The channel is shared
Only one nearby transmitter should occupy the same channel at a time. Carrier sense checks whether the medium looks idle before a frame starts.
Wi-Fi CSMA/CA DCF Workbench
Trace Wi-Fi carrier sense, interframe waits, random backoff, frozen counters, RTS/CTS, ACKs, and retry growth
animation
wifi
csma-ca
dcf
mac-layer
channel-access
wireless
Learner-ready Wi-Fi CSMA/CA workbench with scenario presets, Play/Step/Reset controls, backoff and retry calculations, hidden-node and RTS/CTS comparison, diagnosis cards, formula trace, quick reference, technical notes, and source links.
Wi-Fi MAC
CSMA/CA
DCF backoff
Wi-Fi CSMA/CA DCF Workbench
Step through how a Wi-Fi station wins the shared radio channel: it listens, waits a DIFS or AIFS, counts down a random backoff only while the channel stays idle, transmits, then waits for an ACK. Switch on RTS/CTS to see how virtual carrier sensing can protect large frames from hidden terminals.
Current stageReady to contend
Access estimate0 us
Collision riskLow
OutcomeWaiting for learner input
Beginner ramp
What the learner should notice first
Wi-Fi is a shared half-duplex radio medium. A station cannot reliably listen for a collision while it is transmitting, so the MAC layer spreads attempts in time and treats a missing ACK as the practical failure signal.
Backoff breaks ties
If several stations hear the channel become idle, a fixed wait would make them collide. Random slots spread their attempts.
Busy slots pause the counter
A station counts down only while the channel remains idle. If another station starts first, the loser freezes and resumes later.
Hidden stations need a reservation hint
Stations that cannot hear each other can still collide at the AP. RTS/CTS adds overhead, but CTS and NAV can protect the data exchange.
Interactive workbench
Tune contention and trace one Wi-Fi access attempt
Start with a scenario, then use Step to inspect each stage. The formulas intentionally use a compact teaching model: timing profiles, contention windows, backoff slots, and RTS/CTS overhead are visible, while full PHY aggregation and rate adaptation are out of scope.
Topology and channel access
Backoff slot timeline
CW 15, slot 9 us
focus slot
freeze
exchange
collision
NAV
Station counters
Event trace
Diagnosis
What changed because of your controls?
Each card ties one visual behavior to a concrete MAC-layer idea: contention window size, backoff delay, collision or hidden-node risk, RTS/CTS overhead, ACK-driven retry, and model limits.
Retry growth and access profile will update this range.
Average idle-slot wait plus freeze time appears here.
Station count, hidden nodes, RTS/CTS, and CW determine the teaching estimate.
Large and hidden-node frames benefit differently from the reservation exchange.
Formula trace
The compact math behind the scene
These equations are intentionally small enough to inspect. The page uses them for learning feedback, not for predicting exact throughput on a real AP.
| Quantity | Teaching formula | Current value |
|---|
What this simulator is not doing
It does not model aggregation, MCS selection, guard intervals, spatial streams, frame fragmentation, capture effect, rate fallback, Block ACK, or every EDCA parameter-set variation. Use it to reason about DCF-style access logic before moving to packet-level tools.
Quick reference
Keep these rules in view
Use these compact definitions while experimenting. Open the deeper notes when you need the standards context.
Clear Channel Assessment
Physical carrier sense decides whether energy or a decodable Wi-Fi preamble makes the channel busy.
Virtual carrier sense
Duration fields can reserve the medium in station timers, even when a station cannot decode the whole exchange.
Idle interframe wait
DCF uses DIFS = SIFS + 2 x slot. QoS EDCA uses AIFS values that can differ by access category.
Contention Window
Backoff is selected from 0..CW. After failed attempts, CW grows toward CWmax.
Guided practice and technical notes
Freeze a counter
Use Crowded classroom, press Step to Backoff, then raise neighbor busyness. The timeline should show frozen slots before the exchange.
RTS/CTS trade-off
Use Hidden terminal with RTS/CTS off, then on. The DATA collision risk drops, but the exchange becomes longer.
Retry growth
Increase Retry attempt. The current CW grows approximately by powers of two, spreading stations over more slots.
Wi-Fi waits for ACK feedback
A transmitting station cannot use Ethernet-style collision detection while sending. A missing ACK is treated as the practical failure signal.
Small frames often skip RTS/CTS
RTS/CTS can waste airtime on tiny frames. It is most useful when frame cost or hidden-node risk is high enough to justify the extra control frames.
QoS changes contention parameters
EDCA access categories can use different AIFS and contention windows. This page uses simplified profiles so the direction of the effect is visible.
Microseconds, slots, and bytes
Timing outputs use microseconds. Airtime uses bytes converted to bits and a simplified data/control rate model.
Sources
Primary and technical references used
Links open the standards home pages or detailed technical references used to check terminology, timing relationships, and DCF behavior.
IEEE 802.11 Working Group
Official working group context for the IEEE 802.11 WLAN MAC and PHY standard family.
Open IEEE 802.11 WG pageIEEE SA 802.11 standards page
IEEE Standards Association listing for IEEE 802.11 base standards and amendments.
Open IEEE standards listingIETF RFC 8325
Maps Diffserv to IEEE 802.11 and summarizes CSMA/CA, DCF, slot time, IFS, and CW terms.
Open RFC 8325Cisco 802.11 DCF overview
Vendor technical reference with a concise DCF process description and interframe/backoff examples.
Open Cisco DCF reference