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In this paper, we analyze the performance of mobile real time services in a large-scale IEEE 802.11 multi-hop network. We present our field measurements with discussion of performance bottlenecks for VoIP services under mobile scenarios. We utilized our test bed which is located in Nebraska and supported by the University of Nebraska-Lincoln (UNL), US Federal Railroads Administrations (FRA), and major US railroad companies. Our UNL-FRA test bed consists of 8 outdoor access point towers, which are deployed along 3.5 mile of BNSF railroad. Passive measurement approaches are taken to ensure the integrity of collected data and multi-layer stream based packet analyzer has been implemented to provide a global view of the entire performance of the monitored network from the physical layer to the application layer. Based on our analysis of collected data, we conclude that in a typical outdoor 802.11 environment similar to our UNL-FRA test bed, uncertain handoff latencies and lack of Quality of Service guarantee are main performance bottlenecks for real-time applications. Furthermore, we discuss the enhancement strategies to support mobility in high-speed railway networks. We believe our work is one of the initial efforts in wireless mobile network field measurement in terms of scale, methodology, and analysis.