The MPEG-4 standard encodes a video stream in 3 different frames, I, P and B, where the P and B frames depend on the I frame. Loosing an I frame is especially bad. In this paper we exploit the Partial Reliability features of the SCTP transport protocol to selectively retransmit I frames in presence of congestion, obtaining a better quality of the decoded stream.

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We describe the design and implementation of PEG, a networked system of distributed sensor nodes that detects an uncooperative agent called the evader and assists an autonomous robot called the pursuer in capturing the evader. PEG requires services such as leader election, routing, network aggregation, and closed loop control. Instead of using general purpose distributed system solutions for these services, we employ whole-system analysis and rely on spatial and physical properties to create simple and efficient mechanisms. We believe this approach advances sensor network design, yielding pragmatic solutions that leverage physical properties to simplify design of embedded distributed systems.We deployed PEG on a 400 square meter field using 100 sensor nodes, and successfully intercepted the evader in all runs. While implementing PEG, we confronted practical issues such as node breakage, packaging decisions, in situ debugging, network reprogramming, and system reconfiguration. We discuss the approaches we took to cope with these issues and share our experiences in deploying a large sensor network system.