Multi-Granularity OXC (MG-OXC)

The network considered in this thesis is composed of the MG-OXC architecture proposed in [1], shown in Fig. 1. A MG-OXC mainly comprises fiber-, waveband-, and wavelength-switching boxes. The fiber- and waveband-switching boxes on the left-hand side serve as selectors on the input fibers and wavebands while the fiber- and waveband-switching boxes on the right-hand side serve as OXCs that switch fibers and wavebands.

add drop

Fig. 1 Architecture of an MG-OXC

For the same number of input fibers and output fibers, a MG-OXC costs much less than the traditional OXC. Fig. 2 gives an example. Assume that there are ten wavelengths in a fiber and a node has two fibers coming in and going out. In Fig. 2, assume that there are ten wavelengths in a fiber, and only calculates the ports needed for each switch at the left side. In Fig. 2(a), the traditional OXC uses a 20×20 wavelength switch. However, in Fig. 2(b), the MG-OXC uses a 10×10 wavelength switch and two 4×4 fiber switches. Although cost savings can be achieved by using MG-OXCs, this characteristic reduces the throughput and the performance of the networks. For example, in Fig. 2(b), the traffic in the fiber can be accessed by de-multiplexing only one of the two fibers into wavelengths. The traffic in the other fiber must bypass this node since no redundant wavelength-switching ports left for the wavelengths in this fiber.

Lambda Switching

Box

B

Fiber Switching Box Fiber Switching Box

WB Switching Box WB Switching Box

B

B L

B L

L L

B Waveband B Waveband L Wavelength L Wavelength

De-multiplexer Multiplexer De-multiplexer Multiplexer

add

(a)

add

(b)

Fig. 2 (a) Traditional OXC without hierarchy (b) MG-OXC

In MG-OXC networks, a directional link consists of F fibers in which F1, F2, and F3 fibers are assigned as fiber-switched, waveband-switched, and wavelength-switched fibers respectively (i.e. F = F1 + F2 + F3). On each end of a tunnel, wavelength-switching ports are required so that the traffic can be grouped or de-grouped. For example, in Fig. 3, there is a tunnel between node A and node C. A lightpath from node A to node C can be established by traversing that tunnel. Note that the number of wavelength-switching ports the tunnel consumes at the two ends of the tunnel is equal to the number of the wavelengths that the tunnel carries.

Ports consumed by the

Fig. 3 MG-OXCs with two switching tier, wavelength-switching and waveband-switching

1.2 Tunnel Allocation and Protection Problems

twork costs, some problems also arise. The bundled channels in MG-OXC

aximize network perfor

Although applying MG-OXC can reduce ne

networks form the so-called waveband or fiber tunnels, in which lightpaths can not be wavelength-switched except at the ends of the tunnels. Tunnels complicate the routing and wavelength assignment (RWA) problem and should be allocated carefully to achieve higher network performance.

Additionally, the protection problem in MG-OXC networks should also be examined, since it has not been intensively studied. This work investigates problems related to MG-OXC networks, including the tunnel allocation problem and the protection problem. The remainder of this thesis is organized as follows.

Chapter 2 deals with where to allocate tunnels on the network to m

mance. We consider the following network design problem. Given fixed amount of network resources and a historical traffic matrix that the dynamic requests will follow, the objective is to determine a set of tunnels that minimize the blocking probability for the dynamic traffic requests. To solve the above problem, the

Lambda

Ports consumed by the fiber tunnel

fiber tunnel

heuristic Capacity-Balanced Static Tunnel Allocation (CB-STA) [1] has been proposed that first estimates the amount of traffic traveling through each node by routing the historical traffic matrix on the network. Then the nodes with maximal traffic going out and maximal traffic coming in are selected repeatedly for tunnel allocation. To efficiently utilize the wavelength ports and fibers, each node pair selected for tunnel allocation is required to follow a tunnel length constraint, i.e., each tunnel should be equal to an average hop distance. Since CB-STA does not consider the tunnel length constraint when picking the node pairs, only few of the selected pairs for tunnel allocation comply with the length constraint.

