SUMMARYIn heterogeneous networks, TCP connections that incorporate a terrestrial or satellite radio link are greatly disadvantaged with respect to entirely wired connections, because of their longer round trip times (RTTs). To cope with this problem, a new TCP proposal, the TCP Hybla, is presented and discussed in the paper. It stems from an analytical evaluation of the congestion window dynamics in the TCP standard versions (Tahoe, Reno, NewReno), which suggests the necessary modifications to remove the performance dependence on RTT. TCP Hybla performance is firstly evaluated in the case of an ideal channel, with good correlation between analytical and simulation data. Then, more realistic situations, which require the adoption of a benchmark network topology and a careful ns-2 simulation set-up, are examined. In particular, TCP Hybla performance is compared with that achievable by TCP standard in the presence of congestion and link losses, either separately or jointly considered. In all the examined cases, the superiority of TCP Hybla is evident, as it greatly reduces the severe penalization suffered by wireless, and especially satellite, TCP connections. Finally, it is worth noting that TCP Hybla does not infringe the end to end semantics of TCP and is compatible with other promising enhancements.
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 delayand 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 TCPsplitting 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.
Every mission into deep space has a communications system to carry commands and other information from Earth to a spacecraft or to a remote planet and to return scientific data to Earth [1]. Communications systems are central to the success of space missions. Large amounts of data need to be transferred (for example, nearly 25 TB in 2013 concerning the Mars Reconnaissance Orbiter (MRO)), and the demand will grow in the future [1] because of the employment of more sophisticated instruments that will generate more data. This will require the availability of high network transfer rates. Satellite systems already have to cope with difficult communication challenges: long round trip times (RTTs); the likelihood of data loss due to errors on the communication link; possible channel disruptions; and coverage issues at high latitudes and in challenging terrain. These problems are magnified in space communications characterized by huge distances among network nodes, which imply extremely long delays and intermittent connectivity. At the same time, a space communications system must be reliable over time due to the long duration of space missions. Moreover, the importance of enabling Internet-like communications with space vehicles is increasing, realizing the concept of extended Future Internet, an IP (Internet Protocol) pervasive network of networks including interplanetary communication [2], where a wide variety of science information values are acquired through sensors and transmitted.The Delay-and Disruption Tolerant Network (DTN) architecture [3] introduces an overlay protocol that interfaces with either the transport layer or lower layers. Each node of the DTN architecture can store information for a long time before forwarding it. Thanks to these features, a DTN is particularly suited to cope with the challenges imposed by space communication. As summarized in [4], the origin of the DTN concept lies in a generalization of requirements identified for interplanetary networking (IPN), where latencies that may reach the order of tens of minutes, as well as limited and highly asymmetric bandwidth, must be faced.However, other scenarios in planetary networking, called "challenged networks," such as military tactical networking, sparse sensor networks, and networking in developing or otherwise communications-challenged regions, can also benefit from the DTN solution. Delays and disruptions can be handled at each DTN hop in a path between a sender and a destination. Nodes on the path can provide the storage necessary for data in transit before forwarding it to the next node on the path. In consequence, the contemporaneous end-to-end connectivity that Transmission Control Protocol (TCP) and other standard Internet transport protocols require in order to reliably transfer application data is not required.In practice, in standard TCP/IP networks, ABSTRACTDelay-and Disruption Tolerant Networks (DTNs) are based on an overlay protocol and on the store-carry-forward paradigm. In practice, each DTN node can store information for a...
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
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