Delay-/Disruption- Tolerant Networking (DTN) was first conceived to cope with the impairments of Inter-Planetary Internet. Then its scope was enlarged to cover all “challenged networks”, where long delays, disruption, link intermittency and other challenges prevent, or make difficult, the use of ordinary Internet architecture. To assess DTN performance in such heterogeneous scenarios as space communications (including both satellite and deep space) and wireless sensor networks, it is essential to develop suitable evaluation tools, with highly flexible use. In this paper we present the third major release of DTNperf, a client-server evaluation tool designed to assess goodput and to provide logs in DTN Bundle Protocol (BP) architectures. It has been greatly extended in many respects, including full support of both DTN2 and ION (the BP reference implementation and that developed by NASA JPL, respectively). The aim of the paper is to present both new and enhanced features and a variety of application examples, mainly focused on satellite and space communications, in order to promote its use within the DTN community. DTNperf_3 is to be included in DTN2, as were previous versions, and for the first time also in ION.

Delay-/Disruption-Tolerant Networking, which originated from research on deep space communications, has enlarged its scope to encompass all challenged networks, including LEO satellite communications. Focusing on single satellite or incomplete constellation cases, the advantages of DTN mainly relate to its ability to cope with disruption and intermittent connectivity, typical of LEOs. This, however, requires the adoption of routing solutions specifically designed for DTNs. Among the many proposals, Contact Graph Routing, designed by NASA for deep space, seems particularly appealing, as it takes advantage of the a priori knowledge of “contacts” between DTN nodes, a characteristic peculiar to both deep space and LEO environments. This paper aims to investigate the suitability of CGR in LEO satellite DTN communications, by focusing on two practical application scenarios: Earth observation and data mule. Results, obtained through a Linux testbed running ION, the DTN Bundle protocol and CGR implementation developed by NASA, highlight the advantages of CGR when applied to LEO satellite communications.

Satellite communications are characterized by long delays, packet losses, and sometimes intermittent connectivity and link disruptions. The TCP/IP stack is ineffective against these impairments and even dedicated solutions, such as performance enhancing proxies (PEPs), can hardly tackle the most challenging environments, and create compatibility issues with current security protocols. An alternative solution arises from the delay- and disruption-tolerant networking (DTN) architecture, which specifies an overlay protocol, called bundle protocol (BP), on top of either transport protocols (TCP, UDP, etc.), or of lower layer protocols (Bluetooth, Ethernet, etc.). The DTN architecture provides long-term information storage on intermediate nodes, suitable for coping with disrupted links, long delays, and intermittent connectivity. By dividing the end-to-end path into multiple DTN hops, in a way that actually extends the TCP-splitting concept exploited in most PEPs, DTN allows the use of specialized protocols on the satellite (or space) links. This paper discusses the prospects for use of DTN in future satellite networks. We present a broad DTN overview, to make the reader familiar with the characteristics that differentiate DTN from ordinary TCP/IP networking, compare the DTN and PEP architectures and stacks, as a preliminary step for the subsequent DTN performance assessment carried out in practical LEO/GEO satellite scenarios. DTN security is studied next, examining the advantages over present satellite architectures, the threats faced in satellite scenarios, and also open issues. Finally, the relation between DTN and quality of service (QoS) is investigated, by focusing on QoS architectures and QoS tools and by discussing the state of the art of DTN research activity in modeling, routing, and congestion control.

Satellite communications are essential to provide Internet access to wide areas, helping bridge the "digital divide". However, long RTTs and the possible presence of losses due to satellite channel errors, severely impair standard TCP performance. To overcome this problem several approaches are possible, including the adoption of enhanced versions of TCP. This paper focuses on these, by presenting the results of a comparative performance evaluation carried out in a satellite environment through a Linux testbed. The interest of the analysis lies in the wide variety of TCP variants considered and in the different aspects analyzed, such as performance at start-up, level of RTT unfairness and robustness against link losses. The results are analyzed at length in the paper and give interesting indications about the performance achievable on satellite channels by the most promising TCP variants proposed in recent years.

Satellite communications pose serious challenges to transport layer performance, mainly because of long propagation delays (especially in geosynchronous systems) and the possi ble presence of random errors on the satellite link. Solutions that cope with these impairments usually rely upon either the adoption of enhanced versions of transport protocols, or the insertion of intermediate agents, like PEPs (Performance Enhancing Proxies). An alternative approach is to adopt the DTN (Delay/Disruption Tolerant Networking) architecture based on the introduction of the new "bundle" layer in the protocol stack. The aim of the paper is to discuss the advantages and disadvantages of the DTN approach compared to the more conventional solutions mentioned. DTN performance is assessed by considering both fully connected networks and networks where continuous connectivity cannot be guaranteed during data transfer and/or at start-up. Performance is evaluated by using the TATPA testbed (Testbed for Advanced Transport Protocols and Architectures), which is based on a cluster of Linux PCs running the DTNperf application, developed to this end by the authors and now included in the official DTN package. Comparative results show that DTN, coupled with TCP Hybla, outperforms NewReno, offering a performance very close to PEPsal and end- to-end Hybla, which are among the most effective solutions on satellite channels. In addition to these encouraging results, DTN offers an intrinsic greater robustness in intermittent or disruptive environments.

