Resilient Packet Ring

Resilient Packet Ring (RPR), as defined by

Synchronous Digital Hierarchy 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 IP

services.

Technical details

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.

Class of service and traffic queues

All traffic on the ring is assigned a

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.

Spatial reuse

Another concept within RPR is what is known as spatial reuse. Because RPR strips the signal once it reaches the destination (unlike a SONET UPSR/SDH

SNCP ring, in which the bandwidth is consumed 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 the first version of RPR was that it did not provide spatial reuse for frame transmission to/from

IEEE

802.17b, which defines an optional spatially aware sublayer (SAS). This allows spatial reuse for frame transmission to/from MAC address not present in the ring topology.