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Packet rings aim at metro nets

Posted: 01 May 2001 ?? ?Print Version ?Bookmark and Share

Keywords:rpr? rpr alliance? man? metro nets? mac layer?

Resilient packet ring (RPR) is a new technology that has been optimized for the unique requirements of metropolitan-area networks. This market is expected to grow faster during the next five years than that for long-haul equipment.

Although Sonet/SDH equipment represents the largest share of the MAN market today, its underlying technology was designed for voice traffic and does not scale to the demands of data networks. In response to customer demands for a network that addresses the unique requirements of the MAN, a number of vendors have developed proprietary offerings. However, they understand that a standards-based solution is required and have worked within the LAN/MAN Standards Committee to form a new working group, IEEE 802.17. It will create the IEEE 802.17 standard?RPR.

IEEE 802.17 will define a MAC protocol for use in local-, metropolitan- and wide-area networks, employing existing physical-layer specifications such as ethernet time-to-market" target=_blank>gigabit ethernet and 10 Gigabit Ethernet, as well as OC-48/STM-16 (2.5Mbps) and OC-192/STM-64 (10Gbps), for transfer of data packets at rates scalable to many gigabits per second.

Using predefined, standards-based physical layers will both speed up the standardization process and allow for the straightforward and seamless attachment of IEEE 802.3- and Sonet-based LANs to the RPR-based MAN. Creation of the new MAC layer, optimized for MAN traffic and topologies, will enable RPR to deliver cost-effective, quickly provisioned, secure and reliable data links on a service provider's existing fiber infrastructure. End users will be able to "turn up" bandwidth on demand at rates from 1Mbps to 1Gbps and beyond.

To understand the motivation behind RPR, it is useful to examine the primary needs of the emerging packet MAN networks. The demands of packet-based MANs can be grouped into five categories: survivability, fairness, scalability, performance and efficiency.

Survivability or resiliency describes the ability of the access network to withstand equipment and facility failures, maintaining connectivity between the subscriber and the network. Communication technologies that offer varying degrees of survivability include Sonet, LAN bridges and routers. With its ability to protect circuit traffic service within 50ms of an outage, Sonet has set the bar for network resilience, restoring service 100 times faster than bridges and routers.

Fairness, the ability of the access network to allocate bandwidth in a manner that is consistent with its defined service policies, cannot be satisfied by the contention-based and best-effort methods used in traditional routed and switched networks. The architecture for the new access network must be able to scale to thousands of endpoints and must support thousands of applications. The applications themselves will require bandwidth services that can scale from tens of kilobits per second to gigabits per second. Scalability is a critical requirement for the metropolitan network.

Deterministic performance

Performance is another important characteristic of the access network. For convergence over IP to become a reality, the access network must be capable of supporting services with deterministic and predictable performance.

Last, the access network must be efficient. It should be optimized for the ring topology of the MAN fiber plant and for the predominant traffic type it carries-data. Circuit networks and technologies are not readily adapted to data traffic and are often compromised when data capability is introduced. In a data world, a packet network is the logical infrastructure.

The RPR standards will define a MAC with two network interfaces, a system-level interface and a physical-layer interface. The topology associated with RPR will be a set of switching nodes interconnected along a bidirectional ring.

To connect adjacent nodes on this ring a variety of media can be used, including a pair of dark fiber strands, a pair of WDM-derived wavelengths, or a Sonet add-drop OC-n circuit or other bidirectional connection medium. Based on the topology, each RPR node supports two ring ports; one supports a connection to the adjacent node to the left, the other to the right.

The MAC interface for RPR fundamentally differs from an Ethernet MAC in that it contains elements of a switching protocol, including decision making for storing and forwarding data. This key difference of bringing switching decisions into the MAC provides significant network performance improvements compared with Ethernet protocols.

This cut-through capability of RPR, combined with the RPR fairness algorithms, will enable vendors to develop RPR systems that ensure both bandwidth and delay performance per application. Indeed, we anticipate that this degree of control and service assurance will exceed the capabilities of ATM in the access network.

The IEEE 802.17 RPR standard will be developed on a very aggressive schedule, with its technical requirements established in the next 18 months. To meet this timetable, a number of vendors have banded together and formed a Resilient Packet Ring Alliance, to work cooperatively with the 802.17 Working Group and to sponsor early interoperability testing. Reuse of standardized physical layers will remove a major inhibitor to rapid standards development, allowing the working group to focus on the MAC specification.

? Robert D. Love

LAN Connect Consultants





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