Extended Abstract: Towards Context-Aware Wireless Spectrum Agility Ashwini Kumar and Kang G. Shin

1. Sponsored by Extended Abstract: Towards Context-Aware Wireless Spectrum Agility Ashwini Kumar and Kang G. Shin Real-Time Computing Laboratory, EECS, The University of Michigan, Ann Arbor, MI, U.S.A. {ashwinik, kgshin}@eecs.umich.edu CASA Evaluation Motivation • Consists of a set of decision-making steps with a central theme of correlating recent past observations in channel and traffic conditions to near future. • For each upcoming traffic burst, an applications exports its start time, bandwidth requirement, and maximum tolerable latency. • Operates periodically at the beginning of intervals called “epochs,” and for each upcoming epoch, considers the total bandwidth requirement (Bs) and minimum delay requirement (D) using the application provided hints. • Adjusts scanning parameters (fscan , tscan) in order to accommodate delay and bandwidth requirements. • Accounts for switching to a different channel in a probabilistic fashion (using notion of push factors, Pcurr and Ppast), if requirements are not expected to be met in the current channel. For a channel-switch to occur, better channels with lower utilization than current channel should be available. • Simulation-based: Basic SA modules were implemented in ns-2.29 simulator, as a sub-layer of 802.11 MAC protocol. • Objective: To see if and how much CASA improves performance over conventional SA. • Application-centric metrics used: • Overall application throughput during application run (T). • Fraction of application run-time during which the send bandwidth requirement is satisfied (Fb). • Fraction of application run-time during which the delay requirement is satisfied (Fd). • Overheads of cross-layer exchange as well as sub-optimal SA decisions are accounted for. • Simulation scenario: • 4 licensed channels, each with a primary communication group • 3 secondary communication groups • Random ON-OFF traffic for both type of groups (UDP used) • Epoch = 1 s • fscan = {2,1,0.5,0.25} s-1 • tscan = {0.05,0.025, 0.0125} s • D = [10,100] ms • Conclusion: CASA outperforms traditional SA, especially in stricter requirements. • Spectrum Agility(SA) involves opportunistic utilization of idle licensed spectrum bands, improving efficient spectrum utilization and hence providing more bandwidth to network applications. • Fundamental operations involved in SA, such as channel scans, can adversely affect application performance, offsetting its benefits. • Issues with state-of-art SA protocols: • Agnostic of application requirements, or with very limited context knowledge • Parameters are not adaptive • Focus on “unused” channel rather than “good” channels Approach • Captures a trade-off between SA operations (e.g., sensing) and application requirements (e.g., maximum end-to-end delay). • Incorporates an application-aware adaptive scanning and switching mechanism, called Context-Aware Spectrum Agility(CASA). • CASA uses information exchanged between application and link layer via Cross-Layer Interaction Framework(CLIF). Summary and Future Work • Importance of application-awareness in SA was argued and proven. • Systems-based optimization mechanism to accommodate application needs. • CASA is shown to increase resilience of the SA protocol in supporting higher bandwidth and stringent delay requirements. • Future work: • Further analysis on realistic traffic patterns and applications. • Extend CASA to incorporate more context. • Support for multiple wireless interfaces in CASA. • Uses simple history-based techniques to estimate bandwidth observed on current channel (BS), and also to account for legacy application traffic. • Takes channel utilization information from Spectral Opportunity Map(SOM). • Makes probabilistic channel-switch decisions to prevent overcrowding on a single channel. • The periodic context provided to SA protocols consist of both application hints and channel conditions. • The key component of CASA is the CASA Algorithm that adjusts SA operations based on context. MOBICOM 2007, Montreal, QC, Canada