Cisco RX-SOP: How to Turn Down the Noise

Cisco RX-SOP is a feature that de-tunes the receive sensitivity of an Wi-Fi Access Points.

Essentially what feature is doing is to tell the AP to ignore transmissions it hears below a certain threshold - it's not removing the traffic from the air, but allowing the AP to ignore it and be able to talk over the top of background noise.

RX-SOP - Turn down the Noise

If you find yourself in the unlikely situation that there are too many APs, then this is a feature that might help.

Incorrect AP density is one of the most common issues in Wi-Fi design - worth a read of our eBook, The Top 8 Secrets to Great Wi-Fi, which explains this further.

One option for networks with too many APs (or a high density designed network), or with a high density of clients, or noise is to use RX-SOP.

Here's the run through of what RX-SOP is - and does.

Cisco RX-SOP: How does it Work?

The RX-SOP feature will effectively tell the AP to no longer listen to traffic below a certain threshold – meaning the AP can start transmitting over the top of what is now considered background noise.

In tests, setting for RX-SOP to -90dB only makes a mild difference of around 3% improvement in channel utilisation, so its not a silver bullet or replacement for proper RF design in the first place.

The more aggressive settings will definitely have an improvement on channel utilisation, but to an extent you could consider it as masking an underlying problem.

Cisco RX-SOP: What is it Doing?

Let's take the technology out of it for the minute, and look at it a different way.

Cisco RX-SOP In Action

Wi-Fi is really akin to people at a dinner party where everyone at the table takes turns to talk – it would be rude to talk over the top of each other so if someone wants to speak, they wait until everyone else has stopped for a moment.

If you have a very large group, it could take a long time before everyone gets a chance to talk without anyone else talking.

Quite often in a dinner party, multiple conversation groups can break out on a large table simultaneously because humans are able to ignore the background noise of conversations they aren't participating in.

APs aren't able to do this if the other conversations are loud enough to make out clearly. RX-SOP is sometimes referred to as "ear-muffs" for APs.

It will completely ignore the more distant RF signals and pretend it can't hear them and allow the AP to talk over the top when it would normally wait politely for a moment of silence.

 

Cisco RX-SOP: Sensitivities

RX-SOP will improve the network in the downstream direction (from AP to Client). It will now hear more ‘silence’ giving it more transmit opportunities, as its ignoring the background traffic.

The clients though have not changed their behaviour, so the upstream direction (from Client to AP) remains unchanged (they still can still hear everything as they did before) but because they don't have receive sensitives as high as access points, they already had more perceived "silence" and therefore transmit opportunities than the APs did.

802.11b
(2.4 GHz)
802.11a
(5 GHz)
Auto Radio Default Radio Default
Low -85 dBm -80 dBm
Medium -82 dBm -78 dBm
High -79 dBm -76 dBm

The table above shows the available settings for RX-SOP, in each of the available bands.

When a new client turns on, it will send out a probe. Every AP that hears the client will respond with a set of beacons detailing the SSIDs it supports. The APs with RX-SOP in operation will not respond to weak probe requests, so this will help improve the channel utilisation.

It probably goes without saying that this is a feature that will make no difference unless the network has a dense array of APs. The other alternate is to engineer the network with less APs in the first place.

Cisco RX-SOP: Conclusion

RX-SOP would be useful as a tool in your bag when you have some high channel utilisation areas to take a few percent out.

Beware of the more aggressive settings as you might well affect the way in which clients roam – APs will ignore client probe requests, so as you get more aggressive, you’re more likely to affect the roaming behaviour of a client.

This should only be an issue in low density areas such as outdoors where a client may still need to roam to an AP that can only just "hear" it so just be careful to not apply the same RX-SOP settings across an entire site if the site has some very low density areas (use RF profiles to apply different RX-SOP settings to each AP group).

For a further discussion on Wi-Fi design, have a read of 30 Technical Wi-Fi Thoughts.

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