Resilient packet rings touted for metro



Declaring SONET/SDH is no longer the optimal mode of transport for data-heavy metropolitan-area networks, both the Institute of Electrical and Electronics Engineers (IEEE) and industry have offered "resilient packet rings" (RPRs) as an alternative. The IEEE created a working group to address standards for such rings, while five systems vendors have formed an alliance to support the effort.

The IEEE moved first with the approval last December of a new working group within the IEEE 802 LAN/MAN Standards Committee. The IEEE 802.17 Resilient Packet Ring working group (RPRWG) will tackle a MAC-layer-level standard for RPRs. The activity will include the development of an access protocol for such rings to enable the transfer of data packets at rates scalable to multiple gigabits per second. While the working group is expected to use existing physical-layer (PHY) specifications, it has been given the freedom to develop new PHYs where necessary.

As their name implies, RPRs use a ring-based architecture to provide the kind of protection and reliability associated with SONET/SDH to packet-based networks, without suffering from the inefficiencies encountered when attempting to transport data traffic using a circuit-switching protocol. Such topologies will feature dual counter-rotating rings that obviate the need for reserved protection bandwidth; both rings will carry traffic all the time. Despite the lack of reserved protection bandwidth, the protocol will be expected to provide sub-50-msec protection. It also will promote media independence and "plug and play" operation.

The RPRWG held a preliminary meeting in January but will initiate its work in earnest this month in Hilton Head, SC, during the IEEE 802 Plenary. At the January meeting, a proposed timeline called for a first draft by the end of January 2002, a working group ballot the following July. Target date for completion of the standard is March 2003. Cisco Systems' Michael Takefman will serve as the working group's chair.

While the IEEE says the upcoming RPR standards will be suitable for local-, metropolitan-, and wide-area networks, the industry-sponsored Resilient Packet Ring Alliance ( clearly has the metro in mind. The five charter members-Cisco Systems, Dynarc Inc., Lantern Communications, Luminous Networks, and Nortel Networks-have each developed proprietary versions of RPRs and aimed them at metro service providers looking to accommodate the growing percentage of Internet Protocol (IP) traffic. At press time, two other companies, Corrigent Systems and Riverstone Networks, had also signed aboard.

While the five companies will work together to support the RPRWG and generally promote the concept to service providers, they will continue to push their own versions of RPRs to potential customers. Cisco made an initial splash with its Dynamic Packet Transport, based on its Spatial Reuse Protocol (see Lightwave, May 1999, page 76). However, other offerings soon followed, including Dynamic Synchronous Transfer Mode (DTM) from Dynarc (see Lightwave, September 1999, page 130), Nortel's InterWAN Packet Transport, Luminous's Gigabit IP over Fiber (see Lightwave, December 2000, page 154), and Lantern's Resilient Optical Packet Ring.

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