Frame retransmissions occur when frames must be resent by a client or access point due to an error. Monitoring the retransmission rate will allow the detection of possible configuration or interference problems, but it is not possible to indicate a threshold for proper operation. Depending on the services used, the maximum rate to ensure proper operation will be different, real time applications or VoIP will require a much lower retransmission rate (<10%) than file transfer or web browsing for example. In the rest of this article we will explain what a high retransmission rate implies and why it can become problematic, and then we will look at the most common causes of retransmissions.
When a unicast frame is not acknowledged by the client it is retransmitted by the sender. This can be due to two factors, either the frame was never received, or the CRC is not valid, which may indicate a corruption of the frame and in this case the receiver does not transmit an acknowledgement frame. This can be in the direction of the client to the access point or vice versa. Following a transmission failure, this will have two impacts on the retransmission:
- The transmission rate (or MCS) used by the station will be lowered in order to support a low SNR more easily
- The contention window will double at each attempt in order to detect more easily a possible collision
These actions require time, not only the time of retransmissions of the packet, but also the packet will be sent more slowly with a greater delay.
Here are 3 of the most common reasons for retransmissions:
Interference
Cause:
- Non 802.11 interference
- Co Channel Interference
Identification:
- Spectral analysis
- Wifi passive survey
Resolution:
- Identification of interference sources
- Use of 5GHz or 6GHz (less interference and more available channels)
Weak signal
Cause:
- Weak signal (area not covered)
- Roaming problem / Sticky client
- Incorrect configuration of transmission powers
Identification :
- Wifi survey (active or passive)
- Analysis of packet exchanges
- Audit of configuration and connection logs
Resolution:
- Activation of lower transmission speeds
- Re-adjustment of the configuration of the access points and equipment
- Enable 802.11k and 802.11r functionality to facilitate client roaming
Collisions
Cause:
- Hidden nodes
- Too many devices on one access point
Identification:
- Airtime analysis
Resolution:
- Perform a redesign of the access point implementation and capacity
The retransmission rate is more important in a wireless network than in a wired network due to the transmission medium. The air is a place of shared transmission between clients, but also with other equipment using other modes of communication or using the same frequencies for other purposes. This cohabitation makes the configuration of our equipment particularly difficult and requires both a good knowledge and understanding of the RF environment but also a regular monitoring of this indicator in addition to regular audits to anticipate possible incidents or disturbances.