CSMA/CA and Contention Window

The Problem

In WiFi, everyone shares the same air.

If two devices transmit at the same time, their signals collide and both fail. Neither message gets through.

We need a way to avoid collisions before they happen.

CSMA/CA

CSMA/CA = Carrier Sense Multiple Access with Collision Avoidance

The key word is avoidance. We don’t detect collisions after they happen. We try to prevent them.

The Two Rules

Rule 1: Listen before you talk

Before transmitting, check if the channel is busy. If someone else is transmitting, wait.

Rule 2: Wait a random time

Even when the channel becomes free, don’t transmit immediately. Pick a random number and wait that many time slots.

Why random? If two devices both waited for the channel and it becomes free, they’d transmit at the same instant. The random wait spreads them out.

The Contention Window

Where does the random number come from?

You pick it from a range called the Contention Window (CW).

• Pick a random number between 0 and CW - 1
• Wait that many time slots
• Then transmit

Example: CW = 16

You pick randomly from: 0, 1, 2, 3, … 15

If you pick 5, you wait 5 slots before transmitting.

A slot in WiFi is about 9 microseconds.

What Happens on Collision?

Sometimes two devices still collide. Maybe they picked the same random number, or their waits overlapped.

When collision happens:

1. Both transmissions fail
2. Each device doubles its contention window
3. Pick a new random number from the larger range
4. Try again

The Doubling Rule

The window keeps doubling until it hits CWmax (usually 1024).

This is called Binary Exponential Backoff. Each failure doubles your wait range.

Why Does This Work?

Low traffic: Small CW means short waits. Fast transmission.

High traffic (many collisions): CW grows large. Devices spread out over a wider range, reducing collision probability.

The network adapts automatically. More congestion = longer backoffs = fewer collisions.

The Formula

After n collisions, the contention window is:

$CW_n = \min(CW_{min} \times 2^n, \; CW_{max})$

And the average number of slots you wait:

$\text{Average wait} = \frac{CW - 1}{2}$

Worked Example

Given: Initial CW = 63 slots. What is the CW after the 3rd retransmission?

The 3rd retransmission means 3 collisions have occurred.

Each collision doubles the CW:

Answer: CW = 504 slots after the 3rd retransmission.

Another Example

Given: CWmin = 16, CWmax = 1024, slot time = 9 μs

After 5 failed attempts, what’s the expected backoff time?

Step 1: Calculate CW after 5 failures

$CW = 16 \times 2^5 = 16 \times 32 = 512$

Check against max: 512 is below 1024, so CW = 512

Step 2: Calculate average slots

$\text{Average} = \frac{512 - 1}{2} = 255.5 \text{ slots}$

Step 3: Convert to time

$\text{Time} = 255.5 \times 9 \mu s = 2299.5 \mu s \approx 2.3 \text{ ms}$

Answer: Expected backoff is approximately 2.3 ms.

On Success

When your transmission succeeds:

• CW resets back to CWmin
• Start fresh for the next transmission

The penalty for collisions is temporary. Once you succeed, you’re back to fast access.

Key Points

1. CSMA/CA avoids collisions by listening first and waiting randomly
2. Contention Window is the range you pick your random wait from
3. Binary Exponential Backoff: CW doubles on each collision
4. Formula: $CW_n = CW_{min} \times 2^n$ (capped at CWmax)
5. Average wait: $(CW - 1) / 2$ slots