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What is a Carrier Sense Multiple Access?

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  • Written By: R. Woodard
  • Edited By: Jenn Walker
  • Last Modified Date: 04 November 2016
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In terms of computer networking, Carrier Sense Multiple Access is a part of the data link layer, which is part of the overall scheme of how networks and Internet Protocol (IP) addresses work. Carrier Sense Multiple Access, also known by the acronym CSMA, represents a popular way to send packets; packets are data bits that are sent over a network. Before packets are sent, CSMA checks the line to see if it is being used; if so, it waits until the line is idle before transmitting.

Carrier Sense Multiple Access is one of the more popular ways to send information packets across networks. This method is used by Ethernet networks, which are the majority of networked computers in the world. Carrier Sense Multiple Access helps to prevent computers from exchanging information at the same time. When computers send packets to each other at the same time, it causes a collision, and data doesn't go where it is intended. Carrier Sense Multiple Access holds the data until the line is clear in order to make it more likely that it will get to the right computer and user.

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Even though CSMA waits for the line to be clear before transmitting, a collision can still occur. This method is thus often combined with Collision Detection (CD). When a collision is detected, a jam signal will be sent, alerting the receivers of the collision. The data will then be sent again when the line is clear, until either the data is received or a maximum number of attempts is reached.

The important thing to know about Carrier Sense Multiple Access with Collision Detection is what each portion of the protocol does. The carrier sense portion listens to the network to see if someone is sending information at the moment. If nothing seems to be transmitting, it goes ahead and begins to send the packet; if it does hear something, it waits until that information has been sent and then goes ahead with sending the packet.

Multiple access means that nothing can stop two or more network devices from sending information at the same time. Adding Collision Detection forces the Carrier Sense Multiple Access to wait to see if the packets that were sent crashed into another packet. In that case, Collision Detection waits until no other packets are being sent before it starts to send the packet again. Usually, a second collision doesn't happen.

The combination of Carrier Sense Multiple Access and Collision Detection works very well on small networks, such as a home office or home business; these companies will normally have 30 computers or fewer, allowing for a much smoother transition and sending of packets. When there are more than 30 computers, such as in the case of a large enterprise company, the combination of the two does not perform as well, as there are more collisions because of the constant sending of packets from everyone on the network.

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