The emerging resilient-packet-ring standard will provide LECs with a packet-switched architecture that reduces the need for large digital crossconnects and associated backhaul links.
By VINAY BANNAI, Luminous Networks--A major component of cost and expense for today's local-exchange carrier (LEC) is in the transport of services between metro-area central offices. These transport links, called interoffice facilities (IOFs), are typically deployed in SONET OC-48 (2.5-Gbit/sec) or OC-192 (10-Gbit/sec) ring topologies. The design of the IOF is to accommodate links between the Class 5 end offices and tandem offices for local and long-distance traffic. Subsequently, traffic is backhauled to large, expensive crossconnects for grooming and redistribution into the network. The result is an expensive and inefficient architecture, ill-equipped to accommodate the dynamic nature of today's metro services.
Newer technologies employing packet-based transport in rings are now available, which allow the carrier to streamline the IOF network while offering a seamless migration of their transport network to a more efficient, packet-switched architecture. Resilient packet ring (RPR) is an emerging standard from the IEEE 802.17 working group that provides the benefits of survivable optical rings (SONET/SDH) using packet-switching technologies.
"Nailed up" circuits
Traditional approaches in IOF design require traffic (local or long distance) to be transported to core offices for grooming by large digital crossconnects (DCCs). Local traffic is backhauled to the DCC, groomed, and sent to destination end offices. These "nailed up" circuits must be planned and engineered based on busy-hour metrics and worst-case call volumes. The result is fixed, underutilized IOF links from end offices to core DCCs. These devices traditionally are SONET/SDH multiplexers and lack the Layer 2/2.5/3 intelligence that would permit local grooming and traffic routing decisions.
RPR brings packet awareness to SONET/SDH rings. Packet-based technology lends itself to newer transport technologies such as MPLS, which provides a uniform convergence layer for transport services. MPLS allows dynamic path selection and path setup along with traffic engineering capabilities.
Traffic engineering has traditionally been a challenge for carriers' network planners, due to the circuit-based constraints of SONET/SDH. The larger the network, the more difficult it is to engineer. The exploding Internet traffic of recent years has only increased the challenge facing network planners. With RPR, traffic engineering becomes a dynamic process requiring minimal manual intervention.
In traditional SONET/SDH rings, provisioning of services affects all nodes in the path of the circuit. At the intermediate or "pass" nodes, there might be a "channel" hop if the bandwidth does not exist on the originating channel. The problem of stranded bandwidth faced by circuit-based rings is avoided with RPRs because the entire pipe is shared by all circuits. Provisioning of services in RPR does not affect the intermediate or "pass" nodes. This eliminates the complexity of managing topologies in large multi-ring metro networks.
Intelligent transport networks
Adding another layer of intelligence to the IOF network elements enables dynamic switching of traffic within multiple rings. Apart from providing support for multiple classes of services with strict priority scheduling, there is node-based fairness for opportunistic or best effort traffic. The result is an IOF network that can groom and reroute traffic dynamically to any node on the ring without requiring hubbing at large DCC sites for grooming.
Local traffic does not have to hub into a core DCC but can ingress and egress access rings based on destination address. In lieu of DCC grooming, direct connections are made between the IOF rings via Gigabit Ethernet links or via subtending rings. The RPR standard is PHY-agnostic and can be supported on both "Ethernet-like" PHYs and SONET PHYs. The IOF links between the various access rings and the core point of presence can be reduced since local traffic remains on the local rings rather than backhauled to the core. That is especially useful for deployment of private line and virtual private LAN services.
Since carriers in large cities deploy hundreds of optical rings in their access networks, they can gain significant savings through the ability to dynamically route and switch traffic in the transport network. Adding RPR functionality to the transport elements greatly improves overall efficiency of network traffic flows. Traffic paths are dynamically configured based on metrics such as congestion of spans and path availability.
Other benefits of an RPR-enabled ring include full utilization of the ring bandwidth, compared to the allocation of 50% protection bandwidth in SONET/SDH, as well as the ability to reuse individual ring spans between the ring network elements. Like SONET/SDH, RPR provides sub-50-msec ring restoration in case of a fiber cut or node failure. The RPR implementations using a SONET/SDH reconciliation sublayer automatically provide synchronized timing for voice circuits.
The ability to reuse each span, both east and west, for working traffic is another advantage of adding the RPR protocol to an IOF network. RPR supports oversubscription of the span to maximize efficiency and enables synchronized TDM services as well as plesio-synchronous circuits to be transported, which makes it well suited for an IOF application.
With IP and MPLS increasingly used as a medium to deliver services like data, video, and voice, it is clear that networks optimized for packet transport are well suited for IOF. Voice over IP (VoIP) is now proliferating in the enterprise PBX market. With an RPR-capable transport network and class of service attributes, services such as VoIP can be deployed. Voice packets within a customer's Ethernet link can be assigned a higher priority to ensure latency and jitter specifications are achieved. IP-optimized transport solutions using RPR can support DSL, VoIP, video over IP, and many other services.
RPR allows the LEC to maximize network efficiency, streamline the network, reduce the operational complexity, and utilize packet-switching capabilities in their IOF.
Vinay Bannai is a principal architect at Luminous Networks (Cupertino, CA). He can be reached at[email protected].
