Spectrum Expansion/Contraction and Survivable Routing and Spectrum Assignment problems on EONs with time-varying traffic

Abstract

For serving time-varying traffic on an Elastic Optical Networks (EONs), the spectrum allocated previously for the connection request should be expanded or contracted to meet the traffic requirement. For the survivable connection, both the primary and the backup lightpaths of the connection request should be expanded/contracted at the same time. In this paper, the Survivable Routing and Spectrum Allocation (SRSA) problem and the Spectrum Expansion Contraction (SEC) problem for SRSA on EONs with time-varying traffic are studied. In this article, two protecting schemes, Dedicated Path Protection (DPP) and Shared Backup Path Protection (SBPP) are considered. For each protecting scheme, two dynamic survivable routing algorithms and the respective SEC operations are developed to solve it. In the proposed algorithms, both the weighted hop distance and spectrum fragmentation are considered jointly. These algorithms are examined through simulations to find the best parameter for each algorithm. The results show that the blocked ratio (BR) of requests can be reduced by about 3%, if the spectrum fragmentation is considered. And, using SEC operation in time-varying traffic can reduce the BR of request about 1% and speed up the computation compared to the normal release and add operation.

Introduction

Elastic Optical Networks (EONs) have been proposed to increase the flexibility of traditional Wavelength Division Multiplexing (WDM) optical networks. The spectrum of fiber in EONs can be divided into several Frequency Slots (FSs) and the necessary amount of consecutive FSs are assigned to a lightpath to support the bandwidth request of a connection. Based on the researching results in recent years [1], EONs can provide more efficient spectrum allocation than WDM networks.

In an optical network, each connection can be transmitted by a lightpath (or optical channel) which consists of a central frequency and a size. The size of the channel is determined by the requested bit-rate, the modulation technique applied, the (fixed) slice width, and the Guard Band (GB) [1]. Due to the spectrum continuity constraint, the Routing and Spectrum Assignment (RSA) problem has emerged as the essential problem for spectrum management on EONs [1]. For a given connection request, the goal of the RSA problem is to find a lightpath on the network and assign the required resources to it to minimize the objective function.

Several spectrum allocation (SA) schemes that change the bandwidth dynamically have been studied in [2], [3]. A general policy to allocate FSs to time-varying traffic was presented in [3]. For time-varying traffic demands on EONs, there are three SA schemes of different levels of elasticity [3]. For the most efficient elastic scheme studied in [2], both the assigned central frequency and the size can be subject to change by performing Spectrum Expansion/Contraction (SEC) in each time interval. Recently, several SEC schemes have been considered and the EONs enable to expand/contract the slot width of the channel [4]. Furthermore, future EONs will change the slot width according to time-varying traffic by changing the number of FSs flexibility. Din et al. [5] studied the SEC problem for the multipath routing scheme for Routing, Modulation, and Spectrum Assignment (RMSA) on EONs.

In the traditional WDM networks, network survivability has been extensively studied; several network protection techniques have been proposed [6]. Survivability problem for EONs has also been studied in recent past [7], [8], [9], [10], [11], [12]. For the link-failure problem, the protection techniques can be divided into two categories: Dedicated Path Protection (DPP) and Shared Backup Path Protection (SBPP) [6]. For the single link failure problem, DPP means that there is a dedicated backup capacity to protect primary capacity. In contrast, SBPP means that the protection capacity can be shared among multiple protection lightpaths as long as their corresponding primary lightpaths do not fail simultaneously. Because of backup resources sharing, the shared protection scheme is generally more efficient than the dedicated protection scheme [6].

In this paper, the Survivable Routing and Spectrum Allocation (SRSA) and the SEC problems for SRSA on EONs with time-varying traffic are studied. The previous version of this article has been published in [13]. For the SRSA problem, the goal is to find a link-disjoint primary and backup lightpaths to route the connection request. For a given EON and a sequence of survivable connection requests, the goal of SEC problem is to add/delete/expand/contract (primary and backup) lightpaths and assigns suitable channels to the lightpaths to meet the traffic requirement of the survivable connection request such that the performance measure can be optimized. The RSA model is considered in this paper, that is, the transparent network only with a single modulation format.

In time-varying traffic, the survivable routing problem and the SEC issue have not been considered. Moreover, and the spectrum fragmentation has not been considered together with the path routing and spectrum allocation on EONs. In this revised version, the spectrum fragmentation of the EONs was considered with the survival path-finding algorithms. Moreover, several performance measures are considered and the simulation results of the dynamic traffic are compared.

Two protecting schemes DPP and SBPP are considered in this article. When a new connection request arrives, the Survivable Path Routing Algorithm (SPRA) is performed to find a pair of link-disjoint primary and backup lightpaths. If the required bandwidth cannot be allocated, then the connection request is blocked. Otherwise, these lightpaths are established, the required FSs of lightpaths are allocated. If the connection request is an adjusted request (that is, there exists a pair of primary and backup lightpaths with the same source and destination nodes), based on the selected SEC policy, the allocated FSs of the existing lightpath are adjusted.

In this article, the elastic allocation scheme is used, that is, both the central frequency and the size of the lightpath can be adjusted (expanded or contracted). If the bandwidth variation can be accommodated, then the adjusted connection is updated. Otherwise, the SPRA is performed to route the new connection (after the old lightpaths are torn down). According to a literature analysis by the author, only the SEC problems for single path [3] and multipath [5] were studied. Based on the survey, there is no article considered the SEC problem for survivable routing.

The rest of the paper is organized as follows. First, in Section 2, the related works are presented. In Section 3, the definition and assumptions of the problem are given. In Section 4, the SPRA and SEC algorithms for the DPP scheme are described. In Section 5, the SPRA and SEC algorithms for the SBPP scheme are described. Then, in Section 6, the performance of the proposed methods is examined. The conclusion is drawn in Section 7.