Practicing

Safe RX-SOPBy Jacob Snyder

(Holy Grail Edition)

About Jake

Mobility Solutions

Architect at Velaspan

• CCIE-W #43153

• CWNE #161

• ACMP/ACCP

• W7SNY

• @jsnyder81

• www.todsfromds.com

•

What is RX-SOP?

—How does it... um... how does it work?

Receiver Start of Packet

• To receive a frame, or not receive a frame, that is the

question.

• This decision is made during the preamble, decision is

made on a single criteria (RSSI or SNR)

• Receiving frame: Frame is decoded and AP defers to

incoming frame

• Not receiving frame: frame is discarded, and AP behaves

as if it never heard the preamble. Frame becomes noise

on channel and subject to the contention process

RX-SOP Analogy

RX-SOP Analogy

A Better Analogy

AP

Chan1

AP

Chan1

AP

Chan11

-82 Default RX-SOP threshold

Other AP

Signal

How AP

hears Client

AP Differs (CCI)

AP

Chan6

AP Cell

AP

Chan1

AP

Chan1

AP

Chan6

AP

Chan11

-82 Default RX-SOP threshold

Other AP

Signal

How AP

hears Client

No differing to Green

-78 RX-SOP threshold

AP Cell

AP

Chan1

AP

Chan1

AP

Chan6

AP

Chan11

-82 Default RX-SOP threshold

Other AP

Signal

How AP

hears ClientNo differing to Green

-78 RX-SOP threshold

AP Cell

SNR

Contention to Noise

Interchange

RX-SOP is the effect of trading contention on the

channel into noise on the channel.

Or the effect of trading contention for reduced SINR

RX-SOP Pro vs Con

• Reduce congestion between

cells

• Increase aggregate throughput

• Overcomes increased AP RX

Sensitivity

• Raises noise floor

• Can be disruptive to clients

• Unidirectional, doesn’t affect

clients RX

• No differentiation between

wanted and unwanted frames

(Yet)

Pro Con

Types of RX-SOP

1. Cisco RX-SOP: RSSI Based

2. Cisco Meraki RX-SOP: RSSI Based

3. Aruba Cell Size Reduction: SNR based

Guidelines for RX-SOP

Use RX-SOP to “tune out” adjacent cells on the same channel

Do not attempt to affect clients connected to your cell with

RX-SOP.

Do not attempt to alter the effective cell size of an AP with RX-

SOP.

When in doubt, use less RX-SOP. It’s a wall. A giant, brick

wall.

—I’ll do you for that

The RX-SOP Wall

https://giphy.com/gifs/monty-python-holy-grail-cinematics-Bt10KtczGwaFG

Hitting the wall

Bring out your dead

Be wary of marketing

https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-0/hdx_final/b_hdx_dg_final/

high_density_experience_features_added_in_release_8_0.html

RX-SOP Myth

Myth:

RX-SOP creates channel reuse

Truth:

RX-SOP converts contention to Noise

RX-SOP Myth

Myth:

RX-SOP Reduces how an AP hears

Truth:

RX-SOP changes at what level the AP

that will decode something

RX-SOP Myth

Myth:

RX-SOP only impacts Data frames

Truth:

RX-SOP affects every frame an AP

hears

RX-SOP Myth

Myth:

RX-SOP reduces cell size

Truth:

RX-SOP just limits how far the AP will

decode frames.

Where RX-SOP Fits

• Channels

• Power

• Data Rates

• …

• RX-SOP

Course Tuning

Fine Tuning

Working with RX-SOP

• Design, Design, Design

• Evaluate environment, before/after

• Test with actual devices.

Design Principals

• External Antennas: The single best thing you can do to

enable safer use of RX-SOP.

• Good channel planning

• Reasonable TX power

• Understand how RRM/ARM methods are affected.

(NDP Packets are affected by RX-SOP)

My Clients RSSI Adjacent Cell RSSI

De

lta

be

twe

en

Clie

nts

/Ne

igh

bo

rs

Adjacent Cell RSSI

Slicing clients from Neighbors

My Clients RSSI

De

lta

be

twe

en

Clie

nts

/Ne

igh

bo

rs

Determine how much

RX-SOP

• Determine how well APs hear clients

(Debugs, pcaps, polling, telemetry)

• Determine how well APs heard neighbors

(Debugs, pcaps, polling)

• Determine average MCS, Retry % and channel utilization

• Did you design for it?

Finding the sweet spot

Quantify RX-SOP impact

• Reduce static channel utilization

• Without increasing client retry rates

(OTA Packet Capture)

• Without dramatic decrease client MCS rates

(OTA Packet Capture, NMS)

• Without impacting client roaming times

• You are unable to see info on frames you aren’t

decoding

Moving Forward

• Today, RX-SOP is a single dimension (RSSI/SNR)

• Tomorrow 11ax, will use BSS color to add a second

dimension

Thank you

https://www.imdb.com/title/tt0071853/mediaviewer/rm1599872256