Contribution to techniques in order to improve the performance of internet acces over 2.5g, 3g and beyond 3g networks

Resumen

Resum de la tesi (màxim 4000 caràcters. Si se supera aquest límit, el resum es tallarà automàticament al caràcter 4000) Internet, as a global network, offers services that were unthinkable some years ago. First, it works as a great content provider and distributor of all kind of information. Secondly, it has opened the door to a multitude of services. User needs have also changed. Since current life stiles tend to require the mobility of users, the demand for being always and everywhere connected and available has been increased. In this respect, the concept of ubiquitous Internet access takes shape. The development of wireless technologies and, at the same time, the commercialization of new mobile devices that support these technologies can form the basis to cover the new connectivity needs and requirements of users. However, the fact of providing Internet access or enabling connectivity between compliant devices is not enough; it should be assured that communications occur in an efficient manner, i.e. that a user can profit from the services offered over wireless technologies. The global goal of this Ph.D. dissertation centres on this aspect. Internet is based on the TCP/IP protocols stack; in concrete, applications that require end-to-end reliability mostly depend on TCP as the underlying transport protocol. This protocol was designed to work in fixed networks with a low error bit rate (BER), where most problems are caused by congestion at some network elements. Thus, to prevent or combat congestion, TCP includes congestion control mechanisms, which limit the injection of data to the network. Most problems that occur in wireless networks are barely caused by congestion; instead of this, problems are determined by the special features of the wireless link. Hence, some aspects, such as mobility or the high variability of the channel conditions, can be misunderstood as congestion by TCP, what might notably degrade the performance of communications. The first concern of this thesis is the feasibility of current cellular networks to provide access to Internet services. The author presents a live evaluation of the performance of Internet protocols and services over GPRS, UMTS and HSPA networks. This study discusses the impact of most relevant lower layer mechanisms, the improvement achieved with end-to-end optimization methods and the benefits of solutions based on the use of Performance Enhancement Proxies. Moreover, analytic expressions are provided to quantify the underutilization of the link suffered by TCP and HTTP/1.1 transmissions, where the impact of simultaneous connections, non-ideal pipelining, application buffers and DCH dynamic resource assignment mechanisms are considered. Several proposals and recommendations are provided to maximize the performance of web downloads over cellular networks. Secondly, WLAN multi-hop networks are studied as an example of Multi-hop Wireless Networks (MWN). These networks can be used to cover a multitude of possible use cases: public safety, the creation of rapid deployments or the coverage extension to places that lack of an infrastructure... Due to the existence of multiple scenarios, it is difficult to find a universal solution to optimize the performance of communications. In this respect, the thesis focuses on the investigation of approaches to improve the performance of TCP over MWNs. Concretely, the impact of contention and routing protocols on the performance of the transport protocol is evaluated through simulation and real testbeds. Furthermore, a cross-layer approach is proposed to face up the problems of the transport protocol in these environments. Finally, as an example of the interconnection of heterogeneous networks (in the way to the ubiquity concept of 4G networks), the feasibility of using a cellular network to provide Internet access to a WLAN multi-hop network by means of a proxy-based solution is evaluated. This scenario tries to join the experiences learned from the evaluation of communications over cellular and multi-hop environments.