Catálogo de publicaciones - libros

In this paper we analyze measurements from the Finnish University Network (Funet) and study the effect of spatial aggregation on the origin- destination flows. The traffic is divided into OD pairs based on IP addresses, using different prefix lengths to obtain data sets with various aggregation levels. We find that typically the diurnal pattern of the total traffic is followed more closely by the OD pairs as their volume increases, but there are many exceptions. Gaussian assumption holds well for all OD pairs when the aggregation level is high enough, and we find an approximate threshold for OD pair traffic volume after which they tend to be Gaussian. Also the functional mean-variance relation holds better when the aggregation level is higher.

The development of e-Health services in rural environments, where broadband networks are usually not accessible, requires a specific analysis of available resources to improve Quality of Service (QoS) management. This work quantifies the maximum number of simultaneous users that fulfill the specific QoS levels in common e-Health services, including both store-and-forward and real-time telemedicine applications. The analysis also proposes variations in the modelling of traffic distributions regarding the number of multiplexed users. The results obtained in this study permit an accurate users dimensioning, which is necessary to optimize the performance and to guarantee the QoS requirements in this kind of services where network resources are limited.

This paper provides recent empirical traffic data and observations of telephony traffic patterns in mobile and IP telephony. These are compared with old telephony patterns from Public Switched Telephone Networks (PSTN) to investigate potential evolution and impact on traffic characterisations due to technology changes from fixed to mobile phones, changes in quality from fixed phone to mobile and IP telephone, changes in tariffs from usage based to flat-rate subscriptions, and appearance of alternative message based communication means.

The results show different daily and weekly traffic profiles compared to PSTN telephony. In particular, the profile of international calls is significantly changed. Furthermore, the average call holding times show significant variations over the day in flat-rate subscriptions. Finally, the results indicate that the Short Message Service (SMS) seems to serve as a supplement to phone calls, in particular in the evenings, which might change call holding time distribution and traffic intensities.

We present a comprehensive analysis of the packet loss process in a finite-buffer queue fed by the Markov-modulated Poisson process. In particular, solutions for the number of losses in (0,t], the stationary number of losses and the loss ratio are presented in closed forms. Theoretical results are illustrated via numerical examples based on an IP traffic trace file.

Real-time traffic aggregates are often characterized by time-varying nature. However, resources for this kind of traffic are usually allocated assuming busy hour stationary traffic characteristics. This assumption may lead to inefficient use of network resources when static resource reservation is used. In this paper we propose the algorithm for on-line estimation of the traffic state in terms of piecewise weakly stationary stochastic process. As a basic tool of the algorithm we use change-point statistical test allowing us to dynamically and automatically determine whether the traffic pattern changes and, if so, estimate new parameters of the traffic pattern. Assuming that a network may assign the required resources on-demand on a per-node basis, the proposed procedure helps to decide which amount of resources should be assigned to handle the traffic with given performance metrics. The proposed algorithm is well-suited for non-stationary behavior of the aggregated traffic where statistical parameters change in time. The computational complexity of the algorithm is low and it can be implemented on a traffic class basis making the proposal suitable for backbone routers.

The article investigates the influence of the way packets are chosen to be dropped (end of the tail, head of the tail) on the performance, i.e. response time for in case of RED and DSRED queues - two representative active queue management mechanisms used in IP routers. In particular, the self-similar traffic is considered. The quantitative analysis is based on simulation and Markov chain models solved numerically.

One of TCP’s key tasks is to react and avoid network congestion episodes which normally arise in packet switched networks. A wide literature is available concerning the behaviour of congestion control algorithms in many different scenarios and several congestion control algorithms have been proposed in order to improve performances in specific scenarios. In this paper we focus on the UMTS wireless scenario and we report a campaign of measurements that involved around 3000 flows and more than 40 hours of measurements using three different TCP stacks: TCP NewReno, which is the congestion control algorithm standardized by IETF, TCP BIC which is the default congestion control algorithm adopted by the Linux operating system, and TCP Westwood+ also available in the Linux kernel. The experimental evaluation has been carried out by accessing the public Internet using an UMTS card. Measurements of goodputs, RTTs over time, packet loss ratios, number of timeouts and Jain Fairness Indices are reported through cumulative distribution functions. Moreover, the efficiency of each TCP version in transferring files has been evaluated by varying the file size in the range from 50 up to 500 . The cumulative distribution functions reported in the paper show interesting results: 1) a single downlink flow is far from saturating the channel bandwidth; 2) considered TCP stacks provide similar results; 3) 90 (50) percentile of the goodput of a single downlink flow is less or equal then 230 (120 ) compared to a nominal 384 UMTS downlink channel.

It is well-known that the transmission control protocol (TCP) does not perform well in wireless and satellite environments. We investigate the use of cross-layer design involving the transport and medium access control (MAC) layers in the context of a geostationary bandwidth-on-demand satellite network to simultaneously enhance TCP performance and to improve bandwidth utilization. In this paper, we focus on the slow-start phase of the connection. In essence, we create a bandwidth pipe between the two layers so that through cross-layer interactions, the TCP connections are aware of the satellite resources available to them, thus adjusting their congestion window accordingly. Our proposal includes minimal changes to the original protocol, allowing easier integration and inter-working with existing infrastructure. Our evaluation results show a shorter slow-start duration with better bandwidth utilization. Although the performance gain is higher in a lossy satellite link, we also found that it is dependent on the network load.

The performance of a TCP protocol on MANETs has been studied in a numerous researches. It has been shown that performance of TCP on MANET is poor and that it is caused by two groups of reasons: one inherited from wireless networks and one caused by nodes mobility. One reason of TCP degradation is inability to distinguish between packet losses due to congestion from those caused by nodes mobility and as consequence broken routes. This paper presents the Cluster-Label-based Routing protocol that is an attempt to compensate source of TCP problems on MANETs – multi-hop mobile environment. By utilizing Cluster-Label-based mechanism for Backbone the CLR is able to concentrate on detection and compensation of movement of a destination node. The proposed protocol provides better goodput and delay performance than standardized protocols especially in cases of large network size and/or high mobility rate.

We propose and analyse a discrete-time queueing model for the evaluation of the IEEE 802.16e sleep mode operation in wireless access networks. This mechanism reduces the energy consumption of a mobile station (MS) by allowing it to turn off its radio interface () when there is no traffic present at its serving base station (BS). After a sleep period expires, the MS briefly checks the BS for data packets and switches off for the duration of another sleep period if none are available. Specifically for IEEE 802.16e, each additional sleep period doubles in length, up to a certain maximum. Clearly, the sleep mode mechanism can extend the battery life of the MS considerably, but also increases the delay at the BS buffer.

For the analysis, we use a discrete-time queueing model with general service times and multiple server vacations. The vacations represent the sleep periods and have a length depending on the number of preceding vacations. Unlike previous studies, we take the traffic correlation into account by assuming a D-BMAP arrival process. The distribution of the number of packets in the queue is obtained at various sets of time epochs, as well as the mean packet delay and the mean number of consecutive vacations. We apply these results to the IEEE 802.16e sleep mode mechanism with downlink traffic. By means of some examples, we show the influence of both the configuration parameters and the traffic correlation on the delay and the energy consumption.