What are the principles in high-density Wi-Fi design?

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  • Updated 5 years ago
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Read this whitepaper on High-Density Wi-Fi Design Principles
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FAQ poster, Official Rep

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Posted 6 years ago

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Andrew von Nagy

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Also, be sure to check out the larger Aerohive High-Density Wi-Fi Design and Configuration Guide.
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Alex Stephens

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interesting, thanks
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Joeri De Winter

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Hey Guys,
Regarding co-channel interference in high density areas, the white paper states that cell overlap between two cells on the same channel can be between -67dBm and -85dBm?
I was of the impression that -85dBm was the threshold per cell for co-channel interference and everything above this threshold would not affect CCA.

has anyone tested this?

Greets
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Andrew von Nagy

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Hi Joeri,
Great question! There is not a hard rule on this because if a Wi-Fi station can decode the frame preamble from another Wi-Fi station it MUST defer and mark the medium as busy. This is dependent on the receiving station's Wi-Fi chipset / antenna and its receive sensitivity. Remember that the PHY preamble and header are always encoded at a low data rate defined in the standard so that stations can always detect incoming frames. This is 1 Mbps (11b long preamble), 2 Mbps (11b short preamble), and 6 Mbps (11a/g OFDM and 11n with either Legacy preamble or HT-Mixed preamble). So even if you modify data rates and eliminate lower data rates, other Wi-Fi stations that are at a weaker signals (for instance < -67dBm in this example) can still hear the transmitter and recognize that a frame is being transmitted, its duration, and they should hold the medium as busy for that duration (even if they can't decode the frame contents at the higher data rate). This is part of the NAV (network allocation vector) virtual carrier-sense function.

For anything the receiver cannot decode is treated as raw energy and the IEEE standard has documented minimum threshold levels to mark the medium as busy in such instances. This is called the Energy Detection (ED) threshold.

You can read more on this at the following article:
Understanding Wi-Fi Carrier Sense

Long story short, there is no "correct" answer or specific dBm value that can be referenced. But -85dBm is a general guideline based on typical client receive sensitivity in the marketplace.

Cheers,
Andrew von Nagy
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Keith R. Parsons

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I've previously posts a White Paper on 'Channel Overlap' that might help in this discussion.

http://wirelesslanprofessionals.com/w...

Keith
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Joeri De Winter

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Hi Andrew,

Thx for the reply and posted links. This is interesting stuff :-)
While I am chewing on the info I still struggle with the (overlapping) channels cell overlap of between -67dBm and -85dBm as stated in the white paper.
The radius between -67dBm and -85dBm will be much larger than on the drawing (page 14) especially indoor, even with directional antennas and on -1dBm TX power it will cover a quite big area.

I’m aware of the fact, when doing a site survey for let's say VoWLAN, co-channel interference is unavoidable in the 2.4GHz band. When doing a site survey for High Density especially in relative small areas you really want to avoid it (I guess) in the HD areas.

The 5GHz is no problem, so the focus is on the 2.4GHz band and the challenge will be how do I get that 4th layer (overlapping channel) in the defined High Density area.

You probably will need to strategically positioning the 2 access points/radios (corners, corridors, neighboring offices, antennas,..) with overlapping channels in a way the -85dBm (large cell) meets the -65dBm (smaller cell) of the other access point, that’s a hell of a challenge, even so the contention range area will be big and a lot of clients will not be operating in the highest data rate cell.

So my question is, knowing the modus operandus of CCA, what would be the best HD design (RF wise) between overlapping channels, if the High-density area is an auditorium of 400 seats?
Let's assume that the receiver sensitivity is -85dBm for all clients, and all surrounding access points in the non HD area are tuned for minimum bleed in.

Option 1: 4 enabled 2.4GHz radios, were 2 of them are centrally located (non overlapping channels) and 2 radios strategically located with the cell overlap between -67dBm and -85dBm in the HD area.

Option 2: just 3 enabled 2.4GHz radios located centrally with 3 non overlapping channels.

It would be a trade off between a 4 layer (RF) blanket with a big contention range area on the overlapping channel but with more # connection capacity and a 3 layer (RF) blanket with no contention range area but with less # connection capacity

Another question would be for option 1, what would be the impact of the co-channel interference coming from the radios of the WLAN clients that are operating in the below -85dBm (noise floor range) and are connected on the overlapping channel?

Joeri De Winter