Mobile communications across wired/multi-hop wireless domains: An experimental approach.

摘要

The overall goal of this dissertation is to address the challenges of wired-to-wireless connectivity for a variety of traffic and connection types (ie, uni and multicast; data and real time). The wireless ad hoc networks find their most important applications in untethered, mobile, multihop scenarios where there is no wired infrastructure. Yet, when the wired infrastructure (say, the Internet) is within reach, opportunistic connections to Internet sites may be established across the multihop network to transfer files and update databases. Similarly, in mobile multicast applications it is very possible that some users are connected directed to the wired infrastructure, while others can be reached only via wireless multiphopping The work described can be divided into two parts. First half describes the implementation of stand-alone and wired-to-wireless on-demand network system based on the on-demand Multicast routing Protocol (ODMRP). ODMRP is an effective and efficient routing protocol designed for mobile wireless ad-hoc networks. One of the major strengths of ODMRP is its capability to operate both as a unicast and a multicast routing protocol. This versatility of ODMRP can increase network efficiency as the network can handle both unicast and multicast traffic with one protocol. In view of our wired-to-wireless overarching goal, we have extended ODMRP to provide the Internet connectivity to the ad-hoc network. We describe the unicast functionality and mobility-handling feature of Extended ODMRP and analyze the protocol performance in a real wired-to-wireless ad-hoc testbed environment. We generate various topological scenarios in our heterogeneous testbed by applying mobility to the wireless network hosts and study their impacts on our protocol performance. The impact of various mobility-handling schemes at the router level for ad-hoc node access has been discussed. Measurements in the testbed network were essential in guiding us in the realistic analysis and performance evaluation of the protocol and in identifying future research directions.; The second part of the dissertation studies the performance of the transport protocols in the adhoc environment. File transfers use TCP for reliability and congestion control. However, recent experiments with ad hoc, multihop 802.11 networks have exposed serious instabilities when TCP connections span both wired and wireless domains. In particular, some TCP connections capture the wireless channel and drive the throughput on other connections virtually to zero. This is most surprising in view of the fact that connections between 802.11 (single hop) wireless LAN stations and the Internet are relatively well behaved, stable and fair. In fact they are routinely used in most Campuses, Businesses and research Labs. In this paper, we investigate the unstable behavior of TCP over 802.11 ad hoc networks and trace its causes to some of the 802.11 features including binary backoff. Then, we propose a simple backoff scheme to replace 802.11 binary backoff and reinstate TCP fairness. Simulation results confirm that the proposed scheme does improve TCP behavior and fairness without introducing significant performance degradation. Finally, we proceed to study the behavior of video streaming transport protocols in the wired-to-wireless environment. Of particular interest to us in this case is the interaction and fair sharing of TCP and video streams.