SyncScan: practical fast handoff for 802.11 infrastructure networks
13 Mar 2005-Vol. 1, pp 675-684
TL;DR: SyncScan is described, a low-cost technique for continuously tracking nearby base stations by synchronizing short listening periods at the client with periodic transmissions from each base station and it is demonstrated that it allows better handoff decisions and over an order of magnitude improvement in handoff delay.
Abstract: Wireless access networks scale by replicating base stations geographically and then allowing mobile clients to seamlessly "hand off" from one station to the next as they traverse the network. However, providing the illusion of continuous connectivity requires selecting the right moment to handoff and the right base station to transfer to. Unfortunately, 802.11-based networks only attempt a handoff when a client's service degrades to a point where connectivity is threatened. Worse, the overhead of scanning for nearby base stations is routinely over 250 ms - during which incoming packets are dropped - far longer than what can be tolerated by highly interactive applications such as voice telephony. In this paper we describe SyncScan, a low-cost technique for continuously tracking nearby base stations by synchronizing short listening periods at the client with periodic transmissions from each base station. We have implemented this SyncScan algorithm using commodity 802.11 hardware and we demonstrate that it allows better handoff decisions and over an order of magnitude improvement in handoff delay. Finally, our approach only requires trivial implementation changes, is incrementally deployable and is completely backward compatible with existing 802.11 standards.
Citations
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TL;DR: It is proved that the weighted graph coloring problem is NP-hard and scalable distributed algorithms that achieve significantly better performance than existing techniques for channel assignment are proposed.
Abstract: We propose techniques to improve the usage of wireless spectrum in the context of wireless local area networks (WLANs) using new channel assignment methods among interfering Access Points (APs). We identify new ways of channel re-use that are based on realistic interference scenarios in WLAN environments. We formulate a weighted variant of the graph coloring problem that takes into account realistic channel interference observed in wireless environments, as well as the impact of such interference on wireless users. We prove that the weighted graph coloring problem is NP-hard and propose scalable distributed algorithms that achieve significantly better performance than existing techniques for channel assignment. We evaluate our algorithms through extensive simulations and experiments over an in-building wireless testbed.
394 citations
17 Aug 2008
TL;DR: ViFi is developed, a protocol that opportunistically exploits basestation diversity to minimize disruptions and support interactive applications for mobile clients that doubles the number of successful short TCP transfers and doubles the length of disruption-free VoIP sessions compared to an existing WiFi-style handoff protocol.
Abstract: We ask if the ubiquity of WiFi can be leveraged to provide cheap connectivity from moving vehicles for common applications such as Web browsing and VoIP. Driven by this question, we conduct a study of connection quality available to vehicular WiFi clients based on measurements from testbeds in two different cities. We find that current WiFi handoff methods, in which clients communicate with one basestation at a time, lead to frequent disruptions in connectivity. We also find that clients can overcome many disruptions by communicating with multiple basestations simultaneously. These findings lead us to develop ViFi, a protocol that opportunistically exploits basestation diversity to minimize disruptions and support interactive applications for mobile clients. ViFi uses a decentralized and lightweight probabilistic algorithm for coordination between participating basestations. Our evaluation using a two-month long deployment and trace-driven simulations shows that its link-layer performance comes close to an ideal diversity-based protocol. Using two applications, VoIP and short TCP transfers, we show that the link layer performance improvement translates to better application performance. In our deployment, ViFi doubles the number of successful short TCP transfers and doubles the length of disruption-free VoIP sessions compared to an existing WiFi-style handoff protocol.
290 citations
Cites background from "SyncScan: practical fast handoff fo..."
...the delay associated with handoffs in wireless networks [31 , 2, 34, 6, 20]....
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23 Apr 2006
TL;DR: This work proposes an efficient client-based approach for channel management (channel assignment and load balancing) in 802.11-based WLANs that lead to better usage of the wireless spectrum that is based on a “conflict set coloring” formulation that jointly performs load balancing along with channel assignment.
