802.11 Medium Access Simulator
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.
How to use this tool controls and what to look for
Pick a scenario, press Play (or Next event to go one step at a time) and follow each station's row on the timeline.
Controls to change
- Scenario: work through the numbered lessons, from DCF basics and collisions to EDCA priority, hidden nodes with and without RTS/CTS, and TXOP bursting.
- Access method: switch between DCF (everyone equal) and EDCA (voice, video, best effort and background).
- RTS/CTS: turn it on to reserve the channel before each data frame.
- Next event: step through one event at a time. Skip 10 ms jumps ahead.
- Add station: add a station and set its category, location, payload and PHY rate in the Stations table.
- Retry limit: change how many times a frame is retried before it is dropped.
- Random seed: change it to get a different run of backoff numbers.
What to look for
- A station that hears the channel busy freezes its backoff and carries on from the same number later. It doesn't start again.
- After a collision the contention window doubles, so the next backoff tends to be longer.
- With EDCA, voice waits a shorter AIFS and draws smaller backoffs, so it usually wins the channel.
- Watch the NAV: stations that heard an RTS or CTS stay quiet even when they can't hear the data frame.
About this tool
This simulator follows 802.11 medium access one microsecond at a time: stations wait for a quiet channel, count down random backoffs, collide, retry and transmit. Scenarios cover DCF and EDCA, hidden stations, RTS/CTS and TXOP bursts, with an event log and each station's row on the timeline.
What you'll learn
- How CSMA/CA listen-before-talk works
- How interframe spaces (SIFS, DIFS, AIFS) set who goes first
- How backoff freezes and doubles after collisions
- How the NAV and RTS/CTS act as virtual carrier sense
Common questions
Why does Wi-Fi use CSMA/CA and not CSMA/CD?
A radio can't listen while it transmits, so it can't detect a collision. Wi-Fi avoids collisions with random backoff instead, and learns that a frame was lost when no acknowledgement comes back.
What is the NAV?
The Network Allocation Vector: a timer each station sets from the duration field of frames it overhears, so it stays quiet until the other exchange is finished.
What happens after a collision?
Each station doubles its contention window, picks a new random backoff and tries again, up to a retry limit.
Free for private and educational use. This interactive model is a simplified teaching aid and may contain mistakes. It is provided as is, without warranty. Use it to understand the concepts, not as a substitute for a site survey, vendor documentation or professional design advice.