Media Access

What is CSMA/CD?

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. It refers to the means of media access, or deciding “who gets to talk” in an Ethernet network.

A more elegant term for “who gets to talk” is to refer to the “media access method”, which, in this case, would be “CSMA/CD”

Carrier Sense means that before a station will “talk” onto an Ethernet wire, it will listen for the “carrier” that is present when another station is talking. If another station is talking, this station will wait until there is no carrier present.

Multiple Access refers to the fact that when a station is done transmitting it is allowed to immediately make another access to the medium (a ‘multiple’ access). This is as opposed to a Token-Ring network where a station is required to perform other tasks in-between accessing the medium (like releasing a token or sometimes releasing a management frame).

Collision Detection refers to the ability of an Ethernet adapter to detect the resulting “collision” of electrical signals and react appropriately. In a normally operating Ethernet network, it will sometimes occur that two stations simultaneously detect no carrier and begin to talk. In this case the two electrical signal will interfere with each other and result in a collision; an event which is detected by the Collision Detection circuitry in the transmitting network interface cards.

The process of CSMA/CD is implemented slightly differently in a twisted pair (as opposed to a coaxial) Ethernet network.

• Frame Formats
  The four ways that frames may be structured
• Media Access
  Taking turns accessing the cable using the rules of Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
• Collisions
  The results of simultaneous transmissions on the media: Fragments, Runts, CRC errors
• Propagaton Delay
  The relationship between maximum cable length and minimum frame size is based on the propagation delay of the signal
• Frame Corruption
  Troubleshooting coaxial Ethernet networks by examining the types of corruption patterns that result from specific events
• Interframe Gap
  The 9.6 microsecond interframe gap and an understanding of its purpose
• Signal Encoding
  Manchester Encoding for the electrical Ethernet signal