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CHAPTER 1 INTRODUCTION

1.1 Background:
The nature of the public network has changed. Demand for Internet Protocol (IP) data is growing at a compound annual rate of between 100% and 800%1, while voice demand remains stable. What was once a predominantly circuit switched network handling mainly circuit switched voice traffic has become a circuit-switched network handling mainly IP data. Because the nature of the traffic is not well matched to the underlying technology, this network is proving very costly to scale. User spending has not increased proportionally to the rate of bandwidth increase, and carrier revenue growth is stuck at the lower end of 10% to 20% per year. The result is that carriers are building themselves out of business. Over the last 10 years, as data traffic has grown both in importance and volume, technologies such as frame relay, ATM, and Point-to-Point Protocol (PPP) have been developed to force fit data onto the circuit network. While these protocols provided virtual connections-a useful approach for many services-they have proven too inefficient, costly and complex to scale to the levels necessary to satisfy the insatiable demand for data services. More recently, Gigabit Ethernet (GigE) has been adopted by many network service providers as a way to network user data without the burden of SONET/SDH and ATM. GigE has shortcomings when applied in carrier networks were recognized and for these problems, a technology called Resilient Packet Ring Technology were developed.

Fig. 1.1 Packet Rings: Next step in packet based transport

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1.2 Introduction To Resilient Packet Ring (RPR):
RPR Technology is an emerging solution for metro data transport applications that can fully exploit fiber rings. It retains all the inherent advantages of a packet-based transport mechanism like Ethernet. Above all, this latest technology is suitable for data, voice, and video applications. The adoption of RPR technology is particularly critical to handle the rapid growth of data traffic in metropolitan networks.

RPR technology reduces operational and equipment costs, increases resiliency to faults and provides higher throughput, while allowing for rapid deployment and efficient bandwidth allocation. RPR technology is being designed specifically for metropolitan area networks of fiber optic rings and will be an important part of new metropolitan data networks. This new protocol will allow metropolitan area service providers to create high-speed, survivable ring networks optimized for IP and other packet data.
RPR retains the best attributes of SONET/SDH, ATM, and Gigabit Ethernet. RPR is optimized for differentiated IP and other packet data services, while providing uncompromised quality for circuit voice and private line services. It works in point-topoint, linear, ring, or mesh networks, providing ring survivability in less than 50 milliseconds. RPR dynamically and statistically multiplexes all services into the entire available bandwidth in both directions on the ring while preserving bandwidth and service quality guarantees on a per-customer, per-service basis. And it does all this at a fraction of the cost of legacy SONET/SDH and ATM solutions.
Table 1.1 Resilient Packet Ring Technology Key Features

Resilience Proactive span protection automatically avoids failed spans within 50ms. Services Support for latency/jitter sensitive traffic such as voice and video. Support for committed information rate (CIR) services. Efficincy Spatial reuse: Unlike SONET/SDH, bandwidth is consumed only between the source and destination nodes. Packets are removed at their Scalable Supports topologies of more than 100 nodes per ring. Automatic topology discovery mechanism

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CHAPTER 2 SONET AND ETHERNET

2.1 SONET

Fig. 2.1 SONET Access and fully meshed networks

Most metro area fiber is in ring form. Ring topology is a natural match for SONET-based TDM networks that constitute the bulk of existing metro network infrastructure. However, there are well-known disadvantages to using SONET for transporting data traffic (or point-to-point SONET data solutions, like Packet over SONET [POS]). SONET was designed for point-to-point, circuit-switched applications (e.g. voice traffic), and most of limitations stem from these origins. Here are some of the disadvantages of using SONET Rings for data transport: 2.1.1 Fixed Circuits: SONET provisions point-to-point circuits between ring nodes. Each circuit is allocated a fixed amount of bandwidth that is wasted when not used. For the SONET network that is used for access in Figure 2 (left), each node on the ring is allocated only one quarter of the ring’s total bandwidth (say, OC-3 each on an OC-12 ring). That fixed allocation puts a limit on the maximum burst traffic data transfer rate between endpoints. This is a disadvantage for data traffic, which is inherently bursty.

