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ABSTRACT: In this paper, we study the problem of efficiently computing multiple aggregation queries over a data stream. In order to share computation, prior proposals have suggested instantiating certain intermediate aggregates which are then used to generate the final answers for input queries. In this work, we make a number of important contributions aimed at improving the execution and generation of query plans containing intermediate aggregates. These include: (1) a different hashing model, which has low eviction rates, and also allows us to accurately estimate the number of evictions, (2) a comprehensive query execution cost model based on these estimates, (3) an efficient greedy heuristic for constructing good low-cost query plans, (4) provably near-optimal and optimal algorithms for allocating the available memory to aggregates in the query plan when the input data distribution is Zipf-like and Uniform, respectively, and (5) a detailed performance study with real-life IP flow data sets, which show that our multiple aggregates computation techniques consistently outperform the best-known approach.
Data Engineering (ICDE), 2011 IEEE 27th International Conference on; 05/2011
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ABSTRACT: Long-distance multi-hop wireless networks have been used in recent years to provide connectivity to rural areas. The salient features of such networks include TDMA channel access, nodes with multiple radios, and point-to-point long-distance wireless links established using high-gain directional antennas mounted on high towers. It has been demonstrated previously that in such network architectures, nodes can transmit concurrently on multiple radios, as well as receive concurrently on multiple radios. However, concurrent transmission on one radio, and reception on another radio causes interference. Under this scheduling constraint, given a set of source-destination demand rates, we consider the problem of satisfying the maximum fraction of each demand (also called the maximum concurrent flow problem). We give a novel joint routing and scheduling scheme for this problem, based on linear programming and graph coloring. We analyze our algorithm theoretically and prove that at least 50% of a satisfiable set of demands is satisfied by our algorithm for most practical networks (with maximum node degree at most 5).
INFOCOM, 2010 Proceedings IEEE; 04/2010
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ABSTRACT: In this paper, we propose new "low-overhead" network monitoring techniques to detect violations of path-level QoS guarantees like end-to-end delay, loss, etc. Unlike existing path monitoring schemes, our approach does not calculate QoS parameters for all paths. Instead, it monitors QoS values for only a few paths, and exploits the fact that path anomalies are rare and anomalous states are well separated from normal operation, to rule out path QoS violations in most situations. We propose a heuristic to select a small subset of network paths to monitor while ensuring that no QoS violations are missed. Experiments with an ISP topology from the Rocketfuel data set show that our heuristic can deliver almost a 50% decrease in monitoring overhead compared to previous schemes.
INFOCOM 2008. The 27th Conference on Computer Communications. IEEE; 05/2008
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ABSTRACT: In this paper we present a new channel allocation scheme for IEEE 802.11 based mesh networks with point-to- point links, designed for rural areas. Our channel allocation scheme allows continuous full-duplex data transfer on every link in the network. Moreover, we do not require any synchronization across the links as the channel assignment prevents cross link interference. Our approach is simple. We consider any link in the network as made up of two directed edges. To each directed edge at a node, we assign a non-interfering IEEE 802.11 channel so that the set of channels assigned to the outgoing edges is disjoint from channels assigned to the incoming edges. Evaluation of this scheme in a testbed demonstrate throughput gains of between 50 - 100%, and significantly less end-to-end delays, over existing link scheduling/channel allocation protocols (such as 2P [11]) designed for point-to-point mesh networks. Formally speaking, this channel allocation scheme is equivalent to an edge-coloring problem, that we call the directed edge coloring (DEC) problem. We establish a relationship between this coloring problem and the classical vertex coloring problem, and thus, show that this problem is NP-hard. More precisely, we give an algorithm that, given k vertex coloring of a graph can directed edge color it using xi(k) colors, where xi(k) is the smallest integer n such that (lfloorn/2rfloor/n ) ges k.
INFOCOM 2008. The 27th Conference on Computer Communications. IEEE; 05/2008
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ABSTRACT: IEEE 802.11 WiFi equipment based wireless mesh networks have recently been proposed as an inexpensive approach to connect far-flung rural areas. Such networks are built using high-gain directional antennas that can establish long-distance wireless point-to-point links. Some nodes in the network (called gateway nodes) are directly connected to the wired internet, and the remaining nodes connect to the gateway(s) using one or more hops. The dominant cost of constructing such a mesh network is the cost of constructing antenna towers at nodes. The cost of a tower depends on its height, which in turn depends on the length of its links and the physical obstructions along those links. We investigate the problem of selecting which links should be established such that all nodes are connected, while the cost of constructing the antenna towers required to establish the selected links is minimized. We show that this problem is NP-hard and that a better than O(log n) approximation cannot be expected, where n is the number of vertices in the graph. We then present the first algorithm in the literature, for this problem, with provable performance bounds. More precisely, we present a greedy algorithm that is an O(log n) approximation algorithm for this problem. Finally, through simulations, we compare our approximation algorithm with both the optimal solution, and a naive heuristic.
