Jello: A MAC Overlay

To demonstrate the impact of Papyrus as an experimental platform, we have used it to implement Jello, a distributed MAC overlay for high-bandwidth media streaming applications, for example, in digital homes. In this case, Jello enables simultaneous media sessions to work in parallel on isolated frequencies, avoiding costly wireless interference. As shown in the following figure, these sessions also make use of the spectrum efficiently by adapting their frequency usage to varying traffic demands. Designed for plug-and-play consumer devices, Jello does not require centralized spectrum controllers or dedicated radios for control traffic.

Jello uses Papyrus as its physical layer component, but implements a distributed coordination framework to configure and adapt device’s spectrum usage. Without disrupting others, each Jello sender and receiver pair perform spectrum sensing to identify usable spectrum, and coordinate to determine their initial communication frequency. During the session they continue to sense and determine if and when they should change their communication frequency, and what subcarriers to switch to. Without any extra control radio, the device pair exchange coordination messages in-band on the data channel, and apply a resynchronization mechanism to recover from occasional link failures. Using this robust distributed coordination framework, Jello devices can obtain contiguous use of frequency matching their demands, and support demanding applications such as media streaming.

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Figure 3: Jello’s Per-session FDMA provides contiguous use of spectrum to streaming applications, matching their time-varying demands.

Jello also utilizes Papyrus’s frequency-agility to improve spectrum usage efficiency and address the problem of spectrum fragmentation. As transmissions occupy and release spectrum on the fly, the available spectrum becomes increasingly fragmented. Jello first suppresses the impact of fragmentation directly at the physical layer by devices scavenging multiple frequency fragments to form a high-bandwidth transmission. But because the guardband overhead increases with the number of fragments in use, this solution works well under moderate level of fragmentation. Jello also offers a complementary MAC layer solution by devices self defragmenting the spectrum on-demand. That is, when needed, Jello devices change their frequency usage to self-defragment the spectrum. These frequency moves are performed by individual device pairs independently. They are nearly instantaneous and transparent to neighboring flows. By combining both solutions, Jello effectively minimizes spectrum fragmentation and makes the best use of the spectrum.