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Wireless Throughput issues Troubleshooting

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Introduction

Dhiresh Yadav is a wireless expert and working for the Cisco's High Touch Technical Support (HTTS) team, a team that provides reactive technical support to majority of Cisco’s premium customers. In this document Dhiresh has explained different troubleshooting steps if there is a Client throughput/Speed issues in wireless.

Overview

User complaints that the Wireless speed is not good and wants us to fix the issue.

Troubleshooting Steps 

Test the user claims by checking the speed/throughput using some tool like Iperf, setting up FTP server on the LAN etc  or any other tool. This should tell us the throughput and speed while accessing any internal resources via wireless and will rule out any Internet access related issue as well.

  • Repeat the same set of test for wired users to compare.
  • Analyze the results to see if they are as per normal expectations from Wireless. The number of users while the test  was conducted should be taken into account.
  • If it is per AP problem then test can be done for one Problematic and non-Problematic AP and then compare. Number of clients should be same while testing for AP specific speed issue.

Further Troubleshooting

If you still think after above analysis that this might be throughput/speed issue then,  try to check the following points

  • Proximity to AP—Clients closer to the AP experience higher data rates. As clients move farther away from the AP, signal strength reduces. As a result, data rate decreases steadily. Check“sh client detail <mac-address>”  to see RSSI and SNR values.
  • Radio resets using either show cont d0/1 i Reset or show trace dot11_rst. The radio resets will make the radio unavailable for some time.
  • Check if the issue reported is in 802.11a or 802.11b/g. Have a look at the channel width in 802.11a to confirm if the Customer is using channel bonding. Check Guard Interval and Reduced IFS (RIFS) in both the bands. You can see that using “show  802.11a/b” on the WLC or “sh controller 0/1” on the AP.
  • Check the data Rates which are being used, “debug dot11 dx trace print rates” on the AP. Check that also on WLC under   “sh client detail <mac-address”

Point to Remember

The data rates on the WLC should be set to the rates as per the requirement for example If there are no 802.11b client you can set some higher data rate as mandatory in 2.4 Ghz and disable lower 802.11b data rates. The Management frames like ACK or beacons use lowest mandatory data rates and they eat up most of the airtime (for example if set to 1 Mbps )and if the lowest mandatory rate is itself a high data rate, then it might pull up the throughput of the AP cell. The only thing to consider here is that, disabling lower data rates will reduce cell size and you should be sure that there are enough number of APs for proper coverage.

More Information

  1. Keep an eye over the channel utilization, use “show controller d0/1 | b QBSS” on the AP. You can also use “sh ap auto-rf 802.11a/b <ap-name>”  on the WLC to see not only channel utilization on the AP but also other valuable information for the AP related to the RF.
  2. Check the Packet aggregation method in use. “show cont d0/1 | i MPDU” on the AP, to make sure A-MPDU is enabled. By default it is enabled for priorities 0,4 and 5. You can also check that using “sh 802.11a/802.11b” on the WLC.
  3.  In the sniffer, check if we are doing AMSDU v/s AMPDU. Since AMSDU tried to transfer large chunk of data in one shot and if the channel is noisy, then there is a higher chance of a collision and also with only one CRC, it has to retransmit the whole packet again.
  4. Ethernet overruns via show int g0
  5. CAPWAP fragmentation via show capwap client traffic
  6. To look at the AP queues “show controller d0/1 | b queue”
  7. If it looks like a High density environment, we can use RX-SOP feature present in the latest APs and versions. Receiver Start of Packet Detection Threshold (RX-SOP) RX-SOP determines the Wi-Fi signal level in dBm at which an AP radio will demodulate and decode a packet. The higher the RX-SOP level, the smaller the receiver cell size will be. This will ensure that the clients are connected to the nearest access point using the highest possible data rates. It is supported on the following APs : 1552 ,1600 ,2600, 2700 ,3500, 3600, 3700.
  8. Try to keep the number of SSIDs low. Each WLAN/SSID needs separate probe responses and beaconing, the RF pollution increases as less RF time is available for data transmission.
  9. Disable RRM scanning to prevent any throughput drops when going off channel. This can yield a 1-3% improvement.
  10. Make sure the 802.11n rates are enabled.
  11. In the current deployments of 802.11n APs, we might see mixed environment having a combination of 802.11n and 802.11a/b/g clients .802.11 a/g client base would be a limiting factor on the network as they cannot talk to the 802.11n AP using multiple streams. To enhance overall network capacity by bringing 802.11 a/g clients up to a higher performance level, we should enable Client Link. It will optimize the signal received /SNR for 802.11a/g clients in the downlink direction. This will result in the reduced number of retries and higher data rates and will enhance the overall throughput of the cell.

Tips

  • Check if Proper site survey was done so that Wireless users can get proper RSSI ,SNR etc. The Power levels used and the devices used during the Survey to confirm the AP placement must have already considered the actual requirements and actual devices to be deployed, for example Voice might have different requirements than that of data. A bad RF environment cannot be corrected using WLC configuration i.e RRM.
  • RF environment—Amount of noise and interference in the environment. The less the noise and interference, the greater the bandwidth. Use some wireless sniffer to get an idea of the RF environment.

Reference

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