Combined Analysis of Cost and Traffic Grooming Policies for Hybrid Networks Under Dynamic Traffic Requests

TSINGHUA SCIENCE AND TECHNOLOGY ISSNll1007-0214ll01/16llpp677-684 Volume 14, Number 6, December 2009

Combined Analysis of Cost and Traffic Grooming Policies for Hybrid Networks Under Dynamic Traffic Requests* CAO Yining (Ѓ࿊ુ), Hao Buchta†, Erwin Patzak†, ZHENG Xiaoping (ᄂ໌଼)**, ZHANG Hanyi (჆‫)ྡྷۑ‬ Department of Electronic Engineering, Tsinghua University, Beijing 100084, China˗ † Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut, Berlin 10785, Germany Abstract: The benefit of a two-layer hybrid IP/MPLS (multi-protocol label switching) over a wavelength division multiplexing network has been analyzed considering both the cost and different grooming policies. A detailed cost and performance analysis of hybrid networks is done for three different grooming policies. The hybrid network cost is compared with that of an opaque network for equal traffic demand and equal blocking probability of dynamic requests of label switched paths. An algorithm is given to design optimum hybrid nodes for different grooming policies to provide the desired blocking probability for a given number of dynamic connection requests. The results show that all three applied grooming policies (IP layer first, optical layer first, and one hop first) result in lower costs of the hybrid network architecture than for the opaque network. In addition, an adaptive one hop first method is given to improve the best of the applied grooming policies, which limits grooming in heavily loaded hybrid nodes to achieve load balancing. The simulation results show that the new policy significantly reduces the overall blocking probability. Key words: hybrid networks; cost analysis; traffic grooming; dynamic traffic

Introduction In wavelength division multiplexing (WDM) meshed transport networks, a substantial part of the traffic passes through the node and is not locally dropped. If the network nodes have opaque electrical cross-connects (EXCs), all incoming wavelengths are terminated by the opto-electronic (OE) conversion, leading to high operative expenditures (OPEX) and capital expenditures (CAPEX). Insertion of hybrid nodes in a multilayer, hybrid transport network to simultaneously handle different switching granularities by enabling optical by-pass as well as electrical switching and grooming, Received: 2009-03-18; revised: 2009-06-25

* Supported in part by the National High-Tech Research and Development (863) Program of China (Nos. 2008AA01A327 and 2008AA01A329)

** To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62772670

makes more efficient use of network resources and significantly reduces the electrical switch size and node cost[1]. Most published works relating to multi-layer, hybrid networks have focused on static traffic grooming for the given network topology to minimize resource usage for the fixed traffic demand to reduce the cost[2,3]. The increased dynamic bandwidth driven by a wide range of potential applications such as TV, video-conferencing, and video-on-demand (VoD) change the network performance[4], and the well-designed static grooming policy will not be flexible enough for the dynamic traffic. Dynamic traffic grooming to reduce the blocking probability (BP) to enhance network performance has become an important research problem[5-10]. Several traffic grooming policies have been proposed such as the IP layer first (ILF), optical layer first (OLF)[6], the one hop first (OHF)[7], and path inflation

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control (PIC)[8]. PIC and OHF perform better than the other two policies for large numbers of optical-electronic-optical conversion (OEO) ports, with similar performance for low OEO ports. Guo et al.[9] proposed an effective method to reduce the number of OEO ports by dynamically merging some short lightpaths into one lightpath. However, besides traffic grooming policies, the implementation of the network nodes also strongly impacts the cost and the network performance. Thus, the design of a cost-effective hybrid network for dynamic traffic requires that the selection of the node architecture considers both the cost and the traffic grooming policy. This paper analyzes the benefit of a two-layer hybrid IP/MPLS (multi-protocol label switching) over WDM network which allows optical by-pass as well as electrical switching and grooming, by considering both the cost and the various grooming policies. A detailed cost and performance analysis of hybrid networks is given for a European network. The hybrid network cost is compared with that of an opaque network that uses an electrical based MPLS. Three different grooming policies are compared for the hybrid network, for equal traffic demands and equal blocking probabilities of dynamic label switched path requests. An algorithm is given to design the optimum hybrid node structure for various grooming policies which provides the desired blocking probability for a given number of dynamic connection requests. Since the number of electrical ports in a hybrid node strongly impacts the over-all blocking probability, a dynamic traffic grooming policy called the adaptive one hop first (A-OHF) was developed to improve the best of the applied grooming policies. This new policy limits grooming in hybrid nodes which are already heavily loaded by the add/drop traffic to reduce the overall blocking probability.

