// reference · intermediate
Interframe Spacing (IFS)
The gaps between 802.11 frames are not random — every IFS value has a precise purpose and a strict priority order. Understanding IFS is understanding how CSMA/CA actually works.
— Shankar K. · Source: IEEE 802.11-2020, Clause 10.3 · 1 TU = 1024 μs · Values shown for ERP (802.11g) 2.4 GHz
// priority order — shorter gap = higher priority
SIFS
16 μs
PIFS
25 μs
DIFS
34 μs
AIFS[AC_VO]
34 μs
AIFS[AC_VI]
43 μs
AIFS[AC_BE]
43 μs
AIFS[AC_BK]
70 μs
EIFS
364 μs
SIFS = 16μs · Slot time = 9μs · DIFS = SIFS + 2×slot = 34μs · AIFS[n] = SIFS + AIFSN×slot
// each ifs in detail
SIFS
Short Interframe Space
16 μs (2.4 GHz ERP)
Used between frames in the same exchange where no other station should be allowed to transmit — ACK after data, CTS after RTS, BlockACK after BlockACKReq, subsequent frames in a TXOP burst.
rule
Any station that hears a SIFS-separated exchange knows not to attempt transmission — the channel will be occupied again before their DIFS wait completes.
cwna exam note
SIFS is ALWAYS shorter than DIFS. Any frame that uses SIFS has implicitly higher priority than any frame waiting a full DIFS.
Wireshark: radiotap.channel — look for back-to-back frames with ~16μs gap. Data → ACK sequence is the most common SIFS pair.
DIFS
DCF Interframe Space
34 μs (2.4 GHz ERP)
The standard wait before attempting channel access under DCF (Distributed Coordination Function). After a DIFS idle period, a station enters the random backoff phase before transmitting.
rule
DIFS = SIFS + 2 × SlotTime. After any busy period, stations must detect the channel idle for a full DIFS before starting backoff.
cwna exam note
DIFS is the most important IFS for CWNA. Understand that after a successful frame exchange, the channel must be idle for DIFS before anyone transmits.
Wireshark: Most data frames in a capture are preceded by a DIFS gap. Hard to measure directly in Wireshark — look at inter-frame gaps in the time delta column.
AIFS
Arbitration Interframe Space
SIFS + AIFSN × SlotTime
Used by EDCA (802.11e/WMM) to give different Access Categories different priorities. Voice (AIFSN=2) waits the same as DIFS. Background (AIFSN=7) waits much longer — giving voice a statistical advantage.
rule
AIFS[AC_VO]=34μs, AIFS[AC_VI]=43μs, AIFS[AC_BE]=43μs, AIFS[AC_BK]=70μs (values for 9μs slot time). These are per-AC, per-radio — each queue independently competes.
cwna exam note
AIFS replaces DIFS in QoS-enabled (WMM) networks. Voice gets AIFSN=2, same as DIFS — it does NOT get a shorter-than-DIFS wait. Priority comes from both AIFS and CWmin.
Wireshark: QoS Control field in data frames (wlan.qos.priority) shows which AC the frame belongs to. AIFSN values are in the EDCA Parameter Set IE of beacon frames.
EIFS
Extended Interframe Space
364 μs (2.4 GHz ERP)
Used after receiving a corrupt or undecodable frame. Forces a much longer wait before attempting to transmit — protects against stations transmitting into an ongoing exchange they couldn't decode.
rule
EIFS = SIFS + ACK transmission time + DIFS. A station uses EIFS instead of DIFS if its previous receive attempt failed (bad FCS, wrong preamble etc). Resets to DIFS after a successful receive.
cwna exam note
EIFS kicks in after a bad FCS. It's the longest standard IFS. Purpose: prevent a station from transmitting during an exchange it couldn't hear completely due to range or interference.
Wireshark: High FCS error rate in a capture correlates with EIFS usage — stations back off longer after each corrupt frame. Look for wlan.fcs_bad == 1 frames followed by gaps.
PIFS
PCF Interframe Space
25 μs (2.4 GHz ERP)
Used by the Point Coordinator in PCF (Point Coordination Function) mode. Shorter than DIFS so the AP can seize the channel after SIFS without competing with DCF stations.
rule
PIFS = SIFS + 1 × SlotTime = 25μs. Falls between SIFS and DIFS in the priority order. PCF is effectively unused in modern enterprise deployments — superseded by EDCA/HCF.
cwna exam note
Know PIFS exists and where it falls in priority (between SIFS and DIFS). Understand PCF is legacy — modern QoS uses EDCA not PCF.
Wireshark: PCF frames are extremely rare in modern captures. If you see CF-Poll or CF-End frames (wlan.fc.type_subtype == 0x1A/0x1B), you have a PCF network.
RIFS
Reduced Interframe Space
2 μs
Introduced in 802.11n to reduce overhead between frames in a burst transmission from the same transmitter. Allows back-to-back frame transmission with minimal gap.
rule
RIFS = 2μs. Only used between consecutive transmissions from the same station in HT mode. Not used in 802.11ac/ax — replaced by MPDU aggregation (A-MPDU) which achieves the same goal more efficiently.
cwna exam note
Know RIFS is 2μs, introduced in 802.11n, and that it's largely replaced by aggregation in later standards. The smallest standard IFS value.
Wireshark: RIFS is rare in modern captures. 802.11ac/ax deployments use A-MPDU aggregation instead — look for Block ACK frames (wlan.fc.type_subtype == 0x19) which indicate aggregation in use.
// interactive wireshark frame tree — data frame with ack exchange
Click any row to expand · hover for Wireshark filter
// ifs timing values by phy
| IFS | 802.11b (HR-DSSS) | 802.11g (ERP) | 802.11a/n/ac/ax (OFDM) | Slot time used |
|---|---|---|---|---|
| SIFS | 10 μs | 16 μs | 16 μs | — |
| Slot time | 20 μs | 9 μs | 9 μs | — |
| DIFS | 50 μs | 34 μs | 34 μs | SIFS + 2×slot |
| PIFS | 30 μs | 25 μs | 25 μs | SIFS + 1×slot |
| EIFS | 364 μs | 364 μs | 364 μs | SIFS + ACK + DIFS |
| RIFS | — | — | 2 μs | 802.11n HT only |
See IFS gaps in a real PCAP
WiFi Analyser shows inter-frame timing and flags EIFS conditions from high FCS error rates.
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SK
— Shankar K., Wi-Fi engineer, Irving TX
Building WiFi Analyser V2 · CWNA-109 in progress · one post every two weeks
Building WiFi Analyser V2 · CWNA-109 in progress · one post every two weeks
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