INFOCOM 2008. The 27th Conference on Computer Communications. IEEE; 05/2008
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ABSTRACT: Cisco's NetFlow collector (NFC) is a powerful example of a real-world product that supports multiple aggregate queries over a continuous stream of IP flow records. NFC enables a plethora of network management tasks like traffic demands estimation, application traffic profiling, etc. In this paper, we investigate two computation sharing techniques for enabling streaming applications such as NFC to scale to hundreds of queries. Our first technique instantiates certain intermediate aggregates which are then used to generate the final answers for input queries. Our second technique coalesces the filter conditions of similar queries and uses the coalesced filter to pre-filter stream data input to these queries. Using these techniques, we propose a heuristic to compute a good query plan and perform extensive simulations to show that our heuristic delivers a factor of over 3 performance improvement compared to a naive approach.
Data Engineering, 2008. ICDE 2008. IEEE 24th International Conference on; 05/2008
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ABSTRACT: IEEE 802.11 Wi-Fi equipment based wireless mesh networks have recently been proposed as an inexpensive approach to connect far-flung rural areas. Such networks are built using high-gain directional antenna that can establish long-distance point-point links. In recent work, a new MAC protocol named 2P has been proposed that is suited for the interference pattern within such a network. However, the 2P protocol requires the underlying graph (for each 802.11 channel) to be bi-partite. Under the assumption that 2P is the MAC protocol used in the mesh network, we make the following contributions in this paper. Given K non-interfering 802.11 channels, we propose a simple cut-based algorithm to compute K bi-partite sub-graphs (on each of which the 2P protocol can be run separately). We establish the class of graphs that can thus be completely covered by K bipartite subgraphs. For the remaining set of graphs, we look into the "price" of routing all end-to-end demands over only the bipartite subgraphs. We analytically establish what fraction of the max flow of the original mesh-graph can be routed over the bipartite subgraphs. Finally we look into the problem of mismatch between the load on a link (as computed by max flow) and its effective capacity under a given channel allocation. We propose heuristics to cluster links with similar loads into the same bipartite graphs (channels) and through comprehensive numerical simulations show that our heuristics come very close to the best possible flow.
INFOCOM 2007. 26th IEEE International Conference on Computer Communications. IEEE; 06/2007
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ABSTRACT: In this paper, we develop passive network tomography techniques for inferring link-level anomalies like excessive loss rates and delay from path-level measurements. Our approach involves placing a few passive monitoring devices on strategic links within the network, and then passively monitoring the performance of network paths that pass through those links. In order to keep the monitoring infrastructure and communication costs low, we focus on minimizing (1) the number of passive probe devices deployed, and (2) the set of monitored paths. For mesh topologies, we show that the above two minimization problems are NP-hard, and consequently, devise polynomial-time greedy algorithms that achieve a logarithmic approximation factor, which is the best possible for any algorithm. We also consider tree topologies typical of Enterprise networks, and show that while similar NP-hardness results hold, constant factor approximation algorithms are possible for such topologies.
INFOCOM 2007. 26th IEEE International Conference on Computer Communications. IEEE; 06/2007
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ABSTRACT: VillageNet is a wireless mesh network that aims to provide low-cost broadband Internet access for rural regions. The cost of building the network is kept low by using off-the-shelf IEEE 802.11 equipment and optimizing the network topology to minimize cost. In this paper we describe the over-all operation of VillageNet and discuss two fundamental problems in building such a network. Nodes in VillageNet communicate using long-distance point-to-point wireless links that are established using high-gain directional antenna. VillageNet uses the 2P MAC protocol [?], that is suited for the interference pattern within such a network. However, the 2P protocol requires the underlying mesh graph (for each 802.11 channel) to be bi-partite. Thus, if K channels are available, then an important consideration is how to select K bi-partite subgraphs to activate, such that the demands of the nodes are best met. We formally pose this problem and present some initial results. Second, we observe that the dominant cost of constructing such a mesh network is the cost of constructing antenna towers at nodes. The cost of a tower depends on its height, which in turn depends on the length of its links, and the physical obstructions along those links. Thus to minimize cost, we pose the problem of deciding which links should be established, such that all villages are connected and the cost of constructing antenna towers to establish the selected links is minimized.
Communication Systems Software and Middleware, 2007. COMSWARE 2007. 2nd International Conference on; 02/2007