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fibres with each fibre having the same number of wavelengths (w). The network nodes were either opaque electrical or hybrid architectures as shown in Figs. 1b and 1c. The opaque node (Fig. 1b) consists of an electronic switch with all incoming wavelengths terminated using transponders/colored line cards. The hybrid node consists of a hierarchical assembly of an electronic switch and a transparent optical cross-connect (OXC), as shown in Fig. 1c. The OXC handles all the traffic coming from the other nodes with only part of the traffic going to the electronic switch via OEO ports (p). Therefore, the electronic switch might be much smaller than in the opaque case, depending on the portion of the traffic going to the electric switch and the amount of the add/drop traffic. For hybrid nodes, the optical switching is assumed to be implemented as a micro electromechanical system (MEMS)based transparent OXC.

Node Architectures and Network Model

This analysis uses a multi-layer network simulation platform based on the generalized MPLS (GMPLS) control protocol using the OPNET Modeler to evaluate the network performance. The Europe28 topology (Fig. 1a) with 28 nodes and 41 bidirectional fiber links is used in the simulations. Each link consists of two

Fig. 1 Network topology and node architecture

CAO Yining (Ѓ࿊ુ) et al.ġCombined Analysis of Cost and Traffic Grooming Policies …

1.1

Cost model for the nodes

The costs of the components in the node are normalized to the cost of a 10-Gbit/s transponder for a 1500km transmission line[11]. This normalization reduces the variability of the cost data between different sources, as this partially removes the issue of different vendor discounts offered to customers. For simplification, the total network cost only includes the costs for all the nodes, but not the link cost (e.g., the cost of the optical amplifier, dynamic gain equalizer, and dispersion compensating fibre span). All the costs for the components in the node are considered in the model, unlike other cost models for network nodes which just consider the number and cost of the transponder[4]. Since the average link length is 625 km and the maximum link length is 1500 km in this Europe28 network, this cost calculation corresponds to a specific technology for a 1500-km maximum transmission distance. For this maximum reach, the cost for one colored line card (OE/EO) is assumed to be 1.3. The cost of the dense wavelength division multiplexing (DWDM) system multiplexer/demultiplexer is capacity dependent and for a 40-channel system is assumed to be 4.5. The costs for an electrical switch (0.28/port) and MEMS-based transparent switch (0.33/port) are also considered. The number of OEO ports in the hybrid node is related to the node degree, assigned according to p=2pf Dn, where Dn is the node degree and pf is the number of OEO ports per fiber. 1.2

Network model with dynamic traffic demand

The hybrid network can be represented by a physical topology (determined by the set of physical links) and a virtual topology (determined by the set of lightpaths). In this study, the physical topology is a weighted unidirectional graph, with all the physical links assumed to have the same weight of 1, which corresponds to the physical hop distance. Each fiber carries w wavelength channels and the bandwidth of each channel is B. Each node has p OEO ports as shown in Fig. 1b. The virtual topology is a weighted directional graph, where the link between nodes i and j corresponds to a directional lightpath setup between node pair (i, j). The cost of a virtual link can be fixed to weight 1, as in existing grooming policies or as adapted here. For the dynamic traffic model, the connection request arrivals are a Poisson process with mean rate O

