**2. Network and node architecture**

In WDM optical networks, transparent optical cross‐connects (OXCs) are used in order to pro‐ vide efficient space and wavelength switching functions [2]. An OXC takes as input signals at multiple wavelengths and some of these wavelengths can be dropped locally, while others pass through by switching them to the appropriate output ports. For the implementation of OXCs, wavelength selective switch (WSS) technology is used for the deployment of cost‐effec‐

In transparent optical networks, where data signals remain in the optical domain until they reach their destinations, connections are vulnerable to physical layer attacks. An attack is defined as an intentional action against the ideal and secure functioning of the network. One type of attack in optical networks is high‐power jamming which can affect the signal through in‐band jamming that is the result of intra‐channel crosstalk or out‐of‐band jamming that is the result of inter‐channel crosstalk and nonlinearities [4]. This type of attack propagates through the transparent network affecting several connections, and as a consequence, the localization of this kind of attack is a difficult problem. Due to the high bit rates of optical networks and the interaction of the connections, a jamming attack can potentially cause a huge amount of information loss. Therefore, the limitation of attack propagation is a crucial consideration in optical network planning. An overview of security challenges in communica‐

Physical layer attacks in optical networks have been studied by several researchers [6–10]. In these works, the concept of attack‐aware routing and wavelength assignment (Aa‐RWA) is analyzed. Specifically, in Ref. [6], authors proposed an integer linear program (ILP) formula‐ tion and a tabu search heuristic algorithm for the routing sub‐problem in optical networks in order to minimize the effect of out‐of‐band jamming and the gain competition caused in optical fibers and optical amplifiers, respectively. In Ref. [7], authors proposed ILP formula‐ tion and heuristic algorithms for the wavelength assignment sub‐problem in optical networks in order to minimize the in‐band jamming attack caused in optical nodes. In Ref. [8], authors proposed ILP and heuristic algorithms based on simulated annealing techniques in order to minimize the in‐band and out‐of‐band jamming attacks. Moreover, in Ref. [9, 10], authors proposed a greedy randomized adaptive search procedure (GRASP) heuristic and an ILP formulation, respectively, for the placement of power equalizers in order to limit the jamming

Another important aspect in network planning that usually is not taken into account is the uncertainty of the connection requests. In most cases, the demands are considered to be known before network planning; however, in some cases, network planning must be performed for a period of time where the demand requests can only be forecasted with uncertainty. One approach to deal with demand uncertainty is by overprovisioning, essentially allocat‐ ing many resources that can satisfy any traffic demand. However, this approach requires a high cost investment (capital expenditure—capex) from the network operators [11]. More sophisticated approaches to deal with demand uncertainty are necessary in order to achieve

Stochastic programming (SP) [13] and robust optimization (RO) [14] are the main alternative techniques to deal with uncertain data both in a single period and in a multi‐period decision making process. In SP, the probability distribution functions of the underlying stochastic

tive and dynamic wavelength‐switched networks [3].

50 Optical Fiber and Wireless Communications

tion networks can be found in Ref. [5].

attack propagation in transparent optical networks.

a cost‐effective network investment strategy [12].

An optical network topology is represented by a connected graph *G* = (*V*, *E*), where *V* denotes the set of optical cross‐connects (nodes) and *E* denotes the set of (point‐to‐point) single‐fiber links (edges). Each fiber link is able to support a common set *C*= {1,2,…,*W*} of, *W,* distinct wavelengths. Source‐destination pairs are equipped with transmitter‐receiver pairs, also known as transponders (TSP), in order to transmit/receive data. Optical nodes currently deployed in optical networks are based on two architectures. The first architecture utilizes a broadcast‐and‐select (BS) configuration and the second a route‐and‐select (RS) configuration. Both of these optical node architectures consist of two stages and can remotely configure all transit traffic and only differ in the implementation of their first stage. The building compo‐ nents of these node architectures are the WSSs. A WSS can steer each optical channel present on its input port toward one of its output ports according to the desired routing choice.

