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Welcome to the series of articles on Wi-Fi roaming analysis. In this article, part 1, we will define Wi-Fi roaming, provide background on how a client and access point establish a connection, what roles each play in the control of the connection, and establish the importance of Wi-Fi roaming analysis for engineers to successfully operate a modern wireless LAN (WLAN) environment. Wi-Fi Roaming Analysis Series: • • • • • Part 5 - Analysis with Wildpackets Omnipeek (coming) • Part 6 - Tips for Roaming Performance Improvement (coming) Introduction Advanced protocol analysis is becoming an increasingly important skill for Wi-Fi engineers as networks grow increasingly sophisticated and complex. The wireless LAN market is a tremendously innovative and fast-changing landscape, and the skills necessary to understand and dissect their inner workings are highly valuable.
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One of the most important aspects of building a successful enterprise wireless LAN is ensuring adequate Wi-Fi roaming performance. However, Wi-Fi roaming is a complex subject due to the many variations of Wi-Fi security found in the marketplace and the historical difficulty in being able to easily gather and analyze roaming data. In this series I will provide an overview of Wi-Fi roaming, how it works, and provide readers with guidance on how to capture, measure, and analyze wireless roaming performance of clients within their own environments. In addition, I'll highlight a few professional tools and tricks of the trade to make this process simpler than manual analysis. Wi-Fi Roaming Definition Roaming, in the context of an 802.11 wireless network, is the process of a client moving an established Wi-Fi network association from one access point to another access point within the same Extended Service Set (ESS) without losing connection (e.g.
Within a defined time interval, usually in the range of a few seconds). It is also helpful to distinguish between different wireless connection scenarios that may occur.
Delineation will provide a better understanding of how and when each scenario will occur, why variations in performance between scenarios exist, and aid in establishing performance baselines. • Initial Connection - The client has no previous 802.11 association to the ESS (any AP advertising the same SSID). This situation requires the client to perform all required connection and authentication steps defined in the network policy before network access is achieved. The time required for a client to perform an initial connection will be the same as wireless roaming unless fast roaming or session caching techniques are implemented.
The length of time required to complete full 802.1X authentication in secure wireless environments is considerably longer than in open or pre-shared key (PSK) networks, making implementation of fast roaming techniques highly desirable. It may even be required depending on the network architecture and applications implemented (e.g. Branch / remote office networks with central RADIUS across the WAN increase the time to complete EAP authentication and can render real-time voice applications unusable). • Wireless Roaming - The client has an established 802.11 association to an infrastructure AP and migrates its connection within the same ESS to another AP.
Association to the new AP terminates the previous AP association either implicitly or explicitly (only one association is allowed at a time, per the 802.11 standard). The goal of a wireless roam is to identify an alternate AP that can provide better service to the client than the current AP. Wireless client roaming algorithms are typically optimized to minimize the time required to transition between APs in order to avoid network access disruptions to client applications. This can be accomplished through fast roaming or session caching techniques that eliminate some of the authentication steps. Fast roaming can only occur after an initial connection has been performed to ensure the client has successfully completed all required authentication and authorization required by the network policy. • Connection Termination & Re-Establishment - The client has an established 802.11 association, but the performance severely degrades to the point that the connection is rendered unacceptable. The client and/or AP is required to recognize the degraded connection, which may not be explicitly apparent, then terminate and re-establish a connection from scratch. A connection could degrade for a number of reasons, including interference, multipath (with older 802.11a/b/g clients), excessive packet error rate, out of range, roam not completed within the client's time threshold, etc.