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2.1.2 Waste of Bandwidth for Meshing: If the network design calls for a logical mesh, (right), the network designer must divide the OC-12 of ring bandwidth into 10 provisioned circuits. Provisioning the circuits necessary to create a logical mesh over a SONET Ring is not only difficult but also results in extremely inefficient use of ring bandwidth. As the amount of data traffic that stays within metro networks is increasing, a fully meshed network that is easy to deploy, maintain and upgrade is becoming an important requirement. 2.1.3 Multicast Traffic: On a SONET Ring, multicast traffic requires each source to allocate a separate circuit for each destination. A separate copy of the packet is sent to each destination. The result is multiple copies of multicast packets traveling around the ring, wasting bandwidth. 2.1.4 Wasted Protection Bandwidth: Typically, 50 percent of ring bandwidth is reserved for protection. While protection is obviously important, SONET does not achieve this goal in an efficient manner that gives the provider the choice of how much bandwidth to reserve for protection.

2.2 Ethernet

Fig. 2.2 Ethernet Over Ring Technology

Ethernet does make efficient use of available bandwidth for data traffic, and does offer a far simpler and inexpensive solution for data traffic. However, because Ethernet is optimized for point-to-point or meshed topologies, it does not make the most of the ring topology. Unlike SONET, Ethernet does not take advantage of a ring topology to implement a fast protection mechanism. Ethernet generally relies on the spanning tree protocol to
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eliminate all loops from a switched network. Even though spanning tree protocol can be utilized to achieve path redundancy, it recovers comparatively slowly from a fiber cut since the recovery mechanism requires the failure condition to be propagated serially to each upstream node. Link aggregation (802.1ad) can provide a link level resiliency solution, but it is comparatively slow (~500ms vs.~50ms) and not appropriate for providing path level protection. Ethernet is also not good at implementing global “fairness” policies for sharing ring bandwidth. Ethernet switches can provide link-level fairness, but this does not necessarily or easily translate into global fairness.

2.3 Comparing RPR To Other Solutions
Resilient Packet Ring (RPR) technology was designed to combine SONET’s carrier-class functionalities with Ethernet’s high bandwidth utilization and granularity. Additionally, RPR technology offers fairness that has been lacking in today’s Ethernet solutions. RPR is a new MAC layer technology, being standardized in the IEEE 802.17 workgroup. This employs spatial reuse to maximize bandwidth utilization, provides a distributed fairness algorithm, and ensures high-speed traffic protection similar to SONET Automatic Protection Switching (APS). RPR allows full ring bandwidth to be utilized under normal conditions and protects traffi c in the case of a nodal failure or fi ber cut using a priority scheme, alleviating the need for SONET-based protection. Furthermore, because the RPR MAC layer can run on top of a SONET PHY, RPR-based networks can provide performance-monitoring features similar to those of SONET.RPR technology offers all of these carrier-class functionalities, while at the same time keeping Ethernet’s advantages of low equipment cost, high bandwidth granularity, and statistical multiplexing capability.

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Table 2.1 Comparison between Gigabit Ethernet and RPR Gigabit Ethernet Enterprise-class equipment Data service only Point-to-Point or mesh topology (No rings) RPR Carrier-class equipment Data, circuit or video service Point-to-Point, topology Protection in 50seconds Limited scalability Protection in 50 milliseconds or less 254 nodes per ring, multiple rings linear, ring or mesh

Table 2.2 Comparison between SONET/SDH and RPR SONET Manual topology configuration 16nodes per ring Fixed, dedicated management bandwidth Time division multiplexing RPR Auto topology discovery 254 nodes per ring Management bandwidth used as needed Stastical multiplexing

Manual provisioning of bandwidth and Manual or dynamic provisioning routes No service class awareness Fixed direction traffic routing All traffic is protected Differentiate services in eight classes Least cost traffic routing Per connection protection

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CHAPTER 3 NEED FOR A NEW TECHNOLGY
The rise in more widely distributed IP traffic has created a wealth of unpredictability, as any PC or server can become an end point, generating a multitude of network requests. Inefficiencies associated with carrying increasing quantities of data traffic over voice-optimized circuit-switched network technologies like SONET and ATM makes it difficult to provision new services and increases the cost of building additional capacity beyond the limits of most.

