Results 1 - 10
of
431
SSCH: Slotted Seeded Channel Hopping for Capacity Improvement in IEEE 802.11 Ad-Hoc Wireless Networks
- in IEEE 802.11 Ad-Hoc Wireless Networks,” in ACM Mobicom
, 2004
"... Capacity improvement is one of the principal challenges in wireless networking. We present a link-layer protocol called Slotted Seeded Channel Hopping, or SSCH, that increases the capacity of an IEEE 802.11 network by utilizing frequency diversity. SSCH can be implemented in software over an IEEE 80 ..."
Abstract
-
Cited by 353 (3 self)
- Add to MetaCart
(Show Context)
Capacity improvement is one of the principal challenges in wireless networking. We present a link-layer protocol called Slotted Seeded Channel Hopping, or SSCH, that increases the capacity of an IEEE 802.11 network by utilizing frequency diversity. SSCH can be implemented in software over an IEEE 802.11-compliant wireless card. Each node using SSCH switches across channels in such a manner that nodes desiring to communicate overlap, while disjoint communications mostly do not overlap, and hence do not interfere with each other. To achieve this, SSCH uses a novel scheme for distributed rendezvous and synchronization. Simulation results show that SSCH significantly increases network capacity in several multi-hop and single-hop wireless networking scenarios.
A Measurement Study of Vehicular Internet Access Using
- In Situ Wi-Fi Networks. In 12th ACM MOBICOM Conf
, 2006
"... The impressive penetration of 802.11-based wireless networks in many metropolitan areas around the world offers, for the first time, the opportunity of a “grassroots ” wireless Internet service provided by users who “open up ” their 802.11 (Wi-Fi) access points in a controlled manner to mobile clien ..."
Abstract
-
Cited by 197 (6 self)
- Add to MetaCart
(Show Context)
The impressive penetration of 802.11-based wireless networks in many metropolitan areas around the world offers, for the first time, the opportunity of a “grassroots ” wireless Internet service provided by users who “open up ” their 802.11 (Wi-Fi) access points in a controlled manner to mobile clients. While there are many business, legal, and policy issues to be ironed out for this vision to become reality, we are concerned in this paper with an important technical question surrounding such a system: can such an unplanned network service provide reasonable performance to network clients moving in cars at vehicular speeds? To answer this question, we present the results of a measurement study carried out over 290 “drive hours ” over a few cars under typical driving conditions, in and around the Boston metropolitan area (some of our data also comes from a car in Seattle). With a simple caching optimization to speed-up IP address acquisition, we find that for our driving patterns the median duration of linklayer connectivity at vehicular speeds is 13 seconds, the median connection upload bandwidth is 30 KBytes/s, and that the mean duration between successful associations to APs is 75 seconds. We also find that connections are equally probable across a range of urban speeds (up to 60 km/hour in our measurements). Our end-toend TCP upload experiments had a median throughput of about 30 KBytes/s, which is consistent with typical uplink speeds of home broadband links in the US. The median TCP connection is capable of uploading about 216 KBytes of data. Our high-level conclusion is that grassroots Wi-Fi networks are viable for a variety of applications, particularly ones that can tolerate intermittent connectivity. We discuss how our measurement results can improve transport protocols in such networks.
MultiNet: Connecting to Multiple IEEE 802.11 Networks Using a Single Wireless Card
- in IEEE INFOCOM, Hong Kong
, 2004
"... Abstract — There are a number of scenarios where it is desirable to have a wireless device connect to multiple networks simultaneously. Currently, this is possible only by using multiple wireless network cards in the device. Unfortunately, using multiple wireless cards causes excessive energy drain ..."
Abstract
-
Cited by 193 (5 self)
- Add to MetaCart
(Show Context)
Abstract — There are a number of scenarios where it is desirable to have a wireless device connect to multiple networks simultaneously. Currently, this is possible only by using multiple wireless network cards in the device. Unfortunately, using multiple wireless cards causes excessive energy drain and consequent reduction of lifetime in battery operated devices. In this paper, we propose a software based approach, called MultiNet, that facilitates simultaneous connections to multiple networks by virtualizing a single wireless card. The wireless card is virtualized by introducing an intermediate layer below IP, which continuously switches the card across multiple networks. The goal of the switching algorithm is to be transparent to the user who sees her machine as being connected to multiple networks. We present the design, implementation, and performance of the MultiNet system. We analyze and evaluate buffering and switching algorithms in terms of delay and energy consumption. Our system has been operational for over twelve months, it is agnostic of the upper layer protocols, and works well over popular IEEE 802.11 wireless LAN cards. Keywords: System Design, Experimentation with Real Networks, Network Measurements.
