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Survivability of optical networks is one of the most significant features of optical/wireless communications. Two important problems of survivability can be defined in this context - the first one referring to the simulation of the total survivability of the optical networks with Fiber Span Layout Demand Distribution in the Physical Layer Module and the second one dealing with the Fiber User Service Survivability of the optical networks. The present chapter describes the concept of survivability of optical networks, with particular reference to the Physical Layer Module.
The contribution of Song Ho-Wu et.al. Group [67&68] form the strong foundations for the establishment of technologies and strategies related to survivable fiber optic network architecture. Starting from the Physical Layer characteristics, Song Ho-Wu et.al.  discussed the survivability parameters of the optical communication networks, identifying the constitution of a physical layer and expressing the Physical Layer in terms of different sublayers. They surmised that the existence of each sublayer assures certain service to its next higher layer. Further, some of these sublayer networks provide the required service and operate directly above the physical layer, providing their own survivability through adaptive routing. The configuration of Hubbing Span Architecture for the Physical Layer survivability problems has been suggested by Song Ho-Wu et.al. group  for network survivability problems. However, this Hubbing Span architecture case study considered only simple network configurations with 1x5 nodes in an isolated condition, on a direct span with one link connectivity. Further, the hubbing span architecture suffers from the limitation that it is not applicable for complex networks and it is also not compatible with multi span layout configurations.
It would therefore be of interest to propose a new architecture that can overcome the limitation of Hubbing Span Architecture (HSA) and can also take care of NXN node connectivity. The present discussions propose the configuration of Point-to-Point Span Architecture with multiple layers. The related simulations emphatically account for the applicability of Point-to-Point Span Architecture complex network and N X N node connectivity.
This type of architecture is presented for the first time in the literature, and is very significant from the points of view of routing optimization, restoration mechanism and the integration of both services . A new algorithm, designated as FLSP [Fiber Least Shortest Path] has been proposed and implemented to estimate the shortest path of the Direct Span and the same has been successfully extended for the evaluation of shortest path of Indirect Span  in Point-to-Point Span Architecture in Physical Layer Module [PLM].
The Traffic Pattern Connectivity has been investigated for Digital Signal Level 1 (DS1) and Digital Signal Level 3 (DS3). Traffic distances have been computed for DS1/DS3 signals at bit rates of 1.544 Mbps/44.736 Mbps respectively. The introduction of this Point-to-Point Span architecture enables the fruitful implementation of Digital Cross connectivity System Factor (DCSF) for evaluation of the network survivability parameters in Fiber Span Layout Demand distribution environment.
2.2 PHYSICAL LAYER TOPOLOGY CONSIDERATIONS
The Physical Layer is sometimes called Layer 0. This layer consists of physical resources such as cables, cable ducts, and underground vaults and so on. In this layer, survivability considerations are primarily aimed at physical protection of signal-bearing assets, ensuring that its topology has a basic spatial diversity so as to enable higher layer survivability techniques.
When a cable is severed, higher layers can only restore the affected carrier signals by rerouting over physically diverse surviving systems. Physically disjoint routes must therefore exist in the basic layer. Technically, the physical route structure must provide either connectedness or disconnectedness over the nodes. In a biconnected network, there are at least two fully disjoint paths between each node pair. Inorder to enhance the physical layer diversity, Fiber Span Layout Demand Distribution is effectively used.
2.3 FIBER SPAN LAYOUT AND ITS DEMAND DISTRIBUTION
The present work on fiber network is designed and shown in the Fig.2.1 as a global network to estimate the parameters like connectivity pattern and traffic in each link with respect to different demand distributions.
2.3.1 FLSP [Fiber Least Shortest Path] ALGORITHM
The Fiber Least Shortest Path determines the effective measurement of Fiber Span Layout Demand Distribution in Physical Layer Module.
It consists of Central Offices, Hubs and Spans.
It uses the Non-Centralized Span Layout.
Point-to-Point span system is measured.
Span path i.e. source to destination path is measured with DS3 requirements.
Digital Cross connectivity Factor of Fiber span layout Demand Distribution is achieved.
The fiber network design concepts and methods are shown in the flow chart Fig.2.2 and the program is given in Appendix-A, Survivability in the fiber demand distribution is measured and is shown in tables 2.1 and 2.2 in which The three level hubbing architecture is used, viz., Central Office (CO), hubs, and gateways [47&49]. A gateway is also a hub. Gateways are fully connected to each other by fiber spans [2&3]. A group of COs served by the same hub forms a cluster and a group of clusters served by the same gateway hub forms a sector and is depicted in Fig.2.3.
