This paper proposes a semi-random backoff (SRB) method that enables resource reservation in contention-based wireless LANs. The proposed SRB is fundamentally different from traditional random backoff methods because it provides an easy migration path from random backoffs to deterministic slot assignments. The central idea of the SRB is for the wireless station to set its backoff counter to a deterministic value upon a successful packet transmission. This deterministic value will allow the station to reuse the time-slot in consecutive backoff cycles. When multiple stations with successful packet transmissions reuse their respective time-slots, the collision probability is reduced, and the channel achieves the equivalence of resource reservation. In case of a failed packet transmission, a station will revert to the standard random backoff method and probe for a new available time-slot. The proposed SRB method can be readily applied to both 802.11 DCF and 802.11e EDCA networks with minimum modification to the existing DCF/EDCA implementations. Theoretical analysis and simulation results validate the superior performance of the SRB for small-scale and heavily loaded wireless LANs. When combined with an adaptive mechanism and a persistent backoff process, SRB can also be effective for large-scale and lightly loaded wireless networks.

Video traffic will soon become the majority of the data traffic in mobile networks. However, little is known about how users watch video on mobile devices. Understanding the characteristics of mobile video access can greatly benefit the design of video distribution systems in mobile networks. In this paper, we present an analysis of data logs obtained from a large scale commercial mobile TV system. Our analysis shows that user video accesses can be separated into two distinct categories: browsing and viewing. The user browsing sessions are short video accesses, with its length following a Weibull distribution. The user viewing sessions are considerably longer; its length distribution follows a piecewise model which combines the lognormal distribution and a tail following generalized Pareto distribution. The findings are especially useful for modeling and optimization of Internet based mobile TV systems.

Large scale, Internet based mobile TV deployment presents both tremendous opportunities and challenges for mobile operators and technology providers. This paper presents a measurement based study on a large scale mobile TV service offering in China. Within the one month measurement period, our dataset captured over 1 million unique mobile devices and more than 49 million video sessions. Analysis showed that mobile viewing patterns are different from that of landline based IPTV and VoD systems. In particular, the average viewing time is significantly shorter, and the channel popularity distribution is more skewed towards top ranked channels than that of landline based systems. For the channel sojourn time, the distribution follows a piecewise model, which combines lognormal and pareto distribution. The lognormal part, which fits the majority of video sessions, more closely resembles the mobile phone call holding time, rather than the power law distribution in the landline IPTV case. In comparing the 3G and WiFi access methods, we found that users exhibit different behaviors when accessing from different networks. In 3G networks, where users are subject to data charge, users tend to have shorter channel sojourn time and prefer lower bit-rate channels. The parameters of the distributions are also different. Understanding these user behaviors and their implications on network traffic are critical for the success of future mobile TV industry.

802.11e HCCA (Hybrid coordination function Controlled Channel Access) exhibits good QoS provisioning for constant bit rate (CBR) video streams in a single collision domain. However, its performance degrades significantly for variable bit rate (VBR) video streams particularly in multi-collision domains. In addition, HCCA has the disadvantage of high complexity. In this paper, we introduce a deterministic backoff (DEB) method into the HCCA mechanism, which achieves virtual polling via carrier sense on the wireless channel. DEB inherits the merit of HCCA that achieves near collision-free channel access in single collision domain. It also exhibits robust performance in multi-collision domains. Simulation results show that DEB supports well both CBR and VBR video streams.

QoS provisioning for video streaming over 802.11 home networks is challenging due to the tight bandwidth and delay constraints. Currently, both 802.11e HCCA and EDCA have their disadvantages when delivering video over 802.11 networks. EDCA exhibits degraded QoS performance for increased number of stations, while HCCA has high complexity, and also suffers from the impact of inter-cell interference. In this paper, we propose a new backoff mechanism which is fundamentally different from traditional random backoff methods in 802.11. The mechanism achieves resource reservation by reusing one or multiple time slots for transmission in consecutive backoff cycles. The basic idea is borrowed from R-ALOHA, but several modifications are made to allow it to work in the context of CSMA network. An additive increase multiplicative decrease based algorithm is proposed to control resource allocation for video streaming in the 802.11 network. Theoretical analysis and numerical simulations are conducted to validate the effectiveness of the method and the algorithm.