Idle sense: An optimal access method for high throughput and fairness in rate diverse wireless LANs
- In ACM SIGCOMM
, 2005
"... We consider wireless LANs such as IEEE 802.11 operating in the unlicensed radio spectrum. While their nominal bit rates have increased considerably, the MAC layer remains practically unchanged despite much research effort spent on improving its performance. We observe that most proposals for tuning ..."
Abstract
-
Cited by 175 (12 self)
- Add to MetaCart
(Show Context)
We consider wireless LANs such as IEEE 802.11 operating in the unlicensed radio spectrum. While their nominal bit rates have increased considerably, the MAC layer remains practically unchanged despite much research effort spent on improving its performance. We observe that most proposals for tuning the access method focus on a single aspect and disregard others. Our objective is to define an access method optimized for throughput and fairness, able to dynamically adapt to physical channel conditions, to operate near optimum for a wide range of error rates, and to provide equal time shares when hosts use different bit rates. We propose a novel access method derived from 802.11 DCF [2] (Distributed Coordination Function) in which all hosts use similar values of the contention window CW to benefit from good short-term access fairness. We call our method Idle Sense, because each host observes the mean number of idle slots between transmission attempts to dynamically control its contention window. Unlike other proposals, Idle Sense enables each host to estimate its frame error rate, which can be used for switching to the right bit rate. We present simulations showing how the method leads to high throughput, low collision overhead, and low delay. The method also features fast reactivity and time-fair channel allocation. Categories and Subject Descriptors
New Insights from a Fixed Point Analysis of Single Cell IEEE 802.11 WLANs
, 2004
"... We study a fixed point formalisation of the well known analysis of Bianchi. We provide a significant simplication and generalisation of the analysis. In this more general framework, the fixed point solution and performance measures resulting from it are studied. Uniqueness of the fixed point is esta ..."
Abstract
-
Cited by 132 (25 self)
- Add to MetaCart
We study a fixed point formalisation of the well known analysis of Bianchi. We provide a significant simplication and generalisation of the analysis. In this more general framework, the fixed point solution and performance measures resulting from it are studied. Uniqueness of the fixed point is established. Simple and general throughput formulas are provided. It is shown that the throughput of any ow will be bounded by the one with the smallest transmission rate. The aggregate throughput is bounded by the reciprocal of the harmonic mean of the transmission rates. In an asymptotic regime with a large number of nodes, explicit formulas for the collision probability, the aggregate attempt rate and the aggregate throughput are provided. The results from the analysis are compared with ns2 simulations, and also with an exact Markov model of the back-off process. It is shown how the saturated network analysis can be used to obtain TCP transfer throughputs in some cases.
End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks
- In Proceedings of ACM MOBICOM
, 2004
"... Wireless IEEE 802.11 networks in residences, small businesses, and public "hot spots" typically encounter the wireline access link (DSL, cable modem, T1, etc.) as the slowest and most expensive part of the end-to-end path. Consequently, network architectures have been proposed that employ ..."
Abstract
-
Cited by 132 (4 self)
- Add to MetaCart
(Show Context)
Wireless IEEE 802.11 networks in residences, small businesses, and public "hot spots" typically encounter the wireline access link (DSL, cable modem, T1, etc.) as the slowest and most expensive part of the end-to-end path. Consequently, network architectures have been proposed that employ multiple wireless hops in route to and from the wired Internet. Unfortunately, use of current media access and transport protocols for such systems can result in severe unfairness and even starvation for flows that are an increasing number of hops away from a wired Internet entry point. Our objective is to study fairness and end-to-end performance in multihop wireless backhaul networks via the following methodology. First, we develop a formal reference model that characterizes objectives such as removing spatial bias (i.e., providing performance that is independent of the number of wireless hops to a wire) and maximizing spatial reuse. Second, we perform an extensive set of simulation experiments to quantify the impact of the key performance factors towards achieving these goals. For example, we study the roles of the MAC protocol, end-to-end congestion control, antenna technology, and traffic types. Next, we develop and study a distributed layer 2 fairness algorithm which targets to achieve the fairness of the reference model without modification to TCP. Finally, we study the critical relationship between fairness and aggregate throughput and in particular study the fairness-constrained system capacity of multihop wireless backhaul networks.
