R. Cohen

Technion - Israel Institute of Technology, H̱efa, Haifa, Israel

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Publications (14)10.42 Total impact

  • Source
    R. Cohen · L. Katzir
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    ABSTRACT: OFDMA is one of the most important modulation and access methods for the future mobile networks. Before transmitting a frame on the downlink, an OFDMA base station has to invoke an algorithm that determines which of the pending packets will be transmitted, what modulation should be used for each of them, and how to construct the complex OFDMA frame matrix as a collection of rectangles that fit into a single matrix with fixed dimensions. We propose efficient, and theoretically best possible, algorithms that solves this intricate OFDMA scheduling problem by breaking it down into two sub-problems, referred to as macro and micro scheduling. We analyze the computational complexity of these sub-problems and develop efficient algorithms for solving them.
    INFOCOM 2008. The 27th Conference on Computer Communications. IEEE; 05/2008
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    D. Breitgand · R. Cohen · A. Nahir · D. Raz
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    ABSTRACT: In this work we extend the aforementioned supermarket model by incorporating the management costs into it. In particular, we assume that when a server is polled about its load, it has to allocate resources in order to answer this query. We consider a system that consists of n identical servers. Each server processes its incoming service requests according to the FIFO
    Autonomic Computing, 2007. ICAC '07. Fourth International Conference on; 07/2007
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    R. Cohen · D. Raz
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    ABSTRACT: The rapid growth of wireless and cellular networks, and the high availability of small communication devices, such as PDAs, brings us faster than ever to the point where context aware services (CASs) are becoming a commodity. In order to allow fast and efficient development, deployment, and management of such services, a global system that allows the services to gain access to the context information needs to be created, maintained, and managed. We study the requirements for such a context distribution system. We deal with the architectural decisions regarding the definition of context items and the way context information becomes available to the CASs, and also the algorithmic aspects of disseminating this information. We demonstrate the advantages of the architecture and the proposed information dissemination algorithms by conducting a simulation study under realistic practical assumptions. Our results indicate that a modular approach in which context information is provided in many network locations by brokers through an open simple API is both powerful enough to provide the needed context information, and simple enough to be easily implemented.
    Network Operations and Management Symposium, 2004. NOMS 2004. IEEE/IFIP; 05/2004
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    R. Cohen · L. Katzir
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    ABSTRACT: We present a scheme for allocating unsolicited grants to the end hosts of synchronous applications of a wireless access network, in accordance with the condition of the channel, the importance of each packet and the specific loss recovery mechanism employed in the channel. The proposed scheme is generic in the sense that it maximizes the effectiveness of the channel under various conditions and it can he used along with every FEC-based or retransmission-based error recovery strategy
    INFOCOM 2004. Twenty-third AnnualJoint Conference of the IEEE Computer and Communications Societies; 04/2004
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    R. Cohen · L. Katzir · D. Raz
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    ABSTRACT: Cache pre-filling is emerging as a new concept for increasing the availability of popular Web items in cache servers. According to this concept, Web items are sent by a "push-server" to the proxy cache servers, usually through a broadcast-based or a multicast-based distribution mechanism. One of the most difficult challenges is to design the scheduling algorithm of the push-server. This algorithm needs to determine the "broadcast scheduling map", namely which Web items to broadcast and when. In this paper we study the approach where every constant period of time each proxy cache analyzes the requests it has received in the past and determines which Web item it prefers to receive by broadcast and when. We formalize a related problem, called the "cache pre-filing push" (CPFP) problem, analyze its computational complexity, and describe efficient algorithms to solve it.
    INFOCOM 2002. Twenty-First Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 02/2002
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    E. Bortnikov · R. Cohen
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    ABSTRACT: The paper addresses the problem of designing efficient scheduling policies for the transmission of control messages by hierarchical network protocols. Such protocols encounter a tradeoff between the desire to forward a control message across the tree as soon, as it is received, and the desire to reduce control traffic. Scheduling problems that arise in this context are defined and discussed. The paper mainly concentrates on minimizing the average extra delay encountered by the control messages under an upper bound on the number of outgoing messages a node can send during a fixed period of time. A polynomial-time algorithm is presented for the off-line version of the problem, and then several efficient on-line heuristics are presented and compared
    INFOCOM '98. Seventeenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 04/2001
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    R. Cohen · G. Kaempfer
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    ABSTRACT: Network layer multicast is know as the most efficient way to support multicast sessions. However, for security, QoS and other considerations, most of the real-time application protocols can be better served by upper layer (transport or application) multicast. We propose a scheme called M-RTP for multicast RTP sessions. The idea behind this scheme is to set up the multicast RTP session over a set of unicast RTP sessions, established between the various participants (source and destinations) of the multicast session. We then address the issue of finding a set of paths with maximum bottleneck for an M-RTP session. We show that this problem is NP-complete, and propose several heuristics to solve it
    INFOCOM 2001. Twentieth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 02/2001
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    R. Cohen · G. Kaempfer
