Alexander Klemm

A special-purpose peer-to-peer file sharing system for mobile ad hoc networks

Establishing peer-to-peer (P2P) file sharing for mobile ad hoc networks (MANET) requires the construction of a search algorithm for transmitting queries and search results as well as the development of a transfer protocol for downloading files matching a query. In this paper, we present a special-purpose system for searching and file transfer tailored to both the characteristics of MANET and the requirements of peer-to-peer file sharing. Our approach is based on an application layer overlay network. As innovative feature, overlay routes are set up on demand by the search algorithm, closely matching network topology and transparently aggregating redundant transfer paths on a per-file basis. The transfer protocol guarantees low transmission overhead and a high fraction of successful downloads by utilizing overlay routes. In a detailed ns-2 simulation study, we show that both the search algorithm and the transfer protocol outperform off-the-shelf approaches based on a P2P file sharing system for the wireline Internet, TCP and a MANET routing protocol.

Modeling IP traffic using the batch Markovian arrival process

In this paper, we show how to utilize the expectation-maximization (EM) algorithm for efficient and numerical stable parameter estimation of the batch Markovian arrival process (BMAP). In fact, effective computational formulas for the E-step of the EM algorithm are presented, which utilize the well-known randomization technique and a stable calculation of Poisson jump probabilities. Moreover, we identify the BMAP as an analytically tractable model of choice for aggregated traffic modeling of IP networks. The key idea of this aggregated traffic model lies in customizing the BMAP such that different lengths of IP packets are represented by rewards of the BMAP. Using measured traffic data, a comparative study with the MMPP and the Poisson process illustrates the effectiveness of the customized BMAP for IP traffic modeling by visual inspection of sample paths over several time scales, by presenting important statistical properties as well as by investigations of queuing behavior.

TCP with adaptive pacing for multihop wireless networks

In this paper, we introduce a novel congestion control algorithm for TCP over multihop IEEE 802.11 wireless networks implementing rate-based scheduling of transmissions within the TCP congestion window. We show how a TCP sender can adapt its transmission rate close to the optimum using an estimate of the current 4-hop propagation delay and the coefficient of variation of recently measured round-trip times. The novel TCP variant is denoted as TCP with Adaptive Pacing (TCP-AP). Opposed to previous proposals for improving TCP over multihop IEEE 802.11 networks, TCP-AP retains the end-to-end semantics of TCP and does neither rely on modifications on the routing or the link layer nor requires cross-layer information from intermediate nodes along the path. A comprehensive simulation study using ns-2 shows that TCP-AP achieves up to 84% more goodput than TCP NewReno, provides excellent fairness in almost all scenarios, and is highly responsive to changing traffic conditions.

Traffic modeling and characterization for UMTS networks

In this paper, we present a synthetic traffic model for the Universal Mobile Telecommunication Systems (UMTS) based on measured trace data. The analysis and scaling process of the measured trace data with respect to different bandwidth classes constitutes the basic concept of the UMTS traffic characterization. Furthermore, we introduce an aggregated traffic model for UMTS networks that is analytically tractable. The key idea of this aggregated traffic model lies in customizing the batch Markovian arrival process (BMAP) such that different packet sizes of IP packets are represented by rewards (ie, batch sizes of arrivals) of the BMAP. The effectiveness of the customized BMAP for modeling UMTS traffic is illustrated using the synthetic traffic model previously presented.

Performance analysis of time-enhanced UML diagrams based on stochastic processes

In this paper, we propose extensions to UML state diagrams and activity diagrams in order to allow the association of events with exponentially distributed and deterministic delays. We present an efficient algorithm for the state space generation out of these UML diagrams that allows a quantitative analysis by means of an underlying stochastic process. We identify a particular stochastic process, the generalized semi-Markov process (GSMP), as the appropriate vehicle on which quantitative analysis is performed. As innovative feature the algorithm removes vanishing states, i.e. states with no timed events active, and considers branching probabilities within activity diagrams. Furthermore, we introduce a performance evaluation framework that allows a system designer to predict performance measures at several steps in the design process. The applicability of our approach for practical performance and dependability projects is demonstrated by an UML specification of the General Packet Radio Service, a packet switched extension in GSM wireless networks.

