Laurence Clarac

On the impact of link layer retransmission schemes on TCP over 4G satellite links

We study the impact of reliability mechanisms introduced at the link layer on the performance of transport protocols in the context of 4G satellite links. Specifically, we design a software module that performs realistic analysis of the network performance, by utilizing real physical layer traces of a 4G satellite service. Based on these traces, our software module produces equivalent link layer traces, as a function of the chosen link layer reliability mechanism. We further utilize the link layer traces within the ns-2 network simulator to evaluate the impact of link layer schemes on the performance of selected Transmission Control Protocol (TCP) variants. We consider erasure coding, selective-repeat automatic request (ARQ) and hybrid-ARQ link layer mechanisms, and TCP Cubic, Compound, Hybla, New Reno and Westwood. We show that, for all target TCP variants, when the throughput of the transport protocol is close to the channel capacity, using the ARQ mechanism is most beneficial for TCP performance improvement. In conditions where the physical channel error rate is high, hybrid-ARQ results in the best performance for all TCP variants considered, with up to 22% improvements compared to other schemes.

Enabling realistic cross-layer analysis based on satellite physical layer traces

We present a solution to evaluate the performance of transport protocols as a function of link layer reliability schemes (i.e. ARQ, FEC and Hybrid ARQ) applied to satellite physical layer traces. As modelling such traces is complex and may require approximations, the use of real traces will minimise the potential for erroneous performance evaluations resulting from imperfect models. Our Trace Manager Tool (TMT) produces the corresponding link layer output, which is then used within the ns-2 network simulator via the additionally developed ns-2 interface module. We first present the analytical models for the link layer with bursty erasure packets and for the link layer reliability mechanisms with bursty erasures. Then, we present details of the TMT tool and our validation methodology, demonstrating that the selected performance metrics (recovery delay and throughput efficiency) exhibit a good match between the theoretical results and those obtained with TMT. Finally, we present results showing the impact of different link layer reliability mechanisms on the performance of TCP Cubic transport layer protocol.

Making H-ARQ Suitable for a Mobile TCP Receiver over LEO Satellite Constellations

This paper investigates strategies to carry out delay tolerant services over LEO satellite constellations for mobile receiver. In this context, LEO constellations are characterized by important delay variations where propagation impairments are mostly localized on the Land Mobile Satellite (LMS) channel (i.e. on the last hop). To cope with this issue, distinct reliability schemes can be introduced at the physical or link layers. Although their capacity to cope with transmission errors has been demonstrated, these recovery schemes may induce a high jitter that could severely damage TCP’s internal timers and reliability schemes. As a matter of fact, transport and link layers’ reliability schemes exhibit a clear discrepancy. Following temporal traces representing the delay between a mobile terminal and the last hop satellite from a LEO constellation, we assess how HARQ mechanisms impact on the RTO based retransmission and the duplicate acknowledgments of TCP. Based on ns-2 simulations, we propose a layer-2 buffer that let both link and transport layers to conjointly perform. Our evaluations show an end-to-end data rate increase and more generally illustrate the benefit of re-ordering packets at the link layer when link-layer erasure coding recovery mechanisms are used conjointly with TCP.

Enhancing satellite system throughput using adaptive HARQ for delay tolerant services in mobile communications

In this paper we propose the introduction of adaptive hybrid automatic repeat request (HARQ) in the context of mobile satellite communications. HARQ schemes which are commonly used in terrestrial links, can be adapted to improve the throughput for delay tolerant services. The proposed method uses the estimation of the mutual information between the received and the sent symbols, in order to estimate the number of bits necessary to decode the message at next transmission. We evaluate the performance of our method by simulating a land mobile satellite (LMS) channel. We compare our results with the static HARQ scheme, showing that our adaptive retransmission technique has better efficiency while keeping an acceptable delay for services.

Safety aeronautical communications by satellite: create or recycle? S-UMTS and GMR recycling

Air traffic predictions show a steady increase of flights in the next twenty years. Eurocontrol forecasts a traffic increase between 55% and 90% in the ECAC area. Current ATM procedures cannot cope with the predicted number of traffic. In order to deal with the increasing number of flights, ATM global concept needs to evolve. Strategic air traffic management will replace the tactical procedures. The interaction between the aircrafts and the air traffic control centers will be necessary to organize the traffic flow. Current voice communications cannot support the new communications requirements, so new communications systems must be investigated to fulfill the new communication services requirements. SESAR (Single European Sky ATM Research) proposed the operational concept at the end of the first phase of the program. This operational concept will rely on a dual link communications architecture composed by a terrestrial and a satellite system for EnRoute and TMA (Terminal Maneuvering Area). Current satellite systems do not seem well fitted to provide the requirements in terms of availability or latency, therefore, new systems must be proposed. Creating a new communications protocol could be a lot of work, so we propose to evaluate existing satellite protocols to evaluate their performance face to safety air communications.

Evolution of LDACS protocol to support safety aeronautical communications by satellite

Air traffic growth will probably lead the terrestrial ATM communication means to saturation around 2015-2025. The ATM needs are evolving to propose solutions that allow to cope with the future air traffic. In Europe, the SESAR (Single European Sky ATM Research) program is leading the evolution to the new operational concept for the European ATM. In this framework, new communications systems are under study to ensure the future communication services that will not be supported by the current systems, mainly based on VHF radio communications. Taking into account the safety and security requirements that aeronautical community imposes to any new system, it is necessary to start the definition of the new communication means to be ready in the required timeframe. Satellite communications will probably play a role in the future air communications infrastructure. Current satellite communications standards that are used in air communications do not fit to the requirements announced by the aviation community. In this paper, it is presented how a terrestrial safety communications protocol (AMACS) could be modified to satisfy satellite constraints. AMACS has been proposed as candidate for terrestrial communications in L band within the frame of SESAR.