How does binary exponential backoff change the contention window after a collision?

After each unsuccessful transmission (no acknowledgment received), the station increases its contention window by roughly doubling it: CW_new = 2 times (CW_old + 1) minus 1, capped at CWmax. Starting from CWmin = 15, the window grows through the sequence 15, 31, 63, 127, 255, 511, 1023, then holds at 1023 for further retries. Because the backoff counter is drawn uniformly from [0, CW], doubling the window roughly halves the probability that two backlogged stations pick the same slot, which reduces repeated collisions as the channel gets busier. On a successful transmission the window resets to CWmin. After the retry limit is reached (typically 7 for short frames or the dot11ShortRetryLimit), the frame is discarded and CW resets.

How do you convert a contention window value into an average backoff time?

The backoff counter is a uniformly random integer in [0, CW], so its mean is CW divided by 2 slots. Multiply by the slot time to get the average backoff duration. For an 802.11ax OFDM PHY, the slot time is 9 microseconds, so with CWmin = 15 the average initial backoff is 7.5 times 9 = 67.5 microseconds. At CW = 1023 the average backoff is 511.5 times 9 = 4.6 milliseconds, which is why heavy congestion sharply lowers throughput. Total deferral before a transmission also includes the AIFS or DIFS sensing interval (for example 34 microseconds for DIFS on a 5 GHz OFDM channel) and any frozen backoff time accumulated while the medium was busy.

Wireless Protocols

Contention Window

Q: What are the default CWmin and CWmax values in 802.11?

For legacy DCF, the default contention window bounds on OFDM PHYs such as 802.11a, g, n, ac, and ax are CWmin = 15 and CWmax = 1023 slots, derived from aCWmin = 15. The older DSSS and ERP PHYs (802.11b and 802.11g compatibility mode) instead use CWmin = 31 with a 20 microsecond slot time on pure DSSS. Under EDCA, each access category has its own bounds: voice (AC_VO) uses CWmin = 3 and CWmax = 7, video (AC_VI) uses CWmin = 7 and CWmax = 15, while best effort (AC_BE) and background (AC_BK) use CWmin = 15 or 31 and CWmax = 1023. Smaller windows for voice and video give those categories statistically shorter backoffs and therefore higher priority access to the medium. The backoff counter is chosen uniformly at random from the integer interval [0, CW], and each slot represents 9 microseconds on a modern OFDM PHY.

/kuhn-TEN-shun WIN-doh/

Central to the random-access mechanism of CSMA/CA, this is the integer range of time slots, bounded by CWmin and CWmax, from which an 802.11 station picks a backoff counter at random before it is allowed to transmit on a shared channel. Because every backlogged station draws independently from the same window, collisions become statistically less likely than if everyone transmitted the instant the medium went idle. When a frame fails (no acknowledgment), the window doubles under binary exponential backoff, spreading retries over a wider span of slots; a successful transmission collapses it back to CWmin. The window also distinguishes traffic priority: in EDCA, voice and video access categories use small windows for fast access while background traffic uses large ones, layered on top of the per-category AIFS sensing interval.

Category: Wireless Protocols

Default CWmin / CWmax: 15 to 31 / 1023 slots

Slot time (OFDM): 9 μs

How the Backoff Window Governs Shared-Channel Access

When an 802.11 station has a frame to send, it first senses the channel. If the medium has been idle for an AIFS or DIFS interval, the station does not transmit immediately; instead it selects a random integer from the interval [0, CW], where CW is the current contention window size, and loads that value into a backoff counter. The counter decrements by one for every idle slot time observed. If another station begins transmitting, the counter freezes and resumes only after the medium is again idle for the required sensing interval. The station transmits when its counter reaches zero. This randomization is the core collision-avoidance trick: two stations contending at the same moment are unlikely to choose the same backoff value, so one wins the slot and the other defers.

The window is dynamic. It starts at CWmin (15 slots for best-effort OFDM traffic). When a transmission is not acknowledged, the station assumes a collision and grows the window through binary exponential backoff, roughly doubling it on each retry up to CWmax = 1023 slots. Because the backoff counter is uniform over a larger range, the probability that two backlogged stations collide again drops with each doubling. A successful frame exchange resets the window to CWmin, so a lightly loaded network keeps access latency low, while a congested one automatically spreads stations out over a wider time span. The cost of that stability is throughput: at CW = 1023 the average backoff approaches 4.6 ms, far larger than the airtime of a single frame at high data rates, so the listen-before-talk overhead, not the payload, dominates.

Slot time ties the abstract window to real microseconds. A modern OFDM PHY (802.11a/g/n/ac/ax) uses a 9 μs slot; legacy 802.11b DSSS used 20 μs. Average backoff is simply the mean slot count times the slot time, so the same window value translates to longer deferral on older PHYs. This is why mixed-mode networks that must remain backward compatible with 802.11b suffer disproportionately under contention.

Binary Exponential Backoff and Slot Timing

Random backoff slot count:
backoff = Uniform{0, 1, …, CW}, mean ≈ CW / 2

Window growth after a failed retry r:
CW_r = min( 2^r × (CW_min + 1) − 1, CW_max )

Average backoff time:
T_bo ≈ (CW / 2) × T_slot (T_slot = 9 μs for OFDM)

Where CW_min and CW_max are the window bounds in slots and r is the retry index (r = 0 at first attempt). Example: AC_BE with CW_min = 15 gives T_bo ≈ 7.5 × 9 μs = 67.5 μs on the first try; at CW = 1023, T_bo ≈ 511.5 × 9 μs ≈ 4.6 ms.

Contention Window Values by Access Category

Access Category	Traffic Type	CW_min (slots)	CW_max (slots)	AIFSN	Mean First Backoff
AC_VO	Voice	3	7	2	~13.5 μs
AC_VI	Video	7	15	2	~31.5 μs
AC_BE	Best effort	15	1023	3	~67.5 μs
AC_BK	Background	15	1023	7	~67.5 μs
Legacy DCF	All (no QoS)	15 or 31	1023	DIFS	~67.5 to 139.5 μs

Common Questions

Frequently Asked Questions

What are the default CWmin and CWmax values in 802.11?

Legacy DCF on OFDM PHYs (802.11a/g/n/ac/ax) uses CW_min = 15 and CW_max = 1023 slots (aCWmin = 15); the older DSSS/ERP PHYs use CW_min = 31 with a 20 μs slot. Under EDCA each access category differs: voice uses CW_min = 3, CW_max = 7; video uses 7 and 15; best-effort and background use 15 or 31 up to 1023. Smaller windows give voice and video statistically shorter backoffs and higher access priority. The counter is drawn uniformly from [0, CW], with each slot equal to 9 μs on OFDM.

How does binary exponential backoff change the window after a collision?

Each unacknowledged transmission roughly doubles the window: CW_new = 2 × (CW_old + 1) − 1, capped at CW_max. From CW_min = 15 the sequence is 15, 31, 63, 127, 255, 511, 1023, then it holds. Doubling roughly halves the chance that two backlogged stations pick the same slot. A successful frame resets CW to CW_min; reaching the retry limit (often 7) discards the frame and also resets the window.

How do you convert a contention window into an average backoff time?

The counter is uniform over [0, CW], so its mean is CW / 2 slots. Multiply by the slot time: on 802.11ax (9 μs slots), CW_min = 15 gives 7.5 × 9 = 67.5 μs, while CW = 1023 gives 511.5 × 9 ≈ 4.6 ms, which is why congestion sharply lowers throughput. Total deferral also adds the AIFS or DIFS sensing interval and any backoff frozen while the medium was busy.

Related Terms

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