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Wireless, LAN (WLAN)

Troubleshoot 802.11n Speeds

Document ID: 112055

Updated: Aug 18, 2010

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Introduction

This document covers common issues to consider when troubleshooting wireless throughput issues. This document includes usage of tools to measure performance and throughput of the wireless network, which includes different vendor 802.11n access points (APs) in comparison with the Cisco 1252 AP under similar test conditions.

Prerequisites

Requirements

Cisco recommends that you have these requirements:

  • Tools such as iPerf, and network analyzers such as OmniPeek and Cisco Spectrum Analysis

  • 802.11n supported 1140, 1250, 3500, and 1260 Series APs

Components Used

The information in this document is based on these software and hardware versions:

  • WS-SVC-WiSM Controller running software version 6.0.182

  • AIR-LAP1142-A-K9 APs

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Information

802.11n is born due to a number of changes made on the APs Frame Aggregation: A-MPDU and A-MSDU.

  • Block Ack Size

  • MCS and Channel Bonding

  • MIMO

  • Using 5GHz over 2.4 GHz: also mention Wi-Fi certifies channel bonding on 5GHz

Troubleshoot the Controller for 11n Speeds

Complete these steps:

  1. Verify that 802.11n support is enabled on the controller.

    (WiSM-slot3-2) >show 802.11a
    802.11a Network.................................. Enabled
    11nSupport....................................... Enabled
    802.11a Low Band........................... Enabled
    802.11a Mid Band........................... Enabled
    802.11a High Band.......................... Enabled
    802.11a Operational Rates
    802.11a 6M Rate.............................. Mandatory
    802.11a 9M Rate.............................. Supported
    802.11a 12M Rate............................. Disabled
    802.11a 18M Rate............................. Supported
    802.11a 24M Rate............................. Mandatory
    802.11a 36M Rate............................. Supported
    802.11a 48M Rate............................. Supported
    802.11a 54M Rate............................. Supported
    802.11n MCS Settings:
    MCS 0........................................ Supported
    MCS 1........................................ Supported
    MCS 2........................................ Supported
    MCS 3........................................ Supported
    MCS 4........................................ Supported
    MCS 5........................................ Supported
  2. N rates are attained two ways. Speeds up to Modulation Coding scheme (MCS) 7 can be attained without using channel bonding. For MCS rates above 7 and up to 15, channel bonding needs to be enabled. You can verify if channel bonding is enabled using this show command on the controller:

    (WiSM-slot3-2) >show advanced 802.11a channel
    Automatic Channel Assignment
    Channel Assignment Mode........................ AUTO
    Channel Update Interval........................ 600 seconds [startup]
    Anchor time (Hour of the day).................. 0
    Channel Update Contribution.................... SNI.
    Channel Assignment Leader...................... 00:1d:45:f0:d2:c0
    Last Run....................................... 371 seconds ago
    DCA Sensitivity Level.......................... STARTUP (5 dB)
    DCA 802.11n Channel Width...................... 40 MHz
    Channel Energy Levels
    Minimum...................................... unknown
    Average...................................... unknown
    Maximum...................................... unknown
    Channel Dwell Times
    Minimum...................................... unknown
    Average...................................... unknown
    Maximum...................................... unknown
    802.11a 5 GHz Auto-RF Channel List
    Allowed Channel List.........................
    36,40,44,48,52,56,60,64,149,
    153,157,161
    Unused Channel List..........................
    100,104,108,112,116,132,136,
  3. You can also configure channel width per AP using these commands:

    (WiSM-slot2-2) >config 802.11a disable AP0022.9090.8e97
    (WiSM-slot2-2) >config 802.11a chan_width AP0022.9090.8e97 40
    Set 802.11a channel width to 40 on AP AP0022.9090.8e97
  4. The Guard interval and corresponding MCS rates help determine the data rates that are seen on the 802.11n clients. These are the commands to verify this configuration:

