and it’s proprietary. Maybe you work for an AP vendor that has a relationship with a chipset vendor, and they can explain where their RSSI comes from. However, there’s a huge divergence in RSSI values between different devices. There’s no good answer, and it’s not a reliable metric. That’s why there was a transition into looking at MCS rates rather than putting weight on RSSI.

RSSI is measured during transmissions.

7. How are antenna radiation diagrams really used?

They’re used for design. You want to find the antenna radiation pattern that matches the use case and needs of the area you’re deploying the antenna. You use those to understand the coverage pattern.

8. Are there any situations where a design does something different because of the radiation pattern? (Beyond obvious major differences like planar versus omni.)

The antenna shouldn’t be dictated as part of the design – that’s not ideal. You want to be able to select the appropriate antennas as part of the design process. It’s best to have control over that; however, this could be impossible because of budget constraints or similar. 

Essentially, it’s up to network engineers whether the Wi-Fi will work. Never just accept constraints like this. If the situation sets you up for failure, people will happily blame you when something goes wrong. It’s up to you to say, “No, this antenna won’t work. We have to do something else.” It’s worth it for the costs of making a great design the first time.

Sometimes, though, customers aren’t interested in using external antennas when you might want to. Can you make it work with internal omnis? Yes, sometimes you can, but it takes a big change to the design.

9. From transmitter to receiver, why is the signal at the receiver degraded when passing over water? The water is not in the way. Why would the waves above the water be “pulled down?”

This situation is unique to the 2.4 GHz band. It’s a resonant frequency for water molecules. So, if you have a signal encountering water in the 2.4 GHz band, the water will absorb it. It’s what happens in a microwave oven. The microwave transmits 1000 watts of RF energy at 2.4 GHz, the water molecules in the food absorb it, and it gets converted into heat.

It’s not that the waves are getting pulled down. Particularly indoors, there’s some reflectivity with most materials; you’ll receive multi-path and signals that bounce around, down a corridor, and through a room. You don’t get that effect in a room with a big indoor pool, for example. A lot of the signal will get absorbed as it bounces around.

People can have the same effect from the water in us. This can also happen a lot in warehouses and medical facilities, with IV bags and giant pallets everywhere.

Contact 7SIGNAL to learn more

Remember, you can’t hear or see Wi-Fi, but 7SIGNAL can. Contact us to learn more about our wireless experience monitoring platform and designing a reliable Wi-Fi network.

7SIGNAL® is the leader in wireless experience monitoring, providing insight into wireless networks and control over Wi-Fi performance so businesses and organizations can thrive. Our cloud-based wireless network monitoring platform continually tests and measures Wi-Fi performance at the edges of the network, enabling fast solutions to digital experience issues and stronger connections for mission-critical users, devices, and applications. Learn more at www.7signal.com.