**1. Introduction**

Cloud computing has emerged as a new network paradigm [1]. Built on the success of grid computing applications, cloud computing implements the idea of 'computing as a utility' in a more commercially‐oriented vision. Thus, the customer pays per use of computing facilities

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under the conditions stated in a service level agreement (SLA), having dynamic scaling of resources and transparent access to network services, unaware of the location and hardware/ software characteristics of the required resources [2]. Apart from high bandwidth, cloud computing applications require the following functionalities from the underlying physical network [1]:


Network virtualization, which extends the well‐known concepts of server and storage virtual‐ ization to networks, is envisaged as a key enabling technology for cloud computing services. As such, the benefits of running cloud applications on top of virtual networks (as opposed to on top of virtual servers alone, as usually done [4, 5]), was evidenced by several prelimi‐ nary studies on network virtualization for cloud computing. In Ref. [6], resource allocation of cloud‐based data centres services was proposed by abstracting the service requests as vir‐ tual network requests. In Ref. [7], a network virtualization platform that acts as a mediator between the cloud user requirements and the physical resources was proposed. In Ref. [8], a new network architecture based on network virtualization was proposed for cloud computing applications where the geographic location of servers is relevant. In Ref. [9], a network opera‐ tor perspective was given about the convenience of network virtualization as an enabler for cloud computing. Nowadays, the benefits of network virtualization for cloud services are well identified in terms of cost, agility, resilience and multi‐tenancy [10–12].

The underlying network over which network virtualization takes place is of fundamental importance to guarantee a good service. Arguably, the two most important requirements regarding the underlying network are the bandwidth capacity and the variety in the band‐ width granularity of connections, to allow for a high number of cloud computing applications with different bandwidth requirements. Both requirements would be naturally provided by flexible‐grid optical networks [13, 14]. By overcoming the rigid spectrum allocation of cur‐ rent fixed‐grid wavelength‐division multiplexing (WDM) networks, elastic optical networks would make better use of the band C by allocating each connection the bandwidth just required. Depending on the bit rate and the modulation format, a gain in bandwidth usage between 33 and 100% could be achieved by using flexible‐grid networks instead of a fixed one operating with a spectral width of 50 GHz [15]. Finally, flex grid would allow a wide band‐ width granularity of connections: bit rates from 10 Gbps to 1 Tbps are envisaged [13].

Given the impact that network virtualization is expected to have on the ever‐increasing cloud computing area and the potential for significant bandwidth increase and bandwidth granu‐ larity offered by flexible‐grid optical networks, in this survey, we review the efforts on net‐ work virtualization over optical flexible‐grid networks.

The remaining chapter is as follows: Section 2 reviews the fundamental concepts of network virtualization and flexible‐grid optical networks; Section 3 discusses the main challenges in the area of network virtualization over flexible‐grid optical networks; Section 4 presents a taxonomy of the proposals found in the literature to allocate virtual networks over a flexible‐ grid underlying transport network; and Section 5 concludes the chapter highlighting the open research lines in the area.
