2. An Effect Scheme for Fixed-Length Tunnel Allocation in Hierarchical
2.5 Numerical Results
The topology we use is a 16-node network show in Fig. 8. We assume that each directional link has five fibers. Each fiber contains forty wavelengths which are divided into four wavebands with wavelength 1 to 10 in the first waveband, 11 to 20 the second, …, and 31 to 40 the forth. The traffic is uniformly distributed on all node pairs and each request is for a lightpath. The inter-arrival time between two requests is determined by an poisson distribution function with rate ρ, and the request holds the resources it traverses for a time period determined by an exponential distribution function with rate 1.
Let (F1)F(F2)B(F3)L stand for the experiment with F1 fibers for fiber-switching, F2 fibers for waveband-switching, and F3 fibers for wavelength-switching for each directional link. The following three combinations of switching type are examined:
1F1B3L, 1F2B2L and 2F2B1L
1
Fig. 8 The 16-node network for this simulation
0
Fig. 9 Number of allocated tunnels except makeup process among CLWTA, CB-STA and the ideal number (1F2B2L)
Fig. 9 compares the number of allocated tunnels when CB-STA and CLWTA are used without performing their makeup process. The maximum number of allocated fiber tunnels and waveband tunnels are
D
|Ep| is the number of directional links on the topology. The number of allocated fiber/waveband tunnels without makeup process in CB-STA is considerably smaller
than the maximum number. The reason is that most of the I-E pairs selected in CB-STA do not follow the tunnel length constraint.
(a)
2F2B1L
0 0.05 0.1 0.15 0.2
450 500 550 600 650
Load
Blocking Prob.
CB-STA relaxed CB-STA WTA
(b)
1F2B2L
0 0.05 0.1 0.15
1000 1100 1200 1300 1400
Load
Blocking prob.
CB-STA relaxed CB-STA WTA
(c)
1F1B3L
0 0.05 0.1 0.15
1800 1900 2000 2100 2200
Load
Blocking Prob.
CB-STA relaxed CB-STA WTA
Fig. 10Comparison of blocking probability vs. requests for CLWTA and CB-STA on the 16-node topology
Fig. 10compares different blocking probability of the CLWTA and CB-STA under different load ρ. The relaxed CB-STA relaxes the length constraint D in CB-STA. More specifically, in relaxed CB-STA, tunnels with lengths between D-1 and D+1 are permitted to be allocated. Therefore, more useful tunnels can be allocated in relaxed CB-STA than in CB-STA. The results show that CLWTA has the lowest blocking probability in all switching type combinations. The reason is that CLWTA takes the length constraint into account when allocating tunnels while in CB-STA and relaxed CB-STA, length constraint is not carefully considered in their I-E pair selection stage.
(a)
2F2B1L
0 0.05 0.1 0.15
500 550 600 650 700
Load
Blocking prob.
WTA PC-WTA
(b)
1F2B2L
0 0.05 0.1 0.15
1300 1400 1500 1600 1700
Load
Blocking prob.
WTA PC-WTA
(c)
1F1B3L
0 0.05 0.1 0.15
1800 2000 2200 2400 2600
Load
Blocking prob.
WTA PC-WTA
Fig. 11 Comparison of blocking probability vs. requests for CLWTA and PC-CLWTA on the 16-node topology
PC-CLWTA outperforms CLWTA when each node in the MG-OXC network has only limited wavelength-switching ports (in Fig. 11(a)). That is because tunnels in PC-CLWTA are only allocated between nodes that have sufficient wavelength-switching ports. The link capacity and wavelength-switching ports are more efficiently utilized since most of them are consumed by the auxiliary links with higher weights. However, when there are sufficient wavelength-switching ports, performance of PC-CLWTA is the same as CLWTA (i.e., performance curves of the two algorithms in Fig.11 (b) and (c) overlaps).
Chapter 3
Tunnel-based Protection Schemes in Hierarchical Cross-connect WDM Networks
3.1 Introduction
In this Chapter, we investigate the protection schemes for single-link failure in hierarchical cross-connect WDM networks. Multi-granularity optical cross-connect (MG-OXC), has been proposed to support hierarchical WDM networks. It is attractive for its scalability and cost reason. However, the protection problem has not been intensively studied in MG-OXC networks, which make the mass MG-OXC deployment the risk of large data losses once a link failure occurs.
Our work in this chapter thus aims to provide efficient protection schemes for MG-OXC networks. To provide protection for lightpath requests, an intuitive solution is to allocate tunnels by CLWTA mentioned in Chapter 2 and then find two link-disjoint lightpaths from source to destination for each incoming request.
