Extended inter-frame spacing (EIFS), is a waiting period used in MAC layer of IEEE 802.11-based wireless local area network standard (WLAN standard). It is an additional waiting period used in addition to the mandatory DISF technique in case of corrupted frames.
DISF (Distributed coordination function inter-frame spacing) is inserted before sending frames to avoid collisions in areas where carrier-sense multiple access with collision avoidance (CSMA/CA) is used. EISF is used for erroneous frames.
In normal delivery of frames, DISF waiting period is good enough. However, when a previously received frame is corrupted, the DISF waiting period proves to be insufficient. In this case, the station has to extend the waiting period to EISF time period. This allows the intended receiver to decode the frame, which this station could not decode and has marked as corrupted. If the receiver can decode the frame, it returns an acknowledgement frame (ACK).
Using EISF waiting period, it can be ensured that the ACK can be transmitted by the concerned station without fearing interference from other stations which could not decode the original frame.
Extended inter-frame spacing is given by the following formula −
EFIS = TimeACK + SIFS + DIFS
TimeACK is the transmission time of ACK frame at the physical layer mandatory rate
𝑆𝐼𝐹𝑆 is Short Inter-Frame Space. SIFS is the time interval required by a wireless device in between receiving a frame and responding to the frame.
is Distributed coordination function inter-frame spacing.