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

Gate Smashers
23 Dec 202010:10

Summary

TLDRIn 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.

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関連タグ
CSMA/CAWireless NetworksCollision AvoidanceIEEE 802.11Network ProtocolsExam PreparationComputer ScienceEducational VideoTechnology InsightsSignal Transmission
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