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packet over SONET

A metropolitan area network (MAN) or wide area network (WAN) transport technology that carries IP packets directly over SONET transmission without any data link facility such as ATM in between. Packet over SONET is intended to transmit data at the highest rates possible, because SONET has a smaller packet header overhead than ATM (28 bytes out of an 810-byte frame compared with 5 out of a 53-byte ATM cell). However, ATM has traditionally provided proven management and quality of service (QoS) for long haul transport. See RPR.




Transporting IP
In a LAN, IP generally runs over Ethernet. In a WAN, it typically is transported by ATM, which rides over SONET, and increasingly, directly over SONET. In the future, it is expected that IP will run directly over DWDM fiber (rightmost diagram).






RPR

(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.






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