The rapidly increasing importance of wireless communications (including satellite) together with the rapid growth of high speed networks pose new challenges to TCP. To overcome them, a wide variety of TCP enhancements has been presented in literature. However, because most proposals aim to address different impairments, the choice of "the best" TCP enhancement becomes arduous, given the increasing level of heterogeneity of present and future networks. The TCP adaptive- selection concept, introduced by the authors in this paper, aims at providing an alternative approach, by challenging the idea that only one TCP enhancement must be adopted. In fact, by extending the concept that underlies ACM (Adaptive Coding and Modulation) to Transport layer, it envisages concurrent adoption of different TCP versions on the same server, the better to match the different impairments present on different connections. Preliminary results obtained by considering a simple application example are encouraging, and justify the following remarks on feasibility and the discussion of some implementation proposals.

The paper describes a measurement campaign recently carried out by University of Bologna (UoB), National Inter-University Consortium for Telecommunications-(CNIT) and University of California Los Angeles (UCLA), on the CNIT GEO Skyplex platform. The aim of the experiments is the performance assessment of a wide range of TCP enhancements on network environments that include a real GEO satellite link. Measurements were carried out by means of the integrated UoB-CNIT testbed, fully controlled by remote through a Web interface. Analysis of live experiments data confirm the challenges posed by GEO sat channels, and show a negative influence of the Skyplex BoD technique on TCP start-up performance. Among the TCP enhancements compared in the paper, best results are provided by TCP Hybla and TCP Westwood, with an advantage of the former in heterogeneous environments characterized by a high level of RTT unfairness.

To overcome the unsatisfactory performance offered by standard TCP variants in the challenging satellite environment, the scientific community has presented a wide variety of possible solutions, encompassing both innovative transport protocols and architectures. Among these, the paper focuses on those TCP variants that have been actually implemented in real operating systems and in Linux in particular. The aim of the paper is to present a sound emulation approach for their performance assessment. This approach relies on the set-up of a complete testbed, able to reproduce, in a fully controlled ("clean") environment, the characteristics of satellites networks, and to provide "live" data from actual implementations of TCP variants. After validating this approach by means of ns-2 results comparison (when available), the paper presents the results of a complete series of tests, which are proposed by the authors as significant elements for a possible future common evaluation benchmark.

Although TCP has proved very effective and robust for many years, its performance on emerging heterogeneous networks is challenged by the impairments originated by the presence of radio links. In particular, satellite communications are affected by long RTTs and possibly also by random segment losses, which can severely affect end-to-end performance. To cope with these problems, several TCP enhancements have been presented in the literature. The paper aim is to investigate the effectiveness of such modifications, when applied to TCP satellite connections. In particular, the analysis focuses on some emerging proposals, namely TCP Hybla, developed by the authors, and TCP Westwood, examined here in three variants. They are compared with three well established TCP variants, such as NewReno, SACK and Vegas, taking into account both a pure satellite environment and more challenging, but also perhaps closer to reality, heterogeneous network. Performance is assessed by means of ns-2 simulations considering goodput, fairness and friendliness as performance metrics. Results show that large performance improvements may be achieved by some of the considered TCP enhancements, without infringing the end-to-end semantics of this protocol.

Internet communications with paths that include satellite link face some peculiar challenges, due to the presence of a long propagation wireless channel. In this paper, we propose a Performance Enhancing Proxy (PEP) solution, called PEPsal, which is, to the best of the authors' knowledge, the first open source TCP splitting solution for the GNU/Linux operating systems. PEPsal improves the performance of a TCP connection over a satellite channel making use of the TCP Hybla, a TCP enhancement for satellite networks developed by the authors. The objective of the paper is to present and evaluate the PEPsal architecture, by comparing it with end to end TCP variants (NewReno, SACK, Hybla), considering both performance and reliability issues. Performance is evaluated by making use of a testbed set up at the University of Bologna, to study advanced transport protocols and architectures for internet satellite communications.

In the paper, the authors aim to quantitatively and qualitatively assess the impact of the cellular environment on TCP performance. The analysis is carried out through a test bed platform working in real time. Its core is represented by the GALATEA emulator, which allows the users to include all the characteristics of a real UMTS system. Four cellular environments, derived from the MORANS database developed within the COST 273 action, and two alternative RLC configurations are considered in the paper. The corresponding results, achieved through specific GALATEA settings, highlight the complex cross-layer interactions between the cellular environment and both the TCP and the UMTS-RLC protocols. Although the study confirms the effectiveness of the RLC ARQ mechanism in reducing the channel impairments impact on the upper layers, it also shows that the cellular environment still has a significant influence in determining TCP traffic performance.

