Resilient Packet Ring Offers QoS

With data traffic now surpassing voice traffic on the global public network, a new group of carrier-grade technologies has begun to emerge. Circuit, fixed bandwidth and static provisioning technologies, which have been the WAN mainstay, are being challenged by faster, packet-oriented, dynamic and self-provisioning technologies that are used in LANs.

One promising technology is Resilient Packet Ring (RPR), now being standardized by the IEEE 802.17 working group. RPR defines a new media access control (MAC) layer that is physical-layer independent and is optimized to address multiservice networks over ring topologies, particularly in metropolitan and access networks.

The MAC layer will work with an Ethernet physical layer, which initially operates at 10Gbps or below, or with SONET physical layers, which support transmission monitoring and optical management.

Each MAC layer manages a series of physical or logical ring channels consisting of a transmit or receive link on a full-duplex Gigabit Ethernet link, a concatenated SONET channel or an added (transmit) or dropped (receive) wavelength in a wave division multiplexing system.

In an RPR ring, there are at least four ring channels, with receive and transmit traffic flowing in opposite directions. A ring segment is defined as the portion of the ring between two MAC layers or nodes on the RPR network. The MAC manages traffic around its ring segments, adding transmit traffic if the destination is upstream or removing and terminating traffic if its node is the ultimate destination of the data.

RPR utilizes an Ethernet physical layer to minimize development costs and ride the silicon volume/cost curve, but the MAC is not Ethernet and will not interoperate directly with Ethernet networks. The change from existing Ethernet infrastructure interfaces is the biggest drawback of the standard. However, when using an Ethernet physical layer, Ethernet networks can be bridged at the RPR network edge.

The other key elements of the RPR protocol are the virtual media identifiers, which overlay virtual LANs on the RPR ring. Virtual media identifiers are a part of the RPR frame header and provide traffic management and switching between nodes.

RPR provides four key attributes: