(Resilient Packet Ring) A packet-based protocol that provides fault tolerance and statistical multiplexing for the metropolitan and national SONET and Ethernet networks of the carriers. The RPR architecture is often added to SONET networks to enable them to carry packets. RPR is a data link layer protocol (layer 2) that uses dual counter-rotating rings for fault tolerance.RPR nodes, which contain the RPR interface cards, are typically routers and switches in a packet environment (Ethernet) or add/drop multiplexers (ADMs) in a TDM environment (SONET). The IEEE has standardized RPR with its 802.17 specification.Counter-Rotating RingsRPR nodes are connected in a ring topology by two fibers, each transmitting in the opposite direction. Both inner and outer ringlets are active at the same time. If a failure occurs, traffic capacity is decreased.Packets move through every node in the ring up to the destination node, where they are stripped off. In an RPR, traffic from multiple nodes travels on the ring concurrently.RPR includes a "fairness" algorithm. Since packets pass through all intermediate nodes, a node can notify the others to slow the transmission if it cannot get its fair share of bandwidth on the ring to transmit. This traffic adjustment enhances the statistical multiplexing and ensures fair access during congestion. Since SONET interleaves fixed channels, "RPR over SONET" turns SONET into a more dynamic network. See TDM network, SONET and statistical multiplexor.
The IEEE 802.17 specifications for a medium access control (MAC) layer protocol that uses Ethernet switching and a dual counter-rotating ring topology to optimize the transport of Ethernet/IP packet data traffic over optical fiber rings. RPR is designed to maintain the resiliency of SONET/SDH, but at a much reduced level of overhead.As RPR is independent of the Physical Layer, it can be implemented over existing SONET/SDH physical rings or can run on a standalone basis. RPR calls for dual counter-rotating local ringlets that interconnect nodes where data traffic is intended to drop. RPR also uses statistical multiplexing, which allows bandwidth to be oversubscribed, while establishing Committed Information Rate (CIR) and peak-rate thresholds per application. The nodes negotiate bandwidth requirements among themselves based on fairness algorithms and in consideration of a classification scheme that recognizes and provides higher priority access to traffic sensitive to latency and jitter while ensuring that best effort traffic, such as Internet traffic, is afforded equal access and a fair share of the remaining bandwidth. RPR supports the following class of service (CoS) levels.