Network Survivability: End-to-End Recovery Using Local Failure Information
5.4 Case Studies
3 6
3
Fig. 5.5 An example network
In this example, the greedy approach endeavors to serve each request using the minimum number of previously unused wavelengths. However, in doing so, the greedy approach happens to choose paths with no protection sharing, harming net-work resource utilization. In contrast, though the SPH is not optimal at first, it per-forms better over the call arrivals by encouraging protection sharing.
5.4 Case Studies
The objective of this section is to present two case studies that employ and en-hance some of the previously exposed concepts. The first one analyzes how dif-ferent implementations of failure-dependent protection strategies affect the network resources that must be provisioned, and presents a new model to overcome some
Table 5.1 Resource usage for network employing the failure-dependent path protection scheme implemented by different approaches in Figure 5.5
Primary Protection path Number of taken path (protected link) unit of capacity Greedy 1-2-3-4 1-6-5-4 (1-2-3-4) 6 (no sharing) approach 6-5-3 6-2-3 (6-5-3) 10 (no sharing)
3-5 3-2-5 (3-5) 13 (no sharing) Shortest 1-2-3-4 1-6-2-3-4 (1-2) 7 (share (2-3-4))
path 1-2-5-4 (2-3)
approach 1-2-5-4 (3-4) (share (1,2)) 6-5-3 6-2-3 (6-5) 10 (share (6,2))
6-2-3 (5-3)
3-5 3-2-5 (3-5) 12 (share (2,5))
of the highlighted drawbacks. The second case study is a thorough analysis of the availability measures for connections equipped with shared and dedicated path pro-tection in heterogeneous network topologies. It highlights the limitations of path protection to achieve very high availability even with the most resource-intensive method.
5.4.1 First Case Study: Shortcut Span Protection
This section presents a case study of how different protection strategies (such as global and local-to-egress, described in Section 5.1.2) affect the total capacity that must be provisioned in a network. With the prices for fiber (i.e., capacity) drop-ping [30], capacity usage will become a less important factor for deciding which protection method should be employed, whereas complexity and speed combined with the manageability of the method are expected to be given higher priority in the decision process.
To reduce the complexity of the protection method and the restoration time, we want to investigate a variation of the local-to-egress protection method, which we have termed Shortcut Span Protection (SCSP). In SCSP, the traffic is routed from the node before the failed link directly to the endpoint, i.e., the traffic makes a shortcut.
The idea is illustrated in Figure 5.6. The SCSP method achieves the same quick recovery time as standard span-protection, but is more relaxed with reference to the protection routing and hence more efficient with reference to the needed protection capacity. This improved efficiency, though, comes at the price of a more complex protection routing problem. We compare the efficiency of SCSP with that of span protection.
Bundled local-to-egress
Fig. 5.6 Shortcut Span Protection (SCSP)
5.4.2 The Shortcut Span Protection Model
The routing in the SCSP model quickly becomes complicated, so we are forced to make a couple of simplifications:
• We will only consider single link failures
• We will only consider the relaxed case, i.e., the routing of the paths may be bifurcated, with both the ordinary flow and the protection flow.
• We will assume 100% protection, i.e., all traffic will be protected against any single edge failure.
To perform the routing of the nominal flow and the backup flow, we apply Linear Programming (LP). First we describe an LP model for SCSP and standard span pro-tection. Then we present the result of the different protecion methods and comment on them.
Given a network with N nodes, we index them with several different indexes:
i,j,k,l,q,r∈N. The indexes are used for nodes in different contexts: i,j are used to index the flow, k,l are used to index the nodes of the demand requests, and q,r are used to index the single link errors. Between the nodes there are bi-directional edges E. We will index a specific edge by its end nodes: {i j} for the non-failed edges and{qr}for the failed edges. All the flow is directed, and we hence represent each edge {i j} with two arcs (i j) and(ji). When an edge fails, both arcs (qr) and(rq)fail and the flow on these arcs will have to be rerouted. We assume that a number of communication demands are given, and the volume of the demand is D(kl). For each demand(kl)we represent the nominal flow with a variable x(kl)(i j)∈R+, which corresponds to the flow from node i to node j of the demand request from node k to node l. Whenever an edge{qr}fails, both the flows(qr)and(rq)needs to be restored by a protection flow, starting from node q and node r respectively.
The protection flow is represented by yl,(qr)(i j) , for the flow from node i to node j of the failed arc(qr)which is destined for node l. To ease the formulation of the model, we define an auxilliary variable ul,(qr) which represent the total flow with end destination node l from the failed arc (qr). We are now ready to present the complete LP model.
Shortcut span protection arc-flow LP model:
min
∑
The SCSP LP model consists of an objective function (5.2a), nominal flow con-straints (5.2b), the concon-straints (5.2c) setting the auxilliary variables, the protection flow constraints (5.2d) and capacity constraints (5.2e), and domain definitions (5.2f).The objective function (5.2a) measure the cost of the necessary capacity in the net-work. The nominal flow constraints (5.2b) ensure that the nominal flow is routed from the start node k to the termination node l. The constraint setting the auxilliary varibles (5.2c) simply sums the nominal flow across the failed arc(qr)which shares the same termination node l. The protection flow constraints (5.2d) then use the auxilliary variables to create a protection flow. Notice that if the identification of the termination node in constraint (5.2d) is changed from l to q, then standard span protection is performed.
5.4.3 Results
In order to evaluate the effectiveness of short cut protection, we test the method, by optimizing over a set of four networks for a demand of volume 1 between all pairs of nodes in the network, that is, only one way for each pair: D(qr)=1 and D(qr)=0.
Furthermore, we have set all the costs to unit values: c{i j}=1. In Table 5.2 we summarize data about the networks.
Table 5.2 Network data
Network Name Number of Nodes Number of edges Average node degree
Cost239 [3] 11 26 4.73
PanEuropean 13 21 3.23
USANetwork [8] 28 45 3.21
Italy [11] 33 68 4.12
For the test networks in Table 5.2 we used the LP formulation given by the model (5.2a)–(5.2f) to test four variants of the protection methods:
1. Shortest SCSP: Route nominal flows on shortest paths and then protect the flows