The continuous growth of consumers’ IP traffic fed by the generalization of broadband access (through digital subscriber line and fiber to the home) and the emerging rich-content high-rate and bursty applications, such as video on demand, HDTV, and cloud computing, can only be met with the abundant capacity Planning and Operating Flexible Optical Networks: Algorithmic Issues and Tools provided by optical transport networks. For the future, it is expected that the traffic will not only increase in volume traffic increase of 34 percent on average per year but will also exhibit high burstiness, resulting in large variations over time and direction. Recent research efforts on optical networks have focused on architectures that support variable spectrum connections as a way to increase spectral efficiency and reduce costs. Flexible or elastic optical networks appear as a promising technology for meeting the requirements of next generation networks that will span across both the core and metro segments, Planning and Operating Flexible Optical Networks: Algorithmic Issues and Tools and potentially also across the access, all the way to the end user. A flexible network is based on the flex-grid technology, which migrates from the fixed 100 or 50 GHz grid that traditional wavelengthdivision multiplexing, (D)WDM, networks utilize . Flex-grid has granularity of 12.5 GHz, standardized by the International Telecommunication Union and can combine the spectrum units, referred to as slots, to create wider channels on an as needed basis. Planning and Operating Flexible Optical Networks: Algorithmic Issues and Tools Flexible networks are built using bandwidth variable optical switches that are configured to create optical paths of sufficient spectrum slots. We refer to such a connection as a flexpath, a variation of the word lightpath used in standard WDM networks. Bandwidth variable switches operate in a transparent manner for transit traffic that is switched while remaining in the optical domain. Flexible networks in addition to flex-grid switches assume the use of bandwidth variable transponders . Planning and Operating Flexible Optical Networks: Algorithmic Issues and Tools Various BVT implementations exist , employing single- or multicarrier transmission schemes, and usually having some sort of digital signal processing (DSP) capabilities at the receiver but also at the transmitter side. Several transmission parameters can be controlled in a BVT, including the baud rate, the modulation format (number of bits encoded per symbol), the forward error correction (FEC) used, the spectrum slots employed, and the useful bit rate. Since transmission parameters are controllable, the term software defined optics has also recently been used, implying that optical networks, which currently rely on the slowly changing circuit switching paradigm, become more dynamic. Planning and Operating Flexible Optical Networks: Algorithmic Issues and Tools Deciding the transmission parameters is quite complicated since physical layer impairments (PLIs) such as noise, dispersion, interference, and nonlinear effects accumulate and deteriorate the quality of transmission (QoT) of the flexpaths. In particular, the QoT of a flexpath depends on its BVT transmission parameters, the guardband used from its spectrum-adjacent flexpaths, and their transmission parameters.