    (WiSM-slot3-2) >show 802.11a
    802.11a Network.................................. Enabled
    11nSupport....................................... Enabled
    802.11a Low Band........................... Enabled
    802.11a Mid Band........................... Enabled
    802.11a High Band.......................... Enabled
    802.11a Operational Rates
    802.11a 6M Rate.............................. Mandatory
    802.11a 9M Rate.............................. Supported
    802.11a 12M Rate............................. Disabled
    802.11a 18M Rate............................. Supported
    802.11a 24M Rate............................. Mandatory
    802.11a 36M Rate............................. Supported
    802.11a 48M Rate............................. Supported
    802.11a 54M Rate............................. Supported
    802.11n MCS Settings:
    MCS 0........................................ Supported
    MCS 1........................................ Supported
    MCS 2........................................ Supported
    MCS 3........................................ Supported
    MCS 4........................................ Supported
    MCS 5........................................ Supported
    MCS 6........................................ Supported
    MCS 7........................................ Supported
    MCS 8........................................ Supported
    MCS 9........................................ Supported
    MCS 10....................................... Supported
    MCS 11....................................... Supported
    MCS 12....................................... Supported
    MCS 13....................................... Supported
    MCS 14....................................... Supported
    MCS 15....................................... Supported
    802.11n Status:
    A-MPDU Tx:
    Priority 0............................... Enabled
    Priority 1............................... Disabled
    Priority 2............................... Disabled
    Priority 3............................... Disabled
    Priority 4............................... Disabled
    Priority 5............................... Disabled
    Priority 6............................... Disabled
    Priority 7............................... Disabled
    Beacon Interval.................................. 100
    CF Pollable mandatory............................ Disabled
    CF Poll Request mandatory........................ Disabled
    --More-- or (q)uit
    CFP Period....................................... 4
    CFP Maximum Duration............................. 60
    Default Channel.................................. 36
    Default Tx Power Level........................... 1
    DTPC Status..................................... Enabled
    Fragmentation Threshold.......................... 2346
    Pico-Cell Status................................. Disabled
    Pico-Cell-V2 Status.............................. Disabled
    TI Threshold..................................... -50
    Traffic Stream Metrics Status.................... Disabled
    Expedited BW Request Status...................... Disabled
    World Mode....................................... Enabled
    EDCA profile type................................ default-wmm
    Voice MAC optimization status.................... Disabled
    Call Admission Control (CAC) configuration
    Voice AC - Admission control (ACM)............ Enabled
    Voice max RF bandwidth........................ 75
    Voice reserved roaming bandwidth.............. 6
    Voice load-based CAC mode..................... Enabled
    Voice tspec inactivity timeout................ Disabled
    Video AC - Admission control (ACM)............ Disabled
    Voice Stream-Size............................. 84000
    Voice Max-Streams............................. 2
    Video max RF bandwidth........................ Infinite
    Video reserved roaming bandwidth.............. 0

    Ensure A-MPDU packet aggregation. For best effort, QoS levels are enabled via these commands:

    • config 802.11a 11nSupport a-mpdu tx priority 0 enable

    • config 802.11b 11nSupport a-mpdu tx priority 0 enable

  5. All three antennas on the A radio must be used. Make sure the antennas are the same model.

  6. On the WLAN configured for client connectivity, WMM should be allowed or required, and AES or open encryption only must be used. This can be verified using this command output:

    (WiSM-slot2-2) >show wlan 1
    WLAN Identifier.................................. 1
    Profile Name..................................... wlab5WISMip22
    Network Name (SSID).............................. wlab5WISMip22
    Status........................................... Enabled
    MAC Filtering.................................... Disabled
    Broadcast SSID................................... Enabled
    AAA Policy Override.............................. Disabled
    Network Admission Control
    NAC-State...................................... Disabled
    Quarantine VLAN................................ 0
    Number of Active Clients......................... 0
    Exclusionlist Timeout............................ 60 seconds
    Session Timeout.................................. 1800 seconds
    CHD per WLAN..................................... Enabled
    Webauth DHCP exclusion........................... Disabled
    Interface........................................ management
    WLAN ACL......................................... unconfigured
    DHCP Server...................................... Default
    DHCP Address Assignment Required................. Disabled
    Quality of Service............................... Silver (best effort)
    WMM.............................................. Allowed
    CCX - AironetIe Support.......................... Enabled
    CCX - Gratuitous ProbeResponse (GPR)............. Disabled
    CCX - Diagnostics Channel Capability............. Disabled
    Dot11-Phone Mode (7920).......................... Disabled
    Wired Protocol................................... None
    IPv6 Support..................................... Disabled
    Peer-to-Peer Blocking Action..................... Disabled
    Radio Policy..................................... All
    DTIM period for 802.11a radio.................... 1
    DTIM period for 802.11b radio.................... 1
    Radius Servers
    Authentication................................ Global Servers
    Accounting.................................... Disabled
    Local EAP Authentication......................... Disabled
    Security
    802.11 Authentication:........................ Open System
    Static WEP Keys............................... Disabled
    802.1X........................................ Disabled
    Wi-Fi Protected Access (WPA/WPA2)............. Enabled
    WPA (SSN IE)............................... Disabled
    WPA2 (RSN IE).............................. Enabled
    TKIP Cipher............................. Disabled
    AES Cipher.............................. Enabled
    Auth Key Management
    802.1x.................................. Enabled
    PSK..................................... Disabled
    CCKM.................................... Disabled
    FT(802.11r)............................. Disabled
    FT-PSK(802.11r)......................... Disabled
    FT Reassociation Timeout......................... 20
    FT Over-The-Air mode............................. Enabled
    FT Over-The-Ds mode.............................. Enabled
    CKIP ......................................... Disabled
    IP Security................................... Disabled
    IP Security Passthru.......................... Disabled
    Web Based Authentication...................... Disabled
    Web-Passthrough............................... Disabled
    Conditional Web Redirect...................... Disabled
    Splash-Page Web Redirect...................... Disabled
    Auto Anchor................................... Disabled
    H-REAP Local Switching........................ Enabled
    H-REAP Learn IP Address....................... Enabled
    Infrastructure MFP protection................. Enabled (Global
    Infrastructure
    MFP Disabled)
    Client MFP.................................... Optional
    Tkip MIC Countermeasure Hold-down Timer....... 60
    Call Snooping.................................... Disabled
    Band Select...................................... Enabled
    Load Balancing................................... Enabled
  7. Antenna Diversity: if using only two antennas for any reason, you need to use antenna A and B for transmitter/receiver ports.