In recent years, a series of power saving (PS) protocols has been proposed in the family of 802.11 standards to save energy for mobile devices. To evaluate their performance, many works have been carried out on testbeds or simulation platforms. However, till now, there is a lack of accurate theoretical models to analyze the performance for these protocols. In an effort to fill this gap, we present a Markov chain-based analytical model in this paper to model these PS protocols, with its focus on multicast services in wireless LANs. The proposed analytical model successfully captures the key characteristic of the power saving system: the data delivery procedure starts periodically at the previously negotiated time, but ends at a rather random time with its distribution depending on the end time of data delivery in the last delivery period as well as the arrival rate of incoming traffic. In the situations with light to moderate traffic loads and under the Poisson assumption for incoming traffic, the amount of data delivered between consecutive delivery periods possesses the Markov property, which builds up our Markov chain-based model. For incoming traffic with long-range dependence (LRD), a multistate Markov-Modulated Poisson Process (MMPP) is used to approximate the traffic, making the analytical model valid in more general cases. We verify our model by simulations on ns2 and the results show that the model can faithfully predict the performance of these PS protocols over a wide variety of testing scenarios.

In this paper, we propose a semi-random backoff (SRB) mechanism to combat network collisions for 802.11 DCF/EDCA in wireless LANs. SRB is fundamentally different from traditional random backoff methods because it provides a form of resource reservation at no extra cost. The key idea of SRB is to set the backoff counter to a deterministic value upon a successful transmission. This allows a station to reuse a time slot in consecutive backoff cycles without colliding with other stations, thus achieving resource reservations. In cases of failed transmissions due to network collisions or channel errors, SRB automatically reverts to the standard random backoff method. SRB can be readily applied to both 802.11 DCF and 802.11e EDCA networks with minimum modification to the existing DCF/EDCA implementations. Theoretical analysis and simulations are conducted to evaluate this new method and our results validate its good performance in small to middle-sized 802.11 wireless LANs.

Cricket is a fine-grained indoor location system utilizing beacons transmitting both ultrasound and radio wave signals. However, the system accuracy is greatly impacted by the relative angle and distance between the beacon and listener. This paper proposes a low cost method to improve the system precision by means of incremental error compensation. The ultrasonic detecting error, which directly affects the accuracy of time-difference-of-arrival (TDOA), is first analyzed and verified via experiments. Then an error compensation method is designed based on total least squares (TLS) to further improve the cricket's localization performance. Experimental results show that the average error of localization is reduced to 5.60 cm with compensation, compared with 9.06 cm of the original cricket method. The standard deviation of the errors is reduced from 7.80 cm to 3.99 cm. This method lays a foundation for further improving the existing ultrasound-related distance measurement method.

Power conservation is a general concern for mobile computing and communication. In this paper, we investigate the performance of the current 802.11 power saving mechanism (unscheduled PSM) and identify that background network traffic can have a significant impact on the power consumption of mobile stations. To improve power efficiency, a novel scheduled PSM protocol based on time slicing is proposed in this paper. The protocol adopts the mechanism of time division, schedules the access point to deliver pending data at designated time slices, and adaptively adjusts the power state of the mobile stations. The proposed scheme is near theoretical optimal for power saving. It greatly reduces the effect of background traffic, minimizes the station idle time, and maximizes its energy utilization. Comprehensive analysis and simulations are conducted to evaluate the new protocol. The results show that the new protocol provides significant energy saving over the unscheduled PSM, particularly in circumstances where multiple traffic streams coexist in a network. Moreover, it achieves the saving at the cost of only a slight degradation of the one-way-delay performance.

In this paper, we develop an unsupervised machine learning method for network traffic classification. First, the traffic flows from active hosts are separated into different clusters by sequentially applying the information entropy techniques. The clusters are then classified into broad-based application types by examining the parameters of the clusters, as well as the dynamic properties of the clusters during the clustering process. Experiment results from a campus backbone environment are presented, where the network traffic flows from the top 20 active hosts are identified. The results showed that the identification accuracy is approximately 93.81%. This compares to prior works using unsupervised machine learning methods, where the classification accuracy is generally lower, with the highest being 86.5%. We also show that our method can be combined with supervised learning method to further improve the classification accuracy and perform automatic training on the classification engine.

Balancing energy consumption and prolonging network lifetime are open challenges in Wireless Sensor Networks. In this paper, we design a novel load balanced routing protocol to maximize lifetime of Wireless Sensor Networks (WSNs). In order to balance the energy consumption among sensor nodes, we deploy multiple sinks simultaneously which are connected though wired or wireless infrastructure. We introduce a potential model and propose a routing scheme in which sensor nodes construct routes based on local topology information and the state information from sinks' broadcast messages. Sinks monitor their traffic load and adjust their own parameters to balance the traffic load in the network. Theoretical analysis and simulation results show that our protocol can significantly improve the performance of the system in following aspects: robustness, lifetime and reliability.

