Share on Facebook Share on Twitter Email
Answers.com

Resilient Packet Ring

 
Computer Desktop Encyclopedia: RPR
Home > Library > Technology > Computer Encyclopedia

(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 Rings

RPR 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.

Normal Operation
Under normal operation, data flows in both ringlet 0 and ringlet 1. In case of failure, overall bandwidth is reduced.

Wrapping the Packets
If a line breaks, the ring is reconfigured using ringlet 0 and 1 together. RPR "wrapping" causes the two nodes at either side of the break to become return nodes and provides the fastest recovery.

Steering the Packets
RPR "steering" can restore a damaged network and also shorten the number of hops packets take compared to wrapping. However, steering takes a little bit longer to complete as all nodes in the network have to be reconfigured. Sometimes, wrapping is performed immediately, and steering is performed afterwards.

Download Computer Desktop Encyclopedia to your iPhone/iTouch

Search unanswered questions...

Enter a question here...
Search: All sources Community Q&A Reference topics

Wikipedia: Resilient Packet Ring
Top
Home > Library > Miscellaneous > Wikipedia

Resilient Packet Ring (RPR), also known as IEEE 802.17, is a standard designed for the optimized transport of data traffic over optical fiber ring networks. It is designed to provide the resilience found in SONET/SDH networks (50ms 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.

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 MAC (Media Access Control protocol) 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 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 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 didn't provide spatial reuse for frame transmission to/from MAC addresses not present in the ring topology. This was addressed by 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.

See also

External links


This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
 

 

Copyrights:

Computer Desktop Encyclopedia. THIS COPYRIGHTED DEFINITION IS FOR PERSONAL USE ONLY.
All other reproduction is strictly prohibited without permission from the publisher.
© 1981-2009 Computer Language Company Inc.  All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Resilient Packet Ring".  Read more