Abstract: We propose an efficient client-based approach for channel management (channel assignment and load balancing) in 802.11-based WLANs that lead to better usage of the wireless spectrum. This approach is based on a “conflict set coloring” formulation that jointly performs load balancing along with channel assignment. Such a formulation has a number of advantages. First, it explicitly captures interference effects at clients. Next, it intrinsically exposes opportunities for better channel re-use. Finally, algorithms based on this formulation do not depend on specific physical RF models and hence can be applied efficiently to a wide-range of in-building as well as outdoor scenarios. We have performed extensive packet-level simulations and measurements on a deployed wireless testbed of 70 APs to validate the performance of our proposed algorithms. We show that in addition to single network scenarios, the conflict set coloring formulation is well suited for channel assignment where multiple wireless networks share and contend for spectrum in the same physical space. Our results over a wide range of both simulated topologies and in-building testbed experiments indicate that our approach improves application level performance at the clients by upto three times (and atleast 50%) in comparison to current best-known techniques.
252 citations
Cites background or methods from "SyncScan: practical fast handoff fo..."
...Clientdriven approaches have been used for various optimizations in WLANs [8], [9]....
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...Changing the channel for an AP/client is a relatively lowcost operation ( 1 − 2 ms) which can be implemented mostly as a driver update [9]....
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19 Jun 2006
TL;DR: Virgil scans for all available APs at a location, quickly associates to each, and runs a battery of tests to estimate the quality of each AP's connection to the Internet.
Abstract: This paper presents Virgil, an automatic access point discovery and selection system. Unlike existing systems that select access points based entirely on received signal strength, Virgil scans for all available APs at a location, quickly associates to each, and runs a battery of tests to estimate the quality of each AP's connection to the Internet. Virgil also probes for blocked or redirected ports, to guide AP selection in favor of preserving application services that are currently in use. Results of our evaluation across five neighborhoods in three cities show Virgil finds a usable connection from 22% to 100% more often than selecting based on signal strength alone. By caching AP test results, Virgil both improves performance and success rate. Our overhead is acceptable and is shown to be faster than manually selecting an AP with Windows XP.
250 citations
Cites background from "SyncScan: practical fast handoff fo..."
...SyncScan [24] modifies access points as well as clients, forcing APs to synchronize their beacon frame broadcast schedules....
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...SyncScan: Practical Fast Hando. for 802.11 Infrastructure Networks....
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...This happens because APs broadcast their beacon signals at unpredictable times, and do not always respond to beacon requests in a timely fashion [24]....
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...SyncScan [24] modi.es access points as well as clients, forcing APs to synchronize their beacon frame broadcast schedules....
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...Ideally, Virgil would leverage systems such as MultiNet [8] or SyncScan [24] to continuously scan for new access points, without having to disassociate from its current AP, but we have not implemented this....
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19 Jun 2006
TL;DR: The architecture and protocols of SMesh, a completely transparent wireless mesh system that offers seamless, fast handoff, supporting VoIP and other real-time application traffic for any unmodified 802.11 device, are presented.
Abstract: This paper presents the architecture and protocols of SMesh, a completely transparent wireless mesh system that offers seamless, fast handoff, supporting VoIP and other real-time application traffic for any unmodified 802.11 device. In SMesh, the entire mesh network is seen by the mobile clients as a single, omnipresent access point.Fast handoff is achieved by ensuring that each client is served by at least one access point at any time. Mobile clients are handled by a single access point during stable connectivity times. During handoff transitions, SMesh uses more than one access point to handle the moving client. Access points continuously monitor the connectivity quality of any client in their range and efficiently share this information with other access points in the vicinity of that client to coordinate which of them should serve the client.Experimental results on a fully deployed mesh network consisting of 14 access points demonstrate the effectiveness of the SMesh architecture and its handoff protocol.
198 citations
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TL;DR: RADAR is presented, a radio-frequency (RF)-based system for locating and tracking users inside buildings that combines empirical measurements with signal propagation modeling to determine user location and thereby enable location-aware services and applications.
