CSMA/CD Method - 3.0 English

AXI Ethernet Lite MAC LogiCORE IP Product Guide (PG135)

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3.0 English

A full-duplex Ethernet bus is, by definition, a point-to-point dedicated connection between two Ethernet devices capable of simultaneous transmit and receive with no possibility of collisions.

For a half-duplex Ethernet bus, the CSMA/CD media access method defines how two or more stations share a common bus. To transmit, a station waits (defers) for a quiet period on the bus (no other station is transmitting ( phy_crs = 0)) and then starts transmission of its message after the interframe gap period.

If, after initiating a transmission, the message collides with the message of another station ( phy_col - 1), then each transmitting station intentionally continues to transmit (jam) for an additional predefined period (32 bits for 10/100 Mb/s) to ensure 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. When the collision window has passed, a transmitting station has acquired the bus.

Subsequent collisions (late collisions) are avoided because all other (properly functioning) stations are assumed to have detected the transmission and are deferring to it. The time to acquire the bus is based on the round-trip propagation time of the bus (64 byte times for 10/100 Mb/s).