Dynamic Frequency Selection · 2019-10-15 · ©2019 Extreme Networks, Inc. All rights reserved...

©2019 Extreme Networks, Inc. All rights reserved

David Coleman

Director of Product MarketingOctober 10th, 2019

Dynamic Frequency Selection

©2019 Extreme Networks, Inc. All rights reserved2

Who is this guy?

David ColemanDirector of Product Marketing

CWNE #4

@mistermultipath


Sybex CWNA Study Guide – 5th Edition

Amazon: http://bit.ly/CWNAv5

http://bit.ly/CWNAv5


#WhereisWestcott

4

Co-Author

David Westcott – CWNE #007


Channel reuse

▪Primary goal of channel

reuse patterns is to

prevent co-channel

interference

▪Reduces airtime

consumption by isolating

frequency domains

(channels)

Channel 1

Channel 1

Channel 6

Channel 11


Co-channel interference

▪Does RF just stop?

▪Almost impossible to

prevent CCI at 2.4

GHzChannel 1

Channel 1

Channel 6

Channel 11


Co-channel interference

▪CCI is not static and

always changing

▪Client transmissions are

the top cause of CCI

7

Channel 1

Channel 1

Channel 6

Channel 11


5 GHz channel reuse – preventing CCI

▪Whenever possible, use as

many channels as possible in

5 GHz to reduce CCI.

▪ The more channels that are

used, the greater the odds

that CCI can be prevented,

including co-channel

interference that originates

from client devices.


5 GHz channel reuse – preventing CCI

▪ Notice the spatial distance

between the coverage cells on

both APs using channel 36.

▪ When you also factor in the

attenuation from walls, the odds of

associated clients to either channel

36 AP hearing each other and

deferring transmission, has been

most likely eliminated.

▪ A channel reuse pattern for the

United States could also include

the 5 extra channels available in

the U-NII-3 band.


5 GHz channel reuse – DFS channels

▪ In most cases, you should

use the dynamic frequency

selection (DFS) channels.

▪ The good news is that most

current-day client devices

are being certified to

transmit on the DFS

channels, and the inclusion

of DFS channels in channel

reuse patterns is becoming

more commonplace.


▪ All the channels in the U-NII-

2a band, UNII-2c band are

known as the dynamic

frequency selection (DFS)

channels.

▪ WLAN radios operating in

these 5 GHz bands must

support DFS, to protect WLAN

communications from

interfering with military or

weather radar systems.

▪ If radar pulses are detected in

any of these DFS channels,

access points and clients are

not allowed to transmit on the

same channel.

U-NII-2A

36

40

44

48

52

56

60

64

10

0

10

4

10

8

11

2

11

6

12

0

12

4

12

8

13

2

13

6

14

0

14

4

14

9

15

3

15

7

16

1

16

5

U-NII-1 U-NII-2C U-NII-3

5.15 5.25 5.35 5.47 5.725

5.85

5.825


Weather

radar

DFS


DFS

▪ The rules for 802.11

radios to transmit on the

DFS channels might vary

by region; however, the

goal is to avoid inference

with radar.

▪ Many radar systems are

protected by DFS

regulations, including

radar on boats, weather

radar, and military radar.

▪ Please note that DFS

requirements do not apply

just to Wi-Fi radios.

U-NII-2A

36

40

44

48

52

56

60

64

10

0

10

4

10

8

11

2

11

6

12

0

12

4

12

8

13

2

13

6

14

0

14

4

14

9

15

3

15

7

16

1

16

5

U-NII-1 U-NII-2C U-NII-3

5.15 5.25 5.35 5.47 5.725

5.85

5.825


Weather

radar


History of DFS - Regulations

▪Radar avoidance rules first

implemented in Europe by ETSI

▪FCC followed shortly thereafter


History of DFS – 802.11h-2003

▪ In 2003, the IEEE ratified the 802.11h

amendment which proposed two

capabilities for Wi-Fi radios

▪The IEEE proposed two mechanisms:

▪dynamic frequency selection (DFS)

▪ transmit power control (TPC)


Transmit power control

▪ Most APs support transmit power control (TPC)

▪ Initial 802.11h goal of TPC was to protect the Earth Exploration Satellite Service (EESS)

▪ Clients that also support TPC can adjust their power to match the AP transmit power

