Resilient Packet Ring

Resilient Packet Ring (RPR), also known as IEEE 802.17, is a protocol standard designed for the optimized transport of data traffic over optical fiber ring networks. The standard began development in November 2000 and has undergone several amendments since its initial standard was completed in June 2004. The amended standards are 802.17a through 802.17d, the last of which was adopted in May 2011. It is designed to provide the resilience found in SONET/SDH networks (50 ms protection) but, instead of setting up circuit oriented connections, provides a packet based transmission, in order to increase the efficiency of Ethernet and services.

RPR works on a concept of dual counter rotating rings called ringlets. These ringlets are set up by creating RPR stations at nodes where traffic is supposed to drop, per flow (a flow is the ingress and egress of data traffic). RPR uses Media Access Control protocol (MAC) messages to direct the traffic, which can use either ringlet of the ring. The nodes also negotiate for bandwidth among themselves using fairness algorithms, avoiding congestion and failed spans. The avoidance of failed spans is accomplished by using one of two techniques known as steering and wrapping. Under steering, if a node or span is broken, all nodes are notified of a topology change and they reroute their traffic. In wrapping, the traffic is looped back at the last node prior to the break and routed to the destination station.

All traffic on the ring is assigned a Class of Service (CoS) and the standard specifies three classes. Class A (or High) traffic is a pure committed information rate (CIR) and is designed to support applications requiring low latency and jitter, such as voice and video. Class B (or Medium) traffic is a mix of both a CIR and an excess information rate (EIR; which is subject to fairness queuing). Class C (or Low) is best effort traffic, utilizing whatever bandwidth is available. This is primarily used to support Internet access traffic.

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