3.1 Quality concerns Service Providers today are concerned with several fundamental quality concerns as follows:


Reliability: The network must be reliable, providing redundant hardware, ring topology

support, and fiber protection and restoration capabilities.


Fairness: Fair bandwidth management and allocation must be independent of where a

subscriber sits on the network relative to other subscribers.


Scalability: The architecture must be scalable to thousands of end points and must

support a broad range of applications.


Performance: The network must be capable of supporting services with deterministic

and predictable performance, including real-time packet voice and video applications that require minimal latency and jitter.


Efficiency: The packet access network must incorporate service protection, but it must

be accomplished efficiently. The system must be cost efficient to operate.

3.2 Disadvantages of ATM
 

Less speed of transmission Poor scalability.

3.3 Disadvantages of SONET


Fixed circuits: SONET provisions point-to-point circuits between ring nodes. Each

circuit is allocated a fixed amount of bandwidth that is wasted when not used.

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

Multicast traffic: On a SONET ring, multicast traffic requires each source to allocate a

separate circuit for each destination. A separate copy of the packet is sent to each destination. The result is multiple copies of multicast packets traveling around the ring, wasting bandwidth.


Wasted protection bandwidth: Typically, 50% of ring bandwidth is reserved for

protection. While protection is important, SONET does not achieve this goal in an efficient manner that gives the provider the choice of how much bandwidth to reserve for protection.

3.3 Disadvantages of Ethernet


Ethernet does not take advantage of a ring topology to implement a fast protection

mechanism.


Even though spanning tree protocol can be utilized to achieve path redundancy, it

recovers comparatively slowly from a fiber cut as the recovery mechanism requires the failure condition to be propagated serially to each upstream node.


Link aggregation can provide a link level resiliency solution, but it is comparatively Ethernet is also not good at implementing global “fairness” policies for sharing ring

slow and not appropriate for providing path level protection.


bandwidth. Ethernet switches can provide link-level fairness, but this does not necessarily or easily translate into global fairness.

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CHAPTER 4 RPR OPERATION

4.1 Key Features of RPR technology
1. Efficient support for ring topology. 2. Increases resiliency to faults and provides higher throughput. Fast recovery from fiber cuts and link failures. 3. Data efficiency, simplicity, and cost advantages. 4. Problems such as fairness and congestion control solved. 5. Efficient bandwidth allocation.

4.2 RPR Operation

Fig 4.1 RPR Terminology

RPR technology uses a dual counter rotating fiber ring topology. Both rings (inner and outer) are used to transport working traffic between nodes. By utilizing both fibers, instead of keeping a spare fiber for protection, RPR utilizes the total available ring bandwidth. These fibers or ringlets are also used to carry control (topology updates, protection and bandwidth control) messages. Control messages flow in the opposite direction of the traffic that they represent. For instance, outer ring traffic-control information is carried on the inner ring to upstream nodes. By using bandwidth-control messages, a RPR node can dynamically negotiate for bandwidth with the other nodes on the ring. RPR has the ability to differentiate between low and high priority packets. Just like other Quality of Service (QoS) - aware systems,

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nodes have the ability to transmit high priority packets before those of low priority. In addition, RPR nodes also have a transit path, through which packets destined to downstream nodes on the ring flow. With a transit buffer capable of holding multiple packets, RPR nodes have the ability to transmit high priority packets while temporarily holding other low priority packets in the transit buffer. Nodes with smaller transmit buffers can use bandwidth control messages to ensure that bandwidth reserved for high priority services stays available.

