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Self-healing ring

A self-healing ring is a common configuration in telecommunications transmission systems.

Wide-area SDH and metro WDM systems are often configured in self-healing rings.

The system consists of a ring of bidirectional links between a set of stations, typically using optical fiber communications. In normal use, traffic is dispatched in the direction of the shortest path towards its destination. In the event of the loss of a link, or of a entire station, the two nearest surviving stations "loop back" their ends of the ring. In this way, traffic can still travel to all surviving parts of the ring, even if it has to travel "the long way round".

A second break in the ring will divide it into two sub-rings, but each sub-ring will still remain functional.

Self-healing rings offer high levels of resilience at low cost, since it is often geographically easy to take multiple paths across the landscape and link them up into a ring with very little extra fibre length.

Recent submarine communications cables are typically built in pairs, to function as a self-healing ring.

Very high resilience systems are typically built on interconnected meshes of self-healing rings.

to be written: link capacity allocation, different technologies, synchronous vs. asynchronous, cutover state machines

Resilient packet ring is a new technology for packet-switched self-healing ring networks.

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