BS‐based nodes (**Figure 1**) include a splitter first stage (1 × N) that implicitly provides a broad‐ cast capability toward all outputs. In a BS‐based architecture, the WSS functionality (second stage) resembles a multiplexer (it switches each individual wavelength to a certain output). Although this is a simple and popular architecture, the loss introduced by the power splitters limits its scalability and can only be utilized in network nodes with small degrees.

RS architecture nodes (**Figure 2**) on the other hand have a WSS first stage (1 × N) that provides on‐demand routing to the required output. The basic advantage of the RS‐based architecture with respect to the BS‐based architecture is that the through loss is not dependent on the degree of the node. However, it requires additional WSSs at the input stage, which makes it more costly to be implemented.

Both implementations have a WSS second stage (N × 1) that provides the selection of the wavelengths at the output fibers, allowing full switching flexibility (any wavelength from any incoming fiber can pass through or any wavelength from the add/drop terminals can be added/dropped).

In order to deal with the losses introduced by the power splitters of the BS‐based architec‐ ture and the high cost of the RS‐based architecture, a hybrid architecture can also be used (**Figure 3**). This architecture contains either splitters (1 × N) or WSSs (1 × N) at the input ports as can be seen in **Figure 3**. In essence, hybrid nodes are constructed by replacing splitters with WSSs at the input stage of the BS‐based nodes.

**Figure 1.** Broadcast‐and‐select‐based node architecture.

Multi-Period Attack-Aware Optical Network Planning under Demand Uncertainty http://dx.doi.org/10.5772/intechopen.68491 53

**Figure 2.** Route‐and‐select‐based node architecture.

BS‐based nodes (**Figure 1**) include a splitter first stage (1 × N) that implicitly provides a broad‐ cast capability toward all outputs. In a BS‐based architecture, the WSS functionality (second stage) resembles a multiplexer (it switches each individual wavelength to a certain output). Although this is a simple and popular architecture, the loss introduced by the power splitters

RS architecture nodes (**Figure 2**) on the other hand have a WSS first stage (1 × N) that provides on‐demand routing to the required output. The basic advantage of the RS‐based architecture with respect to the BS‐based architecture is that the through loss is not dependent on the degree of the node. However, it requires additional WSSs at the input stage, which makes it

Both implementations have a WSS second stage (N × 1) that provides the selection of the wavelengths at the output fibers, allowing full switching flexibility (any wavelength from any incoming fiber can pass through or any wavelength from the add/drop terminals can be

In order to deal with the losses introduced by the power splitters of the BS‐based architec‐ ture and the high cost of the RS‐based architecture, a hybrid architecture can also be used (**Figure 3**). This architecture contains either splitters (1 × N) or WSSs (1 × N) at the input ports as can be seen in **Figure 3**. In essence, hybrid nodes are constructed by replacing splitters with

limits its scalability and can only be utilized in network nodes with small degrees.

more costly to be implemented.

52 Optical Fiber and Wireless Communications

WSSs at the input stage of the BS‐based nodes.

**Figure 1.** Broadcast‐and‐select‐based node architecture.

added/dropped).

**Figure 3.** Hybrid node architecture.

Depending on the network traffic, it is envisioned that a fraction of the network nodes will be BS‐based, other nodes will be RS‐based and the rest will be hybrid nodes. The objective of the proposed algorithms of this chapter is to use hybrid nodes in order to minimize the lightpath interactions and at the same time to minimize the network cost. This means that WSSs are placed only in some of the input ports and specifically only at the locations that are neces‐ sary in order to allow only the necessary wavelengths to pass through the WSS and avoid all crosstalk interactions. Thus, by using hybrid nodes and not RS‐based nodes, we can minimize the network cost while at the same time eliminating crosstalk interactions and consequently protecting the network against jamming attacks.