On the Client side:

  1. Supplicant used to control the wireless card, preferred to match the vendor of the supplicant to the wireless card.

  2. Client drivers: you need to make sure the latest client drivers are running on the wireless cards.

  3. Contact your wireless adapter vendor.

  4. Make sure you are using 11n certified adapter to achieve 11n data rates.

Wi-Fi certified products:

http://www.wi-fi.org/certified_products.php

How to Improve Performance:

  1. Channel utilization—Network analyzers report channel utilization in percentage of time spent transmitting and receiving frames. This helps to measure the potential variance in speed due to distance from an access point. This will help monitor and see for example, if a channel is fully occupied transmitting at 1Mbps under ideal conditions would perform at 0.94Mbps under 100% utilization.

  2. The physical medium used in wireless as well dictates the performances. Using 802.11g or 802.11a over 802.11b offers much higher throughputs, often up to 30 mbps over 802.11b where a 6mpbs radio capacity is divided between all the associated stations.

  3. Cell Sizes—It is recommended to shrink the cell sizes to have the clients as closer to the APs as possible. This will benefit the data rates at which the client can connect to the AP. This can be done by reducing the power levels on the AP to the lowest.

  4. Shrinking cell size also decreases co-channel interference. If using RRM, the APs should pick channels dynamically per the deployment. However, if implementing dynamic channel assignment, ensure that you do not have two APs at high power levels on the same channel right next to each other.

  5. Protection also causes throughput hit.

How to Calculate Throughput via iPerf

Iperf Setup Tips

For those customers or testers that do not own Chariot, Iperf can be used instead. This is available at http://www.macalester.edu/crash/software/pc/iperf/kperf_setup.exe.

Measuring TCP Throughput

Run this command on the server side:

Iperf –s -w 256k

Run this command on the client side:

Iperf –c <server IP address> -P 6 –w 256k -r –t 60

tshoot-11n-speeds-01.gif

The first circled number in this image represents the upstream throughput, the second circled number represents downstream (AP to client) throughput.

Measuring UDP Throughput

Close the previous Iperf applications on both the server and client side. Both need to be set up again, but this time for UDP performance testing.

Run this command on the server side:

Iperf –s -u –l 56k

Run this command on the client side:

Iperf –c 
<server IP address>
 -u –b 50M –l 56k –P

This is an example of Omnipeek captures to analyze Aggregate MAC service data unit:

A-MSDU trace shows one packet

tshoot-11n-speeds-02.gif

  • Only the first sub frame is shown.

  • Need to inspect hex dump to see additional sub frames.

A-MSDU next Sub frame shown appended

tshoot-11n-speeds-03.gif

  • An A-MPDU is a structure that contains multiple MPDUs, transported as a single PSDU by the PHY.

  • Indication that packet is Data A-MPDU in Physical layer convergence procedure (PLCP).

tshoot-11n-speeds-04.gif

This is an example of Omnipeek captures to analyze Aggregate MAC protocol data unit:

A-MPDU Setup

tshoot-11n-speeds-05.gif

  • ADDBA—Add Block Acknowledgement

  • ADDBA Request—Contains identifier, Block Ack Policy, Buffer Size, etc.

  • ADDBA Response—Can change policy and buffer size.

A-MPDU Setup

  • ADDBA Request

  • AP1250 uses a timeout of zero to indicate no timeout.

tshoot-11n-speeds-06.gif

A-MPDU Setup

  • ADDBA Response

  • Receiver needs to indicate Block Ack Agreement was successfully established.

tshoot-11n-speeds-07.gif

A-MPDU Data Transfer

  • Block Ack contains compressed bitmap to indicate MPDUs received.

  • Refer to the IEEE 802.11n section 9.10.7 “HT-immediate Block Ack extensions” for information on sending the Block Ack.

tshoot-11n-speeds-08.gif

Capabilities Advertised in Beacons

tshoot-11n-speeds-09.gif

Capabilities advertised in Beacons:

tshoot-11n-speeds-10.gif

Capabilities advertised in Beacons:

tshoot-11n-speeds-11.gif

Association similar with addition of Block Ack setup for A-MPDU:

tshoot-11n-speeds-12.gif

tshoot-11n-speeds-13.gif

tshoot-11n-speeds-14.gif

tshoot-11n-speeds-15.gif

tshoot-11n-speeds-16.gif

tshoot-11n-speeds-17.gif

tshoot-11n-speeds-18.gif

Related Information

Updated: Aug 18, 2010
Document ID: 112055