Wi-Fi Optimization KPIs Explained

Wi-Fi Optimization KPIs Explained

An Architect’s Checklist

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Important KPIs For Validation & Troubleshooting

• RSSI

• Noise Floor

• SNR / SINR

• Airtime/Channel

Utilization (duty cycle)

• Throughput

• Cipher Suites

• Minimum Basic Rate

• SSIDs

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• Retransmissions (retries)

• Co-Channel Interference (CCI)

• Channel Reuse

• Channels

• Channel Widths

• Packet Loss

• Jitter

• Latency (delay)

• MOS

Received Signal Strength Indicator (RSSI)

• RSSI is measured in dBm at the receiver.

• Varies according to position and sensitivity of the receiver.

• Usually assessed by the “least capable, most important” client device.

• Typically -65dBm is a desirable RSSI for 5GHz (when voice/video is

required).

• Typically -67dBm is a desirable RSSI for 2.4GHz (when voice/video is

required).

• Provides the top-line for SNR.

• Used both for Primary and Secondary coverage values.

• Measured by client devices, APs, scanners, and protocol analyzers.


Received Signal Strength Indicator (RSSI)

Typical Output Power

• +20dBm (100mW)

• +17dBm (50mW)

• +14dBm (25mW)

• +13dBm (20mW)

• +10dBm (10mW)

• +7dBm (5mW)

• +3dBm (2mW)

• 0dBm (1mW)


Typical 5G RSSI Values

• In-room -40 to -55dBm

• Multimedia / Voice grade -65 to -67dBm

• Data grade -68 to -72dBm

• Quality degradation until 4dB SNR

Free Space Path Loss (FSPL)

• -40dB at 2.4G for 1m

• -47dB at 5G for 1m

• -6dB in open space for 2D

• -10 to -12dB in closed

space for 2D

Example (math only)

• 4m radius room, AP in center

• +10dBm, -47dB, -6dB, -6dB = -49dBm

Noise Floor

• Measure of overall interference

• Environmentally dependent.

• Used as a bottom line for SNR.

• Typically good numbers of <-95dBm for 5G and <-90dBm for 2.4G in the general enterprise.

• Fixed noise per MHz. Doubles (+3dB) with channel width doubling.

• Example: Lots of Bluetooth transmitters can raise your noise floor in 2.4GHz as much as 25dB in a stadium/arena.


• The background radiation, including

thermal noise and transmission

circuit noise.

Signal to Noise Ratio (SNR)• The difference between the Noise Floor (background radiation plus

circuit noise) and RSSI, measured in dB.

• SNR determines or influences the MCS rate on a per-device-type basis.


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• 25dB+ at cell edge in most cases.

• ~31dB at cell edge for 256QAM.

– Watch your CCI in this case.

• Example:

– TxBF (for 2Tx), max 3dB

– TxBF (for 3Tx), max 4.77dB

– TxBF (for 4Tx), max 6dB

– Realistic gain: 50% of max

– If 3dB gain, then usually +1 MCS rate

Signal to Interference+Noise Ratio (SINR)

• The difference between the Noise Floor (background radiation plus

circuit noise) plus any Interference sources (both modulated and

unmodulated) along the transmission path, and the RSSI,

measured in dB.


Throughput


• Due to the half-duplex nature of the 802.11 protocol, there is always

contention and variable channel loading aspects.

• Marketing often misuses “throughput” as a perf indicator, when it’s often not.

• Throughput tests can serve as a good starting point for roaming tests

• Simple Throughput Example:

– 1 client, up to 55% of Data Rate

– <5 clients, up to 45% of Data Rate

– >5 clients, up to 40% of Data Rate

– 30 iPad Air 2 (2x2:2) clients, One 3x3:3 AP

– 20MHz channel, SGI, 31dB+ SNR for all

– 173Mbps (D.R.) x 40% = 69.2Mbps / 30 =

2.31Mbps (T.P.) each

Airtime is

more

important!

Throughput

• For a good user experience:

– In classrooms and the enterprise, 2Mbps per device is usually enough

– In VHD, 500kbps per device is usually enough

• Should be validated in a real-world (production network) scenario


Per-user

throughput

requirements are

often lower than

expected.

Airtime/Channel Utilization (duty cycle)

• Airtime (or Channel) Utilization (measured in %) is the industry-standard

term used to describe use of the time on the channel. Also: “duty cycle”.

• Voice will be negatively impacted ~20%

• Video will be negatively impacted ~50%

• Data will be negatively impacted ~80%


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• When the airtime utilization negatively

impacts your applications, deployment

corrections should be made.

• Some APs can announce QBSS Load

– But accuracy is unknown.

Minimum Basic Rate (MBR)

• The Minimum Basic Rate is the minimum rate allowed for transmission

by the AP.

– The AP can still receive frames sent at slower rates (lesser MCS)

• Many frames must use the MBR, including beacons & probe responses.

• 12 or 24Mbps are usually best, but are directly linked to AP spacing!


http://divdyn.com/disable-lower-legacy-data-rates/

5 GHz

2.4 GHz

Service Set Identifiers (SSIDs)

• It is generally recommended that there be no more than 5 SSIDs

applied to an AP radio due to airtime overhead.

• An MBR of 12 or 24Mbps is often best in reducing airtime use.


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Cipher Suites

• There are four possibilities for cipher suite selection– CCMP/AES, TKIP/RC4, WEP/RC4, OPEN.

