1. Introduction

The nature of the public network has changed. Demand for Internet Protocol (IP) data is growing at a compound annual rate of between 100% and 800%1, while voice demand remains stable. What was once a predominantly circuit switched network handling mainly circuit switched voice traffic has become a circuit-switched network handling mainly IP data. Because the nature of the traffic is not well matched to the underlying technology, this network is proving very costly to scale. User spending has not increased proportionally to the rate of bandwidth increase, and carrier revenue growth is stuck at the lower end of 10% to 20% per year. The result is that carriers are building themselves out of business.

Over the last 10 years, as data traffic has grown both in importance and volume, technologies such as frame relay, ATM, and Point-to-Point Protocol (PPP) have been developed to force fit data onto the circuit network. While these protocols provided virtual connections”a useful approach for many services”they have proven too inefficient, costly and complex to scale to the levels necessary to satisfy the insatiable demand for data services. More recently, Gigabit Ethernet (GigE) has been adopted by many network service providers as a way to network user data without the burden of SONET/SDH and ATM. GigE has shortcomings when applied in carrier networks were recognized and for these problems, a technology called Resilient Packet Ring Technology were developed.

RPR retains the best attributes of SONET/SDH, ATM, and Gigabit Ethernet. RPR is optimized for differentiated IP and other packet data services, while providing uncompromised quality for circuit voice and private line services. It works in point-to-point, linear, ring, or mesh networks, providing ring survivability in less than 50 milliseconds. RPR dynamically and statistically multiplexes all services into the entire available bandwidth in both directions on the ring while preserving bandwidth and service quality guarantees on a per-customer, per-service basis. And it does all this at a fraction of the cost of legacy SONET/SDH and ATM solutions.

Data, rather than voice circuits, dominates today's bandwidth requirements. New services such as IP VPN, voice over IP (VoIP), and digital video are no longer confined within the corporate local-area network (LAN). These applications are placing new requirements on metropolitan-area network (MAN) and wide-area network (WAN) transport. RPR is uniquely positioned to fulfill these bandwidth and feature requirements as networks transition from circuit-dominated to packet-optimized infrastructures.

Table 1. Resilient Packet Ring Technology Key Features

Resilience Proactive span protection automatically avoids failed spans within 50 ms.
Services Support for latency/jitter sensitive traffic such as voice and video. Support for committed information rate (CIR) services.
Efficiency Spatial reuse: Unlike SONET/SDH, bandwidth is consumed only between the source and destination nodes. Packets are removed at their destination, leaving this bandwidth available to downstream nodes on the ring.
Scalable Supports topologies of more than 100 nodes per ring. Automatic topology discovery mechanism.




2. RPR Operation

RPR technology uses a dual counter rotating fiber ring topology. Both rings (inner and outer) are used to transport working traffic between nodes. By utilizing both fibers, instead of keeping a spare fiber for protection, RPR utilizes the total available ring bandwidth. These fibers or ringlets are also used to carry control (topology updates, protection, and bandwidth control) messages. Control messages flow in the opposite direction of the traffic that they represent. For instance, outer-ring traffic-control information is carried on the inner ring to upstream nodes.

Figure 1. RPR Terminology

By using bandwidth-control messages, a RPR node can dynamically negotiate for bandwidth with the other nodes on the ring. RPR has the ability to differentiate between low- and high-priority packets. Just like other quality of service (QoS)“aware systems, nodes have the ability to transmit high-priority packets before those of low priority. In addition, RPR nodes also have a transit path, through which packets destined to downstream nodes on the ring flow. With a transit buffer capable of holding multiple packets, RPR nodes have the ability to transmit higher-priority packets while temporarily holding other lower-priority packets in the transit buffer. Nodes with smaller transit buffers can use bandwidth-control messages to ensure that bandwidth reserved for high-priority services stays available.

The RPR MAC
One of the basic building blocks of RPR is Media Access Control (MAC). As a Layer-2 network protocol, the MAC layer contains much of the functionality for the RPR network. The RPR MAC is responsible for providing access to the fiber media. The RPR MAC can receive, transit, and transmit packets.

Figure 2. MAC Block Diagram

Receive Decision: Every station has a 48-bit MAC address. The MAC will receive any packets with a matching destination address. The MAC can receive both unicast and multicast packets. Multicast packets are copied to the host and allowed to continue through the transit path. Matching unicast packets are stripped from the ring and do not consume bandwidth on downstream spans. There are also control packets that are meant for the neighboring node; these packets do not need a destination or source address.

Transit Path: Nodes with a non-matching destination address are allowed to continue circulating around the ring. Unlike point-to-point protocols such as Ethernet, RPR packets undergo minimal processing per hop on a ring. RPR packets are only inspected for a matching address and header errors (TTL=0, Parity, CRC).
Transmit and Bandwidth Control: The RPR MAC can transmit both high- and low-priority packets. The bandwidth algorithm controls whether a node is within its negotiated bandwidth allotment for low-priority packets. High-priority packets are not subject to the bandwidth-control algorithm.

Protection: RPR has the ability to protect the network from single span (node or fiber) failures. When a failure occurs, protection messages are quickly dispatched. RPR has two protection mechanisms:

Wrapping: Nodes neighboring the failed span will direct packets away from the failure by wrapping traffic around to the other fiber (ringlet). This mechanism requires that only two nodes participate in the protection event. Other nodes on the ring can send traffic as normal.



Figure 3. Wrapped Traffic Flow

Steering: The protection mechanism notifies all nodes on the ring of the failed span. Every node on the ring will adjust their topology maps to avoid this span.
Regardless of the protection mechanism used, the ring will be protected within 50 ms.