Although the intuitive heuristic provides a protection solution for the MG-OXC networks, it does not consider the protection when allocates tunnels. The lack of protection consideration while allocating tunnels complicates the finding of link-disjoint lightpaths since two different tunnels on the logical topology may actually traverse the same physical link. Thus, we have pay additional attention to the overlapping of tunnels when finding link-disjoint path pair. Therefore, we propose a protection scheme named Tunnel Based Segment Protection (TSP) that takes protection into consideration when allocating tunnels. In TSP, a protection tunnel is always allocated simultaneously with a working tunnel. The channels dedicated for protection can be shared more easily. In addition, performance of the network is
improved since working and protection tunnels use the same wavelength-switching ports in the network with MG-OXC in which port resources are rare. Simulation is conducted to compare two algorithms in MG-OXC networks. The results show that TSP works well in the hierarchical cross-connect WDM networks.
The rest of this Chapter is organized as follows. In Section 3.2, we describe the background and related works of protection. Concept of the protection schemes with MG-OXC is presented in Section 3.3. Performance evaluation between different protection schemes is given in Section 3.4.
3.2 Background and Segment Protection Schemes
This section introduces basic protection techniques in the WDM networks, including link protection and path protection, and the concept of shared protection.
Besides, segment-based protection which takes advantages of both link and path protection is also introduced. Two proposed algorithms, Short Leap Shared Protection (SLSP) [14] and Protection with Multiple Segments (PROMISE) [15] are also presented here as examples of segment-based protection.
We first examine two basic techniques, link and path protection, for survivability in the WDM networks. For path protection, if a fiber failure occurs on the working path, the end nodes of the failed link detects the fault and sends a notification signals to the source and destination of the path to activate a switchover. The source then immediately sends a wake-up packet to activate the configuration of the nodes along the protection path and after receives a confirm message from the destination, traffic are switched over from the working path to the protection path. On the other hand, link protection scheme reroutes all affected traffic over the prescheduled paths between the two ends of the failed link.
Based on whether the sharing of network resources is allowed, a protection
scheme can be further categorized as dedicated protection or shared protection. In dedicated protection, different protection paths do not share any link in the same wavelength plane. In shared protection, multiple protection paths may pass common links and share the same wavelength with each other. For example, in Fig. 12 (a) two working paths, W1 (A→B) and W2 (E→D) do not go through the same physical link.
The protection path P1 can share the same wavelength in link C-B with P2. However, if W1 and W2 go through the same physical link, shown in Fig. 12 (b), the sharing of P1 becomes impossible since the failure of link B-D would interrupt both these two working paths.
B
(a)
(b)
Fig. 12 (a) Two working paths can share the same wavelength in link C-B (b) Two working paths that have common link can not share the same protection resources
Shared protection provides one advantage over dedicated protection by offering higher network utilization. If all the lightpaths in the network need to be protected, the
A W1 D
W2 F
P1
E C
P2
B D
W1
W2 F A
C E
P1
best resource utilization is 50% in dedicated protection. But in shared protection, since the protection paths of link-disjoint working paths can share the resources with each other, network utilization can be higher than dedicated protection.
Compared to link protection, recovery in path protection may be slower since failure notification signal has to reach the source node of the lightpath before restoration is initialized. On the other hand, link protection may use more resources than path protection since in link protection a protection path has to be reserved for each link. To compromise both protection techniques, each working path can be divided into several protection domains, with each domain being protected individually. Segment-based protection can initialize restoration faster than path protection and usually require fewer resources dedicated for protection than link protection. The idea of SLSP is to divide each working path into several overlapped protection domains, each of which contains a working and protection path-pair. In, PROMISE is proposed that provides a dynamic programming based algorithm to find an optimal segmentation of the working path. In PROMISE, each division combination is examined to find out the best way to divide the working path. After the best segmentation is decided, PORMISE finds a link-disjoint protection path for each working segment.
3.3Protection Schemes on MG-OXC networks
In this section, we describe our heuristics to handle the protection problem in MG-OXC networks. We first present an intuitive heuristic, Tunnel Based Path Protection (TPP) that deals with the two sub-problems independently. Then we introduce the improved algorithm, Tunnel Based Segment Protection (TSP) that takes the protection requirement in mind while allocating tunnels.
A. Tunnel Based Path Protection (TPP)
TPP allocates tunnels in the same way as CLWTA. After allocating tunnels, we can start to serve the incoming requests. For each request, both working path and protection path should be found or the request should be blocked. Fig. 13 illustrates TPP. Two tunnels, E-F-G and A-B-C (i.e., the thick lines), are allocated on the network. For the request from A to D, the working path A-B-C-D (i.e., the bottom dashed line) is found where sub-path A-B-C is in the tunnel layer and sub-path C-D in the wavelength-switching layer. The protection path A-E-F-G-D (i.e., the top dashed line) is found where sub-paths A-E and G-D are in wavelength-switching layer and sub-path E-F-G in the tunnel layer.