Sniffing out the correct physical layer capture model in 802.11b
, 2004
"... Physical layer capture (PLC) in 802.11b refers to the successful reception of the stronger (higher signal strength at receiver) frame in a collision. PLC causes significant imbalance in the throughputs of sources. Existing 802.11b simulators, including ns2 and Qualnet, assume that PLC occurs only if ..."
Abstract
-
Cited by 117 (0 self)
- Add to MetaCart
(Show Context)
Physical layer capture (PLC) in 802.11b refers to the successful reception of the stronger (higher signal strength at receiver) frame in a collision. PLC causes significant imbalance in the throughputs of sources. Existing 802.11b simulators, including ns2 and Qualnet, assume that PLC occurs only if the stronger frame arrives first at the receiver. We show empirically that in reality PLC occurs even if the stronger frame arrives later (but within the physical layer preamble of the first frame). Consequently, throughput unfairness in reality can be significantly (up to 15%) higher than with the former PLC model. We have modified the ns2 simulator to account for this and Qualnet will be incorporating a fix in their next release. To identify which frames were involved in collisions, when their transmissions started, and which of them were retrieved, we have devised a novel technique using multiple sniffers and instrumented device drivers to reconstruct from the air interface all tx/rx events in a WLAN to within 4 � × accuracy. This allows us to quantify the causal links from the PHY layer through the MAC layer to the observed application layer imbalance. It also shows that the arrival times of colliding frames routinely differ by as much as 20 � × due to inherent uncertainties of 802.11b firmware clock synchronization and rx/tx turnaround delays, and that the frame to arrive first can be either the stronger or the weaker with equal likelihood. 1.
DOMINO: A System to Detect Greedy Behavior
- in IEEE 802.11 Hotspots,” in Proceedings of MobiSys 2004
, 2004
"... The proliferation of hotspots based on IEEE 802.11 wireless LANs brings the promise of seamless Internet access from a large number of public locations. However, as the number of users soars, so does the risk of possible misbehavior; to protect themselves, wireless ISPs already make use of a number ..."
Abstract
-
Cited by 109 (3 self)
- Add to MetaCart
The proliferation of hotspots based on IEEE 802.11 wireless LANs brings the promise of seamless Internet access from a large number of public locations. However, as the number of users soars, so does the risk of possible misbehavior; to protect themselves, wireless ISPs already make use of a number of security mechanisms, and require mobile stations to authenticate themselves at the Access Points (APs). However, IEEE 802.11 works properly only if the stations also respect the MAC protocol. We show in this paper that a greedy user can substantially increase his share of bandwidth, at the expense of the other users, by slightly modifying the driver of his network adapter. We explain how easily this can be performed, in particular with the new generation of adapters. We then present DOMINO (System for Detection Of greedy behavior in the MAC layer of IEEE 802.11 public NetwOrks), a piece of software to be installed in the Access Point. DOMINO can detect and identify greedy stations, without requiring any modification of the standard protocol at the AP and without revealing its own presence. We illustrate these concepts by simulation results and by the description of our prototype.
Modulation Rate Adaptation in Urban and Vehicular Environments: Cross-layer Implementation and Experimental Evaluation
"... Accurately selecting modulation rates for time-varying channel conditions is critical for avoiding performance degradations due to rate overselection when channel conditions degrade or underselection when channel conditions improve. In this paper, we design a custom cross-layer framework that enable ..."
Abstract
-
Cited by 104 (10 self)
- Add to MetaCart
(Show Context)
Accurately selecting modulation rates for time-varying channel conditions is critical for avoiding performance degradations due to rate overselection when channel conditions degrade or underselection when channel conditions improve. In this paper, we design a custom cross-layer framework that enables (i) implementation of multiple and previously unimplemented rate adaptation mechanisms, (ii) experimental evaluation and comparison of rate adaptation protocols on controlled, repeatable channels as well as residential urban and downtown vehicular and non-mobile environments in which we accurately measure channel conditions with 100-µs granularity, and (iii) comparison of performance on a per-packet basis with the ideal modulation rate obtained via exhaustive experimental search. Our evaluation reveals that SNR-triggered protocols are susceptible to overselection from the ideal rate when the coherence time is low (a scenario that we show occurs in practice even in a nonmobile topology), and that “in-situ ” training can produce large gains to overcome this sensitivity. Another key finding is that a mechanism effective in differentiating between collision and fading losses for hidden terminals has severely imbalanced throughput sharing when competing links are even slightly heterogeneous. In general, we find trained SNRbased protocols outperform loss-based protocols in terms of the ability to track vehicular clients, accuracy within outdoor environments, and balanced sharing with heterogeneous links (even with physical layer capture).
