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Archive    7 Nov 2006              #55


by Vladimir Kaminsky, Contributing Editor & Sr. Consultant

Resilient Packet Ring (RPR) transforms a chain of point-to-point links between packet nodes into a true shared medium with the quick recovery performance of a SONET ring. The basic 802.17 physical topology is of two counter-rotating ringlets (one inner, one outer).  Nodes effectively act as dual add/drop multiplexers (ADMs) that can transmit on either ringlet and receive from both.  Normally, the shorter path would be used, but congestion or fault conditions can cause the alternative path to be selected.  Without dedicating one ringlet to protection bandwidth, as in SONET, RPR avoids putting 50% of the bandwidth on reserve.

RPR allows the full ring bandwidth to be utilized under normal conditions.  It protects traffic in the case of a nodal failure or fiber cut by using the single remaining path.  In addition, a priority scheme (Fairness Control Algorithm) allows nodes to negotiate automatically for bandwidth among themselves.  Under a protection event, FCA lets best-effort traffic be degraded, but best-effort services will not be preempted entirely.

Each node has a topology map of the dual ringlets and can send a packet on the optimal ringlet toward its destination.  Packets are removed by the receiver, not returned to the source as in Token Ring, so only the sections on one ring between the source and destination nodes are used for each packet.  Other transfers can to take place simultaneously between other nodes on the ring.  This feature (spatial reuse) significantly increases the efficiency of RPR and allows a chopped up ring to continue to carry traffic within each segment of contiguous nodes.

Adding to efficiency (compared with static TDM configuration of SONET rings), RPR implements efficient statistical multiplexing of packet traffic--it is optimized for packet networks.

AS defined by IEEE 802.17, RPR is an access control protocol to a shared media ring with carrier-grade characteristics:
--Guaranteed SONET-like restoration time (50 ms or less)
--Competitive cost when compared with SONET rings.

Standardized in 2004 in response to the user demand for reliable IP transport with minimum delay, the transport was extended to other types of traffic, the most important of which was Ethernet.  Normally the rings use fiber, but other physical media are possible.  Networks may have up to 128 nodes.

RPR is essentially neutral with respect to its payload, just adding an 18-byte header to the traffic's native protocol data unit (PDU). The header includes a 2-byte field for RPR functions (such as time to live, ringlet identification, message type, and priority), the destination and source MAC addresses, a protocol-type field, and a header error-checking (HEC) field.

RPR is strictly a layer 2 technology.  Ethernet, SONET/SDH, DWDM, and MPLS are technologies that complement RPR in Metro networks.  As a technique independent of the physical layer, RPR can be carried either over Ethernet or SONET (802.17 defines how RPR maps to them through so-called reconciliation sublayers).  Procedures for RPR are defined to interwork with:
--GFP-Generic Framing Procedure;
--High-level data link control (HDLC) framing alternatives;

RPR supports all existing SONET rates.  RPR's GFP adaptation layer can be built over the SONET path layer (and not directly over a SONET physical medium), and thus enables transparent interworking with underlying SONET multiplexing schemes such as Virtual Concatenation (VCAT) without affecting current SONET traffic.

The reason for developing RPR, as a Layer 2 protocol was simple: cost.  Because RPR runs on top of existing technologies, you don't need to scrap expensive network infrastructures--you can just add RPR equipment (or configure existing routers that have RPR features in upgraded firmware).

RPR is designed primarily for the LAN-MAN environment, leaving SONET to run long-haul networks. In LAN-MAN, RPR can be used separately as a carrier, or it can be implemented as a Layer 2 MAC access technology, relying on underlying SONET transport.

The following are the summary of the RPR technology advantages:
--Efficient bandwidth utilization.  Both ring are used;
--Sub-50 ms protection switching;
--Multiple physical layer options;
--Real-time traffic support.  Constant Bit Rate services such as voice and video are supported at the packet-layer through advanced QoS features and, in the case of the SONET-ring option, through synchronization to the 8 kHz stratum timing provided at the physical layer;
--GFP-mapping option.  RPR packets can be mapped into part of an OC-48 or OC-192 SONET frame, leaving the reminder of the SONET frame for TDM based circuit-switched services.

At the present time, the RPR market in the U.S. is not significant, blocked by declining prices for SONET equipment and rapid development of New Generation SONET ADMs that effectively transports video as well voice.  China is the largest consumer of RPR rings--it did not have the SONET/SDH infrastructure in place, and is attracted to RPR's cost efficiency--something for enterprises to consider as well.

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Flanagan Consulting
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