Inter – frame spaces (RIFS, SIFS, PIFS, DIFS, AIFS, EIFS)

Inter-frame spaces (IFS) are waiting periods between transmission of frames in the medium access control (MAC) sublayer where carrier-sense multiple access with collision avoidance (CSMA/CA) is used. These are techniques used to prevent collisions as defined in IEEE 802.11-based WLAN standard (Wi-Fi).

IFS is the time period between completion of the transmission of the last frame and starting transmission of the next frame, apart from the variable back-off period. Different types of IFS provide priority mechanisms, with shorter durations indicating higher priority access to the wireless medium.

Reduced Inter-frame Space (RIFS)

Reduced Inter-frame Space (RIFS) is the shortest duration inter-frame spacing used for high-priority frames in frame bursts. RIFS was introduced in 802.11n amendment and has a duration of only 2?s.

When a station needs to send multiple frames consecutively, RIFS is used between individual frames in the burst. This ensures that no other station can access the channel during the frame burst, maximizing throughput for high-priority transmissions.

Short Inter-frame Space (SIFS)

Short Inter-frame Space (SIFS) is the time interval required by a wireless device between receiving a frame and responding to it. SIFS is used in the Distributed Coordination Function (DCF) scheme and is mandatory for collision prevention.

The duration of SIFS equals the sum of radio frequency (RF) delay, Physical Layer Convergence Procedure (PLCP) delay, and MAC processing delay. SIFS is maintained before transmission of acknowledgment frames and Clear-To-Send (CTS) frames. The typical duration is 10?s.

PCF Inter-frame Space (PIFS)

Point Coordination Function Inter-frame Space (PIFS) is used in the optional PCF technique for centralized collision prevention in WLANs. The Access Point (AP) waits for PIFS duration to access the channel.

Since PIFS is shorter than DIFS, the AP always has priority over regular stations. PIFS is calculated as: PIFS = SIFS + slot time.

DCF Inter-frame Space (DIFS)

Distributed Coordination Function Inter-frame Space (DIFS) is the mandatory waiting period for stations using DCF in IEEE 802.11 networks. Before transmitting, a station must sense the channel and wait for DIFS duration if the channel is idle.

DIFS provides the basic channel access mechanism for regular data frames. It is calculated as: DIFS = SIFS + 2 × slot time.

Arbitrary Inter-frame Space (AIFS)

Arbitrary Inter-frame Space (AIFS) provides Quality of Service (QoS) by assigning different waiting periods based on Access Categories. Higher priority traffic gets shorter AIFS values, enabling faster channel access.

AIFS is calculated as: AIFS = SIFS + (AIFS number × slot time), where the AIFS number varies based on traffic priority (voice, video, best effort, background).

Extended Inter-frame Space (EIFS)

Extended Inter-frame Space (EIFS) is used when a station receives a corrupted frame or detects transmission errors. EIFS provides additional waiting time to allow for error recovery procedures before attempting channel access again.

Comparison of Inter-frame Spaces

Conclusion

Inter-frame spaces provide an elegant priority mechanism in IEEE 802.11 networks, with shorter durations granting higher priority access to the wireless medium. This system ensures efficient collision avoidance while supporting Quality of Service requirements for different types of traffic.