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Module 5
Lesson 3
The CSMA/CD access method

Assess the characteristics of the CSMA/CD access method.

What is CSMA/CD?

The most common access method in use on LANs today is CSMA/CD, which stands for Carrier-Sense Multiple Access/Collision Detection. To understand what CSMA/CD is, let's break the name down into its three component parts:
  • Carrier-Sense: This means the NIC (or network interface card) on each computer on the network "listens" and senses whether there is traffic on the cable before sending.

  • Multiple Access: This means all computers have access to the cable at any given time (making this a contention method of access control).

  • Collision Detection: This means that collisions may occur, if two computers send data at exactly the same time--but the NICs of the sending computers will detect that a collision has occurred so they can re-send their data.

TIP:Specifications for CSMA/CD are defined in the IEEE 802.3 standard. The Institute of Electrical and Electronic Engineers 802 Project is is discussed in detail later in the Networking fundamentals course series.

How CSMA/CD works

When a computer transmits data on a network that uses the CSMA/CD access method, its NIC first checks the cable to determine if there is already data on the wire.

If the NIC senses that the cable is not in use, it sends its data packet. Once that packet goes out onto the cable, other computers will sense it and will not attempt to send data until the cable is empty again. All goes well--unless two computers listen to the cable at exactly the same time, sense that that there is no traffic, and then send their packets at exactly the same time. If this happens, the two data packets will collide.

The animation below shows how the sending computers manage the collision detection process.


The table below shows the advantages and disadvantages of the CSMA/CD method.
Advantages of CSMA/CD Disadvantages of CDMA/CD
  • Reliable; Collisions are detected and packets are re-sent, so no data is lost.

  • Relatively fast; A computer does not have to wait its "turn" to transmit data.
  • Limited to 2500 meters/11/2 mile; The collision detection mechanism restricts the length of cable segment that can be used.

  • Inappropriate for large/active networks; Collisions slow the network and clog bandwidth with retransmissions.


Next lesson

In the next lesson, you will learn about the CSMA/CA access method.
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