In 802.11 WLANs, arbitration is the process by which devices decide who gets to use the wireless medium (RF channel) next.
Because Wi-Fi is a shared, half-duplex medium, only one device can transmit at a time on a given channel. Arbitration ensures that devices take turns in an organized way to avoid collisions.
What is Slot Time?
- Slot time is the basic time unit used in the CSMA/CA backoff algorithm in Wi-Fi.
- It defines how long a station waits before checking the medium again or decrementing its backoff counter.
Slot Time values in common PHYs
| Standard / PHY | Slot Time |
|---|---|
| 802.11b | 20 µs |
| 802.11a/g/n (OFDM) | 9 µs |
| 802.11g (legacy non-OFDM mode) | 20 µs |
| 802.11n/ac/ax | 9 µs (assuming OFDM mode, short slot enabled |
µs = microseconds
How Does Arbitration Work?
The primary method used is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance):
- Carrier Sense:
A device listens to the channel to check if it is idle (no one else is transmitting). - Wait for DIFS:
If the channel is idle, the device waits for a Distributed Inter-Frame Space (DIFS) time before proceeding. - Backoff Timer:
- If the channel is still idle after DIFS, the device selects a random backoff timer within a defined contention window (CW).
- The backoff timer counts down only while the channel is idle.
- If the channel becomes busy, the timer pauses and resumes when the channel is idle again.
- Transmit When Timer Hits Zero:
The device whose backoff timer reaches zero wins arbitration and transmits its frame. - Collision Avoidance:
Unlike Ethernet (which uses CSMA/CD and detects collisions), Wi-Fi avoids collisions by using this backoff mechanism before transmitting.
Why Is Arbitration Needed?
Prevents simultaneous transmissions that would collide and require retransmissions.
Ensures fair access to the medium among clients and APs.
Maintains network performance and stability in high-density environments.
Arbitration Mechanisms in 802.11 Standards
- DCF (Distributed Coordination Function):
Standard CSMA/CA method described above. Used by default. - EDCA (Enhanced Distributed Channel Access) – 802.11e QoS:
Introduces traffic prioritization with different Arbitration Interframe Spaces (AIFS) and contention windows for traffic categories (voice, video, best effort, background). - PCF (Point Coordination Function):
Rarely used polling-based method for time-sensitive transmissions. Not widely implemented.
Arbitration Example
Scenario: Three clients want to transmit.
- All sense the channel is idle.
- Each sets a random backoff timer (e.g. Client A = 3, B = 7, C = 12).
- Timer counts down. Client A’s timer hits zero first.
- Client A transmits.
- Others pause their timers until the channel is idle again.
Key Takeaway
Arbitration is the process by which Wi-Fi devices decide who transmits next, using CSMA/CA to avoid collisions and ensure fair, efficient medium access.
Interframe Spaces in 802.11
1. SIFS – Short Interframe Space
- Shortest IFS
- Used for highest priority transmissions, such as:
- ACK frames
- CTS frames
- Data frame fragments
- Typical value:
- 10 µs for 802.11b
- 16 µs for 802.11a/g/n/ac/ax
2. DIFS – Distributed Interframe Space
- Standard wait time for DCF (Distributed Coordination Function) data frames before attempting medium access.
- Used by stations when sending regular data frames.
- DIFS = SIFS + (2 \times slot_time)
3. AIFS – Arbitration Interframe Space
- Used in EDCA (Enhanced Distributed Channel Access) for QoS-enabled networks (802.11e/WMM).
- Replaces DIFS to provide different wait times for different traffic categories (e.g. Voice, Video, Best Effort, Background).
- AIFS = SIFS + (AIFSN \times slot_time)
4. EIFS – Extended Interframe Space
- Used when a frame is received with errors (CRC fail).
- Ensures the original recipient has time to send an ACK before others attempt to access the medium.
- Longer than DIFS to avoid collisions in such scenarios.
Summary Table
| IFS | Used By / Purpose | Calculation |
|---|---|---|
| SIFS | ACK, CTS, Fragmentation | Fixed (10 or 16 µs depending on PHY) |
| DIFS | Standard data (DCF) | SIFS + (2 × slot time) |
| AIFS | QoS traffic (EDCA) | SIFS + (AIFSN × slot time) |
| EIFS | Error recovery wait | Long fixed value |