We propose the heuristic, Constant Length Weighted Tunnel Allocation (CLWTA) scheme that aims to improve CB-STA. Instead of finding node pairs for the tunnel allocation without considering the tunnel length constraint, CLWTA only takes node pairs whose hop distance complies with the length constraint into consideration. Only those node pairs possess the potential to be allocated tunnels.

Chapter 3 investigates the problem of single-link failure protection in the multi-fiber network with MG-OXC. We are given fixed amount of network resources and a historical traffic matrix. The objective is to minimize the blocking probability under the constraint that for each request, a working path and protection path must be found simultaneously to guarantee 100% survivability. Since the protection problem has not been intensively studied in the MG-OXC networks, the mass MG-OXC deployment is at the risk of huge data losses once a link failure occurs. This work thus aims to provide an efficient protection scheme for MG-OXC networks.

The protection problem in MG-OXC networks can be divided into two phases, tunnel allocation and finding link-disjoint lightpaths for each incoming request. To provide protection for lightpath requests, an intuitive solution is to allocate tunnels without protection consideration 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 while allocates tunnels. The lack of protection consideration while allocating tunnels complicates the finding of link-disjoint lightpaths since two different tunnels 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 the protection scheme named Tunnel Based Segment Protection (TSP) that considers the tunnel allocation with protection requirement in mind. 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 MG-OXC networks in which port resources are rare.

Chap 4 concludes the results of our works and suggests some possible future works.

Chapter 2

An Effect Scheme for Fixed-Length Tunnel Allocation in Hierarchical Cross-connect WDM Networks

2.1 Introduction

This chapter considers tunnel allocation problem in the hierarchical wavelength-division-multiplexing (WDM) optical networks with varying traffic granularity among wavelengths, wavebands, and fibers. Given fixed amount of network resources and a historical traffic matrix that the dynamic requests will follow, the objective is to determine a set of tunnels that minimize the blocking probability for the dynamic traffic requests. Capacity-Balanced Static Tunnel Allocation (CB-STA) has been proposed but has some problems when selecting node pairs for tunnel allocation. We propose a heuristic algorithm, the Constant Length Weighted Tunnel Allocation (CLWTA), which is based on an auxiliary graph used to rate the preference of tunnel allocation for each node pair to improve CB-STA. Additionally, the Port-Constraint Constant Length Weighted Tunnel Allocation (PC-CLWTA) which considers the constraint of wavelength-switching ports is proposed. Simulation is conducted to compare the performance of CB-STA, CLWTA and PC-CLWTA.

The remainder of this chapter is organized as follows. Section 2.2 introduces the background and related work. Section 2.3 then illustrates the basic concepts related to the problem and assumptions made in this chapter. Heuristics for tunnel allocation including CB-STA, CLWTA and PC-CLWTA are presented in Section 2.4. Finally, simulation results are given in Section 2.5.

2.2 Background and Related Works

This section focuses on the related works that consider multi-granularity traffic.

More flexible and cost-efficient allocation of capacity is required to satisfy the growing demand for bandwidth. A considered method that has been studied intensively is to bundle a group of consecutive wavelength channels together and switch them as a single unit on a specific route to reduce the required resources of intermediate cross-connects along the route [1-6]. In [2], merits of hierarchical or multi-granularity OXC (MG-OXC) were summarized such as small-scale modularity, reduced cross-talk, and the reduced of complexity. [3] showed that the number of ports required when grouping of consecutive lightpaths are applied to the network (excluding grouping the traffic from different source nodes to different destination nodes) can be significantly reduced, compared to a traditional OXC solution. In [4], which employs a two-stage scheme of waveband and wavelength, an integer linear programming (ILP) formulation and a heuristic are given that aim to group lightpaths with the same destination only, while in [5] both the ILP and heuristic were given to handle the more general case.

