Extended Inter-Frame Spacing (EIFS)

Extended Inter-Frame Spacing (EIFS) is a waiting period used in the MAC layer of IEEE 802.11-based wireless local area network (WLAN) standard. It is an additional waiting period used in addition to the mandatory DIFS technique when frames are corrupted or cannot be properly decoded.

DIFS (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. EIFS is specifically used when a station receives erroneous or corrupted frames that it cannot decode properly.

How EIFS Works

In normal frame delivery, the DIFS waiting period is sufficient to prevent collisions. However, when a previously received frame is corrupted, the DIFS waiting period proves to be inadequate. In this case, the station must extend the waiting period to the EIFS time duration.

This extended waiting allows the intended receiver to decode the frame that this station could not decode and has marked as corrupted. If the receiver can successfully decode the frame, it returns an acknowledgement frame (ACK).

By using the EIFS waiting period, it ensures that the ACK can be transmitted by the intended receiver without interference from other stations that could not decode the original frame.

EIFS Calculation

Extended Inter-Frame Spacing is calculated using the following formula:

EIFS = Time_ACK + SIFS + DIFS

Where:

• Time_ACK is the transmission time of ACK frame at the physical layer mandatory rate

• SIFS is Short Inter-Frame Space − the time interval required by a wireless device between receiving a frame and responding to the frame

• DIFS is Distributed Coordination Function Inter-Frame Spacing

Key Benefits

• Collision avoidance − Prevents interference with ACK frames from successfully decoded transmissions

• Network efficiency − Allows proper completion of frame exchanges even when some stations cannot decode frames

• Protocol compliance − Maintains IEEE 802.11 standard behavior in error conditions

Conclusion

EIFS is a crucial mechanism in IEEE 802.11 networks that extends the waiting period when corrupted frames are received. This ensures proper ACK transmission and prevents collisions, maintaining network efficiency even in error-prone wireless environments.