Traffic Modeling of IP Networks Using the Batch Markovian Arrival Process

In this paper, we identify the batch Markovian arrival process (BMAP) as an analytically tractable model of choice for aggregated traffic modeling of IP networks. The key idea of this aggregated traffic model lies in customizing the batch Markovian arrival process such that the different lengths of IP packets are represented by rewards (i.e., batch sizes of arrivals) of the BMAP. The utilization of the BMAP is encouraged by the observation that IP packet lengths follow to a large extent a discrete distribution. A comparative study with the MMPP and the Poisson process illustrates the effectiveness of the customized BMAP for IP traffic modeling by visual inspection of sample paths over four different time-scales, by presenting important statistical properties, and by analysis of traffic burstiness using R/S statistics. Additionally, we show that the BMAP model outperforms MMPP and Poisson traffic models by comparison of queuing performance.

Gateway adaptive pacing for TCP across multihop wireless networks and the Internet

In this paper, we introduce an effective congestion control scheme for TCP over hybrid wireless/wired networks comprising a multihop wireless IEEE 802.11 network and the wired Internet. We propose an adaptive pacing scheme at the Internet gateway for wired-to-wireless TCP flows. Furthermore, we analyze the causes for the unfairness of oncoming TCP flows and propose a scheme to throttle aggressive wired-to-wireless TCP flows at the Internet gateway to achieve nearly optimal fairness. Thus, we denote the introduced congestion control scheme TCP with Gateway Adaptive Pacing (TCP-GAP). For wireless-to-wired flows, we propose an adaptive pacing scheme at the TCP sender. In contrast to previous work, TCP-GAP does not impose any control traffic overhead for achieving fairness among active TCP flows. Moreover, TCP-GAP can be incrementally deployed because it does not require any modifications of TCP in the wired part of the network and is fully TCP-compatible. Extensive simulations using ns-2 show that TCP-GAP is highly responsive to varying traffic conditions, provides nearly optimal fairness in all scenarios and achieves up to 42% more goodput than TCP NewReno.

TCP with gateway adaptive pacing for multihop wireless networks with Internet connectivity

This paper introduces an effective congestion control pacing scheme for TCP over multihop wireless networks with Internet connectivity. The pacing scheme is implemented at the wireless TCP sender as well as at the Internet gateway, and reacts according to the direction of TCP flows running across the wireless network and the Internet. Moreover, we analyze the causes for the unfairness of oncoming TCP flows and propose a scheme to throttle aggressive wired-to-wireless TCP flows at the Internet gateway to achieve nearly optimal fairness. The proposed scheme, which we denote as TCP with Gateway Adaptive Pacing (TCP-GAP), does not impose any control traffic overhead for achieving fairness among active TCP flows and can be incrementally deployed since it does not require any modifications of TCP in the wired part of the network. In an extensive set of experiments using ns-2 we show that TCP-GAP is highly responsive to varying traffic conditions, provides nearly optimal fairness in all scenarios and achieves up to 42% more goodput for FTP-like traffic as well as up to 70% more goodput for HTTP-like traffic than TCP NewReno. We also investigate the sensitivity of the considered TCP variants to different bandwidths of the wired and wireless links with respect to both aggregate goodput and fairness.

Peer-to-Peer Computing in Mobile Ad Hoc Networks

Establishing applications based on the popular peer-to-peer (P2P) paradigm in mobile ad hoc networks (MANET) requires the employment of efficient mechanisms for communication and information exchange among peers. In this paper, we show that P2P systems designed for the wireline Internet cannot be deployed in MANET in a straightforward way. As a major shortcoming, these systems construct an overlay network consisting of application-layer connections that do not reflect the network-layer topology of the MANET. Furthermore, overlay construction and maintenance does not consider node mobility. We present two solutions to cope with these shortcomings: P2P communication based on cross-layer cooperation and P2P information exchange by epidemic dissemination. As an example for a system that implements the first concept, we describe the Optimized Routing Independent Overlay Network (ORION). As an example for a system that implements the second concept, we describe Passive Distributed Indexing (PDI). Detailed simulation studies show that both ORION and PDI provide efficient building blocks for the deployment of P2P computing applications in MANET.

Quantitative system evaluation with DSPNexpress 2000

This paper describes the software package DSPNexpress 2000, a tool for the quantitative evaluation of systems specified in stochastic Petri nets, the Unified Modeling Language (UML), or other specification languages for discrete-event systems. Linking the DSPNexpress software to commercial UML design packages allows the effective computation of performance measures like throughput and delay for UML system specifications. The unique feature of DSPNexpress constitute numerical solvers for transient and steady-state analysis of generalized-semi Markov processes with exponential and deterministic events, which may be concurrently active. The applicability of the DSPNexpress software for practical performance and dependability projects is demonstrated by an UML specification of an alternating bit protocol. The computational effort required by DSPNexpress for transient and steady-state analysis is plotted for varying model size. Furthermore, a performance curve for a measure of interest is presented.