Impact of Vertical Handovers on Cooperative Content Distribution Systems

In this paper, we evaluate the performance of a P2P- based content distribution system in heterogeneous, wireless networks. The mobile users coordinate each other with cooperation strategies enabled by the multi-source download mechanism, as in eDonkey or BitTorrent. Due to the mobility, vertical handovers between different wireless access technologies are required which may result in transmission delays and IP address changes of the switching peer. Hence, connections among users have to be reestablished and downloading users are requeued at a providing peer's waiting queue. In detail, we investigate the impact of requeueing with each VHO as well as the use of mechanisms that preserve the IP address and connections beyond VHOs, like MobilelP. Another important phenomenon occurring with VHOs is the abrupt change of the available bandwidth, e.g., from a fast WLAN connection to a rather slow UMTS connection. We evaluate the download times for files by means of simulation while considering different load scenarios in today's and future network layouts of the B3G network. As a result of the performance evaluation, we derive a new time-based cooperation strategy that counters the impact of mobility. Instead of downloading individual blocks of a file, a user gets a time slot at a providing peer. We show that this leads to a significant performance gain.