Bridging in RPR networks : evaluation of an enhanced bridging algorithm

Abstract

Resilient Packet Ring (RPR) is an emerging standard for a packet based ring network. The MAC protocol defined by RPR is primarily intended for use in a metro (MAN) or wide area (WAN) environment, supporting link capacities of multiple gigabits per second.

Being an IEEE 802 standard, RPR is required to support transparent bridging. Transparent bridging was originally designed for shared medium networks, like Ethernet. RPR is not such a shared medium network, hence some new problems are encountered.

Two different solutions for bridging in RPR are discussed in this work. The first, named basic bridging, is the one defined in the existing draft standard. This bridging strategy relies heavily on broadcasting on the rings, thus giving a poor utilisation of the network resources. The second strategy, enhanced bridging, reduces and eventually eliminates this broadcasting. The price to pay is a need for more memory in the stations on the ring to keep mapping tables.

A simulation model of RPR networks has been developed and used in this work to evaluate the performance of the two bridging strategies. Through three simulated network scenarios, different aspects of the enhanced bridging algorithm are highlighted and compared to the basic strategy.

This work shows that enhanced bridging at least halves the load on the network produced by a bridged traffic flow compared to basic bridging. This is done by eliminating the need for broadcasting. Enhanced bridging depends on mapping tables in the RPR stations. Simulations are performed that show the effect of limiting the size of these tables. The last scenario shows how local traffic on a ring is affected by the choice of bridging algorithm. The simulations show that by reducing the traffic load in the network, the enhanced bridging algorithm gives improved latency and jitter characteristics for local traffic.