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and the holding times are exponentially distributed with mean 1/P . The capacity demand, c, for each request is assumed to be fixed. The source and destination nodes are randomly selected in the network and the traffic demands between each node pair are equal. The capacity of wavelength channel is B and the network has N nodes, then the normalized traffic load at each node is U O c /( P BN ) (Erlang), and the corresponding load for overall networks is L (O /P )c (bit/s). For this analysis, the capacity of a wavelength channel, B, is assumed 40 Gbit/s and capacity demand, c, for each request is 2.5 Gbit/s. For the Europe28 network with N=28, for instance with O 28 and P 90, the normalized traffic load at each node is 5.625 Erlang (157.5 Erlang for the overall network) and the corresponding overall network load is 6.3 Tbit/s. In hybrid networks, when a low-speed request (e.g., optical carrier (OC)-3, or OC-12) arrives, the network calculates the shortest path on the virtual topology and sets up an end-to-end connection through single or multiple established lightpaths to groom the traffic. The Dijkstra algorithm is used to find the shortest path on both the physical and virtual topologies. This analysis uses first-fit wavelength assignment and dynamic grooming. In an opaque network, the virtual and physical topologies are the same with each lightpath corresponding to a wavelength link in the physical layer. The lowspeed traffic is switched into optical signals at the source node by transponders/colored line cards and goes through an OEO exchange and an EXC switch in each intermediate node, until it arrives at the destination. 1.3

Dynamic traffic grooming

In a hybrid network, traffic grooming can effectively multiplex a set of low-speed connection requests onto high-capacity channels (OC-48 or OC-192) with intelligent switching at intermediate nodes. The grooming policy determines how to carry the traffic in a given situation. When a connection request arrives, the network admits it by setting up an end-to-end label switched path (LSP) on established lightpaths (LP) in the optical layer. Generally, there are four possible operations to groom the low-speed traffic: (1) Route the traffic onto an existing lightpath,

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which directly connects source s and destination d. (2) Route the traffic through multiple established lightpaths. (3) Set up a new lightpath, which directly connects s and d, and then route the traffic onto this path. (4) Set up one or more lightpaths and route the traffic through these new lightpaths and existing lightpaths. Since operation (4) requires that the IP layer knows the utilization of wavelength channels, extra link state information has to be exchanged which leads to heavy flooding of the cross-layer control, so only the first three operations are considered. ILF uses operation (1) first. If operation (1) fails, it tries operation (2), and then operation (3). OLF carries out operations (3), (1), and (2) in sequence. OHF always tries to route the traffic in a single-hop, and implements operations (1), (3), and (2) in order. Since OLF and OHF tend to set up new light paths before blocking occurs in the IP/MPLS layer, they have smaller blocking probabilities than ILF. The tradeoff is that the signaling load is heavier in OHF and especially in OLF due to more attempts to set up new lightpaths.

2 Benefit of Hybrid Network for Different Traffic Grooming Policies The opaque network uses a fixed number of wavelengths per fiber (w) to calculate the offered load for the target BP. For a reasonable comparison, the hybrid network should have an equal network performance in

Fig. 2

term of the blocking probability as the opaque network for the same load. The sizes of the hybrid nodes assume that the OEO ports are the dominant cost in the networks, so the objective is to find the minimum number of OEO ports, p, and the corresponding number of wavelength per fiber, w, for hybrid node. As shown in Fig. 2, the inputs are the target BP and the reference load achieved for the opaque network. The outputs are the number of ports per fiber, pf , and the number of wavelengths per fiber, w, in the hybrid node. Parameter pf starts from 2, with w increasing as pf increases. If pf is small, not all terminating traffic can go to the EXC, so the blocking is mainly caused by insufficient OEO ports and increasing the number of wavelength cannot reduce BP. Then, the algorithm adds another OEO port and continues searching for a suitable w until the hybrid network can carry the reference load with the target BP. In this study, the reference load is defined as the load in an opaque network with 8 OEO ports and 8 wavelengths per fiber working at 1% as the target BP. The size of the hybrid nodes (the number of OEO ports and wavelengths per fiber) is then calculated for the various traffic grooming policies. The node sizes in terms of the number of OEO ports and wavelengths per fiber and the overall network cost are shown in Fig. 3 for networks with opaque nodes and hybrid nodes. With the ILF grooming policy, the hybrid node needs 3 OEO ports per fiber to handle the traffic sent from the optical layer to the EXC with 20 wavelengths per fiber in the optical layer. Although the number of wavelengths per fiber increases in the hybrid solutions, the