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    ABSTRACT: A virtual private network (VPN) is a private data network that uses a nonprivate data network to carry traffic between remote sites. An “Intranet VPN” establishes network layer connectivity between remote Intranet sites by creating an IP overlay network over the nonprivate network, using various tunneling mechanisms. There are two approaches for establishing such tunnels: a “CPE-based approach” and a “network-based approach.” In the first approach, tunnels are established only between the CPE devices, whereas in the second approach tunnels are also established between the routers of the core nonprivate network. In this paper we address the problem of determining a CPE-based and a network-based layout of VPN tunnels while taking into account two factors: the cost of the links over which the VPN tunnels are established and the cost of the core routers that serve as end points for the VPN. We define related graph algorithm problems, analyze their complexity, and present heuristics for solving these problems efficiently
    IEEE/ACM Transactions on Networking 01/2001; DOI:10.1109/90.893873 · 1.99 Impact Factor
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    R. Cohen · Y. Hamo
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    ABSTRACT: TCP suffers from low performance over asynchronous transfer mode (ATM) networks. This is mainly because during phases of congestion, ATM drops cells without taking into account the effect this has on the upper layer protocols. Two main algorithms, called PPD and EPD, were proposed in the past for improving TCP performance. However they address one aspect of the problem, that has only small effect on the final performance. In this paper we propose an enhanced method for packet discard, called balanced packet discard (BPD), that improves TCP performance dramatically on congested networks and guarantees fairness among multiple connections. We show that BPD increases TCP throughput by more than 25% compared to EPD/PPD
    INFOCOM 2000. Nineteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 04/2000
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    R. Cohen · E. Felstaine · R. Emek
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    ABSTRACT: We propose a framework for the creation and maintenance of multicast trees in hierarchical ATM networks. This framework aims at coping with an inherent difficulty of topology aggregation in such networks. The main idea of the proposed framework is to distribute the tree topology information among a set of hierarchical multicast group servers (MGS) nominated for each multicast tree, while keeping regions that do not have a member in the multicast group unaware of the tree. The framework can be employed with every multicast routing algorithm designed for non-hierarchical networks
    INFOCOM 2000. Nineteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 04/2000
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    E. Felstaine · R. Cohen
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    ABSTRACT: ATM private network-to-network interface (PNNI) is a hierarchical and dynamic link-state routing protocol, designed to scale to the largest possible ATM networks, encompassing thousands of nodes. This paper investigates the route computation load imposed by the PNNI routing scheme, and shows that this load is unevenly distributed among the network nodes. More specifically, the routing computation load associated with the setup of a single virtual path grows exponentially with the hierarchy level. As a result, some of the network nodes-mainly those that function as border nodes of high levels-may be overloaded with route computation, while other nodes are rarely involved in this process. This paper also proposes a possible scheme for spreading the route computation burden more evenly. According to this scheme, heavily loaded nodes transfer route computation tasks to lightly loaded nodes
    IEEE/ACM Transactions on Networking 01/2000; DOI:10.1109/90.811455 · 1.99 Impact Factor
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    R. Cohen
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    ABSTRACT: Asymmetric digital subscriber loop technology is a new platform for delivering broadband services to homes and small businesses, thus bringing the information highway to the mass market. Successful deployment of an ADSL-based access network is not only a matter of delivering faster rates, but also offering solutions that are cost-effective and easy to use and manage. As deregulation and competition spread globally, an ADSL-based access network should also enable end users to choose their service providers dynamically, and to switch from one provider to another easily and rapidly, even on a real-time basis. In light of these requirements, this article addresses the issue of service provisioning in an ATM-over-ADSL access network. It concentrates on the provisioning of services using the PPP protocol. PPP is the common protocol for service provisioning in circuit-switched telephone networks. However, it is also considered a good choice for the delivery of broadband services since it has built-in mechanisms for IP address assignment, layer-2 security, and AAA (authentication, authorization and accounting). This article presents the problem of initiating a PPP connection at an Ethernet-based host. It presents several solutions for this problem and discusses the advantages and disadvantages of each solution
    IEEE Communications Magazine 11/1999; DOI:10.1109/35.795594 · 4.46 Impact Factor
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    E. Felstaine · R. Cohen · O. Hader
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    ABSTRACT: When an ATM node discovers that it cannot continue the setup of a virtual channel under the requested QoS, it initiates a back-tracking procedure called “crankback”. We propose a novel scheme, referred to as crankback prediction, that decreases the crankback overhead. Under the proposed scheme, nodes check during the connection admission control procedure whether the establishment of a virtual channel has a good chance to be admitted over the entire designated route. If this is not the ease, crankback is initiated even before a certain QoS parameter is exceeded
    INFOCOM '99. Eighteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 04/1999
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    Ehud Aharoni · Reuven Cohen
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    ABSTRACT: The paper addresses the issue of minimizing the number of nodes involved in routing over a multicast tree and in the maintenance of such a tree in a datagram network. It presents a scheme where the tree routing and maintenance burden is laid only upon the source node and the destination nodes associated with the multicast tree. The main concept behind this scheme is to view each multicast tree as a collection of unicast paths and to locate only the multicast source and destination nodes on the junctions of their multicast tree. The paper shows that despite this restriction, the cost of the created multicast trees is not necessarily higher than the cost of the trees created by other algorithms that do not impose the restriction and therefore require all nodes along the data path of a tree to participate in routing over the tree and in the maintenance of the tree
    IEEE/ACM Transactions on Networking 07/1998; 6(3):286-297. DOI:10.1109/90.700892 · 1.99 Impact Factor