Understanding SIFS, PIFS, DIFS, EIFS, and AIFS in WLAN

This article explains the differences between Short Interframe Space (SIFS), PCF Interframe Space (PIFS), DCF Interframe Space (DIFS), Extended Interframe Space (EIFS), and Arbitration Interframe Space (AIFS) as defined in the IEEE 802.11 WLAN standard. Interframe spacing is crucial for coordinating access to the shared wireless medium, which carries 802.11-compliant Ethernet packets.

The IEEE 802.11 WLAN standard defines these interframe spaces to manage access to the medium.

The fundamental principle in 802.11 MAC for collision avoidance is delaying transmission until the medium is idle. Different types of traffic require different priority levels, which can be achieved by varying the spacing between frames. The goal is to provide less delay for high-priority traffic, allowing it to access the network before lower-priority traffic as soon as the medium becomes available. To ensure interoperability between different data rates and vendor products, interframe space durations are fixed. Let’s delve into the basics of each type: SIFS, PIFS, DIFS, and EIFS. These interframe spacing timings represent time gaps on the medium. Except for AIFS, the interframe spacing timings are fixed for each PHY layer.

SIFS (Short Interframe Space)

SIFS is used for RTS/CTS (Request to Send/Clear to Send) exchanges and positive acknowledgment-based high-priority transmissions. Once a SIFS duration has elapsed, the transmission can start immediately. Because the medium will be busy after the SIFS period, this type of transmission has higher priority over others initiated after a longer time than SIFS. Extended rate physical layers conforming to Clause 19, such as CCK, DSSS, and OFDM, use a SIFS value of 10 µs.

The following table lists SIFS times for various physical layer configurations:

PIFS (PCF Interframe Space)

PIFS is used by the Point Coordination Function (PCF) during contention-free operations. After the PIFS period elapses, stations with data to be transmitted in the contention-free period can initiate transmission. This preempts any contention-based traffic. PIFS can be used either by:

• Stations (STAs) operating under PCF to gain priority access to the medium at the start of the Contention-Free Period (CFP).
• A Station (STA) to transmit a channel switch announcement frame.

The PIFS duration is calculated as follows:

$PIFS = aSIFSTime + aSlotTime$

Where $aSIFSTime$ and $aSlotTime$ are both constants per physical layer. The $aSIFSTime$ period and $aSlotTime$ are calculated using the following parameters:

$aSIFSTime = aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime$

$aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime + aMACProcessingDelay$

DIFS (DCF Interframe Space)

DIFS is the minimum idle time for the medium and is used for contention-based services/applications. WLAN-compliant Stations can access the medium immediately if it has been free for a period longer than the value defined as DIFS.

The DIFS duration is calculated as follows:

$DIFS = aSIFSTime + 2 * aSlotTime$

EIFS (Extended Interframe Space)

EIFS does not have a fixed interval value. It is used only when there is an erroneous frame transmission.

The EIFS duration is calculated as follows:

$EIFS = aSIFSTime + DIFS + ACKTxTime$

Where:

$ACKTxTime$ is the time (in microseconds) required to transmit an ACK frame, including the preamble, PLCP header, and any additional PHY-dependent information, at the lowest PHY mandatory rate.

AIFS (Arbitration Interframe Space)

AIFS is used by QoS STAs to transmit all data frames, management frames (e.g., MMPDUs), and control frames (e.g., PS-Poll, RTS, CTS).