4.3

The RPR MAC
One of the basic building block of RPR is Media Access Control(MAC). As a Layer-2 network protocol, the MAC layer contains much of the functionality for the RPR network. The RPR MAC is responsible for providing access to the fiber media. The RPR MAC can receive, transit, and transmit packets.

Fig 4.2 RPR MAC Block Diagram

4.3.1 Receive Decision: Every station has a 48-bit MAC address. The MAC will receive any packets with a matching destination address. The MAC can receive both unicast and multicast packets. Multicast packets are copied to the host and allowed to continue through the transit path. Matching unicast packets are stripped from the ring and do not consume bandwidth on downstream spans. There are also control packets that are meant for the neighboring node; these packets do not need a destination or source address.

4.3.2 Transit Path: Nodes with a non matching address are allowed to continue circulating around the ring. Unlike point-to-point protocols such as Ethernet, RPR packets undergo minimal
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processing per hop on a ring. RPR packets are only inspected for a matching address and header errors.

4.3.3 Transmit And Bandwidth Control: The RPR MAC can transmit both high and low priority packets. The bandwidth algorithm controls whether a node is within its negotiated bandwidth allotment for low priority packets. High priority packets are not subjected to the bandwidth control algorithm.

4.3.4 Protection: RPR has the ability to protect the network from single span (node or fibre) failures. When failure occurs, protection messages are quickly dispatched. RPR has two protection mechanisms.

1 Wrapping: Nodes neighboring the failed span will direct packets away from the failure by wrapping traffic around to the other fibre (called ringlet). This mechanism requires that only two nodes participate in the protection event. Other nodes on the ring can send traffic as normal.

Fig. 4.3 Wrapped Traffic Flow

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2 Steering: The protection mechanism notifies all nodes on the ring of the failed span. Every node on the ring will adjust their topology maps to avoid this span. Regardless of the protection mechanism used, the ring will be protected within 50ms.

4.3.5 Topology Discovery RPR has a topology discovery mechanism that allows nodes on the ring to be inserted/removed without manual management intervention. After a node joins a ring, it will circulate a topology discovery message to learn the MAC addresses of the other stations. Nodes also send these messages periodically (1 to 10 seconds). Each node that receives a topology message appends its MAC address and passes it to its neighbor. Eventually, the packet returns to its source with a topology map (list of addresses) of the ring. Routers are able to use the Address Resolution Protocol (ARP) mechanism to determine which RPR MAC address belongs to the destination address of an IP packet. RPR switches and bridges will have a list of stations that they can reach through a RPR MAC address. The topology map will be used to determine which direction on the ring will provide the best path to the destination.

4.3.6 Physical Layer RPR packets can be transported over both SONET and Ethernet physical layers. The SONET/SDH physical layer offers robust error and performance monitoring. RPR packets can be encapsulated within the Synchronous Payload Envelope (SPE) using a High-level Data Link Control (HDLC) like or Generic Framing Protocol (GFP) encapsulation. A robust Layer-1 protocol, SONET/SDH provides information such as loss of signal and signal degrade for use by the RPR protection mechanism. When using a SONET/SDH physical layer, RPR can be carried over SONET/SDH TDM transport or dark fiber. Ethernet provides an economical physical layer for RPR networks. RPR packets are transmitted with the required inter-packet gap (IPG). RPR Systems using the SONET physical layer will not interoperate with Ethernet physical-layer-based systems on the same ring.

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4.4 MAC Frame Format

Fig. 4.4 RPR MAC Frame

4.4.1 Destination Address: This one or six byte address (see dual mode addressing) is the MAC address of the ring node to which the frame is being transmitted, and therefore does not change from link to link on the ring. This address can also be a broadcast address.

4.4.2 Source Address: This one or six byte address is the MAC address of the ring node from which the frame is being transmitted, and therefore also does not change from link to link.

