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3 Operation Theory
PCS-9785 Satellite-Synchronized Clock
3-4
Date: 2019-07-05
3.3.2 High-availability Seamless Redundancy (HSR)
HSR nodes send a copy of data to application layer. The data is copied at data link layer and is
transmitted from Port A and Port B via different physical link.
HSR nodes have two identical interfaces arranged in a ring. Source nodes send packets over both
ports. Each node relays unreceived frames from port A to port B and vice versa. The source node
removes frames it receives that it injected into the ring.
HSR ring transmission through two-way link ensures the redundancy of data. When a link fails, a
message can be transmitted to the receiving device from another loop, and there is no network
reconstruction time. There is no switching network, and forwarding through the device. However,
forwarding device has a forwarding delay time per level, so the total forwarding delay is great.
Each HSR node receives two copies of the same packet, and accepts the first packet and discards
the second. The accepted packet is transmitted to application layer.
For P2P message (for example, TCP message), receiving node will stop transmitting after
receiving the message. For multicast or broadcast message, if the message is from itself,
receiving node will stop transmitting after receiving the message. If the message is from other
node, receiving node will transmit it from another port after receiving the message, i.e., receiving
message from Port A and transmitting message from Port B.
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