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PHY-Level Collision Avoidance (PLCA) is a component of the Ethernet reconciliation sublayer (between the PHY and the
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 "coordinator" node that during the idle intervals transmits the synchronization ''beacon'' (a special heartbeat frame). After the beacon (within ''PLCA cycle'') each node gets its transmission opportunity (TO). Each opportunity interval is very short (typically 20 bits), so overhead for the nodes that do not have anything to transmit is low. If the PLCA circuitry discovers that the node's TO cannot be used (the other node with a lower ID have started its transmission and the media is busy at the beginning of the TO for this node), it asserts the "local collision" input of the MAC thus delaying the transmission. The condition is cleared once the node gets its TO. A standard MAC reacts to the local collision with a backoff, however, since this is the first and only backoff for this frame, the backoff interval is equal to the smallest possible frame - and the backoff timer will definitely expire by the time the TO is granted, so there is no additional loss of performance.
MAC
Mac or MAC may refer to:
Common meanings
* Mac (computer), a line of personal computers made by Apple Inc.
* Mackintosh, a raincoat made of rubberized cloth
* Mac, a prefix to surnames derived from Gaelic languages
* McIntosh (apple), a Canadi ...
) defined within IEEE 802.3
IEEE 802.3 is a working group and a collection of standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of the Institute of Electrical and Electro ...
clause 148. The purpose of PLCA is to avoid the shared medium
In telecommunication, a shared medium is a medium or channel of information transfer that serves more than one user at the same time.
In order for most channels to function correctly, no more than one user can be transmitting at a time, so a c ...
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
Internet of things (IoT) describes devices with sensors, processing ability, software and other technologies that connect and exchange data with other devices and systems over the Internet or other communication networks. The IoT encompasse ...
and low throughput
Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel in a communication network, such as Ethernet or packet radio. The data that these messages contain may be delivered ov ...
, noise
Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
-tolerant, industrial deployment use cases.
In order for a multidrop 10BASE-T1S standard to successfully compete with CAN XL, some kind of arbitration was necessary. The linear arbitration scheme of PLCA somewhat resembles that of the Byteflight, but PLCA was designed from scratch to accommodate the existing shared medium Ethernet MACs with their busy sensing mechanisms.
Operation
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 "coordinator" node that during the idle intervals transmits the synchronization ''beacon'' (a special heartbeat frame). After the beacon (within ''PLCA cycle'') each node gets its transmission opportunity (TO). Each opportunity interval is very short (typically 20 bits), so overhead for the nodes that do not have anything to transmit is low. If the PLCA circuitry discovers that the node's TO cannot be used (the other node with a lower ID have started its transmission and the media is busy at the beginning of the TO for this node), it asserts the "local collision" input of the MAC thus delaying the transmission. The condition is cleared once the node gets its TO. A standard MAC reacts to the local collision with a backoff, however, since this is the first and only backoff for this frame, the backoff interval is equal to the smallest possible frame - and the backoff timer will definitely expire by the time the TO is granted, so there is no additional loss of performance.
See also
*Internet of things
Internet of things (IoT) describes devices with sensors, processing ability, software and other technologies that connect and exchange data with other devices and systems over the Internet or other communication networks. The IoT encompasse ...
(IOT)
References
Sources
* * {{Ethernet Ethernet Computer networking Algorithms Internet of things