Automating Cross-Layer Diagnosis of Enterprise Wireless Networks

Yu-Chung Cheng, Mikhail Afanasyeve, Peter Benko, Patrick and Chiang Verkaik Jennifer , Alex Snoeren, Stefan Savage and Geoff Voelker
March 9, 2007

Modern enterprise networks are of sufficient complexity that even {\em simple} faults can be difficult to diagnose --- let alone transient outages or service degradations. Nowhere is this problem more apparent than in the 802.11-based wireless access networks now ubiquitous in the enterprise. In addition to the myriad complexities of the wired network, wireless networks face the additional challenges of shared spectrum, user mobility and authentication management. Not surprisingly, few organizations have the expertise, data or tools to decompose the underlying problems and interactions responsible for transient outages or performance degradations. In this paper, we present a set of analysis techniques and models to precisely determine all sources of data transfer delay due to media access and mobility in 802.11 networks --- from the physical layer to the transport layer --- as well as the interactions among them. While some sources of delay can be directly measured, many of the delay components, such as AP queuing, backoffs, contention, etc., must be inferred. To infer these delays from measurements, we develop a detailed model of MAC protocol behavior, both as it is described in the 802.11 specification as well as how it is implemented in vendor hardware. Combined with comprehensive traces of wireless activity taken from an enterprise network, we produce a complete delay breakdown for packet transmissions and pinpoint problems that constrain connectivity or limit performance.

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