PHY-Level Collision Avoidance

PHY-Level Collision Avoidance (PLCA) is a component of the Ethernet reconciliation sublayer (between the PHY and the MAC) defined within IEEE 802.3 clause 148.

[1] The purpose of PLCA is to avoid the shared medium collisions and associated retransmission overhead.

PLCA is used in 802.3cg (10BASE-T1), which focuses on bringing Ethernet connectivity to short-haul embedded internet of things and low throughput, noise-tolerant, industrial deployment use cases.

[3] Under a PLCA scheme all nodes are assigned unique sequential numbers (IDs) in the range from 0 to N. Zero ID corresponds to a special "master" node that during the idle intervals transmits the synchronization beacon (a special heartbeat frame).

Each opportunity interval is very short (typically 20 bits), so overhead for the nodes that do not have anything to transmit is low.

10BASE-T1S Physical Level Collision Avoidance (PLCA) operation. The picture contains two oscilloscope-like traces with beacons in green and data frame in yellow. The top trace depicts the absence of traffic, beacons are sent by the master at fixed intervals. The bottom trace shows the situation when the node with an ID 11 uses its transmission opportunity (TO, TOs are marked using the grid scale). The data frame expands the regular interval between two beacons.