Networking

Csma/cd

Shashi Shekhar's photo
Shashi Shekhar
·

3 min read

Breaking Down CSMA/CD

  1. Carrier Sense (CS):

    • Before sending data, a device (node) listens to the network to check whether the medium is idle (no other device is transmitting).

    • If the medium is idle, the device proceeds with the transmission.

  2. Multiple Access (MA):

    • Multiple devices share the same network medium.

    • Each device has equal access to attempt transmission when the medium is available.

  3. Collision Detection (CD):

    • While transmitting, a device monitors the medium to check if another device is also transmitting (resulting in a collision).

    • If a collision is detected, both devices stop transmitting and follow a backoff algorithm to retransmit the data after a random delay.

How CSMA/CD Works (Step-by-Step):

  1. Listen for Idle Medium:

    • A device wanting to transmit first listens to check if the network is idle (carrier sense).

  2. Transmit Data:

    • If the medium is idle, the device begins transmitting its data.

  3. Monitor for Collisions:

    • While transmitting, the device checks for collisions by monitoring signals on the medium.

  4. Collision Handling:

    • If a collision is detected:

      • The device sends a jam signal to notify all other devices that a collision has occurred.

      • Transmission stops immediately.

  5. Backoff and Retransmission:

    • After a collision, devices enter a backoff period:

      • Each device waits for a random time (determined by an exponential backoff algorithm) before attempting to retransmit.

      • This reduces the chances of repeated collisions.

  6. Repeat the Process:

    • After the backoff period, the device retries transmission by listening to the medium again.

When CSMA/CD is Used:

  • CSMA/CD is primarily used in half-duplex Ethernet networks, where devices cannot send and receive data simultaneously.

  • It is effective in networks with shared mediums, such as hub-based Ethernet or early coaxial-cable networks (like 10BASE2 or 10BASE5).

Why CSMA/CD Is No Longer Commonly Used:

With the evolution of Ethernet technology, CSMA/CD has become largely obsolete because of the following advancements:

  1. Switches:

    • Modern Ethernet networks use full-duplex communication and Ethernet switches.

    • Full-duplex mode allows simultaneous sending and receiving of data, eliminating collisions.

  2. Dedicated Links:

    • Each device connected to a switch has its own dedicated link, so there's no shared medium to cause collisions.

  3. Higher Speeds:

    • High-speed networks (like Gigabit and 10-Gigabit Ethernet) require full-duplex communication to maximize efficiency.

Summary of CSMA/CD:

  • Purpose: To manage shared medium access and avoid collisions in half-duplex Ethernet networks.

  • Key Mechanism: Listen → Transmit → Detect Collisions → Backoff → Retry.

  • Limitation: Not suitable for full-duplex or modern switched Ethernet networks.

  • Obsolescence: Modern Ethernet (switched networks and full-duplex communication) has eliminated the need for CSMA/CD.

In short, CSMA/CD played a critical role in early Ethernet networks but is no longer used due to the adoption of full-duplex Ethernet and switching technologies.