▪ Now used in all bands for other reasons such as reducing contention interference caused by

clients

Warning: Some legacy clients may have connectivity issues when TPC is enabled


DFS – Regulatory certification - Europe

▪Wi-Fi radios must be certified that they can

detect radar and avoid interfering with radar

▪ In Europe the certification process is fast and

DFS channels are available for transmission

when new enterprises-class APs hit the

market


DFS – Regulatory certification - USA

▪ In the USA there is a 4-6 month backlog to get

APs certified to transmit on the DFS channels

▪ This means that when APs first ship in the

USA, they only support the nine non-DFS

channels that are available in the USA

▪Many customers never updated their channel

plan once DFS certification is complete and

firmware is available

40

44

48

36

14

9

15

7

16

1

15

3

16

5


DFS – Regulatory certification - USA

Why is this puppy so sad?

▪Right about when 802.11n debuted in

2009, the FCC suspended DFS

certification for Wi-Fi APs

▪ Because of one one naughty vendor,

nobody could get their APs certified

for DFS for about 2 years

▪ As a result many client devices were

manufactured during that same time

period and they never bothered to

code drivers/firmware on the client

side to support DFS


DFS mechanisms

▪ Master device: Any device (like an AP) that features radar

detection and has the ability to control other individual components

in a coordinated system.

⎻ DFS certification is required for master devices.

▪ Client device: A device that requires authorization by a master device to initiate communication on a channel. ⎻ Certification required to prove to RF regulatory body that the client device obeys the

master device

⎻ No radar detection is required if the client device is controlled by master device

⎻ This also applies to ad-hoc communications


DFS - CAC

▪Channel availability check (CAC): DFS

function that monitors a channel to determine

if a radar waveform above the DFS

detection threshold is present.

▪Channel availability check (CAC) Time: The

period of time during which a CAC is

performed on the channel before initiating

communication

DFS Detection Threshold: The minimum signal level that

must be present before the system is triggered to move to

another channel. Interference detection threshold is

calculated based on a 0dBi antenna.


Pulse radar waveforms

▪Six radar waveforms types are

tested for DFS certification

▪Five short pulse and one long

pulse

▪Minimum percentages of

successful detection are

required based on defined

repetitive tests

▪Testing varies per region


DFS – Radar Waveform


DFS - CAC▪ Whenever an AP boots up for the first time on a

DFS channel, the AP’s radio must listen for a

period of 60 seconds before being allowed to

transmit on the channel.

▪ If any radar pulses are detected, the AP cannot

use that channel and will have to try a different

channel.

▪ If no radar is detected during the initial 60-second

listening period, the AP can begin transmitting

beacon management frames on the channel.

▪ In Europe, the CAC rules are even more restrictive

for the Terminal Doppler Weather Radar (TDWR)

channels of 120, 124, and 128. An AP must listen

for 10 full minutes before being able to transmit on

the TDWR frequency space.

Channel 100

Do I hear radar pulses?


DFS – radar detected

▪ If APs and clients are already

operating on a DFS channel and a

radar pulse is detected above the

DFS detection threshold, the AP and

all the associated clients must leave the channel.

▪ If radar is detected on the current

DFS frequency, the AP will inform all

associated client stations to move to

another channel using a channel switch announcement (CSA) frame.

Channel 100

Oh no….. radar detected!

We have to move to channel 36


DFS – channel move time▪ The AP and the clients have a

channel move time of 10 seconds to

leave the DFS channel. The AP may

send multiple CSA frames to ensure

that all clients leave.

▪ The CSA frame will inform the clients

that the AP is moving to a new

channel and that they must go to that channel as well.

▪ In most cases, the channel is a non-

DFS channel and very often is channel 36 but it varies as you will

see later in this presentation

Channel 100



Channel switch announcement (CSA) information element

CSA information element information is found in

three types of management frames:

▪Action frames

▪Beacon frames

▪Probe Responses

Element ID LengthChannel Switch

Mode

New Channel

Number

Channel

Switch Count


Channel switch announcement (CSA)


CSA – channel switch mode

Clients cannot transmit during the channel move time

Channel 64



CSA – channel switch count

▪Switch count is the number of TBTTs before the channel jump

▪A value of 1 indicates that the channel switch occurs

immediately before the next TBTT


OK… we all have to leave channel 100!

30

In-service monitoring

▪ If a client device is performing In-Service Monitoring and detects a radar

waveform above the DFS detection threshold, it will inform the AP

▪ This is equivalent to the AP detecting the radar waveform

Channel 100

I hear radar dude!