4.4.3 Payload Type: This two-byte field tells the system what type of payload follows the RPR fields. For example. MPEG or ATM or Ethernet.

4.4.4 Class of Service (CoS): The three bit CoS field allows the identification of up to eight Classes of Services, including Expedited Forwarding (EF), six levels of Assured Forwarding (AF1 through AF6), and Best Effort (BE).

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4.4.5 Extension (E) Bit: This field indicates that there is an extension to the RPT header. This allows for fields that may be added in the future. 4.4.6 Time To Live (TTL): The one byte TTL field is included to allow the RPT ring topology. It ensures that under no circumstances do RPT frames continue to circulate in a loop indefinitely. 4.4.7 Flow ID (optional): The 20-bit flow ID field maps a virtual connection from ingress to egress over the RPT ring. It allows the simple manual or automatic setup of connection oriented services such as Time Division Multiplexed (TDM) circuit emulated services and Ethernet virtual leased lines through the packet-switched network. 4.4.8 Header Error Check (HEC): Borrowing a concept from ATM, the two byte Header Error Check (checksum) provides a way to test the integrity of the header, allowing for persistent delivery of frames despite errors in the payload. 4.4.9 Cyclic Redundancy Check (CRC): The four byte (32 bit) Cyclic Redundancy Check (CRC-32) works differently in RPT than it does for standard Ethernet.

4.5 Using RPR with SONET and Ethernet
Ethernet is the most common interface within enterprise networks. Although the optimal customer interface and LAN solution, Ethernet does not work well within metro networks.

Fig. 4.5 RPR over Ethernet Customer Interfaces

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Most RPR edge systems offer an Ethernet interface as the customer interface. SONET/SDH can be used to transport RPRs that use the SONET/SDH physical layer. A 2.5G RPR can be carried as two optical carrier (OC)–48/synchronous transfer mode (STM)–16 channels (inner and outer rings) inside an OC–192/STM–64 SONET/SDH ring. This allows customers to utilize their existing transport infrastructure to deliver legacy TDM services, while RPR delivers data services efficiently. New SONET advancements such as virtual concatenation and the link-capacity adjustment scheme (LCAS) allow more precise and dynamic allocation of bandwidth dedicated to RPR services.

4.6 Resilient Packet Ring – Emerging Metro Network Architecture
SONET does take advantage of the ring topology, but does not handle data traffic efficiently, wasting ring bandwidth. Ethernet, while a natural fit for data traffic, is in fact difficult to implement on a ring and does not make the most of the ring’s capabilities. Resilient Packet Ring is an emerging network architecture and technology designed to meet the requirements of a packet-based metropolitan area network. Unlike incumbent architectures based on Ethernet switches or SONET ADMs (Add-Drop MUXes), RPR approaches the metro bottleneck problem with a clean slate. In the past few years there have been fiber ring deployments in most metro areas. The challenge for service providers is to tap into the latent capacity available on these fiber rings and carve out as many profitable, revenuegenerating services as possible. This problem of effectively managing a shared resource (in this case the fiber ring is the resource that needs to be shared across thousands of subscribers in a metro area) is most efficiently solved at the MAC layer of the protocol stack. RPR (IEEE 802.17) will be a new MAC protocol designed for metro fiber ring networks. By creating a MAC protocol for ring networks, RPR attempts to find a fundamental solution to the metro bottleneck problem. Other solutions attempt to make incremental changes to existing products but do not address the basic problem and hence are inefficient. Neither SONET ADMs nor Ethernet switches address the need for a MAC layer designed for the metropolitan area environment. SONET employs Layer 1 techniques (point-to-point connections) to manage capacity on a ring. Ethernet switches rely on Ethernet bridging or IP routing for bandwidth management. Consequently, the

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network is either underutilized in the case of SONET or non-deterministic in the case of Ethernet switches.