Calculation of the hybrid node size

CAO Yining (Ѓ࿊ુ) et al.ġCombined Analysis of Cost and Traffic Grooming Policies …

Fig. 3 Node size (number of OEO ports, pf , and wavelengths per fiber, w) and the overall network cost

overall cost is reduced 16% through by-passing of the electrical layer. The average wavelength usage is 50% in the opaque network and 38% in the hybrid network. The wavelength utilization in the hybrid network is increased by the OLF and OHF grooming policies. Thus, the OLF and OHF policies further reduce the node size, since they set up new light paths before blocking occurs in the IP/MPLS layer, which leads to more effective use of the wavelengths. The wavelength utilizations for these policies are over 60%, even better than in the opaque network. Thus, as shown in Fig. 3, the networks need 2 OEO ports and 10 wavelengths per fiber, and the overall cost saving reaches 37%. Comparison of the control complexity of the OLF and OHF policies shows that the hybrid network with OHF control achieves the best results. Thus, the following studies mainly focus on the OHF policy but the applications are not limited to this.

3 A-OHF Traffic Grooming Policy Generally, the transit traffic grooming in each node tends to be about equal. The drawback of this uniform grooming is that hybrid nodes with a low node degree may exhibit a higher probability of blocking local add/drop traffic due to the lack of available OEO ports. Note that in this model the number of OEO ports is proportional to the number of fibers per node. Such blocking of the local add/drop traffic then leads to a high overall blocking probability. One solution would be to implement more OEO ports per fiber for low-degree nodes, which would increase the cost of the hybrid nodes. Another solution used here is to balance the loads between nodes with an adaptive grooming

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scheme which limits the groomed transit traffic over OEO ports in nodes with low node degree, leaving the port capacity to the add/drop traffic, which, thereby, guarantees high admission probability. This proposed policy prevents the transit traffic from using the electrical OEO ports in low-degree nodes if the ratio between the carried transit traffic and the local add/drop traffic is above a preset threshold, T (0 - T - 1). This method is based on an improved link cost model for the virtual topology which does not change the shortest cost path routing. The existing model generates the basic virtual topology by setting the link cost to 1 if the link has enough available capacity to admit the request; otherwise, the cost is infinite. Then, the link cost would be changed to infinite if the following conditions are met concurrently: (1) at least one end-node of this link has a low degree, (2) the lowdegree node is neither the source nor the destination of the request, and (3) the ratio of the groomed transit traffic at the low-degree node is larger than the threshold. With the cost to adjacent links set to infinity, the low-degree nodes are isolated in the virtual topology so no extra traffic would transit these nodes and pass through the electrical layer. These rules enhance the route computation constraint to improve the load balance between nodes. With this virtual link cost model, when a connection request arrives, the adaptive traffic grooming algorithm calculates the ratio of the groomed transit traffic to the add/drop traffic for nodes with degree 2 and constructs the corresponding virtual topology according to the network state. Then, the general grooming policy is used to compute the route path. Since the virtual topology generation is independent of the grooming operations, it can be combined with any existing grooming policy.

4

A-OHF Policy Performance

The performance of the A-OHF traffic grooming policy in a hybrid network can be analyzed in terms of the blocking probability and resource utilization by controlling only the transit traffic for nodes with node degree 2 in the first step. The Europe28 topology with 28 nodes and 41 bidirectional fiber links are again used for the simulations. The number of wavelengths per fiber was assumed to be 16. As the threshold, T, for the ratio between the carried transit traffic and the local