Off-channel CAC time

▪ ETSI defines an optional

method for off-channel

scanning radar detection

▪Off-channel CAC time is

performed by a number of non-

continuous checks spread over

a period of time

▪ For channels 120-124 the off-

channel CAC time is 1-24 hours

▪ For all other channels the off-

channel CAC time is 6 minutes

to 4 hours

Channel 100

I am going to go listen to radar

on other DFS channels


DFS – non-occupancy time

New Channel 40

▪Once an AP and clients

switch to a non-DFS

channel, they cannot

return to the previous

DFS channel for at least

30 minutes.

▪This is known as the non-

occupancy time


Zero-Wait-DFS

▪One challenge in returning to the

original DFS channel is that after

the 30-minute waiting period, the

AP will once again have to

monitor on the DFS channel for

60 seconds before transmitting

again.

▪ This means there will be at least

a 60 second interval when the

AP will not be servicing clients.

Channel 36


Zero-Wait-DFS

▪ One chipset vendor, Broadcom, offers a

solution called zero-wait DFS to solve

this problem, using the MIMO radio

chains of the 5 GHz access point radio.

▪ For example, a 3×3:3 AP could be

listening on DFS channel 100 with a

single MIMO radio chain while still

providing access to clients on non-DFS

channel 36 with the two remaining radio

chains.

▪ If channel 100 is clear, the AP can send

a new channel switch announcement to

all the clients on channel 36, telling them

to return to the original channel 100.

Channel 36

Let’s move to channel 100


False positives

▪Historically, the biggest

problem with using the DFS

channels has been the

potential for false-positive

detections of radar.

▪ In other words, the APs

misinterpret a spurious RF

transmission as radar and

begin changing channels even

though they do not need to

move.

Channel 100

I heard a bird - We have to move to channel 36


DFS – AP vendor problems

Sad puppy:

▪False positives

▪Radar pulse thresholds too sensitive

▪APs all jump to the same channel

▪APs never return to a DFS channel

▪Vendor implementations have

improved over the years

▪Or have they?


DFS

▪Why is this puppy so

happy?

▪Because it is time for a

DFS demo


What is David holding in his hand?

WiFiMETRIX Channel Analyzer

AirHORN signal generator

WifiPROBE


WifiMETRIX – Radar Simulator Beta

New radar waveform simulator

www.wifimetrix.com

http://wifimetrix.com/



▪Selectable DFS channels

▪6 radar waveform types

▪Single or Traverse modes

▪Low/High power

www.wifimetrix.com

http://wifimetrix.com/



Available soon from…

www.nutsaboutnets.com

http://www.nutsaboutnets.com/


DFS – Example 1

Vendor A:

▪ Two Action frames

▪ Chanel Switch Mode: 1

▪ Channel Switch Count 6

▪ 6 Beacons before the jump

▪ Moved from channel 52 to channel 149 (non-DFS)

▪ Clients followed


DFS – Example 2

Vendor B:

▪ Three tests and no Action frames – Only Beacons




▪ Moved from channel 64 to channel 48 (DFS)



DFS – Example 3

Vendor B:

▪ Three tests and no Action frames – Only Beacons







DFS – Example 4

Vendor C – AP model #1:




▪ Only counted down 2 Beacons before the jump




DFS – Example 5


▪ One Action frames

▪ Channel Switch Mode: 0


▪ 5 Beacons all with a switch count of 0

▪ Moved from channel 64 to channel 40 (DFS) – seemed to always go to channel 40 on multiple tests


DFS – Example 6


Unresponsive to all radar signatures at both low and high power levels


DFS – Example 7

Vendor D – old firmware:





▪ Moved from channel 52 to channel 100 (DFS) – 80 MHz – holy smokes!



DFS – Example 8

Vendor D - new firmware





▪ Moved from channel 52 to channel 161 (non-DFS) – 20 MHz ☺



Observations

▪Vendors implement channel switch announcements (CSAs) and

channel move times differently

▪AP models don’t always yield the same results

▪Different AP models form the same vendor may perform

differently

▪Changes in firmware may affect CSAs and move time


Everyone loves this….

