Extension of segment protection for bandwidth efficiency and differentiated quality of protection in optical/MPLS networks

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Abstract

This paper investigates the problem of dynamic survivable lightpath provisioning against single-node/link failures in optical mesh networks employing wavelength-division multiplexing (WDM).

We unify various forms of segment protection into generalized segment protection (GSP). In GSP, the working path of a lightpath is divided into multiple overlapping working segments, each of which is protected by a node-/link-disjoint backup segment. We design an efficient heuristic which, upon the arrival of a lightpath request, dynamically divides a judiciously selected working path into multiple overlapping working segments and computes a backup segment for each working segment while accommodating backup sharing. Compared to the widely considered shared-path protection scheme, GSP achieves much lower blocking probability and shorter protection-switching time for a small sacrifice in control and management overhead.

On the basis of generalized segment protection, we present a new approach to provisioning lightpath requests according to their differentiated quality-of-protection (QoP) requirements. We focus on one of the most important QoP parameters—namely, protection-switching time—since lightpath requests may have differentiated protection-switching-time requirements. For example, lightpaths carrying voice traffic may require 50 ms protection-switching time while lightpaths carrying data traffic may have a wide range of protection-switching-time requirements. Numerical results show that our approach achieves significant performance gain which leads to a remarkable reduction in blocking probability.

While our focus is on the optical WDM network, the basic ideas of our approaches can be applied to multi-protocol label switching (MPLS) networks with appropriate adjustments, e.g., differentiated bandwidth granularities.

Introduction

In a wavelength-routed optical network, the failure of a network element (e.g., fiber, crossconnect) can cause the failure of several lightpaths, thereby leading to large data and revenue loss. Protection—a proactive procedure in which spare capacity is reserved during lightpath set-up [4], [6], [20], [21], [22], [28]—is essential for recovering from such failures in a short time period, e.g. 50 ms. Protection schemes can be classified according to the type of routing used (link based versus path based) and the type of resource sharing (dedicated versus shared). A path carrying traffic during normal operation is known as a working path.1 When a working path fails, the lightpath is rerouted over a backup path. High bandwidth efficiency and short protection-switching time are two of the most important and desirable features of a protection scheme [14], where the protection-switching time for a lightpath is the time duration the network takes to properly signal/configure the nodes along the backup path before switching traffic to the backup path after a failure occurs on the working path [28].

We consider the problem of dynamic survivable lightpath provisioning against single-node (crossconnect) and single-link (fiber) failures. Specifically, we focus on shared protection (because of its desirable resource efficiency) with the assumptions that existing lightpaths cannot be disturbed and no knowledge of future arrivals is available at the time of provisioning the current lightpath request. While we consider full wavelength-convertible networks here, the extension to the wavelength-continuous case is straightforward.

Much work has been conducted on dynamic shared protection [8], [23], [25], [34] in optical WDM networks and on dynamic routing of restorable bandwidth-guaranteed connections in MPLS networks [11], [12], [13], [26]. A widely considered approach—shared-path protection [28]—is bandwidth efficient due to backup sharing. Consequently, how to increase backup sharing based on different cost models and route-computation techniques is of particular interest and has been reported in [3], [12], [16], [19], [30], [31], [33]. The complexity of shared-path protection is high as it is NP-complete to find a working path and a backup path for a new lightpath request when backup sharing with existing backup paths is allowed [5], [25]. As a result, practical heuristics are usually employed.

One possible limitation of shared-path protection is that backup paths may sometimes become longer due to backup sharing [3]. Consequently, protection-switching time may increase because of longer backup paths. The relation between backup sharing and backup-path hop distance for path protection has been shown to be that one trades off another in [3], [34].

Furthermore, lightpath requests may have differentiated protection-switching-time requirements. For example, lightpaths carrying voice traffic may require 50 ms protection-switching time while lightpaths carrying data traffic may have a wide range of protection-switching-time requirements. Due to the pathwise node-/link-disjoint nature of path protection, shared-path protection may not provision lightpath requests according to their protection-switching-time requirements effectively in practical-sized networks [17], [24]. Clearly, proper mechanisms are needed to provision such lightpath requests in a resource-efficient manner.

Motivated by the above considerations, we first unify various forms of segment protection into generalized segment protection (GSP) and propose an effective heuristic in Section 2. Then, on the basis of GSP, we present a new and effective approach to provisioning lightpath requests according to their protection-switching-time requirements while taking into account backup sharing in Section 3.

While our focus is on the optical WDM network, in which the bandwidth requirement of a lightpath request is one wavelength, our approaches can also be directly applied to MPLS networks for provisioning restorable, bandwidth-guaranteed connections of differentiated bandwidth granularities with appropriate adjustments.

Section snippets

Generalized segment protection

Below, we unify various forms of segment protection into generalized segment protection (GSP) in Section 2.1, design an efficient heuristic in Section 2.2, and demonstrate the effectiveness of GSP in Section 2.3.

Providing differentiated quality of protection (QoP) based on generalized segment protection

GSP can be employed for provisioning differentiated quality of protection (QoP). Here, we focus on one of the most important QoP parameters, namely protection-switching time. The protection-switching time of a shared-path protected lightpath can be calculated from the hop count of the working/backup paths, as shown in [2], [24], [28]. Therefore, we consider QoP in terms of hop count.

Below, we first argue why new mechanisms are needed to provision differentiated QoP in Section 3.1; in Section 3.2

Conclusion

This paper considered the problem of dynamic survivable lightpath provisioning against single-node/link failures in optical WDM mesh networks. We unified various forms of segment protection into generalized segment protection (GSP). We designed an efficient heuristic which, upon the arrival of a lightpath request, dynamically divides a judiciously selected working path into multiple overlapped working segments and computes a backup segment for each working segment while accommodating backup

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    This work was supported in part by NSF Grant No. ANI-98-05285. Part of this work was presented at the IFIP Networking 2004 conference, Athens, May 2004.

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