4.7 Technical aspects of RPR
Technical aspects of RPR are multicast, spatial reuse, fairness, fast protection and quality of service. 4.7.1 Multicast One RPR multicast packet can be transmitted around the ring and can be received by multiple nodes. Mesh topologies require multicast packets to be replicated over all possible paths, wasting bandwidth. 4.7.2 Spatial Reuse RPT has the ability to switch traffic over multiple spans of the ring simultaneously. Hence, the bandwidth on a particular span between ring nodes is utilized asynchronously with regard to the bandwidth on other spans. This allows bandwidth to be added or dropped on a span-by-span basis, which ensures maximum utilization of bandwidth in the ring, especially when traffic patterns are highly distributed. For example, traffic passing from node A to node B will be dropped at node B, allowing new traffic to be inserted at node B for transmission to downstream nodes C, D, etc.

4.7.3 Fairness Fairness is one of the most important features in carrier-class networks. Fairness is achieved when the traffic characteristics of two service flows that have the same service level agreement (SLA) are identical, regardless of their network source and destination.

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The RPR protocol can guarantee fairness across the metropolitan network. Each node on the metro ring executes an algorithm designed to ensure that each node on the ring will get its fair share of bandwidth. However, ring nodes use the spatial reuse capability to use additional available bandwidth, which is greater than their fair share on local ring segments, as long as it doesn’t affect other ring nodes. More specifically, the ring supports weighted fairness, proportional to the bandwidth each user buys. Figure 4.7 demonstrates the operation of the fairness algorithm. Before employing the fairness algorithm, all nodes transmit at their peak usage rate. At some point Node 1 experiences congestion and requests upstream nodes to reduce their transmission rate to the allowed rate. After convergence, each node will receive its fair share of bandwidth, while nodes that can locally transmit higher rates, like nodes 2 and 3, without generating congestion on other nodes, continue to transmit their peak usage rate. Other data optimized technologies, such as Ethernet, do not provide the carrierclass fairness guaranteed in RPR-based networks. Ethernet switches prioritize traffic locally, on every interface on the ring, thus creating unfair conditions for traffic that has to traverse through several nodes on its way to the destination node.

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4.7.4 Fast Protection and Restoration The feature that more than any other makes RPR a carrier-class technology is its fast self-healing capability which allows the ring to automatically recover from a fiber cut or node failure. This is done by wrapping traffic onto the alternate fiber within the SONET-class 50msec restoration timeframe. RPR technology provides carriers with more than SONET-class fast protection and performance monitoring capabilities; it also enables them to achieve it without dedicating protection bandwidth, as in the case of SONET, thus eliminating the need to sacrifice 50% of the ring bandwidth for protection. Two mechanisms are proposed for fast protection in the RPR MAC - Wrap and Steer. Each of these mechanisms has its own advantages and limitations, and both can be used on an RPR ring utilizing the Selective Wrap Independent Steer (SWIS) scheme.

