Wireless Protocols

CW Max

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

For legacy DSSS and HR/DSSS (802.11b) PHYs the default best-effort values are CWmin = 31 and CWmax = 1023 slots. For OFDM PHYs (802.11a/g/n/ac/ax) the defaults are CWmin = 15 and CWmax = 1023. With EDCA QoS access categories the values differ per category: voice (AC_VO) uses CWmin = 3 and CWmax = 7, video (AC_VI) uses CWmin = 7 and CWmax = 15, while best effort and background keep the larger windows. CWmax is always one less than a power of two because the window represents the inclusive range [0, CW] of integer backoff slots.

Q: How does the backoff window grow from CWmin to CWmax?

After each unsuccessful transmission the contention window roughly doubles following CW(i) = min(2^i times (CWmin + 1) minus 1, CWmax). Starting from CWmin = 15 the sequence is 15, 31, 63, 127, 255, 511, 1023, then it clamps at CWmax = 1023 for any further retries. The station draws a uniformly random integer backoff counter from [0, CW] and decrements it once per idle slot. Once CW reaches CWmax it stops growing, so additional collisions no longer widen the window; this bounds the worst-case average backoff at CWmax/2 slots.

Q: How does CWmax affect latency and throughput in a busy network?

A larger CWmax spreads retransmission attempts over a wider range of slots, which lowers the collision probability when many stations contend and protects aggregate throughput. The cost is higher and more variable access latency, since the average backoff after several collisions approaches CWmax/2 slots; at a 9 microsecond OFDM slot time, CWmax = 1023 yields up to roughly 4.6 ms of backoff delay. Voice access categories therefore use a small CWmax (7) to keep jitter low, while best-effort traffic uses CWmax = 1023 to maximize collision resilience at the expense of latency.

/see-dub-uhl-yoo maks/ (CWmax)

The upper limit of the contention window used by the CSMA/CA medium access scheme. When a transmission fails because a frame is not acknowledged, binary exponential backoff doubles the window after each unsuccessful attempt, but it never exceeds CWmax; a successful transmission resets the window back to CWmin. The station draws a uniformly random integer backoff counter from the range [0, CW] and counts down one slot per idle slot interval, so CWmax bounds the largest random delay any node will insert before access. In OFDM 802.11 PHYs the default best-effort CWmax is 1023 slots, while voice traffic under EDCA uses a CWmax of just 7 to keep latency low. Choosing CWmax trades collision resilience in dense networks against worst-case access delay and jitter.

Category: Wireless Protocols

Default (OFDM): 1023 slots

EDCA Voice: 7 slots

CWmax and the Exponential Backoff Ceiling

In a shared-medium wireless network, several radios may want the channel at the same instant. CSMA/CA manages this by forcing each station that finds the medium busy to wait a randomized number of idle slots before it transmits. That random number is drawn from the contention window, an integer range [0, CW]. The contention window starts small at CWmin so that lightly loaded networks have low access delay, then grows after every failed attempt to spread retransmissions out in time. CWmax is the hard ceiling on that growth: once the window reaches CWmax, further collisions no longer widen it.

The backoff procedure is a binary exponential algorithm. After the i-th consecutive failure the window is set to the smaller of CWmax and roughly twice the previous window plus one. Because the window is expressed as an inclusive count of slots, both CWmin and CWmax are always one less than a power of two (15, 31, 63, up to 1023). The station then picks a fresh uniform random backoff counter in [0, CW] and decrements it each time it senses the medium idle for one slot time. When the counter reaches zero, transmission begins. Freezing CW at CWmax prevents the window from diverging under heavy contention, which would otherwise stall the network entirely.

Tuning CWmax is a network-engineering decision. A high CWmax (1023) is appropriate for best-effort data in dense cells because the wide random range minimizes the chance that two stations choose the same slot. A low CWmax (7 to 15) is used for latency-sensitive access categories such as voice and video, where bounded jitter matters more than maximizing collision avoidance. The 802.11e EDCA framework exposes CWmin, CWmax, AIFSN, and TXOP per access category so that operators can shape this trade-off directly.

Backoff and Window Equations

Window growth (after i-th failure):
CW_i = min( 2ⁱ × (CW_min + 1) − 1, CW_max )

Random backoff counter:
Backoff = Uniform[0, CW] × aSlotTime

Mean backoff at the ceiling:
E[Backoff] ≈ (CW_max / 2) × aSlotTime

Where CW_min and CW_max are slot counts (one less than a power of two), i is the retry index, and aSlotTime is the PHY slot duration (9 μs for OFDM, 20 μs for DSSS). Example: CW_max = 1023, 9 μs slot → worst-case mean backoff ≈ 4.6 ms.

Standard CWmin / CWmax Values

PHY / Access Category	CW_min (slots)	CW_max (slots)	aSlotTime	Typical Use
802.11b (DSSS)	31	1023	20 μs	Legacy 2.4 GHz data
802.11a/g/n/ac/ax (OFDM)	15	1023	9 μs	Best-effort data
EDCA AC_BK (background)	15	1023	9 μs	Bulk / background
EDCA AC_BE (best effort)	15	1023	9 μs	General traffic
EDCA AC_VI (video)	7	15	9 μs	Streaming video
EDCA AC_VO (voice)	3	7	9 μs	Latency-critical VoIP
802.15.4 (Zigbee, slotted)	macMinBE 3 → 7	macMaxBE 5 → 31	320 μs symbol-based	Low-rate IoT mesh

Common Questions

Frequently Asked Questions

What are the default CWmin and CWmax values in 802.11?

Legacy DSSS/HR-DSSS (802.11b) uses CWmin = 31 and CWmax = 1023 slots; OFDM PHYs (a/g/n/ac/ax) use CWmin = 15 and CWmax = 1023. Under EDCA the values vary by access category: voice (AC_VO) uses CWmin = 3, CWmax = 7; video (AC_VI) uses 7 and 15; best-effort and background keep CWmax = 1023. CWmax is always one less than a power of two because the window is the inclusive slot range [0, CW].

How does the backoff window grow from CWmin to CWmax?

After each failed attempt the window roughly doubles via CW_i = min(2ⁱ(CW_min+1) − 1, CW_max). From CWmin = 15 the sequence is 15, 31, 63, 127, 255, 511, 1023, then it clamps at CWmax = 1023 for every later retry. The station draws a fresh uniform backoff counter from [0, CW] and decrements once per idle slot. Freezing at CWmax bounds the worst-case mean backoff at CWmax/2 slots.

How does CWmax affect latency and throughput in a busy network?

A larger CWmax spreads retransmissions over more slots, cutting the collision probability under heavy contention and protecting aggregate throughput. The cost is higher, more variable access latency: the mean backoff approaches CWmax/2 slots, so at a 9 μs OFDM slot, CWmax = 1023 adds up to roughly 4.6 ms. Voice uses CWmax = 7 to keep jitter low; best-effort uses 1023 to maximize collision resilience.

Related Terms

← CVD CW Min →