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: Spectrum Fragmentation Management Approaches in 1+1 Protected Elastic Optical Networks

Elastic optical networks (EONs) carry highly reliable traffic and failure of any component or any interruption of traffic flow in network causes massive loss of data and revenue. Therefore, the survivability against the failure has become a crucial requirement for EONs. 1 + 1 protection is considered as one of the most reliable data transfer techniques in survival EONs, where suppressing spectrum fragmentation is always challenging. This chapter presents and analyzes defragmentation schemes in 1 + 1 path protected EONs to suppress the call blocking in the network.


Several techniques, namely restoration [144], p-Cycles [209], and protection [210], have been considered for survivability purposes in EONs [136,211]. As the backup resources are reserved by a protection technique prior to fault occurrence, it assures a prompter recovery than other techniques do. Thus, to design a faster recovery system, the protection technique is more preferable.

The protection techniques are typically classified into shared and dedicated protection techniques. Shared protection techniques enhance the resource utiliza?tion efficiency but they cannot cover multiple link failures completely. To provide instantaneous recovery and support reliability against multiple link failures, 1 + 1 protection techniques are considered in survival EONs, where suppressing bandwidth fragmentation to enhance spectrum utilization is always challenging. Chapter 8 has already confirmed that the performance of defragmentation approaches in terms of suppressing fragmentation and call blocking is better than non-defragmentation approaches. Therefore, a defragmentation approach [212,213] is considered here to handle fragmentation problems in 1 + 1 protected EONs, which is explained below.

Demonstration of Defragmentation Scheme Using Path Exchanging

The presented defragmentation scheme is intended to offer increased traffic load in resilient EONs. For network resiliency, a 1 + 1 path protection scenario is considered to offer protection against link failures. With 1 + 1 path protection, each established signal is duplicated and both signals are transmitted to the destination through disjoint paths. This allows the receiver to select the incoming data from any of the two signals. Thus, if one path suffers link failure or is disconnected, the data reception is continued through the other path.

The presented scheme considers that both paths of the 1 + 1 path protection can be alternately primary and backup paths. In order to perform spectrum de- fragmentation on primary paths, we simultaneously toggle them and their respective protection paths from primary to backup paths and from backup to primary paths respectively. Toggling a primary path to become a backup path changes its function from being the primary path through which the data is transmitted to become the backup path on standby. Thus, we exchange the function of the primary path to its backup path and vice versa. We allow backup paths on standby to be reallocated, for defragmentation, while the data is being transmitted through the primary path. We suppose that the period of release during the reallocation process is short enough to guarantee the 1 + 1 protection at almost every moment.

The presented scheme is able to achieve hitless defragmentation on 1 + 1 path protected networks without requiring any additional equipment. We take advantage of the availability of a by default alternate signal to reallocate lightpaths considering spectrum fragmentation. Since the data is being received through the primary paths, we can afford to reallocate the signals of backup paths during the defragmentation process without disrupting the data transmission. The defragmentation is performed without traffic disruption, provided that no failure occurs on a primary path while its corresponding backup path is being reallocated.

In terms of eliminating spectrum fragmentation, the advantage of the presented scheme is to be able to reallocate both paths of the 1 + 1 protection for hitless defragmentation without restriction. With the designated primary and backup paths where data from backup paths are used only if there is some impediment on the corresponding primary paths, only backup paths can be reallocated in a hitless defragmentation [214]. The ability to reallocate both primary and backup paths permits a flexible defragmentation that can be performed thoroughly.

Figure 9.1 illustrates the principle of the presented defragmentation scheme with the function of exchanging the primary and backup paths in 1 + 1 protection network. Consider the network ABCD with four active signals 51 — 54. The network and its corresponding spectrum before proceeding to any defragmentation is presented in Fig. 9.1(a). In the illustration examples, the links are considered bidirectional for simplicity. The primary and backup paths of each signal are respectively represented by solid lines and dotted lines, and their corresponding spectrum by plain boxes and hatched boxes. On link AB, 53 and 54 are primary- path signals and 52 is a backup-path signal.

Figure 9.1(b) presents the network when the designated primary and backup paths with spectrum retuning is used. After moving backup path 52, primary paths 53 and 54 are retuned. Then, the backup paths are reallocated using the first fit allocation. We can see that, in this particular example, spectrum retuning does not improve the spectrum fragmentation due to the end-of-line situation preventing 54 from being retuned over 53.

Figures 9.1(b) and 9.1(d) show the defragmentation process with the primary and backup paths exchanging. In Fig. 9.1(c), 54 signal through link AC, which is in the backup state is reallocated without path exchanging operation. Then, in Fig. 9.1(d), 54 through link AB is toggled to the backup state while its corresponding backup path on the 1 + 1 protection through links AC and BC becomes the primary path (see the network). While it is in the backup state, the light- path 54 is reallocated to remove the spectrum fragmentation on link AB (see the spectrum).

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