A fiber-hubbed network can be represented by a span layout, which consists of a set of fiber spans. Two types of fiber spans are generally used in fiber network design, the point-to-point span/single hop architecture and the hubbing span. A point-to-point span terminates at two nodes belonging to the same facility hierarchy. An example of the point-to-point span is a fiber span that terminates at two non-hubs through CO. A hubbing span is any span that is not a point-to-point span. An example of hubbing span is a span that terminates at a CO and its hub.
A hubbing span always terminates at DCS of the hub and a point-to-point span bypasses the DCS at the hub. A span layout is called a centralized span layout if all spans are hubbing spans, whereas it is called a non-centralized span layout if its spans include at least one point-to-point span [51&53].
Survivability setup in an optical network is by provisioning both a primary path and a backup path for each connection. The data is typically transmitted only on the primary path during normal network operation and switched to the backup path when failure occurs. Typically, routing for networks is based on pre-computed static routes . The benefit of dynamic route computation is based on optical networking specific link information. This problem makes an enhancement of dynamic route computation mechanism. The routing scheme for primary paths is termed as conversion free primary routing
(CFPR). The objective of CFRR is to eliminate conversion delay, possible signal degradation and also to reduce the number of converters needed in the network.
Different survivability approaches are broadly classified as static or dynamic routing schemes. In the static routing scheme, the route between each node pair is fixed and when a request arrives, the specific routing length is determined by some routing reservation protocol. In the dynamic routing scheme, such as Digital Cross connectivity Factor (DCSF) is estimated by number of links - link sequence / maximum number of links of different networks in the Fiber Span Layout and its Demand Distribution.
2.4 NUMERICAL RESULTS
Numerical results have been evaluated for 1 X 5, 3 X 3, 5 X 5 and
9 X 9 node connectivites are through Point - to - Point Span Architecture and as shown in the tables 2.3[a] Traffic requirements between Node Pairs 1 X 5 PPSA., 2.3[b] Determination of Digital Cross connectivity Pattern 1 X 5, 2.3[c] DCS - Fiber Span Layout Distribution 1 X 5, 2.4[a] Traffic requirements between Node Pairs 3 X 3 PPSA, 2.4[b] Determination of Digital Cross connectivity Pattern 3 X 3 PPSA, 2.4[c] DCS - Fiber Span Layout Distribution 3 X 3, Traffic requirements between Node Pairs 1 X 5 PPSA. 2.5[a] Traffic requirements between Node Pairs 5 X 5 PPSA, 2.5[b] Determination of Digital Cross connectivity Pattern 5 X 5 PPSA, 2.5[c] DCS - Fiber Span Layout Distribution 5 X 5, 2.6[a] Traffic requirements between Node Pairs 9 X 9 PPSA, 2.6[b] Determination of Digital Cross connectivity Pattern 9 X 9 PPSA, 2.6[c] DCS - Fiber Span Layout Distribution 9 X 9 are presented. It is obvious that this method can be easily extended to N X N node connectivity. The theoretical value of DCS factor is 100%, with 9 X 9 node connectivity and better DCS factor was achieved as compared to the previous results.
In the previous work Song Ho-Wu et.al. Group worked the 1 X 5 node connectivity by using Hubbing Span Architecture as shown in Fig.2.4 and tables 2.7, 2.8 and 2.9. The Digital Cross connectivity Factor was very less and as well as they are unable to extend their work with multiconnectivity network architectures.
The DCS Factor by this method for 1 X 5 node connectivity is 88.8%, where as it is 20% in the work reported earlier.
The Hubbing Span Architecture Proposed by Song-Ho-Wu group examined the fiber network from a single user point of view. It established the node-to-node connectivity for a simple network. The survivability of the network in a given Hubbing span Architecture is poor and it cannot be used from a multi user point of view.
The new point - to - Point Span Architecture proposed, overcomes the limitations of Hubbing Span Architecture. This architecture assures satisfactory survivability parameter estimations for multi-node-to-node connectivity of complex networks.
The Fiber Span Layout Demand Distribution provides the computing paths with restoration mechanism, from network survivability point of view. An effective FLSP algorithm has been proposed for the centralized/non-centralized span layout to compute DCSF in FDDB algorithm.
Introduction of the new Point - to - Point Span Architecture is thus justified, and the implementation with FLSP algorithm provided the necessary proof and accountability to assure normal operation mode in complex networks with multiple connectivities, with enhancement link estimates and performance characteristics.
Survivability parameters of Fiber Span Layout Demand Distribution in the Physical Layer Module have been estimated in this chapter, and the procedure is extended to compute the Fiber Network User Service Survivability (FNUSS) parameters of the optical networks as described in the next chapter.