While [3]-[5] discussed aim to dimension the network resources given the set of lightpath requests to be established, in [1] a novel switching architecture, MG-OXC was proposed to minimize the blocking probability for the dynamic requests given the limited network resources. In [1], the tunnel-like passage created by the bundled wavelength channels was defined as a waveband/fiber tunnel. If any residual capacity is left in the tunnel, it can be used to accommodate future lightpath requests. On the other hand, when no lightpath is traversing the tunnel, it can be torn down to release the resources including the link and the wavelength-switching ports as well as the multiplexer and de-multiplexer along the tunnel path for the future use. To illustrate this, the network in Fig. 4 is considered. Each link is assumed to have ten wavelengths,

λ1 to λ10, and can be divided into two wavebands with λ1 to λ5 being waveband 1 and λ6 to λ10 being waveband 2. Assume that a tunnel set up already exists from B to D on waveband 1 which is traversed by a lightpath from A to D on λ1. If the next lightpath request is from B to D, the established tunnel can be utilized to reach D. On the other hand, if the original lightpath is dropped, the tunnel can be torn down and the resources dedicated for this tunnel can be released for future use.

B C

Fig. 4 A network which has a tunnel with a lightpath traversing it

2.3 Basic Assumptions and Tunnel Allocation Characteristics

This section characterizes the tunnel allocation problem. Each node is assumed to be equipped with sufficient wavelength conversion capability in the wavelength-switching layer. Therefore, a lightpath in the wavelength-switching layer can be converted into any other wavelength if necessary. The tunnels are restricted to traverse only on their shortest paths from their ingress to egress node thus increasing the efficiency network resource consumption. The tunnels are restricted to traverse only on their shortest paths from their ingress to egress node thus increasing the efficiency network resource consumption.

A tunnel can be allocated between a node pair, if there is free capacity on each link along its route. Note that for the waveband tunnel, it has to use the same waveband on each link along the route. To bring up an allocated tunnel, wavelength-switching ports are further required at the two ends of the tunnel.

Fig. 5 illustrates an example of two possible tunnel allocations if the length of A

E D

tunnels is restricted to two. Fig. 5(a) is part of the physical network. Four fibers are used for tunnel allocation, includingAB, BD, DC and CA. Fig. 5(b) and (c) show the two possible ways of tunnel allocation. The total traffic trend should be considered when deciding which tunnel set is suitable. For example, if most traffic is between node A and node D, the tunnel set in Fig. 5(b) is more suitable. If most traffic is between node B and node C, the tunnels are allocated in Fig. 5(c).

(a) (b) (c)

B B B

D

A D A

A D

C C

C

Fig. 5 (a) Physical links to allocate tunnels (b) and (c) Two possible tunnel allocation

2.4 Heuristics for Tunnel Allocation

We first briefly introduce Capacity-Balanced Static Tunnel Allocation (CB-STA) proposed in [1]. Then we present our heuristic Constant Length Weighted Tunnel Allocation (CLWTA) that aims to improve CB-STA.

Capacity-Balanced Static Tunnel Allocation (CB-STA)

CB-STA aims to allocate tunnels off-line before start serving the lightpath requests.

The process comprises three stages: (a) tunnel ingress-egress (I-E) pair selection, (b) tunnel allocation and (c) makeup process. In (a), a series of I-E pairs are selected sequentially for the tunnel allocation stage in (b). To select I-E pairs, CB-STA estimates the amount of traffic traveling through each node by routing a historical traffic matrix in the network. Then the nodes with maximal traffic going out and maximal traffic coming in are selected repeatedly for tunnel allocation. In (b),

CB-STA tries to allocate a tunnel for each I-E pair selected in (a). After (a) and (b), the makeup process (c) is performed to further utilize the remaining resources to fill the fiber- and waveband-switching layer with as many tunnels as possible.

The tunnel allocated at stage (b) is required to follow a tunnel length constraint which is set to the minimum integer that is larger than the average physical hop distance between each node pair in the network. This is because when the tunnel length is too small, although the short tunnels are flexible and easily utilized by most of the lightpaths, the wavelength-switching ports are used up easily since the wavelength-switching ports are required at the ingress and egress nodes of each tunnel.

When the tunnel length is too large, although wavelength-switching ports can be greatly saved, the tunnels may not be suitable for the requests since most of the lightpath requests are shorter than the tunnels. We observe that the I-E pairs selected in stage (a) of CB-STA does not consider the tunnel length constraint, therefore most of the tunnels are allocated at the stage (c), leaving the performance of CB-STA some space to be improved.

