Steven S. W. Lee

Link weight assignment and loop-free routing table update for link state routing protocols in energy-aware internet

In order to lessen the greenhouse effects and diminish environmental pollution, reducing energy usage is important in designing next generation networks. Shutting down the network devices that carry light load and redirecting their traffic flows to other routes is the most common way to reduce network energy consumption. Since traffic demands among node pairs vary in different time periods, an energy efficient network has to dynamically determine the optimal active links to adapt itself to network traffic changes. However, in current IP networks, shutting down and/or turning on links would trigger link state routing protocols to reconverge to a new topology. Since the convergence time would take tens of seconds, routing table inconsistencies among routers would result in network disconnection and even worse, generating traffic loops during the convergence interval. Removing routing images inconsistent among routers to prevent loops is a critical issue in energy efficient network and this issue is still not considered in the green network design yet. The contribution of the paper is presented in two parts. First, we propose a comprehensive approach to determine a network topology and a link metric for each time period. Traffic engineering is considered in our design such that flows going on the energy-aware network are within a predetermined percentage of the link capacity such that no congestion occurs in a statistical manner. Second, to avoid transient loops during time period changes, we propose a Distributed Loop-free Routing Update (DLRU) scheme to determine the correct sequence for updating the routing table. A scrupulous proof was also presented to ensure the loop-free property of the DLRU. In this paper, we formulate an integer linear programming to determine this multi-topology and link weight assignment problem. Due to its NP-hard property, we propose an efficient algorithm, termed Lagrangian Relaxation and Harmonic Series (LR&HS) heuristic. Numerical results demonstrate that the proposed LRHS approach outperforms the other approaches on several benchmark networks and random networks by providing up to 35%-50% additional energy saving in our experimental cases.

Design and Analysis of a Novel Energy Efficient Ethernet Passive Optical Network

Ethernet Passive Optical Network (EPON) has become an important access network for realizing FTTX. Due to its broadcast nature in EPON, each Optical Network Unit (ONU) needs to examine every downstream frame delivered from Optical Line Terminal (OLT) to determine whether the frame is destined to it or not. This results in a waste of energy for all ONUs in a PON system. To reduce power consumption, we propose a new MAC that turns a PON system to be a slotted one when the system is operating in light load state. By doing so, an ONU can turn off its transmitter, receiver, and even part of the MAC controller to reduce power consumption. We present a Markov model for the system. Numerical results reveal that an ONU can still have 20 Mbps bandwidth even if only 3% of its time is in working state.

A Lagrangean relaxation approach to routing and wavelength assignment for multi-granularity optical WDM networks

In this paper, we propose an efficient approximation approach, called Lagrangean relaxation with heuristics (LRH), aimed to resolve routing and wavelength assignment (RWA) for multi-granularity WDM networks facilitating fiber, waveband, and lambda switching capabilities. The task is first formulated as a combinatorial optimization problem in which the bottleneck link utilization is to be minimized. The LRH approach performs constraint relaxation and derives a lower-bound solution index according to a set of Lagrangean multipliers generated through subgradient-based iterations. In parallel, using the generated Lagrangean multipliers, the LRH approach employs a new heuristic algorithm to arrive at a near-optimal upper-bound solution. With lower and upper bounds, we delineate the performance of LRH with respect to accuracy and convergence speed under different parameter settings. We further draw comparisons between LRH and a typical linear programming (LP) approach via experiments over the widely-used NSFNET and three randomly generated networks. Numerical results demonstrate that LRH outperforms the LP approach in both accuracy and computational time complexity particularly for larger sized networks.

