Case Study: A STA Enters the Power-Saving Mode, Leading to a High Delay

Symptom

The network delay occurs on the STA. When the IP address of the STA is pinged from the gateway, the delay is high regularly.

Relevant Alarms and Logs

None

Cause Analysis

When no wireless service is running on a STA, the STA periodically enters the power-saving mode to save power.

Procedure

1. Run the display station ap-id x command on the AC to check the uplink signal strength and negotiated rate of the STA. The command output shows that the uplink signal strength and negotiated rate are normal.
2. Run the display radio ap-id x command on the AC to check the channel utilization of the AP that the STA connects to. The command output shows that the channel utilization is normal (2%).
3. Run the station-trace sta-mac sta-mac command in the diagnostic view of the AC to check ICMP packets from the STA. The command output shows that the forwarding delay on the AP side is long.

4. Ping the STA from the AC to check the delay.
   1. Run the rf-ping -c 100 sta-mac command. The command output shows that a large number of packets are lost when the interval is short, but most packets can be transmitted with a short delay.
   2. Run the ping -c 100 sta-ip command. The command output shows that the STA delay increases regularly and the elapsed cache delay in station-trace is large.

   3. Run the ping -system -c 100 -m 10 command to perform fast ping operations at an interval of 10 ms to keep the network adapter of the STA in working state. This prevents the STA from entering the power-saving state. The command output shows that the delay of the STA is stable at x ms, which is normal.

   4. Run the display station sta-mac sta-mac command on the AC to ping the STA. During the fast ping, the value of Power save mode enabled is always No. When the STA is pinged normally, the value of this field changes from No to Yes.

5. Based on the preceding analysis, the STA enters the power-saving mode. As a result, the delay for the gateway to ping the STA is long regularly. In this case, disable the power-saving mode of the STA.

Suggestion and Summary

When a STA does not run any wireless services, it periodically enters the power-saving mode to save power consumption of the network adapter. Then, the STA periodically wakes up to listen to the Beacon frames sent by the AP. Based on the TIM field in the Beacon frames, the STA determines whether the AP buffers data packets for the STA when the STA is in power-saving mode. If the AP buffers data packets for the STA, the STA sends a PS-Poll frame to obtain the buffered packets. However, the interval from power-saving to waking-up of the STA is longer than the Beacon interval. This leads to a large delay of data exchange between the STA and AP.