An Introduction to Resilient Packet Ring Technology A White Paper by the Resilient Packet Ring Alliance October 2001 An Introduction to Resilient Packet Ring Technology Table of Contents 1.0 Introduction ......................................................................................................................................................3 2.0 Ethernet in the Metro .....................................................................................................................................3 3.0 The Limitations of SONET and Ethernet in Metro Rings ..................................................................5 3.1 SONET ............................................................................................................................................................5 3.2 Ethernet ..........................................................................................................................................................6 4.0 Resilient Packet Ring – Emerging Metro Network Architecture ....................................................7 5.0 RPR Characteristics.........................................................................................................................................8 5.1 Packet ADM Architecture........................................................................................................................8 5.2 Physical Layer Versatility.......................................................................................................................10 5.3 Resiliency.....................................................................................................................................................10 5.4 Bandwidth Fairness..................................................................................................................................10 5.5 Broadcast or Multicast Traffic...............................................................................................................12 5.6 Simplified Service Provisioning...........................................................................................................13 6.0 Packet Rings in Application.......................................................................................................................14 7.0 Packet Ring Standard Development in IEEE 802.17..........................................................................15 8.0 Conclusion .......................................................................................................................................................16 RPR Alliance White Paper www.rpralliance.org Page 2 An Introduction to Resilient Packet Ring Technology 1.0 Introduction An important trend in networking is the migration of packet-based technologies from Local Area Networks to Metropolitan Area Networks (MANs). The rapidly increasing volume of data traffic in metro networks is challenging the capacity limits of existing transport infrastructures based on circuit-oriented technologies like SONET and ATM. Inefficiencies associated with carrying increasing quantities of data traffic over voice-optimized circuit-switched networks makes it difficult to provision new services and increases the cost of building additional capacity beyond the limits of most carriers’ capital expense budgets. Packet- based transport technology is considered by many to be the only alternative for scaling metro networks to meet the demand. 2.0 Ethernet in the Metro Defined simply, an Ethernet service is any data service offered via an Ethernet interface (10 Mbps, 100 Mbps, 1 Gbps Ethernet port). A key difference between Ethernet services and legacy data services such as leased lines, Frame Relay or ATM is the scalability of the service interface. With legacy data services, physical interface requirements vary with the speed of the service. Thus hardware required for a T1 service is completely different from that required for DS-3 or OC-3 services. With Ethernet service, on the other hand, a service provider can drop a Fast Ethernet (100 Mbps capacity) or Gigabit Ethernet (1000 Mbps capacity) port to a subscriber once and upgrade many times, without additional truck rolls beyond the initial installation. Bandwidth and other service changes can be administered remotely, simplifying and accelerating service provisioning. Ethernet services are widely viewed as an offering that holds promise for rapid acceptance in the marketplace. The question remains as to what infrastructure can cost effectively scale to meet this demand. Ethernet has evolved Figure 1: Packet Rings: The Next Step in Packet-Based Transport over the past 25 years from 10 Mbps to Transport 100 Mbps to 1 Gbps and now to 10 Gbps. These and other Circuit: Packet SONET/ATM changes adopted by the IEEE make Optical Mesh: Ring: Gigabit Ethernet Ethernet ??? RPR Alliance White Paper www.rpralliance.org Page 3 technology, capable of supporting fiber spans of more than 50 miles, now emerge as a viable alternative for data transport in public networks. As nearly all data packets begin and end their trip across the Internet as Ethernet frames, carrying data in a consistent packet format from start to finish throughout the entire transport path eliminates the need for additional layers of protocol and synchronization that result in extra costs and complexities. In addition to efficient handling of IP packets, Ethernet has the advantages of familiarity, simplicity, and low cost. Gigabit Ethernet, however, is only the first step in the evolution of packet-based transport in the MAN. Though well suited for point-to-point and mesh network topologies, it is difficult to deploy Ethernet in ring configurations and as a shared media. Rings act as a shared media and need media access control (MAC) mechanisms to manage access across multiple users. Ethernet has evolved to support full duplex switched infrastructures and lacks this MAC. Yet, most of the existing fiber plant in metro areas is in ring form, because the incumbent transport technology, SONET, is typically deployed over fiber rings. Ring topologies also enable SONET to implement a fast (sub 50ms) protection mechanism that can restore connectivity using an alternate path around the ring in case of fiber cuts or equipment failure. Unlike SONET, Ethernet does not have a built-in fast protection mechanism. There are, therefore, great benefits in a new technology that can fully exploit fiber rings (in particular, ring resiliency) while retaining all the inherent advantages of a packet-based transport mechanism like Ethernet. The emerging solution for metro data transport applications is Resilient Packet Ring (RPR) technology. It offers two key features that have heretofore been exclusive to SONET: efficient support for ring topology and fast recovery from fiber cuts and link failures. At the same time, Packet Ring technology can provide data efficiency, simplicity, and cost advantages that are typical to Ethernet. In addition, RPR solves problems such as fairness and congestion control that have not been addressed heretofore by incumbent technologies. Several vendors are already developing and introducing RPR technologies to address this emerging market. This paper introduces RPR networking, explains its advantages in the metro environment, and gives some examples that illustrate applications that can make the best use of Packet Ring technology. RPR Alliance White Paper www.rpralliance.org Page 4 3.0 The Limitations of SONET and Ethernet in Metro Rings 3.1 SONET Most metro area fiber is in ring form. Ring topology is a natural match for SONET-based TDM networks that constitute the bulk of existing metro network infrastructure. However, there are well-known disadvantages to using SONET for transporting data traffic (or point-to-point SONET data solutions, like Packet over SONET [POS]). SONET was designed for point-to-point, circuit-switched applications (e.g. voice traffic), and most of limitations stem from these origins. Below are some of the disadvantages of using SONET Rings for data transport. Figure 2: SONET Access and Fully Meshed Networks SONET SONET Ring Ring Optic Fiber Logical Circuit · Fixed Circuits. SONET provisions point-to-point circuits between ring nodes. Each circuit is allocated a fixed amount of bandwidth that is wasted when not used. For the SONET network that is used for access in Figure 1 (left), each node on the ring is allocated only one quarter of the ring’s total bandwidth (say, OC-3 each on an OC-12 ring). That fixed allocation puts a limit on the maximum burst traffic data transfer rate between endpoints. This is a disadvantage for data traffic, which is inherently bursty. · Waste of Bandwidth for Meshing. If the network design calls for a logical mesh, (right), the network designer must divide the OC-12 of ring bandwidth into 10 provisioned circuits. Provisioning the circuits necessary to create a logical mesh over a SONET ring is not only difficult but also results in extremely inefficient use of ring bandwidth. As the amount of data traffic that stays within metro networks is RPR Alliance White Paper www.rpralliance.org Page 5 increasing, a fully meshed network that is easy to deploy, maintain, and upgrade is becoming an important requirement. · Multicast Traffic. On a SONET ring, multicast traffic requires each source to allocate a separate circuit for each destination. A separate copy of the packet is sent to each destination. The result is multiple copies of multicast packets traveling around