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add/drop traffic varied from 0 to 1, the overall blocking probability of the hybrid network will first decrease as the low-degree nodes are used more to groom traffic. However, increasing congestion in the low-degree nodes then causes the overall blocking probability to increase as the threshlod is further increased. For the Europe28 network, the simulation results indicate that the lowest overall blocking probability is obtained for a threshold approximately equal to 0.4; therefore, T = 0.4 was used for all the following analyses. For the following analysis, a port ratio, r, is defined as the ratio of the number of electrical OEO ports, p, to the total number of wavelengths, wf in Fig. 1c for the hybrid nodes. The port ratio may take any value between 0 and 1. A larger port ratio means more electrical OEO ports, p, in a hybrid node. Figure 4 shows the overall blocking probabilities in the hybrid network versus the traffic load for a fixed port ratio r = 0.15 using the OHF and A-OHF grooming policies. For hybrid nodes with a small number of electrical OEO ports, the A-OHF (with 0.4 T) policy has less blocking than the OHF policy for various traffic loads. For a load of 157.5 Erlang (6.3 Tbit/s) for instance, the blocking probability is reduced from 20% to 10% by the A-OHF policy. The improvement is due to the limitations of the transit traffic through the 2-degree nodes in the electrical OEO ports which increases the admissions in these nodes. The bar diagram in Fig. 4 shows that with the A-OHF policy for the 2-degree nodes, 53% less transit traffic passes through the hybrid node than for the OHF. This leads to a reduction of the average blocking probability of these nodes from 24% to 6%. In addition, this connection, which would consume excess resources with the OHF policy, is blocked by Table 1 Offered load (Tbit)

the A-OHF policy which would reduce the number of alternative ports and lightpaths for grooming. The more effective utilization of electrical OEO ports by the A-OHF policy also reduces the blocking probability in high-degree nodes.

Fig. 4 Overall blocking probabilities in the hybrid network for a port ratio r = 0.15 for the OHF and A-OHF policies

Table 1 shows the average resource consumption (number of used OEO ports and number of used wavelengths per fiber for one connection) and the resource utilization for the overall network at different loads. The A-OHF policy gives higher OEO port utilization and lower wavelength utilization than the OHF policy. In the A-OHF policy, establishing of one connection for a unit capacity request consumes less OEO ports and wavelengths, which means that A-OHF reduces the requirements for the OEO ports and wavelengths to achieve the target BP for a given traffic load. This further indicates how the A-OHF performance benefits from more effective resource utilization with a limited number of OEO ports.

Average consumed resource utilization

Average consumed resources for one connection

Utilization for overall network (%)

OEO ports used

OEO port

Wavelengths used

Wavelength

OHF

A-OHF

OHF

A-OHF

OHF

A-OHF

OHF

A-OHF

4.9

3.29

3.25

5.92

5.67

89.58

92.51

54.30

51.42

5.6

3.51

3.42

6.17

5.90

90.79

91.26

64.56

61.87

6.3

5.26

4.41

8.63

7.14

90.84

91.30

81.61

75.39

Since the port ratio, r, determines the number of electrical OEO ports in the hybrid node which strongly impacts the performance of hybrid networks, the overall network blocking probabilities are shown in Fig. 5

for r from 0.1 to 0.4 and a traffic load of 157.5 Erlang (6.3 Tbit/s). The A-OHF policy reduces the overall BP significantly when the port ratio is smaller than 0.15, where the lack of electrical ports, p, is the main reason

CAO Yining (Ѓ࿊ુ) et al.ġCombined Analysis of Cost and Traffic Grooming Policies …

for the blocking. For a port ratio of 0.15 for example (Fig. 5), compared to OHF, the blocking probability is reduced 9.9% by the A-OHF policy, which means that the network can admit 10% (0.63 Tbit/s) more traffic. With a very small number of port (e.g., a 0.1 port ratio), the efficiency of groomed traffic transit for nodes with node degree 2 is reduced because the add/drop traffic alone exceeds the electric port capacity. Moreover, with the grooming restriction in nodes with node degree 2, the high-degree nodes have to take groomed transit traffic which then exceeds these port capacities and reduces the admissions. The overall blocking probability is then not significantly improved by the A-OHF policy. With many OEO ports (port ratio higher than 0.17), enough ports make good use of the wavelength resource which sharply reduces the blocking probability. When the lack of enough wavelength becomes the bottleneck, the saving ports in the low degree nodes is not necessary.

