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CSMA/CD and CSMA/CA–Media Access Methods

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Media Access method is nothing but a cable access method, which defines a set of rules to any NIC (Network Interface Card) for how to put data on and retrieve it from a network cable.

Media Access Methods:

  1. CSMA/CD -  Carrier Sense Multiple Access with Collision Detection
  2. CSMA/CA – Carrier Sense Multiple Access / Collision Avoidance
  3. Token Passing or Token Ring
  4. Polling

CSMA/CD (IEEE 802.3)

  • In CSMA/CD method, every computer can transmit at any time. Hence, this method is a contention-based method.
  • Detects collision after transmitting data.
  • Efficient in medium-sized networks (less collisions)
  • Ethernet (Mostly Wired LAN) works on IEEE 802.3 CSMA/CD standard.

How collision occurs in CSMA/CD? How collision is detected in CSMA/CD?
Step by step process:

  1. Carrier Sense: NICs that use CSMA/CD listen or sense the cable whether there is a carrier signal indicating that the network is currently being used.
  2. If the node hears a signal, it waits for a random period of time.  If no carrier signal is detected, the node starts sending its data frames onto the network.
  3. Another node might perform the above 2 steps at the same time. if so, the two messages are transmitted simultaneously and a collision occurs.
  4. Collision detection: When a node hears the collision, it sends a jam signal.
  5. Retransmission: The nodes stop transmitting for a random amount of time, then attempt the transmission again.

CSMA/CA (IEEE 802.11)

  • In CSMA/CA method, nodes can transmit only after confirming the traffic by successfully transmitting a ready signal.
  • Cannot detect collisions as it avoids collision.
  • CSMA/CA is used mostly in wireless networks.
  • CSMA/CA is comparatively slower than CSMA/CD.

How collision is avoided in CSMA/CA media access method?
Step by step process:

  1. Instead of sensing the cable ‘s traffic, CSMA/CA allows the node to send a ready signal that it is ready to transmit data.
  2. If the ready signal transmits without an issue, the node then transmits the data.
  3. Collision Avoidance: If the ready signal is not transmitted successfully, the node waits and tries again after a random period of time.

Token Passing:

  • Token passing method gives every node to have equal access to the cable. Hence this method is a contention-free method.
  • There will be no collisions but it has drawbacks in the speed, token overhead, single point of failure etc.,
  • Token Ring was mostly used by IBM in 1980’s.


How Token Ring method transmits data without collisions?
Step by step process:

Media Access method
  1. A special chunk of data called a token circulates through the ring from node to node.
  2. Any node that has data to send waits for a free token and takes it off the ring.
  3. After taking the token, this node modifies it to inform other nodes that it has the token.
  4. Then, the transmitting node places the token along with the data it requires to transmit on the ring.
  5. The placed token then travels around the ring until it reaches the destination node.
  6. The destination node takes the token along with the data off the cable, modifies the token (showing it has received the data) and puts the token back on the cable.
  7. After the original sender receives  the  token back and verifies that the destination node has received the data, the sender modifies the token to set it free.
  8. It then places the token back on the ring and waits until it has any other data to transmit.

Polling:

  • In Polling access method, there is a central master node or unit to  ask each node in turn if it has any data to transmit.
  • If any of the nodes answers “yes”, the master controller node allows the node to transmit its data.
  • Polling process has many drawbacks while expanding the network and cost wise.


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