HOPSMAN: An Experimental Testbed System for a 10-Gb/s Optical Packet-Switched WDM Metro Ring Network

For future WDM MANs, optical packet-switching has been considered to be a promising paradigm that efficiently supports a wide range of Internet-based applications having time-varying and high bandwidth demands and stringent delay requirements. This article presents the design of an experimental testbed system for a high-performance optical packet-switched WDM metro ring network, HOPSMAN. HOPSMAN boasts three crucial features. First, it has a scalable architecture in which the number of nodes is unconstrained by the number of wavelengths. Second, HOPSMAN nodes are equipped with high-speed photonic hardware components, including fast tunable receivers and optical slot erasers, capable of performing speedy optical packet-switching operations. Third, HOPSMAN incorporates a MAC scheme that embodies efficient and dynamic bandwidth allocation, resulting in exceptional delay-throughput performance. The article presents the key hardware components by highlighting the challenging issues we faced and the solutions we proposed for the testbed implementation. Finally, to demonstrate the feasibility of HOPSMAN, the article describes the experimental setup and presents the results obtained from running a commercially available remote media player application on the system.

A 40-Gb/s OFDM PON System Based on 10-GHz EAM and 10-GHz Direct-Detection PIN

This letter demonstrates a 40-Gb/s optical double-sideband (ODSB) orthogonal frequency-division multiplexing passive optical network (OFDM-PON) system using a cost-effective 10-GHz-bandwidth electroabsorption modulator (EAM). By employing a subcarrier-adaptive modulation format and pre-emphasis, we successfully achieve 20-km EAM-based single-mode-fiber (SMF) transmission.

A Lagrangean relaxation-based approach for routing and wavelength assignment in multigranularity optical WDM networks

Optical wavelength-division multiplexed (WDM) networks often include optical cross-connects with multigranularity switching capability, such as switching on a single lambda, a waveband, or an entire fiber basis. In addition, it has been shown that routing and wavelength assignment (RWA) in an arbitrary mesh WDM network is an NP-complete problem. In this paper, we propose an efficient approximation approach, called Lagrangean relaxation with heuristics (LRH), aimed to resolve RWA in multigranularity WDM networks particularly with lambda and fiber switches. The task is first formulated as a combinatorial optimization problem in which the bottleneck link utilization is to be minimized. The LRH approach performs constraint relaxation and derives a lower-bound solution index according to a set of Lagrangean multipliers generated through subgradient-based iterations. In parallel, using the generated Lagrangean multipliers, the LRH approach employs a new heuristic algorithm to arrive at a near-optimal upper-bound solution. With lower and upper bounds, we conduct a performance study on LRH with respect to accuracy and convergence speed under different parameter settings. We further draw comparisons between LRH and an existing practical approach via experiments over randomly generated and several well-known large sized networks. Numerical results demonstrate that LRH outperforms the existing approach in both accuracy and computational time complexity, particularly for larger sized networks.

Path layout planning and software based fast failure detection in survivable OpenFlow networks

In an OpenFlow network, the controller is responsible to control and manage the whole network. Although such central control paradigm is easier to achieve efficient network resources usage, the controller becomes the bottleneck when the network size is large. In this paper, we propose using pre-provisioned paths to mitigate the burden on a controller. The basic idea is to setup some background paths in the network. When an ordinary flow coming into the network, the controller only needs to setups flow entries at the ingress and egress switches. An admitted flow follows a preplanned path toward its destination. Except the ingress and egress switches, the flow entries in the remaining switches on the path kept unchanged. Since routing paths for ordinary flows are pre-planned, the whole admission and connection setup process are simplified. Jointly considering load balancing, network survivability, fast failure recovery, and flow entry consumption are the major challenges in this problem. The contribution of this paper includes two parts. First, we propose a path preplanning scheme that takes advantage of interface specific forwarding (ISF) to resolve the problem. We formulate the path preplanning problem as an integer linear programming problem. Optimization based heuristic algorithms are proposed to obtain a near optimal solution. In the second contribution, we propose a software based failure detection and failure location identification scheme. Since some existing switches take longer time to detect a network failure, a reliable lightweight software based failure detection scheme that can fast pinpoint the failure location is needed. We have performed simulations and experiments to evaluate the performance of the proposed schemes. The experimental results indicate that the proposed software failure detection scheme generates very limited overhead on the OpenFlow controller. It can be used to achieve fast failure recovery even when the network is in heavy load state. The simulation results also show that the proposed ISF based path preplanning scheme can reduce controller load and is able to achieve 100% survivability against any single link failure with low bandwidth consumption.

