IEEE 802.15.4 defines a specific time that is allotted for the MAC sublayer to process received data. The interframe spacing (IFS) is used to prevent two frames from being transmitted too close together. If the transmission is requesting an acknowledgement, the space before the second frame must be at least one IFS period.
IFS is determined to be one of the following:
- IFS = macMinSIFSPeriod (12 symbols) if MPDU ≤ aMaxSIFSFrameSize (18 octets) octets
- IFS = macMinLIFSPeriod (40 symbols) if MPDU > aMaxSIFSFrameSize
Using the efficient assisted modes in RADIO, the TIFS will be programmed with the correct value based on the frame being transmitted. If the assisted modes are not in use, the TIFS register must be updated manually. The following figure shows what IFS period is valid in both acknowledged and unacknowledged transmissions.