MAC Layer: Carrier Sense Multiple Access with Collision Avoidance Simulator

This project will cover the simulation of Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) focusing on Distributed Coordination Function (DCF) with contention window and basic Inter-Frame Space (IFS). You can find this project on my Github profile MAC Layer - CSMA/CA Technique.

1. Understanding CSMA/CA with DCF

CSMA/CA is a MAC layer protocol used primarily in wireless networks (WiFi - IEEE 802.11) to manage how multiple stations share a common communication medium. CSMA/CA focuses on avoiding collisions before they occur.

The Distributed Coordination Function (DCF) is the fundamental access method in IEEE 802.11. It relies on following principles:

Carrier Sensing

A station listens to the wireless medium before attempting to transmit information bits. If the medium is sensed as busy, the station defers its transmission.

Inter-Frame Space (IFS)

After the medium becomes idle, a station does not transmit immediately. It waits for a specific period called the IFS.
Most relevant: Distributed IFS (DIFS): This waiting period ensures that higher-priority traffic (like acknowledgements using Short IFS - SIFS) gets access first.

Collision Avoidance (CA) with random backoff

If the medium is sensed idle after the DIFS, the station does not transmit right away. Instead, it enters a contention window and chooses a random backoff time.

Backoff Time = Random(0,Contention Window) X Slot Time
Slot Time = Defined by the standard (e.g., 9 µs in 802.11g)

The backoff timer starts decrementing only when the channel starts being in idle state.
If the channel becomes busy (e.g., another station starts transmitting), the backoff timer is paused.
Once the channel becomes idle again and stays idle for a DIFS, the countdown resumes.

Acknowledgement (ACK)

After a successful data frame transmission, the receiving station sends an ACK frame after a short SIFS.
The absence of an ACK within a certain timeout period indicates a potential collision, and the transmitting station will attempt retransmission.

Binary Exponential Backoff (BEB)

If a transmission fails (no ACK received), the transmitting station increases its contention window size (CW) for the next retransmission attempt.
The CW typically doubles with each failed attempt, up to a maximum size.
This exponential increase in the backoff range further reduces the chances if repeated transmissions.

2. CSMA/CA algorithm steps

2.1 Brief Overview

Step-1: Start
Step-2: Packet is ready for Transmission
Step-3: Channel Sensing
Step-4: Channel Idle

Step-4.1: If No, obtain Back-off

Step-4.2.2: After backoff timer ends, send packet

Step-4.2: If Yes, send packet

Step-5: Collision

Step-5.1: If No, Successful transmission.
Step-5.2: If Yes, Go back to Step-3

2.2 In-Depth Analysis

2.3 CSMA/CA Mathematical Flowchart

3. Request for Feedback

Are there additional topics you'd like me to cover? Here are some potential topics:

3.1 Carrier Sensing Mechanisms: How stations determine if the transmission medium is busy?

Wireless stations employ two complementary methods to assess channel availability:

Physical Carrier Sensing (PHY Layer)

Performed by radio hardware through Energy Detection
Measures RF signal strength on the channel
Channel declared busy if detected power exceeds threshold (e.g., -82 dBm)
Key advantage: Works without packet decoding - detects any RF activity
Example: Network interface defers transmission when detecting nearby WiFi signals

Virtual Carrier Sensing (MAC Layer via NAV)

Analyzes packet headers (RTS/CTS/data frames) for Duration fields
Maintains a Network Allocation Vector (NAV) timer
Medium considered busy while NAV > 0, regardless of RF detection
Critical benefit: Solves hidden terminal problems

RTS/CTS Protocol Deep Dive

Collision avoidance in hidden terminal scenarios

Control Frame Functions:

RTS (Request to Send): Reserves channel for transmission
CTS (Clear to Send): Confirms channel reservation

Hidden Terminal Scenario:

Scenario:

    A ----> AP <---- C
         |
         B
Station A wants to send to an Access Point (AP).
Station C also wants to send to the same AP.
A and C can’t hear each other (they’re out of range), but both can hear the AP.
If both A and C transmit at the same time, a collision occurs at the AP, but A and C don’t know because they can’t sense each other.

Step-by-Step implementation:

A sends RTS (Request to Send) to the AP:

Includes how long it wants the channel (duration field).

AP replies with CTS (Clear to Send):

Broadcasts to all stations that A can now transmit.
This CTS also contains the same duration info.

Other stations (like C) hear the CTS:

Even if they didn’t hear A’s RTS, they now know someone else is using the channel.
They update their NAV (Network Allocation Vector) and defer transmission.

A sends the data.
AP sends an ACK (acknowledgment) if received successfully.

3.2 Inter-Frame Spacing Hierarchy ?

The timing architecture that governs transmission priority

After channel clearance, stations observe prioritized waiting periods:

Type	Name	Purpose	Duration Formula	Priority
SIFS	Short IFS	High-priority responses	Fixed (~10µs)	Highest
PIFS	Point IFS	AP-controlled access	SIFS + 1 slot	High
DIFS	Distributed IFS	Standard data frames	SIFS + 2 slots	Medium
EIFS	Extended IFS	Error recovery	Longest	Lowest

Key Applications:

SIFS: ACK/CTS responses (ensures uncontested replies)
DIFS: Regular data transmission attempts
EIFS: Recovery after corrupted frame detection

3.3 Slot Times ?

In IEEE 802.11 wireless networks (Wi-Fi), the slot time is a fundamental timing parameter that varies depending on the physical layer (PHY) standard being used. Here's the breakdown:

802.11 Standard	Slot Time (μs)	Notes
802.11a/g/n/ac/ax (5 GHz)	9 μs	OFDM-based standards
802.11b	20 μs	DSSS/CCK (2.4 GHz legacy)
802.11n (2.4 GHz)	9 μs or 20 μs	Depends on legacy compatibility
802.11ax (HE)	9 μs	High Efficiency (HE)

You can find this project on my Github profile MAC Layer - CSMA/CA Technique.

Search This Blog

Bits and Bytes of Wireless