Stations defer, count down, collide, retry and transmit on a shared channel. Pick a scenario, press Play, and follow each station's row on the timeline.
Click any point on the timeline to see every station's status at that moment, then drag the blue time label or its line to scrub through time. Hover a block for its details. Drag the timeline or use the slider to look back. Changing any setting restarts the run.
Every station hears the AP. West and East stations are out of range of each other, which creates hidden nodes. Stations near the AP hear everyone.
Newest first. Select an entry to move the timeline there.
Defaults are the 802.11 values an AP advertises to client stations. Timing uses 5 GHz OFDM at 20 MHz: slot 9 µs, SIFS 16 µs, DIFS 34 µs. Control frames go at 6 Mbps: RTS 52 µs, CTS and ACK 44 µs. ACK and CTS timeout is 50 µs (SIFS + slot + 25 µs PHY receive start). EIFS = SIFS + ACK time + DIFS or AIFS.
A radio can't listen while it transmits, so Wi-Fi can't detect a collision as it happens the way wired Ethernet did. It tries to avoid one instead.
SIFS (16 µs) is shorter than DIFS or AIFS (34 µs and up), which is shorter than EIFS. Responses such as CTS, ACK, and the DATA that follows a CTS wait only SIFS, so a station starting fresh contention can never cut into an exchange already in progress.
The backoff counter only drops while the medium stays idle, one 9 µs slot at a time. If someone else transmits, the counter freezes and resumes from the same number after the medium has been idle for DIFS or AIFS again. Losing one round doesn't reset the count, so stations that have waited longest tend to win next.
No ACK or CTS within the timeout means the sender assumes its frame was lost. It doubles its contention window (15, 31, 63, and so on up to CWmax), draws a new random backoff from the wider range, and tries again. Spreading the retries out makes it less likely the same stations collide again. After the retry limit the frame is dropped and the window resets to CWmin.
Every frame carries a Duration field saying how long the rest of the exchange will take. Stations that decode it set a timer called the NAV and treat the medium as busy until it expires, even when they hear nothing. In the timeline it's the grey bar along the bottom of a row.
EDCA gives voice, video, best effort, and background traffic separate AIFSN, CWmin/CWmax, and TXOP limits. Voice waits a shorter AIFS and draws from a smaller window, so it wins more often. That's priority by probability, not a reservation.
A station that wins access for video or voice may keep the medium for several DATA and ACK exchanges back to back, separated only by SIFS, up to its TXOP limit. One win of the contention buys a short run of frames instead of one. Best effort and background have a TXOP limit of 0, meaning one frame per win.
Two stations on opposite sides of the AP can't hear each other, so carrier sense reports an idle medium while the other one is mid-frame, and both frames arrive at the AP together. With RTS/CTS the sender asks first with a short RTS. The AP's CTS is heard by everyone, including the hidden station, which sets its NAV and stays quiet. Collisions still happen, but only on the 52 µs RTS instead of the full DATA frame.
A station that receives a frame it can't decode doesn't know that frame's Duration value. To avoid stepping on an ACK that might follow, it waits EIFS instead of DIFS or AIFS before contending. The amber band in the timeline marks this.
Traffic is uplink only: stations send to the AP, and the AP sends only CTS and ACK. Each station carries one access category, so internal collisions between queues inside the same station aren't shown. Any overlap at a receiver destroys both frames (no capture effect). Every new frame draws a backoff, so immediate access on a long-idle medium isn't modelled. One retry counter replaces the separate short and long retry limits. Frame timing is legacy OFDM (802.11a/g style) with no A-MPDU, Block Ack, rate adaptation, or 802.11ax trigger-based access.