The media access controller (MAC) is the hardware implementation of the protocol defined for the media access control which is used in the case of networks shared with several nodes available for communication using a single physical support. The method has evolved from ALOHA Ethernet and two variants have been defined to deal with different scenarios. Among them, CD/CSMA and CSMA/CA are widely deployed in many networks such as Ethernet and wireless. These techniques senses before sending the data over different type of networks.

Difference between CSMA/CD and CSMA/CA

  1. The CSMA/CD is used in wired local area networks and CSMA/CA used in wireless local area networks and other types of wireless networks.
  2. The CSMA/CD is standardized in IEEE 802.3 and CSMA/CA is standardized in IEEE 802.11 .
  3. CSMA/CD will take no action to prevent the transmission collision until it has occurred, while CSMA/CA takes measures to avoid any collision, the latter having no way of knowing whether a collision occurred.

What is CSMA/CD?

CSMA/CD known as Carrier Sense Multiple Access with Collision Detection. This multiple access method is used to detect a collision in a network.

CSMA/CD is IEEE 802.3 Ethernet networks standard in which a node monitors the traffic on the line. In case of idle condition a particular node transmit the data over network. When 2 nodes transmit the data at the same time this is called collision and data may corrupt.

How CSMA/CD works

  • Each node(computer) sense if the line is idle to send data.
  • When a node founds the line idle, it releases the data packets.
  • If two computers send the data at the same time this is condition of collision.
  • In the condition of collision, the node try to send the data again with variable time interval (the process continues if collision happens again).

When the collision occurs, the stations will wait till variable random time to transmit data again. If a collision occurs again, the random time taken is increased and waits again. This is the procedure used in CSMA/CD networks and the method has no deterministic capacity.

  • CSMA/CA used on early networks.
  • Technique utilize in half duplex network ( coaxial / twisted pair cables).

What is CSMA/CA?

CSMA/CA know as Carrier Sense Multiple Access with Collision Avoidance. This is the multiple access scheme used in the Layer 2 access method, in which the nodes try to transmit data simultaneously in a shared network.

A node must first listen to the medium for a predetermined period to transmit data. When channel is inactive, the node is able to transmit. Otherwise, channel is said to be busy and the node must wait for the channel to be standby mode.

How CSMA/CA works

  • A node first sense either any other transmission happening on the network.
  • This method does it’s best to avoid collision all together.
  • If any other transmission is happening, it will wait short amount of time before checking the channel again.
  • Once the destination device receives the data, it will respond back with acknowledgment.
  • If the sender computer does not receive an acknowledgment from the destination, it will assume that it did not get the data.

RTS/CTS used with CSMA/CA

Another method CSMA/CA uses to avoid collision is optional RTS/CTS (ready to send/ clear to send) protocol.

  • This method allows a node to send RTS signal to wireless network requesting opportunity to send out its data.
  • Once the node receives the clear to send signal it sends out the data.
  • The wireless network automatically stop all the transmission except the node which is allowed.
  • This is also know as wireless network traffic controller protocol.

This is implemented in IEEE 802.11 wireless networks, which is preferable since wireless networks cannot detect collisions during transmission, like wired networks. Thus, the implementation of the CSMA certification authority will improve the deletion of packets in wireless networks.

  • Used on wireless networks.