This paper presents an experimental study of the IEEE 802.11 power saving mechanism on PDA in a wireless local area network (WLAN). The power consumptions of the PDA in both continuous active mode (CAM) and power saving mode (PSM) are measured under various traffic scenarios, and the impact of both internal and external factors on power performance, such as beacon/DTIM period and background multicast traffic, are examined. Our findings are: 1) The impact of beacon/DTIM period is only significant when their values are small and the imposed traffic rate is high. 2) The presence of background traffic can significantly reduce the effectiveness of the PSM. Analysis of these findings may provide important guidance to the design of future power saving mechanisms.

The need to quickly and accurately classify Internet traffic for security and QoS control has been increasing significantly with the growing Internet traffic and applications over the past decade. Pattern recognition by learning the features in the training samples to classify the unknown flows is one of the main methods. However, many methods developed in the previous works are too complicated to be applied in real-time, and the prior probabilities based on the training samples are severely biased. This paper uses the SVM (support vector machine) method to train 7 classes of applications of different characteristics, captured from a campus network backbone. A discriminator selection algorithm is developed to obtain the best combination of the features for classification. Our optimized method yields approximately 96.9% accuracy for un-biased training and testing samples. For regular biased samples, the accuracy is about 99.4%. Furthermore, all the feature parameters are computable in real time from captured packet headers, suggesting real time network traffic classification with high accuracy is achievable.

Power conservation is a general concern for mobile computing and communication. In this paper, we investigate the performance of the current 802.11 power saving mechanism (unscheduled PSM) and demonstrate that background network traffic can have a significant impact on the power consumption of mobile stations. To improve power efficiency, a scheduled PSM protocol based on time slicing is proposed in this paper. The protocol adopts the mechanism of time division, schedules the access point to deliver pending data at designated time slices, and adaptively adjusts the power state of the mobile stations. The proposed scheme is near theoretical optimal for power saving in the sense that it greatly reduces the effect of background traffic, minimizes the station idle time, and maximizes its energy utilization. Comprehensive analysis and simulations are conducted to evaluate the new protocol. Our results show that it provides significant energy saving over the unscheduled PSM, particularly in circumstances where multiple traffic streams coexist in a network. Moreover, it achieves the saving at the cost of only a slight degradation of the one-way delay performance.

Rational and selfish nodes in P2P systems usually lack effective incentives to cooperate, contributing to the increase of free-riders, and degrading the system performance. Various attacks such as whitewashing, collusion, and software cracking pose great challenges on distributed reputation management. To tackle these problems, we propose to build a social network on P2P system, and use the strength of social connections to facilitate transactions in P2P system. The' small world' character of social networks makes it feasible for nodes to locate resources and conduct transactions while maintain limited local memory history. Such distributed memory combined by relationship between peers constructs a powerful reputation management network, which could have better performance than shared history system and is more robust under various attacks. Our simulation and analysis show that the social network model can greatly incent cooperation in P2P networks and enormously reduce the memory cost.

A prototype microcellular wireless asynchronous transfer mode network (WATMnet) capable of providing integrated multimedia communication services to mobile terminals is described in this paper. The experimental system's hardware consists of laptop computers (NEC Versa-M) with WATMnet interface cards, multiple VME/i960 processor-based WATMnet base stations, and a mobility-enhanced local-area ATM switch. The prototype wireless network interface cards operate at peak bit-rates up to 8 Mb/s, using low-power 2.4 GHz industrial, scientific, and medical (ISM)-band modems. Wireless network protocols at the portable terminal and base station interfaces support available bit rate (ABR), variable bit rate (VBR), and constant bit rate (CBR) transport services compatible with ATM using a dynamic time-division multiple-access/time-division duplex (TDMA/TDD) MAC protocol for channel sharing and data link control (DLC) protocol for error recovery. A custom wireless control protocol is also implemented between the portable and base units for support of radio link related functions such as user registration and handoff. All network entities including the portable, base and switch use a mobility-enhanced version of ATM ("Q.2931+") signaling for switched virtual circuit (SVC) connection control functions, including handoff. In the first stage of the prototype, the application-level API is TCP/UP over ATM ABR service class using AAL5. Early experiments with the WATMnet prototype have been conducted to validate major protocol and software aspects, including DLC, wireless control, and mobility signaling for handoff, Selected network-based multimedia/video applications requiring moderate bit-rates (/spl sim/0.5-1 Mb/s) in the ABR mode have been successfully demonstrated on the laptop PC.