In a half-duplex system, the CSMA/CD media access method defines how two or more stations share a common medium.
- Even when it has nothing to
transmit, the Ethernet MAC monitors the Ethernet medium for traffic by watching the
carrier sense signal (CRS) from the external PHY. Whenever the medium is busy (CRS =
1), the Ethernet MAC defers to the passing frame by delaying any pending transmission of its own. - After the last bit of the passing frame (when the carrier sense signal changes from TRUE to FALSE), the Ethernet MAC starts the timing of the interframe gap.
- The Ethernet MAC resets the interframe gap timer if the carrier sense becomes TRUE during the period defined by “interframe gap part 1 (IFG1).” IEEE Std 802.3-2008 states that this should be the first 2/3 of the interframe gap timing interval (64-bit times) but it can be shorter and as small as zero. The purpose of this option is to support a possible brief failure of the carrier sense signal during a collision condition and is described in paragraph 4.2.3.2.1 of the IEEE standard.
- To ensure fair access to the bus, the Ethernet MAC does not reset the interframe gap timer if carrier sense becomes TRUE during the period defined by “interframe gap part 2 (IFG2).” IEEE Std 802.3-2008 states that this should be the last 1/3 of the interframe gap timing interval.
After initiating a transmission,
if the message collides with the message of another station (COL =
1 ), each transmitting station
intentionally continues to transmit (jam) for an additional
predefined period (32-bit times for 10/100 Mbbs) to ensure the
propagation of the collision throughout the system. The station
remains silent for a random amount of time (back off) before
attempting to transmit again.
A station can experience a collision during the beginning of its transmission (the collision window) before its transmission has had time to propagate to all stations on the bus. After the collision window is passed, a transmitting station acquires the bus. Subsequent collisions (late collisions) are avoided because all other (properly functioning) stations are assumed to have detected the transmission and trying to defer it.