Resilient Packet Ring
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Resilient Packet Ring (RPR), also known as IEEE 802.17, is a standard designed for the optimized transport of data traffic over fiber rings. Its design is to provide the resilience found in SONET/SDH networks (50ms protection) but instead of setting up circuit oriented connections, providing a packet based transmission. This is to increase the efficiency of Ethernet and IP services.
RPR works on a concept of dual counter rotating rings called ringlets. These ringlets are setup 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 MAC (Media Access Control protocol) messages to direct the traffic, which traverses both directions around the ringlet. 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 CIR (Committed Information Rate) 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 EIR (Excess Information Rate - 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.
Another concept within RPR is what is known as “spatial reuse”. Because RPR “strips” the signal once it reaches the destination (unlike SONET/SDH which consume the bandwidth around the entire ring) it can reuse the freed space to carry additional traffic. The RPR standard also supports the use of learning bridges (IEEE 802.1D) to further enhance efficiency in point to multipoint applications and VLAN tagging (IEEE 802.1Q).
One drawback of RPR is that it does not provide spatial reuse for frame transmission to/from MAC addresses not present in the ring topology. This is being addressed by Amendment 1 in 802.17b.
