Raffaello Secchi

Simulating dynamic bandwidth allocation on satellite links

In the last years, DVB-RCS has emerged as a flexible technology offering broadband Internet access to a large community of users at a relatively low cost. At the same time, the spreading of networked multimedia applications has highlighted the need to investigate mechanisms that guarantee a certain level of Quality of Service (QoS) to the end users. In particular, the DVB-RCS standard specifies different capacity request categories to support QoS at the link layer.We describe Tdma-bod, an ns-2 improvement that implements generic bandwidth-on-demand allocation in TDMA satellite systems; the patch is available as free software. This simulator has been validated through experimental tests performed on the Skyplex satellite system. Specifically, we run CBR UDP flows to measure the characteristics of the satellite link in terms of throughput and delay and to verify that the simulative model output matches the experimental dynamic throughput and one-way delay behaviour. The simulations and experiments show that bandwidth-on-demand allocation mechanisms may cause large delays when sudden variations in the incoming traffic rate occur, a behaviour typical of multimedia flows.

An Analysis of TCP Startup over an Experimental DVB-RCS Platform

Satellite systems are evolving towards higher available bandwidths and dynamic allocation based on instantaneous traffic rates offered at the stations, so called BoD (bandwidth on demand) channel sharing. This trend is paired with more and more powerful error correcting schemes, like those adopted in the recent DVB-S2 standard, which promise to make the channel virtually immune from packet errors. These factors, together with the significant round-trip delay of geostationary satellites, combine so that most TCP connections would send all of their data during the slow start phase. We investigate the performance of TCP during startup on recent BoD system by observing and explaining the behavior of different TCP flavors on different systems when transmitting data over the Eutelsats Skyplex data satellite system. We make recommendations for choosing and improving TCP implementations and for future BoD allocation schemes

Suitability of DAMA and Contention-Based Satellite Access Schemes for TCP Traffic in Mobile DVB-RCS

The problem of optimizing access and bandwidth sharing among transmission control protocol (TCP) connections in the mobile digital video broadcasting return channel via satellite (DVB-RCS) is tackled in this paper. After sketching the general system architecture, we explicitly deal with the dynamic assignment of bandwidth to TCP connections on the return link, which is accomplished by a network control center (NCC) placed onboard the satellite. Mobile users access the satellite in multifrequency time-division multiple access (MF-TDMA), whereas they receive data from the NCC in time-division multiplexing (TDM). Two different techniques, based on deterministic and random access, are compared in terms of bandwidth usage and average completion time per connection, when the mobile user acts as both server and client. In the server case, to increase the TCP throughput, both packet-level forward error correction (FEC) on data sent by mobile users and a duplicated and delayed acknowledgment technique for TCP acknowledgment traffic from the NCC to the mobile users are applied. An analysis of the packet losses and a simulation campaign of file transfers by employing a realistic channel model has been carried out. The results of the analysis show the convenience of adopting a technique, in addition to the optimal data redundancy in different cases, such as the server or client role of users, their willingness to pay, the file size, and the environment type.

On the use of pipesize estimators to improve TCP transient behavior

This paper presents a simulative analysis of a modification to the TCP congestion control mechanism called ESSE (early slow start exit), designed to improve the TCP startup phase by setting the slow start threshold according to a pipesize estimation based on the observation of few ACK arrival times. We evaluate the performance of ESSE by using various methods to estimate the pipesize as the ratio between the round trip time and the spacing between ACK. This algorithm is easy to implement and preserves the compatibility with the standard protocol since it requires changes to the sender side only. Simulative experiments show that ESSE allows us to speed-up TCP connections and drastically reduce the packet drop rate under several working conditions and load levels. Better performance of TCP can be observed for any of the considered estimators, which indicates that the algorithm is robust against estimation errors. Further, the characteristics of fairness and friendliness (towards Newreno) of the algorithm are investigated. According to our simulations, ESSE-modified protocols guarantee fair utilization of bandwidth among homogeneous and heterogeneous (Newreno) connections sharing a common link.