This paper applies the theoretical framework for the evaluation of code acquisition in the presence of fading developed in the companion Part I paper. Code synchronization for code-division multiple-access (CDMA) cellular networks, such as IS-95 and cdma2000, is considered. A multidwell testing procedure is adopted. Notably, a procedure for the optimization of the multidwell parameters (number of dwells, dwell lengths, thresholds) to achieve minimum acquisition time is introduced and enforced. The effects of fading, interchip interference (ICI), frequency offset, multiple-access interference, and noise are taken into account in the determination of false alarm and detection probabilities. In particular, anomalous ICI despreading and internal interference-cancellation effects are described. Numerical and simulation results, in terms of time for correct acquisition, confirm the accuracy of the proposed approach and show how it can be effectively used in the design and evaluation of a code-acquisition subsystem.

This paper deals with the analysis of the code-acquisition process performed by a mobile terminal in a code-division multiple-access cellular system. The fundamental contribution is the setting of a theoretical framework for code-acquisition analysis, accounting for the presence of fading and shadowing with specified coherence time. A multidwell architecture is considered, accounting for different verification strategies. The MAX/TC detection criterion is used, and the acquisition performance is analyzed, following both the direct and flow-graph approaches. Expressions for the acquisition time probability density function under different assumptions are given, as well as closed-form expressions for the mean and variance of the acquisition time. Practical applications and numerical results are reported in the Part II companion paper.

W-CDMA and SW-CDMA (satellite W-CDMA) air interfaces require that a user equipment in a given cell acquires slot and frame synchronization, and identifies the primary scrambling code used by the target cell before starting communications. This synchronization procedure is identified as cell search procedure in W-CDMA and beam search procedure in SW-CDMA. Notwithstanding the extensive commonalities between the two air interfaces, the two procedures are actually different. The paper aims at the evaluation of the false acquisition probability achievable by the two procedures in a scenario in which the receiver may experience both satellite and terrestrial radio propagation conditions. This is a sensible evaluation scenario since, in the delivery of multicast-broadcast multimedia services, the satellite UMTS network represents an efficient complement to terrestrial UMTS, since its coverage is extended to indoor and urban areas through, for example, the use of terrestrial intermediate repeaters. Rayleigh fading, Rice fading, and vehicular multipath propagation channels are used for the performance assessment.

The paper addresses the exploitation of satellite diversity in a satellite mobile network. In particular, we focus on the impact of diversity on service availability and on system capacity, considering the forward link of a CDMA system with a multisatellite and multibeam architecture. The analysis includes the effects of path blockage, intrabeam and interbeam interference, imperfect power control, and fading correlation in the time-domain due to nonideal interleaving. A closed-form solution is given for the estimation of system capacity. We show that satellite diversity is essential in providing service availability in urban and suburban areas, while the impact of satellite diversity on system capacity may be positive or negative, mainly depending on the fading channel characteristics. In particular, diversity becomes more and more beneficial for increasing fading time-domain correlation (i.e., for low mobile speed and/or limited interleaving depth). The analytical results have been validated by means of Monte Carlo simulation.

Subband perceptual audio coding aims at providing the best subjective quality by decomposing the input signal into adjacent subbands. Every frequency component is then separately coded with a different number of bits in order to properly shape the quantization noise density power spectrum. In this letter, an optimum bit allocation algorithm is presented. As well as being applicable to either uniform or nonuniform frequency decompositions, it allows us to take into account the nonideal selectivity of the reconstruction filters. The effects of leakage for both short and long FIR filters have been investigated referring to a wavelet packet subband decomposition.

The paper addresses the exploitation of satellite diversity in a satellite mobile network. In particular, we focus on the impact of diversity on service availability and on system capacity, considering the forward link of a CDMA system with a multi-satellite and multi-beam architecture. The analysis includes the effects of path blockage, imperfect power control, and fading correlation. We show that satellite diversity is essential in providing service availability in urban and suburban areas. Also, we show that the impact of satellite diversity on system capacity may be positive or negative, mainly depending on the transmission channel characteristics. In particular, diversity becomes very beneficial whenever fading correlation is considered. The analytical results have been validated by means of Monte Carlo simulation.

Several multisatellite and multispot systems have been recently proposed for provision of mobile and personal services with global coverage, adopting GEO or non-GEO (i.e., MEO, LEO) satellite constellations. The paper addresses an in-depth analysis of these constellations, evaluating both geometrical performance measures and cochannel interference levels caused by extensive frequency reuse. The geometrical analysis yields the statistics for coverage, frequency of satellite hand-overs, and link absence periods. The interference analysis is based on a general model valid for all access techniques, which is here applied to the case of FDMA. The outage probability as a function of the specification on carrier-to-interference power ratio is evaluated for four selected constellations. Several techniques are introduced for interference reduction in non-GEO systems, in which the satellites coverage areas may intersect: spot turnoff, intraorbital plane frequency division, and interorbital plane frequency division. The effects of Rice fading have also been analyzed by means of an analytic approximated method. The overall analysis allows a fair comparison between LEO, MEO, and GEO constellations.>

An EHF satellite system for land-mobile applications to be integrated with a terrestrial cellular system is described. An approach to evaluate the carrier-to-cochannel interference occurring in multispot satellite coverage adopting frequency reuse is introduced, and results from the analysis are shown. Criteria for spectrum efficiency evaluation are also outlined along with traffic and link budget estimates. Possible options for payload implementation and mobile terminal design are presented.>