– TKIP & WEP are deprecated by the WiFi Alliance & IEEE.• If you use them, you’ll be limited to legacy data rates (≤54Mbps) only.

• Use of TKIP or “Auto” may sometimes corrupt ARP table on Apple devices, stopping all traffic flow, while the client device remains associated.

• CCMP should be used for corporate WLANs.

• OPEN should be used for many (but not all) guest/hotspot scenarios.

• Announced in the RSN IE in beacons and probe responses.– For pairwise and group cipher suites.


A cipher suite is a named combination of

authentication, encryption, message

authentication code and key exchange

algorithms used to negotiate security settings.

Retransmissions (“retries”)

• Retransmissions (aka retries) are a second (or subsequent) transmission of a data frame due to non-acknowledgement from the receiver.

• Retries can be due to collisions, power asymmetry in a link, hidden nodes, ACI, or other causes.

• L2 can affect L4 TCP Windowing, reducing throughput.

• Below 5% retries is normally acceptable. Anything higher than 10% warrants investigation.

• Higher SNR, balanced links, less ACI, and less hidden nodes help.

• Retries are NOT your friend. They eat airtime unnecessarily. They eat airtime unnecessarily. They eat airtime unnecessarily. ;)

• Tracked by APs, some client utilities, protocol analyzers, and some WIPS


Co-Channel Interference (CCI)

• 3 Kinds of CCI

• CCI is the #1

WiFi network

capacity killer.

• In VHD, you

may have

downlink reuse

but not uplink

reuse.


1. Clients contending within a BSS

2. Two or more BSSs on the same channel

3. Clients of two or more BSSs that can hear each other or other APs

Channel Reuse

• Channel Reuse is the ability to reuse one

or more channels without CCI.

– Yields added capacity in medium & large

networks

• Channel Reuse is difficult to achieve in

open spaces or with thin walls/floors.


165

140 64

52

116100

108

48

136

60

10414944

56

112

120124 128149

161

36157

153 40

144

Channels

• Channel availability depends on regulatory domain.

• Minimum channel widths produce the highest over capacity.

• Use all channels available to you in VHD environments.

• Consider removing channels not supported by important clients.

• Use of channel 144 may need to be delayed.

• Remove DFS channels that are experiencing significant problems.

• Use as wide of a channel as possible without introducing CCI.

• SNR is reduced by 3dB for every channel width doubling.

• Avoid OBSS conditions (e.g. 36+ overlapping 40-)

• Avoid using dynamic channel widths if RRM mixes +/- secondaries.

• In Dual-5GHz (D5G), pair DFS and non-DFS channels within an AP.

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Channel Widths• Use as wide of a channel that you can, without introducing CCI.

• Wide channels are inefficient. – Primary is always used, and may saturate, preventing use of secondaries.

• Wide channels limit channel reuse, and may therefore limit capacity.


BE

FO

RE

AF

TE

R

Packet Loss

• Acceptable loss values depend on the applications in use.

• For general data, <3% is normally acceptable.

• For VoWiFi, acceptable packet loss is codec-dependent and varies depending on who you listen to, e.g.

– 0% (perfect)

– 0.5% (good)

– 1% (modest)

– 3% (border-line acceptable)


Packet loss occurs when one or more packets of

data travelling across a computer network fail to

reach their destination – often due to congestion.

To remediate:

1. Increase bandwidth

2. Increase infrastructure device

performance

3. Implement QoS

4. Increase SNR

5. Remove interference sources

6. Choose a better channel

7. Fix software bugs

8. Replace bad Ethernet cabling

Jitter

Jitter is a variation in packet transit delay

caused by queuing and contention on the

path through the network.


• In general, higher levels of jitter are more likely to occur on either slow or

heavily congested links.

• High jitter can cause packet loss at the jitter buffer.

• Users may not experience jitter, but instead would see packet loss & delay.

• Jitter buffers (used to compensate for varying delay) add to end-to-end delay,

and are usually only effective on delay variations <100ms.

• Acceptable jitter for VoWiFi ranges from <5ms to <20ms, depending on who

you talk to.

Latency (aka “Delay”)


Latency is an expression of how much time

it takes for a packet of data to get from one

designated point in the network to another.

• When the application is VoWiFi, the connection should permit a

tempo of speech similar to that of a face-to-face conversation.

• Acceptable end-to-end (one-way) latency across a VoWiFi network

ranges from 50 - 150ms, depending on who you talk to.

WiMOS

• A call with a WiMOS of 4 or greater is considered to have excellent call quality.

• A call with a WiMOS of 2 or less is considered to have poor call quality.

• Lower WiMOS may indicate choppy calls, one-way radio, dropped calls, etc.


The Wireless Mean Opinion Score

(WiMOS) is a numerical

measurement of call quality.

Additional Considerations

• System Configuration Items

– Band Steering (primary to be used in HD/VHD areas)

– Beacon Interval (don’t touch it!)

– DTIM Interval (1 or 2 normally)

– Voice Enterprise (11k/r/v, enable it if possible!)

– Probe Suppression - sometimes in SNR (dB), sometimes in RSSI (dBm)


WiFi Toolset For Understanding KPIs

AirCheck G2

Wireless TesterAirMagnet

Spectrum XT


AirMagnet WiFi

Analyzer PRO

AirMagnet

Enterprise

AirMagnet

Survey PRO

Wi-Fi Optimization KPIs Explained

Technology

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