Communications and networking over satellites: SatNEx experimental activities and testbeds

One of the aims of the SatNEx (Satellite Network of Excellence, http://www.satnex.de) European project is to provide the scientific community that operates on satellite communications with measurement campaign results and evaluation tools, in order to support research activities that investigate problems in satellite communications at different levels of the OSI stack. In fact, the research activities developed in SatNEx address the most relevant aspects of satellite communications, such as channel modulation and coding schemes, access strategies, advanced networking, end-to-end quality of service, transport performance, delay tolerant networks, and new applications. Even if SatNEx is mainly devoted to satellite communications, integration of satellite and terrestrial wireless segments has also been taken into account. This paper presents the results obtained in the specific research activity devoted to trials and testbeds developed during the SatNEx project. The high number of authors of this paper is an indication of the strong collaborative work sustained, the numerous measurement campaigns, and the complexity in developing integrated testbeds. Copyright

A flexible QoS architecture for DVB-RCS2

SUMMARY This paper presents a QoS, architecture for the Higher Layers Satellite in the second generation Digital Video Broadcasting with Return Channel via Satellite standard. It describes the flexibility offered by integrating link and network functions to support internet protocol-based differentiated QoS. The operation of key mechanisms is outlined and analysed to show how these may be combined to realise differentiated services by defining mappings between per-hop behaviours, to queues in the traffic plane and return channel satellite terminal request classes in the control plane. Simulation is used to show that an appropriate mapping can be used to can tune the offered service to different traffic classes and that an appropriate assignment can significantly improve the satellite bandwidth efficiency and/or the performance/QoS supplied to a traffic class. Copyright

Evaluating Web Traffic Performance over DVB-RCS2

The web has undergone a radical change over time. Changes not only in the volume of data transferred, but also the way content is delivered to the user. Current web server architectures are often highly distributed and adapted for user interaction, with transactions characterised by multiple connections to multiple servers. This paper discusses the implication of this new web on next generation two-way satellite systems. It seeks to answer the question of whether classical resource provisioning remains suitable for this traffic. It first presents a more representative simulation model that captures the key features of modern web traffic. It then uses simulation to evaluate the performance over the second generation of DVB-RCS, assessing the impact on performance for a range of bandwidth on demand methods. This paper may be used to formulate recommendations for how to support web traffic in DVB-RCS2.

Using Quick‐Start to enhance TCP‐friendly rate control performance in bidirectional satellite networks

SUMMARY In the not so distant future, we envisage an Internet where the biggest share of capacity is used by streaming applications. To avoid congestion collapse from unresponsive flows calls for a robust and ubiquitous end-to-end multimedia congestion control mechanism, such as TCP-friendly rate control (TFRC), which provides fair sharing with the other Internet traffic. This paper therefore analyses the implications of using rate-adaptive congestion control over satellite links that utilize demand allocation multiple access (DAMA) to maximize satellite transponder utilization. The interaction between TFRC and DAMA is explored using simulations supported by fluidic flow models. The analysis shows that DAMA slows down the start-up phase of TFRC, causing non-negligible delays. To mitigate this problem, we propose a new cross-layer method based on the Quick-Start mechanism. This can accelerate the start-up of multimedia flows by a judicious allocation of additional capacity derived from cross-layer signalling. Copyright� 2009 John Wiley & Sons, Ltd.

Analysing TCP for Bursty Traffic

The Transmission Control Protocol has been designed to support interactive and bulk applications, with performance tuned to support bulk applications that desire to continuously send data. In contrast, this paper analyses TCP performance for a class of applications that do not wish to send continuous data, but instead generate bursts of data separated by application-limited periods in which little or no data is sent. In this context, the paper evaluates an experimental method, Congestion Window Validation (CWV), proposed to mitigate the network impact of bursty TCP applications. Simulation results show that TCP-CWV exhibits a conservative behaviour during application-limited periods. The results also show that TCP-CWV is able to use the available capacity after an idle period over a shared path and that this can have benefit, especially over long delay paths, when compared to slow-start restart specified by standard TCP. The paper recommends the development of CWV-like algorithms to improve the performance for bursty applications while also providing an incentive for application designers to use congestion control.

Optimization of Scheduling Algorithms Parameters in a DiffServ Environment

In this paper, we evaluate the performance achieved by DWRR and WFQ, the widespread implementations of frame-based and sorted-priority schedulers respectively, in a high speed DiffServ environment. Using deterministic upper bounds of the end-to-end delay, we evaluate the schedulers behavior for a wide range of bandwidth configurations. Our analysis highlights the drawbacks of using frame-based scheduling with respect to sorted-priority when dealing with multiple classes of service. As a consequence, we propose a configuration of scheduler weights, which allows to contain the loss in performance of frame-based schemes, while minimizing the end-to-end delay for premium classes. Extensive simulations confirm our theoretical propositions and show the effectiveness of our method in satisfying hard quality of service requirements even in highly congested networks.