Split spectrum-enabled route and spectrum assignment in elastic optical networks

https://doi.org/10.1016/j.osn.2014.04.002Get rights and content

Abstract

The Split Spectrum Approach (SSA) in elastic optical networks is based on splitting a demand into smaller sub-signals when a blocking situation arises, either due to the lack of spectral resources or transmission reach reasons. In this paper, we propose a mechanism based on a Mixed Integer Linear Programming (MILP) formulation for efficiently serving incoming demands in SSA-based elastic optical networks. Moreover, we also present a lightweight heuristic mechanism for scenarios where the MILP mechanism may suffer from scalability issues. The benefits of the proposal are highlighted through illustrative results. Furthermore, we compare two SSA implementations available in the literature in terms of relative transponder cost.

Introduction

In dynamic elastic optical networks [1], [2], [3], spectral resources are assigned to incoming demands as a set of contiguous Frequency Slots (FSs), tightly adjusted to their bandwidth needs. However, the random connection arrival and departure process over time and the heterogeneous bandwidth requirements of the demands can lead to high fragmentation of the available optical fiber spectrum [4], [5]. This spectrum fragmentation phenomenon arises as a main limitation to successfully serve new incoming demands, since available FSs become scattered along the spectrum in multiple spectral gaps, which complicates the spectrum contiguity constraint to be met.

The recently proposed SSA [6], [7], [8], [9], [10] becomes a promising solution to mitigate the pernicious effects of the aforementioned spectrum fragmentation. The rationale behind SSA is as follows: if a demand cannot be allocated due to the lack of enough contiguous FSs in the candidate paths from source to destination, it can be split into multiple lower data-rate sub-signals, which require a smaller number of contiguous FSs and can better fit in the available spectral gaps. As a result, demands that would otherwise be blocked due to the lack of contiguous FSs can eventually be served. Moreover, SSA can be used to avoid demand blocking due to transmission reach reasons, since lower data-rate sub-signals generally support longer transmission distances.

Looking at the literature, SSA can be implemented using multi-flow transponders (referred as MF-TSPs), which allow sending multiple signal flows using a unique device (e.g., see [8], [9]), or using multiple bandwidth variable transponders (i.e., BV-TSPs), one per sub-signal after the splitting procedure [10]. Both solutions can apply various strategies to route demand sub-signals (hereafter simply referred as parts) [11], [12], [13]: (a) single path strategy, where all parts are routed over the same physical path; (b) multipath strategy, where parts can be routed over different paths. In this paper we follow the single path strategy. Reasons behind this are (a) the avoidance of delay variations among parts, which can lead to packet reordering problems while requiring additional hardware (buffers) at the receiver side, and (b) the reduction of the control plane burden associated to the setup, management and release of the lightpaths supporting the parts.

Although the benefits of SSA have already been highlighted [8], [10], no attempts have been made so far (to the best of our knowledge) on addressing the optimal Route and Spectrum Assignment (RSA) in SSA-based dynamic elastic optical networks. Hence, with such a scenario in mind, we present a novel MILP-based mechanism that finds the RSA for incoming demands, minimizing the number of parts into which a demand is split, jointly with the undesired spectrum fragmentation. Using the proposed mechanism, we illustrate the benefits of SSA against conventional elastic optical network scenarios where no demand splitting is enabled. Additionally, we present a lightweight heuristic mechanism capable of producing good solutions in a short time for network scenarios where the scalability of the aforementioned MILP-based mechanism becomes challenged. Furthermore, we also compare CAPEX associated to SSA implementation solutions based on MF-TSPs or BV-TSPs as a function of the relative MF-TSP cost against that of a BV-TSP.

The rest of the paper is structured as follows: Section 2 defines the scenario that we are targeting along with the main assumptions, elaborates on the approach followed to address the RSA problem in a SSA-enabled dynamic elastic optical network and describes the details of the presented mechanisms. Section 3 evaluates the performance of the presented mechanisms through illustrative results and discusses about the mentioned cost analysis. Finally, Section 4 draws up the main conclusions regarding the presented work.

Section snippets

Efficient RSA with SSA

Without loss of generality, we assume that the data-rate requested by a demand can be translated into a specific bandwidth B in GHz, given the modulation format used to successfully reach the destination. Ideally, the number of FSs needed to serve a demand should be the ceiling of B divided by the width of a single FS, denoted as Fw. However, Bandwidth Variable Wavelength Cross Connects (BV-WXCs) require guard bands between demands to properly operate [14]. Let G be the guard band size in GHz,

Results and discussion

In this section we evaluate the performance of the SSRSA mechanism. In order to quantify the benefits of optimally splitting demands, we benchmark it to a scenario where no splitting is performed (hereafter referred as 1-SSRSA), forcing Mmax=1 and α=0 in the formulation. Two network scenarios have been considered, namely, the EON16 (16 nodes, 23 links) [16] and DT (14 nodes, 23 links) [17] network topologies. Besides, two situations with 160 and 320 FSs per fiber link have been assumed, with Fw

Conclusions

In this paper, we proposed a mechanism based on a formal MILP formulation for optimal SSA operation in dynamic elastic optical networks. Using this mechanism, we highlight the benefits of applying SSA, which can decrease the blocked bandwidth by one order of magnitude when compared against a standard RSA mechanism. We additionally presented a heuristic mechanism capable of producing good results in a very short time for network scenarios where scalability becomes an issue to be taken care of.

Acknowledgments

This work has been supported by the Government of Catalonia and the European Social Fund through a FI-AGAUR research scholarship grant and by the Spanish National project ELASTIC (TEC2011-27310).

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