1. The Maryland Optics Group Multi-Hop View: Interfaces not available between (s, d): Try to create multi-hop path. Link Selection: Local Optimization: Select Link that causes minimum increase in congestion in current cluster. Time Complexity O(N 2 ) Performance: Better than Single-Hop heuristic. Terrestrial Optical Links + Peer-to-Peer and Base-Station RF “subnets” Airborne Free Space Optical Backbone Hybrid Battle-Space Networks Near-Surface, Surface and Sub-Surface user Systems Jaime Llorca, Aniket Desai, *Stuart Milner, and Christopher C. Davis Department of Electrical and Computer Engineering, *Department of Civil and Environmental Engineering Algorithms for Dynamic Topology Optimization in Free Space Optical Networks Free space optical networks are emerging as a cost-effective, rapidly deployable and portable technology, which provides secure, extremely high data rate communication. Civil Sector –Last mile problem –High quality video-based surveillance Military Sector –Dynamic Backbone: secure, high data rate communication Dynamic Nature –Atmospheric obscuration –Node mobility We present algorithms and heuristics for topology reconfiguration as a means to dynamically optimize network performance. Link State Examination Collection of Link State Information Distribution of Topology Solution Solution Computation Deployment/Reconfiguration The Autonomous Reconfiguration Process Minimizing Atmospheric ObscurationMinimizing Network Congestion Topology control involves a number of interacting processes that provide the network with autonomous reconfiguration capability. Local topology information needs to be dynamically gathered at a central node, Global network information is then processed, an optimized topology is computed, and the new network configuration is disseminated. In most cases, finding a topology which optimizes a given physical or network layer metric (obscuration, congestion, delay, etc) is an NP-complete problem. Heuristics have been developed to achieve near-optimal scalable solutions. Key Idea: Arrange a network to evenly distribute the traffic, thus avoiding congestion or delays. Traditional Approach: Routing (TCP/IP). The topology is given. If a path is broken, nothing can be done. Assumptions: –Traffic matrix is known. –Every node has 2 transceivers. Ring topologies Objective: Minimize congestion. Congestion is defined as the traffic-rate on the maximally loaded link. The obscuration factor is a measure of the attenuation of the optical signal as it travels through the atmosphere, caused by the presence of clouds, fog, haze, rain, snow, etc. A cost measure is assigned to every possible link in the network as a function of the obscuration factor. The objective is to find a topology with minimum aggregate network cost. We require the network to be bi-connected, which assures the presence of at least one path between any pair of nodes, even after a single link break. Every node has 2 optical transceivers. Spanning Ring: MST based heuristic with degree constraint. Reconfigurable Spanning Ring: additional processing provides more flexibility to find feasible topologies in adverse conditions. Every node has 3 optical transceivers. 3-MST + Reconfigurable Spanning Path (degree3 old in the graph) : –Step 1: a MST with degree<=3 is built. –Step 2: nodes with degree<2 are connected using a simple path (reconfigurable spanning ring concept). 3-MST + Bridge Covering (degree3 new in the graph) : –Step 1: a MST with degree<=3 is built. –Step 2: edges between nodes with degree<3 are added in order of greater bridge covering capability. –Achieves closer to optimal solutions in terms of cost minimization. Run-time of Spanning Ring and Reconfigurable Spanning Ring heuristics Performance of Spanning Ring and Reconfigurable Spanning Ring heuristics Performance of 3-degree heuristics Run-time of Single-Hop and Multi-Hop Heuristics Average Performance Improvement (Congestion reduction) of Multi-Hop over Single-Hop Heuristic 0 1( s ) 2 3 4( d ) 5 Link-1Link-2 Link-3Link-4 Multi-Hop Heuristic 01 2 3 R 01 = 100 MBPS, R 02 = 90 MBPS R 03 = 75 MBPS, R 13 = 60 MBPS R01 is maximum, add a link between (0, 1) Next R02, add a link between (0, 2) Next is R03. Interfaces at 0 are not available. Pass. Next is R13, add the link between (1, 3) Drawback: R 03 left unassigned by the algorithm. As topology finished, R 03 may be routed on congested path. Single-Hop View: Choose an SD (s, d) pair and try to create a single-hop path by adding a new link between s and d. Higher traffic needs to be routed on fewer hops. Sort traffic matrix in decreasing order of traffic. Links are added in order of higher SD pair traffic until the topology is created. Time Complexity: O(N 2 ) Single-Hop Heuristic Scalability: both heuristics have polynomial time complexity, as shown in the following graph. Scalability: Both heuristics have polynomial time complexity. Reconfiguration times range from 13us (8 nodes) to 50 ms (50 nodes). Ring Networks3-Degree Networks