What actually is EIFS?

Question

There are a couple of things on the Internet about EIFS intervals.

One way I've found to help people understand EIFS is the following: When a station receives a corrupted frame, it has no way of knowing which part of the frame was corrupted. Therefore, it must assume that the frame never made it to its intended receiver. If the frame never made it to the intended receiver, that means that a Retry must happen. The EIFS forces stations to backoff (not transmit) for an extended period of time while a Retry happens after a corrupted frame.

I'm not necessarily sure this is correct, especially considering the following diagram.

As far as I can tell. EIFS is for stations in the carrier sensing zone of the source station. They are out of the transmission zone of the source node meaning there will be errors in the packet received. When there is an error it waits for a NAV duration EIFS. Although it is not known which packet has been received, the EIFS interval is long enough such that if it is a data packet that is received with errors in then it will wait for NAV at least SIFS + ACK + DIFS as it doesn't know the NAV of the actual packet and while the packet is actually being received it just stalls and waits for the end of the transmission.

In the case of the destination receiving a collision it doesn't know whether the frame was for it or another node in the transmission zone or from a node for which it is in the carrier sensing zone. There may be another destination that has received the frame correctly and therefore it delays for EIFS which will be enough time to either align with all the other devices on the beginning of the contention window (destination is the only station that waits for SIFS, sends ACK and DIFS if it recognises the packet otherwise it will wait for EIFS and not send an ACK) if it was a data packet, or enough time to delay it until the medium is busy if it was an RTS/CTS.

Also I'm not sure, but at the end of the second EIFS on the diagram I think it knows it is in the process of receiving a frame so it doesn't do anything until it sets a NAV. It doesn't sense for DIFS intervals or attempt to send in this time frame.

Answer 1

The EIFS is the extended interframe space. The 2020 802.11 standard specifies :

A DCF shall use EIFS before transmission, when it determines that the medium is idle immediately following reception of a frame for which the PHY-RXEND.indication primitive contained an error or a frame for which the FCS value was not correct. […] The EIFS [is] defined to provide enough time for another STA to acknowledge what was, to this STA, an incorrectly received frame before this STA commences transmission.

So you're right, the quoted text is false.