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