MODERN RECOVERY MECHANISMS FOR DATA TRANSPORT NETWORKS

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1 MODERN RECOVERY MECHANISMS FOR DATA TRANSPORT NETWORKS IT Specialist Dorina LuminiŃa Copaci Gorj Court IT Expert Constantin Alin Copaci ANRCTI Bucharest Abstract: A large number of recovery mechanisms for the different network technologies are standardized or published in conferences and journals. The provisioning of protection flexibility, service granularity and resilience manageability are important objectives of network resilience mechanisms in addition to the optimization of performance metrics like resource efficiency and recovery time. The network has to be resilient against failures. It must be able to detect the failure and recover affected services very fast, ideally without the services realizing the outage and disconnecting. Due to the complexity of the transport network architectures sophisticated resilience mechanisms are needed. These may operate in multiple network technologies (or layers). The network technologies Multiprotocol Label Switching (MPLS), Asynchronous Transfer Mode (ATM), Synchronous Digital Hierarchy (SDH), and Optical Transport Networks (OTN) offer such resilience mechanisms. 1. INTRODUCTION The recovery mechanisms are further classified depending on which layer they operate. Four layers are defined in this context: physical layer, system layer, logical layer and service layer. The physical layer includes the physical components and structures of the network, i.e. the ducts, cables, fibers, node sites (houses) and network elements. Survivability techniques for the physical layer are geographical diversity, redundancy (e.g., redundant power supply) and protection against physical damage (like fire). The system layer represents the network transmission systems, and terminating and full-rate interface equipment. Typical system layer components are STM-N transmission channels, Add/Drop Multiplexer and Terminal Multiplexer. The logical layer includes lower layer transmission systems and their interface equipment. The logical and the system layer can be combined to the transport layer. The service layer contains user service network such as voice and public and private data. The type of traffic transported in the service layer is telephone calls, data packets and cells. A typical survivability mechanism in the service layer is dynamic rerouting. In this paper the focus is on the transport layer. 2. RECOVERY MECHANISMS Several categorization schemes exist to classify network survivability mechanisms. The most common classification is to divide recovery mechanisms into protection switching and restoration mechanisms. Protection switching mechanisms use predefined alternative paths, while for restoration mechanisms alternative paths are calculated on demand after the detection of a failure. ATM recovery mechanisms are classified into protection switching, rerouting and self-healing mechanisms. Rerouting mechanisms are restoration mechanisms with centralized control, while distributed restoration mechanisms are called self-healing. To use an unambiguous naming scheme in the recovery framework the recovery mechanisms are divided in protection switching, (distributed) restoration, reconfiguration (centralized restoration), and rerouting (at the service level). Figure 1 summarizes all options of the recovery framework. In this work the focus is set on protection switching and restoration mechanisms. Figure 1. Recovery framework

2 2.1 Recovery Model Protection Switching In the case of protection switching, an alternative connection is pre-established and pre-reserved (preprovisioned). Therefore, protection switching realizes the shortest disruption of the traffic, since no routing and resource allocation is required after failure detection. In the SDH standardization the maximum allowed switching time of protection switching mechanisms is defined to be 50ms. Depending on the recovery scope, the alternative connection is either switched at the source and target network element (global protection or path protection), or locally at the network element adjacent to the failure (local protection or link protection) Dedicated Protection In case of dedicated protection, the protection resources are used dedicatedly to the corresponding working connections. There are two dedicated protection schemes: 1+1 (one plus one) and 1:1 (one for one) protection. In Figure 2 both dedicated protection schemes are compared. The primary path is called working or active path (a). The secondary, alternative path is called protection or backup path (b). Figure 2: 1+1 and 1:1 protection switching In 1+1 protection, the traffic is simultaneously transported over the working and protection path. In case of the failure, the target node only has to select the incoming traffic from the alternative path. With this combination, hitless recovery is possible. In [Iselt-1999] several protocols for hitless switching are analyzed. In case of 1:1 dedicated protection, the traffic is switched to the backup path V only after a failure is detected on the active path 'a'. Under normal conditions, the backup resources can be used for the transport of low-priority preemptive traffic, so-called extra traffic Shared Protection With shared protection the spare resources are not dedicated for the recovery of a specific connection, but can be shared by multiple connections for different failure scenarios. Figure 3 illustrates the concept of shared protection. Figure 3: Dedicated and shared protection On the link E-F the sum of the capacity of the two working connections A-B-C-D and G-H-I-D has to be reserved for the dedicated protection. In case of shared protection, only the larger capacity of A-B-C-D or G-H- I-D has to be reserved. In connections with equal capacity C are used, dedicated protection requires 2 C spare resources on link E-F and 6 C spare resources altogether for the protection of the two connections. With shared protection, the required spare resources are 1 C on the link E-F, and 4 C for the full connections. Because of the sharing of the spare resource, shared protection has better resource efficiency than dedicated protection. On the other hand, it requires a more complex signaling mechanism for the activation of the alternative connection. Shared protection mechanisms are common for ring topologies, where the spare resources are provided by additional fibers used only for protection traffic and extra traffic Restoration In the case of restoration, an alternative path is calculated and established on-demand after the detection of a failure. Since the calculation of alternative routes and the signaling and resource reservation of a new connection are time-consuming, restoration mechanisms are considerably slower than protection mechanisms.