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all need nearly the same size electrical switches, which is much smaller than the opaque switches, but they need more wavelengths per fiber. The ILF policy needs about 2.5 times more wavelengths than the opaque network, whereas the OLF and OHF policies need only 1.5 times more wavelengths. However, the hybrid networks are less expensive than the opaque networks due to the smaller number of required electrical ports and the smaller size of the electronic switches. The second part introduces the A-OHF grooming policy for the hybrid networks, which limits grooming in hybrid nodes where the add/drop traffic already heavily uses the electric ports. The analysis applies the policy to the nodes with node degree 2 because the number of electrical ports is proportional to the node degree. The A-OHF policy outperforms the OHF policy in all cases where the electrical ports limit the blocking probability, and is only slightly worse when the number of wavelengths limits the performance. Furthermore, the A-OHF policy provides higher OEO port utilization and lower wavelength utilization than the OHF, which indicates that the A-OHF benefits from more effective resource utilization with a limited number of OEO ports. References [1] Melle S, Perkins D, Villamizar C. Network cost savings from router bypass in IP over WDM core networks. In: Proceedings of Conference on Optical Fiber Communica-

Fig. 5 Overall network blocking probability for different port ratios at a load of 157.5 Erlang (6.3 Tbit/s)

tion/National Fiber Optic Engineers Conference (OFC/ NFOEC 2008). San Diego, USA, 2008. [2] Zhu K, Mukherjee B. Traffic grooming in an optical WDM

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Conclusions

This paper analyzes the cost and grooming policies for a two-layer IP/MPLS over WDM networks. The first part compares the costs of an opaque network with switching only on the packet level with hybrid networks, where only part of the traffic is switched on the packet level and part of the traffic is switched on the wavelength level. The comparison is based on an equal blocking probability for the requests of label switched paths at equal loads. Different hybrid network grooming policies are compared with respect to the size and cost of the required nodes. The detailed cost model contains all the component costs of the nodes. The three compared grooming policies, ILF, OLF, and OHF,

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Berkeley Lab and Tsinghua University to Tackle Building Energy Efficiency Berkeley Lab and Tsinghua University forged ties on Aug. 12ˈ2009 to promote the development and implementation of building energy efficiency, a move intended to reduce energy consumption and greenhouse gas emissions in the U.S and China. A memorandum of understanding (MOU) was entered into between the University of California, which manages Berkeley Lab, and Tsinghua University. The MOU was signed by Arun Majumdar, Division Director of Berkeley Lab’s Environmental Energy Technologies Division, and Jiang Yi of Tsinghua University’s Building Energy Research Center. The MOU will strengthen and coordinate the efforts of Berkeley Lab and Tsinghua University scientists as they pursue energy efficiency gains in the building sector, which holds enormous potential for slashing energy consumption and greenhouse gas emissions. “We know this MOU will lead to future work and success because we have worked together in the past,” says Mark Levine, who heads the Environmental Energy Technologies Division’s China Energy Group, which he created in 1988 to collaborate with Chinese organizations in furthering energy-efficiency policy in China. “We are working with the best research group in China on building energy efficiency, and we are one of the leading groups here in the U.S,” Levine says. “The intensification of our collaboration, in the form of this MOU, will help both nations tremendously.” “China and the U.S. lead the world in terms of energy consumption, carbon emissions, and number of buildings. Improving the energy efficiency of our buildings will not only help our two nations, but also the entire world,” adds Jiang Yi. Future collaborations facilitated by the MOU could include the development of international building energy policies and standards, energy data collection and analysis, building technology and system innovation, a comparison of energy use between Chinese and U.S. buildings, and joint planning for the development of a next-generation building energy simulation model.

(From http://news.tsinghua.edu.cn, 2009-08-17)