Protection path
E F G
A B C D
Working path
Fig. 13 Example of path protection with MG-OXC
Note that two tunnels for different node pairs on the logical topology may actually traverse the same link on the physical topology, which may cause both tunnels disconnected simultaneously if fiber link failure occurs on that common link.
In Fig. 14 (a), the two tunnels, A-E and B-F may be used for the working and protection path of a request. But in Fig. 14 (b), these two tunnels traverse the same link C-D and may fail simultaneously if a fiber cut occurs on link C-D. Thus, in hierarchical cross-connect network, we must make sure that working and protection paths for a request is physically link-disjoint.
(a)
(b)
A E
C D
B F
A E
C D
B F
Fig. 14 (a) Two tunnels, A-E and B-F in logical topology (b) Physical route of the two tunnels
B. Tunnel Based Segment Protection (TSP)
Although TPP provides a simple protection solution, it complicates the finding of link-disjoint lightpaths since two different tunnels on the logical topology may actually traverse the same physical link. Thus, we have pay additional attention to the overlapping of tunnels when finding link-disjoint path pair.
The main difference of TSP and TPP is that TSP takes protection into consideration when allocating tunnels. TSP divides the working path into segments according its switching types along the route and each segment is protected in its corresponding switching layer. Fig. 15 illustrates the idea of our approach. In Fig. 15 (a), the working path goes though the path A-B-C-D (i.e., the top dashed line) where sub-path A-B-C is in tunnel layer and C-D is in wavelength-switching layer. A-B-C-D is then divided into two segments, A-B-C and C-D. Segment A-B-C is protected by A-E-F-C in the tunnel layer and segment C-D by C-F-D (i.e., the bottom dashed line) in the wavelength-switching layer. Fig. 15 (b) depicts the layered concept of the
protection in each layer.
(a)
C B
A D
E F
(b)
Fig. 15 (a) Division of the working path according to the switching type (b) Finding a protection path for each working segment in each switching layer
Therefore, each time we try to allocate a tunnel between a node-pair, the working and protection tunnels for this node pair are allocated simultaneously. Note that if only the working tunnel can be found, we would abandon this working tunnel since it con not be protected.
CT BT
Tunnel layer DT
AT
ET FT
CW BW
Wavelength-switching
AW DW
layer
EW FW
Tunnel Based Segment Protection (TSP)
Step1. Form the auxiliary graph by adding all possible tunnels to the physical network.
Step2. Compute weight for each possible tunnel by routing the traffic matrix on the auxiliary graph
Step3. If the weight for each auxiliary link is smaller or equal to 0, go to Step 6.
Step4. Try to allocate fiber tunnels (working & protection) for the auxiliary link with maximum weight. If successful, decrease the weight of this auxiliary link by δF and go to Step 3. Otherwise, go to Step 5.
Step5. Try to allocate waveband tunnels (working & protection) for this auxiliary link. Decrease the weight of this auxiliary link by δB. Go to Step 3.
Step6. Wait for the lightpath request. When it comes, go to Step 7.
Step7. Find a working path for the request. If successful, find the protection path for each sub-path in wavelength switching layer. If the working path or any of the protection paths for the sub-path in wavelength switching layer can not be found, block the request.
Step8. Go to Step 6.
In Step 1, we construct the auxiliary graph. In Step 2, the importance of each auxiliary link is computed. Step 1 and Step 2 are the same as the method mentioned in CLWTA. Step 3 to Step 5 allocate tunnels on the network. We pick the auxiliary link with the maximum weight and try to find a link-disjoint path pair in the fiber-switching layer. The working tunnel is found prior to the corresponding protection tunnel. If the fiber tunnel pair (working and protection) are allocated successfully, the weight of the corresponding auxiliary link is decreased
by L F D
δ , where Wi,j is the weight of the auxiliary link connecting node i and
j, L the number of directional links in the original network topology, FT the number of fibers dedicated for tunnel allocation in each directional link and D the length constraint. Otherwise, we find a link-disjoint path pair in the waveband-switching
layer for this auxiliary link. Note that whether the waveband tunnel pair can be
allocated or not, weight of this auxiliary link is decreased by
D
B is the number of wavebands in a fiber. The process repeats until the weight of all auxiliary links are smaller or equal to 0. After all tunnels are allocated, we start to serve each coming lightpath request.
