Why Do Horses Wear Blinders? Or: What is Rx-SOP And How Do I Turn It On?

horses_2Horses 2” by Steve – originally posted to Flickr as Horses 2. Licensed under CC BY 2.0 via Commons.

I learned recently that horses having eyes on the sides of their heads means they were historically hunted in nature.  Like rabbits, they rely on peripheral vision to detect dangers.

When a horse is racing or pulling a carriage, they can get easily get distracted or startled.  So this is why they are outfitted with blinders (sometimes called blinkers).  To focus their vision to the front, so they don’t get distracted. This reminded me of a problem we tend to have in the WiFi world.

Yes, I’m saying AP’s are kinda like horses. -Jason Grant

Don’t believe me?  Consider the following example:


Here we have a decent sized floor with WiFi designed to support a large number of users and devices.  Just for this example, we’ll use 2.4 GHz, where we only have 3 non-overlapping channels:  1, 6, & 11.  To make it easy to see, I’ve colored the AP’s with the same channel.

See that part I circled in red?  This is an area that is covered by multiple AP’s that are using the same channel. Andrew von Nagy  in a blog article described it well.

Remember, an AP is like a horse. It has great peripheral vision. It can hear client devices, on the same channel it’s on, that are actually connected to a different AP!  See the trouble? An AP needs to read in all client transmissions on it’s channel, even if it’s a transmission intended for a different AP.

Eeek!  What do we do?  In comes another Cisco Wireless feature called Rx-SOP.  This stands for Receiver Start of Packet. Since the marketing folks haven’t come up with a fanciRx-SOPer name, I’ve come up with a cool-looking logo for it.  What do you think?

How is Rx-SOP like putting blinders on a horse?  Great question. Just like blinders restrict a horses vision to just what the operator thinks is important (straight ahead), Rx-SOP allows an AP to ignore incoming packets that aren’t strong enough.  This effectively allows you to shrink your AP cell size to minimize co-channel interference.

Cool, eh?

How to Turn Rx-SOP On

First thing to remember is that Rx-SOP shrinks your effective coverage area.  If you shrink it too much you’ll make coverage holes.

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

To configure the RX-SOP threshold go to Wireless >Advanced > Rx Sop Threshold.  By default Rx-SOP is on “auto” which means it’s effectively off.  Each radio is set to Auto, Low, Medium, or High.

The level you set hear instructs the AP to literally ignore incoming packets that are quieter than the configured level.

If you want to use Rx-SOP for only a few AP’s you can use RF profiles.  How to do that is described in the High Density Experience (HDX) Design Guide.

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