clients.mikealbano.com

http://clients.mikealbano.com/


Announcing the DFS Project

www.thedfsproject.com

http://thedfsproject.com/


Tools

▪WifiMetrix radar simulator or other radar

pulse simulator tools such as HackRF

▪Airtool to capture from your MacBook radio

▪Wireshark

▪Use filter wlan.fc.type_subtype == 13 to find

the Action frames used for channel switch

announcements

▪Create a custom column for channel switch

count

▪Spectrum analyzer


DFS Monitoring – ExtremeCloud IQ

▪WLAN vendors traditionally have provided very little visibility into DFS –

mostly info can only be seen in AP log files

▪ ExtremeCloud IQ (formerly HiveManager) now offers 30 days of visibility



View APs with the most DFS events



View DFS events at the individual AP level


DFS – Machine Learning and AI


DFS and 802.11k

▪Clients cannot use active scanning (probe requests on

DFS channels unless they hear an AP transmitter)

▪802.11k neighbor reports useful for VoWiFi and roaming

between DFS


DFS Resources

▪NTS whitepaper: http://bit.ly/2AP9YwV ⎻ Dynamic Frequency Selection (DFS) in 5GHz Unlicensed Bands

⎻ An Overview of Worldwide Regulatory Requirements

▪ http://wifinigel.blogspot.com/2013/05/5ghz-missing-3-

channels-in-europe.html

▪ https://www.adriangranados.com/blog/practical-intro-dfs

http://bit.ly/2AP9YwV

https://www.adriangranados.com/blog/practical-intro-dfs


Wi-Fi 6

802.11ax802.11ac802.11n

2009 2013 2019

▪Wi-Fi Alliance has introduced a new generational Wi-Fi naming system

that helps users better understand the experience they can expect

▪ Wi-Fi 6 is the next generation of Wi-Fi based on 802.11ax technology:

www.wi-fi.org/wi-fi-6

http://www.wi-fi.org/wi-fi-6


Call to action

www.thedfsproject.com

http://thedfsproject.com/


Questions


5 GHz channel design – best practices

▪ In most cases, you should use the

dynamic frequency selection (DFS)

channels. The good news is that most

current-day client devices are being

certified to transmit on the DFS

channels, and the inclusion of DFS

channels in channel reuse patterns is

becoming more commonplace.



▪ The only reason not to use the DFS

channels is if the bulk of your client

population consists of legacy devices

that do not support the DFS channels.

▪ If a nearby radar transmission is

causing your APs and clients to switch

to a non-DFS channel, simply eliminate

the problematic DFS channel from the

5 GHz channel reuse design.


5 GHz channel reuse

▪Depending on the region, and other

considerations, 8 channels, 12

channels, 17 channels, 22 channels,

or other combinations may be used

for 5 GHz channel reuse patterns.

▪ For the best 5 GHz reuse design, the

key is to use as many channels as

possible.



5 GHz channel design best practices:

▪ The first factor to consider is which

channels are available legally in your

country or region.

▪ In Europe, a pattern utilizing most of

the channels in the U-NII-1, U-NII-2,

and U-NII-2E bands is quite common.

▪ The U-NII-3 channels are rarely used

in the pattern because of regulatory

domain restrictions.



Although by the IEEE’s definition, all 5 GHz

channels are considered non-overlapping, in

reality there is some frequency sideband

overlap from adjacent channels.

▪ It is a recommended practice that any

adjacent coverage cells use frequencies that are at least two channels apart and not use adjacent frequencies.

▪ In other words, do not provide coverage with

an AP transmitting on channel 36 adjacent to an AP transmitting on channel 40.

▪ However, an AP transmitting on channel 36

adjacent to an AP transmitting on channel 48 is acceptable. Following this simple rule will

prevent adjacent channel interference from

the sideband overlap.



▪ A second recommended practice for 5

GHz channel reuse design is that

there should be at least two cells of

coverage space distance between any

two access points transmitting on the same channel.

▪ Following this rule should minimize co-

channel interference from APs.

▪ However, this may not necessarily

prevent co-channel interference from

clients—and remember that client transmissions are the main cause of

CCI.


Dual 5 GHz WLAN design

Dual 5 GHz design rules:

▪ 60 to 100 MHz of frequency

separation between these two 5 GHz radios on each AP.

▪ If possible, pair DFS channels with

non-DFS channels

▪ Careful 2-dimesional consideration

for 5 GHz channel plan

Radio #1: SDR radio Radio #2: Fixed 5 GHz Radio

5 GHz: Channel 405 GHz: Channel 100


WWW.EXTREMENETWORKS.COM

Dynamic Frequency Selection · 2019-10-15 · ©2019 Extreme Networks, Inc. All rights reserved...

Documents

Transcript of Dynamic Frequency Selection · 2019-10-15 · ©2019 Extreme Networks, Inc. All rights reserved...