IEEE Communications Surveys & Tutorials • Third Quarter 2005 22d hoc networks are complex distributed systems that consist of wireless mobile or static nodes that can freely and dynamically self-organize. In this way they form arbitrary, and temporary, "ad hoc" network topologies, allowing devices to seamlessly interconnect in areas with no pre-existing infrastructure. Recently, the introduction of new protocols such as Bluetooth [1], IEEE 802.11 [2], and Hyperlan [3] are making possible the deployment of ad hoc networks for commercial purposes. As a result, considerable research efforts have been made in this new challenging wireless environment. For simplicity, in this article we will use the term MANETs instead of mobile ad hoc networks, and SANETs instead of static ad hoc networks. Also we note that the term ad hoc networks will represent both mobile ad hoc networks (MANETs) and static ad hoc networks (SANETs).TCP (Transmission Control Protocol) [4] was designed to provide reliable end-to-end delivery of data over unreliable networks. In theory, TCP should be independent of the technology of the underlying infrastructure. In particular, TCP should not care whether the Internet Protocol (IP) is running over wired or wireless connections. In practice, it does matter because most TCP deployments have been carefully designed based on assumptions that are specific to wired networks. Ignoring the properties of wireless transmission can lead to TCP implementations with poor performance.In ad hoc networks, the principal problem of TCP lies in performing congestion control in case of losses that are not induced by network congestion. Since bit error rates are very low in wired networks, nearly all TCP versions nowadays assume that packet losses are due to congestion. Consequently, when a packet is detected to be lost, either by timeout or by multiple duplicated ACKs, TCP slows down the sending rate by adjusting its congestion window. Unfortunately, wireless networks suffer from several types of losses that are not related to congestion, making TCP not adapted to this environment. Numerous enhancements and optimizations have been proposed over the last few years to improve TCP performance over one-hop wireless (not necessarily ad hoc) networks. These improvements include infrastructure-based WLANs [5][6][7][8], mobile cellular networking environments [9,10], and satellite networks [11,12]. Ad hoc networks inherit several features of these networks, in particular high bit error rates and path asymmetry, and add new problems caused by mobility and multi-hop communications, such as network partitions, route failures, and hidden (or exposed) terminals. We note that the following TCP versions -Tahoe, Reno, Newreno, and Vegas -perform differently in ad hoc networks [13]. However, all these versions suffer from the same problem: the inability to distinguish between packet losses due to congestion and losses due to the specific features of ad hoc networks. For more details about TCP versions see the Appendix; for a survey of...

Abstract. In this paper, we will develop a general framework to analyze polling systems with either the autonomous-server or the time-limited service discipline. We consider Poisson batch arrivals and phase-type service times. It is known that these disciplines do not satisfy the well-known branching property in polling system. Therefore, hardly any exact results exist in the literature. Our strategy is to apply an iterative scheme that is based on relating in closed-form the joint queue-length at the beginning and the end of a server visit to a queue. These kernel relations are derived using the theory of absorbing Markov chains.

This paper addresses the delay analysis and resource consumption in mobile ad hoc networks (MANETs) equipped with throwboxes. Throwboxes are stationary, wireless devices that act as relays, and that are deployed to increase the connectivity between mobile nodes. Our objective is to quantify the impact of adding throwboxes on the performance of two routing protocols, namely the Multicopy Two-hop Routing protocol and the Epidemic Routing protocol, in the cases where the throwboxes are fully disconnected or mesh connected. To this end, we use a Markovian model where the successive meeting times between any pair of mobile nodes (resp. any mobile node and any throwbox) are represented by a Poisson process with intensity λ (resp. µ). We derive closed-form expressions for the distribution of the delivery delay of a packet and for the distribution of the total number of copies of a packet that are generated, the latter metric reflecting the overhead induced by the routing protocol. These results are then compared to simulation results. Through a mean-field approach we also provide asymptotic results when the number of nodes (mobile nodes and/or throwboxes) is large.

Abstract. We evaluate the performance of a class of two-hop relay protocols for mobile ad hoc networks. The interest is on the multicopy two-hop relay (MTR) protocol, where the source may generate multiple copies of a packet and use relay nodes to deliver the packet (or a copy) to its destination, and on the twohop relay protocol with erasure coding. Performance metrics of interest are the time to deliver a single packet to its destination, the number of copies of the packet at delivery instant, and the total number of copies that the source generates. The packet copies at relay nodes have limited lifetime (time-to-live TTL). Via a Markovian analysis, the three performance metrics of the MTR protocol are obtained in closed-from in the case where the number of the copies in the network is limited. Also, we develop an approximation analysis in the case where the inter-meeting times between nodes are arbitrarily distributed and the TTLs of the copies are constant and all equal. In particular, we show that exponential intermeeting times yield stochastically smaller delivery delays than hyper-exponential inter-meeting times, and that exponential TTLs yield stochastically larger delivery delays than constant TTLs. Finally, we characterize the delivery delay and the number of transmissions in the two-hop relay protocol with erasure coding and compare this scheme with the multicopy scheme.