3 However, the restoration is also more resource efficient, since the spare resources can be used for the recovery of different working connections, provided these don't share the same working resources. The recovery path is established using distributed restoration schemes after detecting the failure. There are several restoration mechanisms published, like the Self healing Network (SHN), FITNESS, or RREACT. In general, restoration mechanisms search for a suitable backup path using distributed flooding mechanisms. Depending on the scope of the recovery mechanism, local or global, the node upstream of the failure or the source nodes of affected connections broadcast reservation messages on all outgoing links with enough spare capacity. When a broadcast message reaches the destination node, this node responds with an acknowledgement message. Either the restoration is complete, when the acknowledgement message reaches the source node (2-phase algorithm), or the source node has to send a confirm message downstream to the destination node (3-phase algorithm). Figure 4. Protection and restoration As an alternative to a flooding procedure, the upstream switching node can use a constraints based routing mechanism to calculate the full restoration route Recovery Topology The recovery mechanisms can operate in different network topologies. Ring networks are well suited for protection mechanisms, since they are the simplest form of a two-connected network. One ring direction is used for the working traffic between source and destination, while the protection path is routed in opposite directions. In addition to two-fiber ring systems also four-fiber ring recovery mechanisms are possible. Protection mechanisms can also be used in mesh networks. Additionally, mesh network topologies also support restoration mechanisms. It was introduced the concept of p-cycles, which is based on protection cycles (overlay ring structure) working in a mesh network, utilizing the advantages of both, ring and mesh topologies. Figure 5 illustrates the three recovery topology alternatives Recovery Switching Operation Modes Figure 5 Recovery topology Switching type: Unidirectional versus bidirectional switching Two operation modes for recovery mechanisms exist for the recovery switching in case of failures affecting only one transmission direction. Such failures occur for example due to transmission laser outages. With unidirectional switching, only the affected direction of the failed traffic is recovered in case of unidirectional failures. In case of bidirectional switching, both, the affected and the unaffected direction of traffic affected by a unidirectional failure are recovered. For bidirectional switching, a protection switching protocol is required to control the switching operation. For ATM and SDH networks, this protection switching control protocol is called Automatic Protection Switching (APS) protocol. In case of unidirectional switching, only the sink node controls the switching operation, so no switching protocol is required. Therefore, unidirectional switching is less complex to implement and can operate faster. Unidirectional and bidirectional switching are also termed single ended vs. dual ended switching, respectively. Figure 6 illustrates the two switching modes.

4 3. ATM RECOVERY MECHANISMS Figure 6. Unidirectional and bidirectional switching The Asynchronous Transfer Mode (ATM) was defined as transmission technology for B-ISDN. Figure 7 shows the reference model and the associated transport network layers. The reference model is divided in the physical layer, the ATM layer, an ATM adaptation layer and higher layers. For each layer a control plane and management plane is defined. Figure 7. B-ISDN reference model Figure 8 illustrates the relationship between virtual channels and virtual paths. Figure 8. Relationship between virtual channel, virtual path and transmission path In this section the main characteristics of ATM protection switching and restoration mechanisms are presented, and the failure detection and signaling methods specified. The defect and failure detection and notification and the activation of recovery mechanisms in the ATM layer are realized using specific OAM (Operation, Administration, and Maintenance) cells. According to the five layers of the B-ISDN reference model, five hierarchical OAM flows F1 top F5 are defined are defined (see Table 1). OAM level Network level Network layer F5 F4 F3 F2 Fl Virtual channel level Virtual path level Transmission path level Digital section level Regenerator section level Table 1 OAM levels ATM layer Physical layer Table 2 shows the ATM OAM cells: OAM cell Type Function CC Continuity Check Failure detection

5 LB Loopback Failure localization AIS Alarm Indication Signal Failure notification RDI Remote Defect Indication Failure notification APS Automatic Protection Switching Recovery protocol Table 2 OAM cell types Figure 9 shows the temporal model to evaluate ATM protection switching performance. The model is based on a general temporal model for restoration times. CONCLUSIONS The different resilience mechanisms like protection and restoration have all their specific advantages and disadvantages. The benefit of a resilience strategy depends on the specific network scenario and how much weight a network operator puts on a specific performance metric. In this paper we present several types of protection and recovery mechanisms, especially the mechanisms of recovery ATM. REFERENCES 1. ANSI T1, "A Technical Report on Enhanced Network Survivability Performance", Committee T1 Technical Report No. 68, 2001; 2. Autenrieth, Achim, Brianza, Carlo, Clemente, Roberto, Demeester, Piet, Gryseels, Michael, Harada, Yohnosuke, Jajszczyk, Andrej, Janukowicz, D., Kalbe, Gustav, Ohta, S., Ravera, Mauro, Rhissa, A. G., Signorelli, Giulio, Van Doorselaere, Kristof, "Resilience in a multi-layer network", CSELT Technical Reports, vol. 26, no. 6, 1998; 3. Autenrieth, Achim, Differentiated Resilience in IP-Based Multilayer Transport Networks,2002; 4. Grover, W.D., Stamatelakis, D., "Cycle-oriented distributed pre-configuration: ring-like speed with mesh-like capacity for self-planning network restoration," in Proc. IEEE International Conf. Commun. (ICC '98), Atlanta, June 8-11; 5. Grover, W.D., "The Selfhealing Network, A Fast Distributed Restoration Technique for Networks Using Digital Cross-connect Machines", Proceedings of IEEE GLOBECOM '87, Tokyo, Nov. 1987; 6. ITU-T Recommendation I.731, " Types and general characteristics of ATM equipment", October 2000; 7. ITU-T Recommendation I.732, " Functional characteristics of ATM equipment", October 2000; 8. ITU-T Recommendation M.495, "Transmission restoration and transmission route diversity - terminology and general principles", November 1988; 9. ITU-T Recommendation I.630, "ATM protection switching", February 1999; 10. ITU-T Recommendation I.610, "B-SIDN operation and maintenance principles and functions", November 1995; 11. ITU-T Recommendation M.495, "Transmission restoration and transmission route diversity - terminology and general principles", November 1988.