• How to Troubleshoot Low Throughput Issues on a Wireless Network

 
 
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Introduction

Data Rate is not the same as the actual Throughput achieved for a wireless network. The term Data Rate usually specifies the theoretical bit transfer rate of a particular implementation of Radio Frequency (RF) transmission. Whereas, Throughput is the actual amount of data per second that can be pushed across the link. Some spreading technologies are more effective than others, so the throughput will vary.

802.11 RF medium is a shared medium, meaning that in any discussion about throughput, it should be thought of as aggregate throughput. For example, if the data rate is 54 Mbps, because of CSMA/CA overhead, the aggregate throughput will be around 20 Mbps. This will reduce further, depending upon the number of clients connected simultaneously.

Due to the half-duplex nature of the RF medium and the overhead generated by CSMA/CA, the actual aggregate throughput is typically 50% or less of the data rates for 802.11a/b/g legacy transmissions, and 60-70% of the data rates for 802.11n/ac transmissions.

Typically, throughput of 2.4 GHz devices will be less than 5 GHz devices, due to the higher level of RF noise present on the 2.4 GHZ ISM band.

Prerequisites

  • Administrative access to Wireless Manager.
  • Knowledge of the physical location/placement of the APs.

Solution

Optimize the Radio Settings

For maximum WiFi performance with both radios in AP mode, verify the following settings in the Device Template.

  1. On Wireless Manager, go to Configuration > Device Configuration > Device Template.
  1. Check if the Background Scanning is enabled for Auto-Channel Selection to work.
  1. Channel width should be 20 MHz for a 2.4 GHz radio and 20/40 MHz for a 5 GHz radio.
  1. Based on the requirement, Operating Channel can be set to Manual or Auto.
    • If it is set to auto, the selection interval can be lowered if there are a lot of other WLANs in the vicinity spread across all channels.
    • If it is set to manual per radio on a specific template, then channel crowding may be observed.
    • The operating channel settings can also be set by selecting Auto here, while enabling Allow Device Specific Customization and then selecting the channel manually per AP.
  1. Check the Spectrum Load Balancing Threshold, based on which, the band steering will be managed so that a single radio is not too crowded.

Check Bandwidth and QoS Settings

Check the SSID bandwidth in the SSID profile. It should not have any bandwidth limit set (upload and download).

  1. Log in to Wireless Manager and go to Configuration > Device Configuration > SSID Profiles.
  1. Make sure no bandwidth limit is set for the SSID.
  2. Check the box to Enable Band Steering, so that dual-radio clients are automatically steered to the 802.11a band radio and higher speeds are achieved.
  1. Check if the QoS settings in the SSID are set to default values.

Tests to Validate the Throughput

  • Regardless of the result shown by online speed tests, please check the Throughput by using any tool like iPerf while performing a large file transfer on the local network. This will provide the throughput and speed while accessing any internal resources via wireless and can rule out any Internet access related issues as well.
  • Repeat the same set of test for wired users to compare.
  • Analyze the results to see if they are as expected. The number of simultaneous users should also be taken into account.
  • If the issue appears to be localized to a specific AP, the tests can be done for one problematic and non-problematic AP and then compare the results. The number of clients should be same while in both these test scenarios.
  • Also refer to this article on Troubleshooting WiFi Throughput issues with iPerf on Arista APs.

Environment Checklist

  • Check whether the reported issue is location specific or if it was reported throughout the site. Based on this observation, troubleshooting efforts should be focused on the specific location(s).
  • Is the issue client-specific or do all the entire clients experience low throughput? If it is a client-specific problem, check whether the client is a 802.11b/g band client (does it support .11n speeds) or an 802.11a band client (does it support .11ac speeds).
  • Were there any recent environment changes? For example, any addition/removal of APs, office remodeling, recent increase in WiFi users or any configuration changes made on the wireless network. Remodeling the office like new walls, glass or more furniture could have added new barriers in the RF airspace.
  • Verify the physical placement of the APs. For optimal performance, APs should be ceiling mounted and the Arista or Mojo Networks logo should facing toward the ground. Low signal can cause the clients to operate at lower data rates, therefore delivering low throughput.
  • Check for any same channel/adjacent channel interference. Crowding of APs on any specific channel can bring down the throughput of the whole channel as well.
  • Check for any legacy wireless devices in the vicinity that might be causing the low throughput of the whole BSS e.g. clients supporting FHSS, 802.11b-only devices.
  • Search for sources of WiFi and non-WiFi interference in the vicinity of the affected AP(s) or client(s).
  • Sources of Interference on 2.4 GHz:
    • Microwave ovens
    • 2.4 GHz cordless phones
    • Fluorescent bulbs
    • 2.4 GHz video cameras
    • Elevator motors
    • Cauterizing devices
    • Plasma cutters
    • Bluetooth radios
    • Nearby 2.4 GHz WLANs
  • Sources of interference on 5 GHz:
    • 5 GHz cordless phones
    • RADAR
    • Perimeter sensors
    • Digital satellite
    • Nearby 5 GHz WLANs
    • Outdoor wireless 5 GHz bridges
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