Figure 4.8 shows an example of the data paths taken before and after a fiber cut event when Wrap is being used. Before the fiber cut, node 3 sends traffic to node 1 via node 2 (Figure 9a). When the fiber cut occurs (between node 1 and node 2), node 1 will wrap the inner ring to the outer and node 2 will wrap the outer to the inner ring. After the wrap, traffic from node 3 to node 1 initially traverses through the non-optimal path (passing through node 2, back to node 3 and into the originally assigned port on node 1 Figure 9b). This is immediately done and can inherently meet the 50ms criteria, as it is a local decision of the nodes that identify the fault (in this case, nodes 1 and 2). Subsequently, higher layer protocols discover the new ring topology and a new optimal path is used (Figure 9c). This can take several seconds to converge in a robust manner, after which the double ring capacity is restored (even though not evenly distributed, depending on the location of the fault and the ring traffic pattern). Thus, unlike SONET rings that always pay the redundancy tax to achieve protection, after a failure; an RPR ring reduces its capacity only for several seconds.
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Other data optimized protocols, such as Ethernet, do not provide the SONET-class restoration time. Typically, spanning tree tools are used for protection in these cases. In a mesh topology, these tools provide protection time of the order of 1 second, but in ring topologies, protection time is degraded to the order of 10 seconds or more. In summary, RPR’s fast protection and restoration capability prevents service loss for high priority critical traffic. And just as SONET does, RPR enables carriers to continue to support their existing critical traffic over a data optimized network, without compromising their guaranteed 99.999% service availability. 4.7.5 Quality of Service Quality of Service (QoS) is required in order to let a carrier effectively charge for the services it provides. ATM promised to deliver multiple services due to its rich QoS feature set. However, a carrier’s service offering should be simple. Customers should clearly understand the service differentiators for which they’re required to pay. Often, a too-rich QoS feature set causes a complicated and incomprehensible service offering. Furthermore, different service quality features are required for different types of applications. Data transfer applications require low packet loss rate, while realtime applications, such as voice, require low latency and low delay variation. There are several parameters that more than others govern the characteristics of a delivered service: Service availability, delay, delay variation and packet loss rate. Service availability depends on the reliability of the network equipment, as well as on the network survivability characteristics. Delay occurs in a network when a packet waits in a switch queue for other packets to pass. Delay variation is the difference in delay of several packets belonging to a common traffic flow. High-frequency delay variation is called jitter while low-frequency delay variation is called wander. The most important parameter for a carrier selling bandwidth services is service availability. In order to remain competitive, a carrier must guarantee “five-nines” (99.999%) service availability, irrespective of other service characteristics. The respective requirement from the network is that all traffic flows should remain active under any circumstances, including during a protection event. During a protection event, and until high layer protocols converge, the available bandwidth on an OC-192 RPR ring reduces to 10Gbps. In order to guarantee service for every traffic flow under these circumstances, a portion of each flow’s bandwidth should be allocated on this guaranteed throughput. This way, even during a protection event, service availability is guaranteed and none of the services is preempted.
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Traffic delay is a minor issue in high throughput rings. An end-to-end delay of several tens of milliseconds is usually regarded as acceptable, while in an OC-192 ring, the expected delay for the longest Ethernet frames (1518 bytes) is shorter by three orders of magnitude (about 5(sec). Delay variation control is essential to ensure proper transport of TDM services over an RPR ring. Control can be achieved by synchronizing the nodes on the RPR ring by a common timing source.

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CHAPTER 5 ADVANTAGES, FEATURES AND BENEFITS RING NETWORK

5.1 Advantages

SONET does take advantage of the ring topology, but does not handle data traffic efficiently, wasting ring bandwidth. Ethernet, while a natural fit for data traffic, is in fact difficult to implement on a ring, and does not make the most of the ring’s capabilities. Packet Ring protocols, on the other hand, promise the best of both worlds. Packet Ring protocols create a full, packet-based networking solution that avoids the provisioning complications and inflexibility of SONET and provides the ring protection and global fairness features missing from Ethernet. The basic advantage of a Packet Ring is that each node can assume that a packet sent on the ring will eventually reach its destination node regardless of which path around the ring has taken. Since the nodes “know” they are on a ring, only three basic packethandling actions are needed: insertion (adding a packet into the ring), forwarding (sending the packet onward), and stripping (taking the packet off the ring). This reduces the amount of work individual nodes have to do to communicate with each other, especially as compared with mesh networking where each node has to make a forwarding decision about which exit port to use for each packet.

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5.2 Resiliency
Packet Rings have a natural resiliency advantage. A ring that is built using switches needs to distribute failure information across an entire network to recover fully from a fiber cut. In the Ethernet case, this can be accomplished using a spanning tree protocol. On the other hand, a Packet Ring protocol can use a “ring wrap” at the nodes surrounding the cut (see Figure 5.2).

Only nodes that are adjacent to the failure need to take any action. Wrapped traffic can reach the original destination by going around the ring in the opposite direction. Ring fail-over is often described as “self-healing” or “automatic recovery.” In practice, ringbased transport systems have reliably achieved