Abstract: The proliferation of mobile computing devices and local-area wireless networks has fostered a growing interest in location-aware systems and services. In this paper we present RADAR, a radio-frequency (RF)-based system for locating and tracking users inside buildings. RADAR operates by recording and processing signal strength information at multiple base stations positioned to provide overlapping coverage in the area of interest. It combines empirical measurements with signal propagation modeling to determine user location and thereby enable location-aware services and applications. We present experimental results that demonstrate the ability of RADAR to estimate user location with a high degree of accuracy.
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01 Mar 1992
TL;DR: This document describes the Network Time Protocol (NTP), specifies its formal structure and summarizes information useful for its implementation and describes the methods used for their implementation.
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1,057 citations
"SyncScan: practical fast handoff fo..." refers background in this paper
...By far the simplest approach is to leverage the wide availability of Network Time Protocol (NTP) service over the Internet [13], since it provides a standard out-of-band means for synchronizing APs to absolute time reference (in fact, we find that most enterprise access point products already support NTP)....
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...Inexpensive 802.11-based access points (APs) provide transparent connectivity to the wired Internet at low cost and with minimal configuration overhead....
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01 Apr 2003
TL;DR: This paper presents an empirical study of this handoff process at the link layer, with a detailed breakup of the latency into various components, showing that a MAC layer function - probe is the primary contributor to the overall handoff latency.
Abstract: IEEE 802.11 based wireless networks have seen rapid growth and deployment in the recent years. Critical to the 802.11 MAC operation, is the handoff function which occurs when a mobile node moves its association from one access point to another. In this paper, we present an empirical study of this handoff process at the link layer, with a detailed breakup of the latency into various components. In particular, we show that a MAC layer function - probe is the primary contributor to the overall handoff latency. In our study, we observe that the latency is significant enough to affect the quality of service for many applications (or network connections). Further we find variations in the latency from one hand-off to another as well as with APs and STAs used from different vendors. Finally, we discuss optimizations on the probe phase which can potentially reduce the probe latency by as much as 98% (and a minimum of 12% in our experiments). Based on the study, we draw some guidelines for future handoff schemes.
954 citations
06 Jun 2004
TL;DR: The use of a novel and efficient discovery method using neighbor graphs and non-overlap graphs that reduces the total number of probed channels as well as the total time spent waiting on each channel and reduces the overall probe time significantly.
Abstract: The 802.11 IEEE Standard has enabled low cost and effective wireless LAN services (WLAN). With the sales and deployment of WLAN based networks exploding, many people believe that they will become the fourth generation cellular system (4G) or a major portion of it. However, the small cell size of WLAN creates frequent hand-offs for mobile users. If the latency of these hand-offs is high, as previous studies have shown, then the users of synchronous multimedia applications such as voice over IP (VoIP) will experience excessive jitter. The dominating factor in WLAN hand-offs has been shown to be the discovery of the candidate set of next access points. In this paper, we describe the use of a novel and efficient discovery method using neighbor graphs and non-overlap graphs. Our method reduces the total number of probed channels as well as the total time spent waiting on each channel. Our implementation results show that this approach reduces the overall probe time significantly when compared to other approaches. Furthermore, simulation results show that the effectiveness of our method improves as the number of non-overlapping channels increases, such as in the 5 GHz band used by the IEEE 802.11a standard.
330 citations
07 Mar 2004
TL;DR: A novel and efficient data structure, neighbor graphs, is described, which dynamically captures the mobility topology of a wireless network as a means for prepositioning the station's context ensuring that the station’s context always remains one hop ahead.
Abstract: User mobility in wireless data networks is increasing because of technological advances, and the desire for voice and multimedia applications. These applications, however, require fast handoffs between base stations to maintain the quality of the connections. Previous work on context transfer for fast handoffs has focused on reactive methods, i.e. the context transfer occurs after the mobile station has associated with the next base station or access router. In this paper, we describe the use of a novel and efficient data structure, neighbor graphs, which dynamically captures the mobility topology of a wireless network as a means for prepositioning the station's context ensuring that the station's context always remains one hop ahead. From experimental and simulation results, we find that the use of neighbor graphs reduces the layer 2 handoff latency due to reassociation by an order of magnitude from 15.37ms to 1.69ms, and that the effectiveness of the approach improves dramatically as user mobility increases.
322 citations