Constant Length Weighted Tunnel Allocation (CLWTA)

CLWTA is proposed to overcome the problem in CB-STA. CLWTA allocate tunnels off-line and is based on an auxiliary used to rate the preference of tunnel allocation for each node pair. The process comprises four stages: (a) construction of auxiliary graph, (b) weight calculation for edges in the auxiliary graph, (c) weighted auxiliary graph based tunnel allocation, and (d) makeup process.

(a) construction of auxiliary graph Let G(V, Ep) be the original topology where V denotes the set of nodes and Ep represents the set of all physical links connecting the nodes. The auxiliary graph G’(V, E’) is constructed by adding auxiliary links El between the node pairs that have their shortest physical hop length

follow the length constraint (i.e., E’ = Ep + El). The auxiliary links represent the potential tunnels that could be allocated on the network.

(a)

D B

A F

C E

(b

ig. 6 gives an example of construction of auxiliary graph where Fig. 6(a) is the original topology with the average hop dist

Network Link S ach auxiliar

)

B D

A F

C E

Fig. 6 An example of auxiliary graph

F

ance equal to two and Fig. 6(b) is the corresponding auxiliary graph, in which dashed links represent the auxiliary links.

(b) weight calculation for edges in the auxiliary graph The Weighted tate (W-NLS) [1] is applied to determine the weight of e y link in the auxiliary graph. The weight of an auxiliary link is the predicted loads for the two nodes at the ends of that link. The larger the weight of an auxiliary link, the higher priority the node pair for that link gains to be allocated tunnels.

s d

Fig. 7 An example of deriving the W-NLS for each link in the network

Fig. 7 gives an example of how the weights are derived. There are three shortest paths from node s to d. The load from s to d is assumed to be equally distributed on the three paths. The weight of each link traversed by the shortest paths is thus increased by one third of the load from s to d. The weight of all the auxiliary links can be derived by applying the above procedure for all the node pairs in the network.

(c) weighted auxiliary graph based tunnel allocation This stage applies a greedy approach to allocate a set of tunnels according to the weight derived in the previous stage. The auxiliary link in G’ with the maximum weight is first selected, and an attempt is made to allocate a fiber tunnel for this auxiliary link. If the fiber tunnel can be allocated successfully, the weight of the corresponding auxiliary link is

decreased by

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 try to allocate a waveband tunnel for this auxiliary link. If a waveband tunnel can be successfully allocated, the weight of this auxiliary

link is decreased by

If both fiber and waveband tunnels fail to be allocated, the weight of this auxiliary

link is set to 0. The above procedure is repeated until all of the weights of the auxiliary links in G’ are equal to or less than 0.

(d) makeup process This process is used to further utilize the remaining resource after stage (c). The tunnels allocated in this stage do not have to follow the length constraint.

The whole algorithm of CLWTA is summarized as follows.

Constant Length Weighted Tunnel Allocation (CLWTA)

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. Stop if the weight for each auxiliary link is smaller or equal to 0.

Step4. Try to allocate fiber tunnel 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 tunnel for this auxiliary link. Decrease the weight of this auxiliary link by δB. Go to Step 3.

In CLWTA and CB-STA, a tunnel can be allocated if free link capacity on the route between the ingress and egress of the tunnel is available. An allocated tunnel needs to be further brought up to be utilized by lightpaths. When a tunnel is brought up, wavelength-switching ports are needed so that wavelengths can be group or de-group at two ends of the tunnel. The number of wavelength-switching ports consumed at each end of the tunnel so that the tunnel can be brought up is equal to the capacity (in wavelength) of that tunnel.

We also propose another heuristic, Port Constraint- Constant Length Weighted Tunnel Allocation (PC-CLWTA) with slight modification on CLWTA. In PC-CLWTA,

after a tunnel is allocated, wavelength-switching ports at the ingress and egress nodes of the tunnel are dedicated to the tunnel. That is, a tunnel can not be allocated if any on the two ends of the tunnel has insufficient wavelength-switching ports.

PC-CLWTA improves the performance when the wavelength-switching ports is significantly fewer than the resources in the fiber-switching and waveband-switching layers. The performances of schemes described above are evaluated in the following section.

2.5 Numerical Results

The topology we use is a 16-node network show in Fig. 8. We assume that each

The topology we use is a 16-node network show in Fig. 8. We assume that each