Dijkstra’s shortest path algorithm is applied to find routes for each request. The cost of each channel-link used to find a working path for the request is as follows.
⎪⎪ tunnels more easily than wavelength-switching channels.
After finding working path, it is divided according to the switching type. We do not have to consider the protection path in a tunnel since a protection tunnel has been allocated during tunnel allocation stage. Thus we only have to find a protection path for each segment in wavelength-switching layer. The cost of each channel-link in finding protection path can be classified into three categories. If a channel-link is occupied and cannot be shared, its cost is set to infinite. If a channel-link has been used by other protection path and can be shared, the cost is set to zero to increase the sharing efficiency. Otherwise, the channel-link cost is assigned the same way as the cost of a working channel-link is assigned. Following is the cost function for channel-links used to find protection path.
⎪⎩
A request is satisfied only if the working path and the corresponding protection paths are found. Either the working path or a protection path for a working segment can not be found, the request is blocked.
The performance of the protection schemes, TSP and TPP, in hierarchical cross-connect WDM networks are evaluated in the next section.
3.4 Numerical Results
In this section, we evaluate the performance of the Tunnel Based Segment Protection (TSP) on the 16-node topology in Fig. 16. The intuitive protection scheme, Tunnel Based Path Protection (TPP) is also implemented to compare with TSP. We assume that each directional link has five fibers. Each fiber contains forty wavelengths which are divided into four wavebands. That is, each waveband has 10 wavelengths and the first to the tenth wavelengths are in the first waveband, the eleventh to the twenty-first wavelengths are in the second waveband, …, and the thirty-first to the fortieth wavelengths are in the forth waveband. Each node is assumed to have enough wavelength conversion capability.
(F1)F(F2)B(F3)L stands for the experiment with F1 fibers for fiber-switching, F2 fibers for waveband-switching, and F3 fibers for wavelength-switching. The traffic is uniformly distributed in this simulation and each request is for a lightpath.
1
Fig. 16 The 16-node topology for this simulation
(a)
200 250 300 350 400
Number of requests
Blocking Prob.
TSP TPP TSP-PTLC
(b)
600 700 800 900 1,000
Number of requests
900 1,000 1,100 1,200 1,300 Number of requests
Blocking Prob.
TSP TPP TSP-PTLC
Fig. 17 Simulation results of TSP and path protection with different number of lightpath requests
Fig. 17 shows the simulation results in terms of blocking probability. TSP-PTLC stands for the TSP case where puts the length constraint on the protection tunnels. The results show that TPP is outperformed by TSP in all switching type combinations.
There are two reasons for the better performance of TSP. First, the resources of channels in fibers are used more efficiently in TSP. The channels dedicated for protection paths can be shared more easily if the working paths are divided into segments since two working paths passing the same physical link are not allowed to
share the same protection resources. Secondly, the protection tunnels in TSP can use the same wavelength-switching ports with working tunnels since the ingress and egress nodes of working and protection tunnels are the same. Once a link failure occurs and affect the traffic in a tunnel, we only have to reconfigure the fiber- or waveband-switching box to switch the affected traffic to the protection tunnel while using the original switching ports on the two ends of the affected tunnel.
Wavelength-switching ports are critical resources in MG-OXC networks, so we can get better performance and accommodate more lightpath requests in TSP. The defect in TPP is that wavelength-switching ports are required for each tunnel. Fig. 18 illustrates this concept of port saving.
Ports dedicate for a tunnel
Fig. 18 MG-OXC only reconfigures the fiber-switching box to switch the traffic in working tunnel to protection tunnel
Besides, the results also show that TSP-PTLC has higher blocking probability than TSP and TPP since it is difficult to find two link-disjoint paths that both follow the length constraint for a tunnel. But when there are more link resources dedicated for tunnel allocation (Fig. 17 (a)), TSP-PTLC has better performance than TPP since more tunnels can be allocated successfully.
To verify the significance of the wavelength-switching ports, we conduct the
WB-switching box
WB-switching box Fiber-switching box
Fiber-switching box
Working tunnel Protection tunnel
Wavelength-switching box
simulation under different switching combinations in TSP. The results are shown in Fig. 19. It shows that the more the wavelength-switching ports are, the less the traffic is blocked. The wavelength-switching ports influence the performance of the networks critically, thus it make sense to derive an algorithm that can save ports. TSP
simulation under different switching combinations in TSP. The results are shown in Fig. 19. It shows that the more the wavelength-switching ports are, the less the traffic is blocked. The wavelength-switching ports influence the performance of the networks critically, thus it make sense to derive an algorithm that can save ports. TSP