Link adaptation is the process by which the transmitter selects the optimal MCS with which to send data to a particular receiver. Link adaptation algorithms are implementation specific, however, they are generally based on the measured packet error rate (PER). Most algorithms monitor the PER and adjust the MCS to track an optimal long term average that balances the reduced overhead from sending shorter packets with a higher MCS with the increased overhead from retransmissions due to the increased PER from the higher MCS.

Determining the PER by necessity means monitoring packet errors over a period that is long in comparison with the duration of a packet. For example, to very roughly measure a 10% PER requires that the transmitter send ten packets of which one is in error. Because of this, link adaptation based on PER adapts slowly to changing channel conditions. In many environments the channel is changing with time as the stations move or with changes in the environment itself, such as the 50 Hz or 60 Hz ionizing cycle in fluorescent bulbs, the movement of objects in the environment, or changes in external noise sources. These changing conditions may occur on time scales faster than PER can be measured. As a result the link adaptation algorithm is choosing an MCS that is the long term optimal MCS and not the instantaneously optimal MCS.