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Feature Article
January 2003

Resilient Packet Ring: Enabling VoIP Delivery


Service providers are faced with increasing demands from enterprise customers to reduce costs for transporting their voice and data services. While voice over IP is growing as an expedient solution to streamline enterprise and carrier networks, Resilient Packet Ring is a key enabling technology for rapid delivery of VoIP.

For years, voice and data services have existed on separate, dedicated networks, resulting in added costs and resource inefficiencies. Further complicating matters for the carrier, network equipment, such as traditional Class 5 voice switches, is costly to maintain and inflexible for adding new services or customizing features. To address these problems, next-generation technologies like voice softswitches are now emerging.

By bridging legacy signaling systems like ISDN (Integrated Services Digital Network) and SS7 (Signaling System 7) to new protocols such as MGCP (Media Gateway Control Protocol), SIP (Session Initiation Protocol), and H.323 for packet-based services, softswitches enable carriers to migrate from circuit to packet architectures. Softswitches are now being deployed in several carrier applications including long-distance, tandem switching, Internet offload, PBX access, and IP voice. Gartner Group predicts that 75 percent of new voice switch shipments will be IP-based systems by 2005, bolstering the view that this is growth market.

As voice services migrate to packet switches, carriers are challenged to incorporate IP/packet-based services into the transport systems used in the public network. The current transport systems used for connecting Class 5 end offices to tandem offices are inefficient for delivering IP-based services in metro area networks. Current metro connections, commonly called Inter-Office Facilities (IOF), are typically SONET OC-48/192 spans connecting Class 5 voice switches to each other and to a local tandem switch. Since most IOF links use circuit-based SONET transport, there is now significant focus on packet transport architectures that are better suited to carry IP traffic. Softswitches, combined with packet transport elements, optimize carrier networks with a single multi-service platform that accommodates both circuit and packet requirements.

A significant new approach for packet transport is the emerging IEEE 802.17 Resilient Packet Ring (RPR) standard. RPR is optimized to support packet-based services such as Voice over IP (VoIP). Key advantages of RPR include the ability to support Quality of Service (QoS), survivable ring topologies for Inter-Office Facilities (IOF), and synchronization to ensure the critical latency and delay requirements for voice traffic. RPR elements can either operate as standalone, or can be integrated into current legacy SONET networks for added carrier flexibility.


Softswitches interact with the existing public telephone network via standard interfaces such as DS1 and DS3 links, and via standard systems, including SS7. These new softswitches also support key telephony functions such as directory assistance, numbering plans, and custom calling features.

A packet backbone enables direct IP interfaces to the network elements. Class 4/5 switches evolve to become media gateways (GW) that utilize a centralized softswitch for such functions as call control and feature access. Carriers can evolve to a packet network by migrating their TDM-based Inter-Office Facilities from a SONET to a packet transport solution such as RPR that supports timing and synchronization. RPR enables a convergence of both legacy circuit and emerging packet technologies over a common transport facility -- thus assisting the migration of the inter-office network from circuit to packet. The result is that Gigabit Ethernet will now be used to interconnect the VoIP Gateways to the IOF network rather than DS1, DS3, or OC-N circuits. This provides improvements over circuit-switched transport in areas such as resource utilization, since the IOF facilities no longer must be allocated by traffic type or constrained by inherent circuit limitations. Since IOF facilities are traditionally engineered for busy hour call volumes, the actual usage can be as low as 50 to 75 percent of the link capacity. In contrast, RPR opens the entire link is for any type of traffic -- voice or data -- for transport as needed to accommodate network traffic. RPR provides for oversubscription of services, such as Internet data traffic for maximum link utilization and efficiency.

IOF links based on RPR network elements allow complete utilization of the transport facility, with more efficient use of bandwidth than SONET/SDH. By packing these IOF links more efficiently, metro service providers can significantly increase network efficiency and reduce overall transport costs.


For carriers desiring to offer new packet-based services like VoIP, RPR is an ideal solution due to its unique capability to support current legacy interfaces, such as TDM circuits, as well as softswitch interfaces such as Gigabit Ethernet. RPR provides a seamless evolution to this next-generation architecture by supporting Quality of Service, TDM transport, and traffic management.

Quality of Service

RPR matches the reliability and quality of service provided by SONET and ATM technologies. This includes QoS support for various traffic types, such as voice and video. RPR supports a survivable optical ring topology and sub-50 millisecond restoration times in case of fiber cuts, on a par with SONET. RPR supports classification of traffic packets according to priority. For example, voice packets are classified at the highest priority among the different traffic classes, thus ensuring minimal delay and jitter when congestion is encountered on the network. By utilizing Class of Service (CoS) to classify packets and Multi-Protocol Label Switching (MPLS) for traffic engineering, an RPR/MPLS solution provides an all-packet transport and switching solution in a softswitch network with complete quality of service and full survivability.

TDM Transport

Migrating from current circuit-switched architecture to a packet-based architecture is simplified by RPR�s ability to support both traffic types. T1 or DS3 voice circuits, such as Inter-Machine Trunks (IMT) from a Class 4/5 switch, can be packetized and transported over an optical RPR ring, along with VoIP, data, and video traffic. Carriers can migrate voice trunks from existing transport solutions such as SONET to an RPR solution during their evolution to a softswitch architecture. With RPR, these voice trunks provide the same level of quality and reliability as legacy SONET equipment. By using wavelength division multiplexing, RPR elements can transport SONET transparently over a new RPR ring, adding more design flexibility during the transition to softswitch architectures.

Traffic Management

Traffic management on the RPR-based VoIP network is improved with its ability to monitor and process traffic on a �per-packet� basis. Service Level Agreements can be monitored for specific service class and bandwidth usage. Powerful bandwidth utilization tools enable carriers to perform trend analysis to pinpoint both service problems and potential revenue opportunities. Network efficiency is improved via the bandwidth monitoring capabilities of the RPR ring management system. Traffic can be rerouted and bandwidth increased in real time to accommodate changing customer requirements. Customer service is improved by exposing potential bottlenecks before they occur.


The ability to carry multiple traffic types, such as voice and data, over a common transport solution is optimal for emerging softswitch-based VoIP networks. Resilient Packet Ring transport solutions enable service providers to extend RPR from the metro core to the enterprise edge, over a common packet-based network. RPR allows the carrier to transition Inter-Office Facilities from expensive and inflexible circuit transport to more efficient packet transport. This allows carriers to deploy all-packet softswitch networks for VoIP using Resilient Packet Ring optical solutions.

Ken Lancaster is on the marketing team at Luminous Networks, Inc., a leader in packet-based solutions for metropolitan area networks. For more information, visit the company online at www.luminous.com.

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