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physical layer collision avoidance (PLCA)

Physical LayerCollision Avoidance (PLCA) is a generic reconciliation sublayer (gRS) used in Single-Pair Ethernet (SPE). This sublayer is used to run deterministics to ensure high performance from the CSMA/CD access method for different transmission types.

The goal of the PLCA method is to achieve high transmission performance with a small number of nodes using half-duplex and multidrop transmissions. Since the CSMA/CD access method causes random, indeterminable latency, it is of limited use for real-time applications in industrial and automotive applications. In contrast, PLCA has guaranteed latency and higher throughput, like a Time-Sensitive Network (TSN).

PLCA relies on CSMA/CD and maps the Media Independent Interface (MII) signals to the MAC layer primitives in such a way that collisions are avoided at the physical layer. PLCA dynamically searches for transmission opportunities where one node at a time is allowed to send only one data packet over the transmission medium. Each node is assigned a unique identification number (ID). The identification number with the digit zero is assigned to the PLCA coordinator, who starts the cycle with a beacon. If no data is transmitted, the transmit capability is passed to the next node. The Reconciliation Sublayer (RS) detects when data is pending transmission. Since the cycles are very short, the RS sublayer buffers the few data for a short time. Since the PLCA method avoids collisions on the physical media, the data flow is continuous without interruption. So that nodes with higher identification numbers do not have to wait longer than others, the transmission option is passed on from one node to the next.

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Englisch: physical layer collision avoidance - PLCA
Updated at: 26.11.2019
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