A Lagrangean relaxation based near-optimal algorithm for advance lightpath reservation in WDM networks

Advance lightpath reservation is a new research topic for connecting high-speed computer servers in lambda grid applications and for dynamic lightpath provisioning in the future optical internet. In such networks, users make call requests in advance to reserve network resources for communications. The challenge of the problem comes from how to jointly determine call admission control, lightpath routing, and wavelength assignment. In this paper, we propose an efficient Lagrangean relaxation (LGR) approach to resolve advance lightpath reservation for multi-wavelength optical networks. The task is first formulated as a combinatorial optimization problem in which the revenue from accepting call requests is to be maximized. The LGR approach performs constraint relaxation and derives an upper-bound solution index according to a set of Lagrangean multipliers generated through subgradient-based iterations. In parallel, using the generated Lagrangean multipliers, the LGR approach employs a new heuristic algorithm to arrive at a near-optimal solution. By upper bounds, we assess the performance of LGR with respect to solution accuracy. We further draw comparisons between LGR and three heuristic algorithms—Greedy, First Come First Serve, and Deadline First, via experiments over the widely-used NSFNET network. Numerical results demonstrate that LGR outperforms the other three heuristic approaches in gaining more revenue by receiving more call requests.

Traffic Grooming and Delay Constrained Multicast Routing in IP over WDM Networks

In this paper, we investigate delay constrained multicast routing for supporting QoS guaranteed point to multi-point communications in IP over WDM networks. To achieve high bandwidth utilization, packets coming from different multicast connections are groomed and carried together over a single wavelength. Lightpath scheme is adopted in this paper that unicast lightpath is provisioned to support the multicast traffic in the IP network. Hop count constraint is introduced to deal with and queueing delay from traffic grooming. The challenge of the problem comes not only from considering delay constrained multicast routing but also WDM lightpath routing and wavelength assignment (RWA). We formulated the problem as an integer optimization problem in which the revenue from admitting multicast groups is to be maximized. The problem constraints include hop count constraint for end-to-end QoS requirements, tree constraint for multicast routing, IP link capacity and WDM fiber link capacity constraints, and wavelength continuity constraint. We apply Lagrangean relaxation technique to perform constraint relaxation and propose optimization-based heuristics (LGR) to tackle this problem. We draw performance comparisons between the LGR and the minimum hop (MH) heuristics. Numerical results demonstrate that LGR outperforms MH algorithm under all experimental cases.

Design and Implementation of a GPON-Based Virtual OpenFlow-Enabled SDN Switch

Passive optical networks (PON) have become a promising solution for accessing networks because of the advantages they offer, such as high efficiency, security, and cost reduction. However, network management in PON is not yet automated and needs network operator intervention. In recent years, software-defined networking (SDN) has become an emerging technology. Through the separation of control and data plane in SDN switches, SDN provides dynamically fine-grained traffic control that enhances total network controllability and manageability. In this paper, we leverage the benefits of gigabit-capable passive optical network (GPON), while enhancing its capabilities on traffic management to the same level as an SDN switch. More specifically, we abstract the underlying physical GPON into an OpenFlow-enabled virtual SDN switch. The virtual switch can be used to connect multiple sites in widespread geographic locations. Similar to a real OpenFlow switch, a GPON virtual switch can be controlled by a standard OpenFlow controller. In our design, an embedded OpenFlow agent resides in the optical line termination (OLT) of the underlying GPON. The agent communicates with the external OpenFlow controller and simultaneously uses optical network unit management and control interface inside the OLT to manage ONUs. We created a prototype system based on a commodity GPON network. In the virtual switch, we implemented all the OpenFlow functions, including packet forwarding, bandwidth metering, statistical data collection, and status reporting. The experimental results show that the GPON virtual switch can correctly perform all the functions defined in the OpenFlow 1.3 specification. Its performance on flow entry modification time, dynamic bandwidth control, and switch status monitoring are comparable to the performance of a real OpenFlow switch.