Lec-36: CSMA/CA in Computer Network | Full Explanation

Summary

In this video, the presenter explains Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), a key protocol used in wireless LANs, such as Wi-Fi. Unlike wired networks that utilize Collision Detection (CSMA/CD), CSMA/CA focuses on avoiding collisions by making devices wait before transmission. The video outlines the strategy involving key steps like sensing the carrier, waiting for Inter Frame Space (IFS), sending a Ready To Send (RTS) signal, and receiving a Clear To Send (CTS) signal. The process emphasizes the importance of timing and backoff strategies to minimize collisions, providing essential insights for competitive and academic exams.

Takeaways

• 📡 CSMA/CA stands for Carrier Sense Multiple Access with Collision Avoidance, primarily used in wireless LANs like Wi-Fi.
• ⚡ Collision Detection is used in wired networks (CSMA/CD), while Collision Avoidance is the focus for wireless networks due to signal energy loss.
• 👂 Before transmission, a station senses whether the channel is idle. If it detects activity, it waits and senses again.
• ⏳ After sensing an idle channel, the station waits for a specified time known as Inter Frame Space (IFS) before attempting to transmit.
• 🎲 The station selects a random number (R) from an increasing range to determine its wait time after failed attempts, helping to reduce collision chances.
• 📩 A 'Ready To Send' (RTS) signal is sent to indicate the station's intention to transmit data.
• 🕹️ The station waits for a 'Clear To Send' (CTS) signal from the receiver/modem before proceeding with data transmission.
• ✅ After transmitting, the station expects an acknowledgment (ACK) to confirm successful transmission; failure to receive it indicates a collision.
• 🔄 If a collision occurs, the attempt count (K) increases, and the station repeats the process, waiting longer before its next attempt.
• 🚫 There is a maximum limit (e.g., 10 attempts) for transmission; exceeding this limit leads to an abortion of the transmission process.

Q & A

What does CSMA/CA stand for?

CSMA/CA stands for Carrier Sense Multiple Access with Collision Avoidance, a protocol used in wireless networking.

In what type of networks is CSMA/CA primarily used?

CSMA/CA is primarily used in wireless LANs, commonly referred to as Wi-Fi, and is part of the IEEE 802.11 standards.

What is the main difference between CSMA/CD and CSMA/CA?

The main difference is that CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is used in wired networks to detect collisions, while CSMA/CA avoids collisions in wireless networks by using waiting and sensing mechanisms.

Why is collision detection difficult in wireless networks?

Collision detection is difficult in wireless networks because the transmitted signal loses energy as it travels, making it hard for a device to recognize a simultaneous transmission.

What does IFS stand for and why is it important in CSMA/CA?

IFS stands for Inter Frame Space. It is important because it provides a waiting period for devices before they attempt to transmit, helping to prevent collisions by allowing other devices to finish their transmissions.

How does a station determine the waiting time before transmitting in CSMA/CA?

A station determines the waiting time by selecting a random number R from the range 0 to 2^K - 1, where K is the number of previous transmission attempts.

What are RTS and CTS signals, and what roles do they play in CSMA/CA?

RTS stands for Ready To Send, and CTS stands for Clear To Send. The RTS signal indicates that a device is ready to transmit data, while the CTS signal is sent in response to grant permission for the transmission, helping to manage access to the channel.

What happens if a station does not receive an acknowledgment (ACK) after sending data?

If a station does not receive an acknowledgment (ACK) after sending data, it presumes a collision has occurred and increments the value of K, which represents the number of attempts, before attempting to retransmit.

What is the Backoff Time (TB) in CSMA/CA and how is it calculated?

The Backoff Time (TB) is the time a station waits before retrying to transmit after a collision. It is calculated by multiplying a randomly chosen number R by the Slot Time, which is typically equivalent to the round-trip time.

What is the maximum number of transmission attempts a station can make in CSMA/CA?

The maximum number of transmission attempts is determined by a predefined limit. If the value of K exceeds this limit, the station will abort its transmission attempts.