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Approaches for solving the routing subproblem in the EON fall into two main groups, namely—(i) routing without elastic characteristics, and (ii) routing with elastic characteristics. In the following, we explain these two routing approaches.

Routing Without Elastic Characteristics

This subsection focuses on different routing policies [79-84], namely—(i) fixed routing, (ii) fixed alternate routing, (iii) least congested routing, and (iv) adaptive routing, with no consideration given to the elastic characteristics of optical networks. These routing approaches are mainly intended to discover suitable routes between source-destination pairs. These algorithms are discussed below.

Fixed Routing

In fixed routing (FR) [6,80], a single fixed route is precomputed for each source- destination pair using some shortest path algorithms, such as Dijkstra’s algorithm [85]. When a connection request arrives in the network, this algorithm attempts to establish a lightpath along the predetermined fixed route. It checks whether the required slot is available on each link of the predetermined route or not. If even one link does not have the slot desired, the connection request is blocked. In the situation when more than one required slot is available, a spectrum allocation policy is used to select the best slot.

Fixed Alternate Routing

Fixed alternate routing (FAR) [6,80] is an updated version of the FR algorithm. In FAR, each node in the network maintains a routing table (that contains an ordered list of a number of fixed routes) for all other nodes. These routes are computed off-line. When a connection request with a given source-destination pair arrives, the source node attempts to establish a lightpath through each of the routes from the routing table taken in sequence, until a route with the required slot is found. If no available route with required slot is found among the list of alternate routes, the connection request is blocked. In the situation when more than one required slot is available on the selected route, a spectrum allocation policy is used to choose the best slot. Although the computation complexity of this algorithm is higher than that of FR, it provides comparatively lower blocking probability than the FR algorithm. However, this algorithm may not be able to find all possible routes between a given source-destination pair. Therefore, the performance of the FAR algorithm in terms of blocking probability is not optimum.

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