Siwaruk Siwamogsatham

An empirical study on achievable throughputs of IEEE 802.11n devices

The empirical performance studies on the emerging IEEE 802.11n technology by an independent and vendor-neutral party have not really been explored. In this paper, we conduct performance measurements for the IEEE 802.11n network using a mixture of commercially available IEEE 802.11n devices from various manufacturers. With the same standard 20-MHz channel width configuration, the results demonstrate that IEEE 802.11n significantly outperforms the IEEE 802.11g network. The performance improvements of IEEE 802.11n are measured to be roughly about 85% for the downlink UDP traffic, 68% for the downlink TCP traffic, 50% for the uplink UDP traffic, and 90% for the uplink TCP traffic. We also observe that the UDP throughputs are largely imbalanced for the uplink and downlink traffics in most test networks, while the downlink and uplink TCP throughput results are quite balanced for all test networks. In addition, the 40-MHz channel configurations only provide marginal performance improvements. Unlike other existing work, here we also capture and analyze the IEEE 802.11n packets transferred during the performance tests in order to technically explain the measured performance results. It is observed that when the frame aggregation and block acknowledgement mechanisms are utilized, the superior performance results are achieved. However, the decisions on how and when to use these mechanisms are very hardware dependent.

Achieving Uplink/Downlink Fairness in WLANs via Multiple Backoff Timers

The IEEE 802.11 Distributed Coordination Function (DCF) scheme suffers from a well-known uplink/downlink throughput imbalance problem when operating in the infrastructure mode. A common existing approach is to assign higher priority to the access point by properly tuning the backoff parameters (e.g., the contention windows size and the inter frame space) of each station so that the access point obtains a larger share of network bandwidth. However, it is quite difficult to map and adjust these parameters to accurately offset arbitrary uplink/downlink throughput imbalance. Recently, the IEEE 802.11e standard additionally provides several parameters that can be properly adjusted to effectively offset the imbalance problem. However, this simple solution also takes away some of the IEEE 802.11e capacity to support flexible prioritized services. In this work, we propose a new simple and effective technique to solve the uplink/downlink throughput imbalance problem in an infrastructure WLAN. The idea is to have the access point employ multiple independent backoff timers, each of which is associated with the downlink traffic flows for each client station. The proposed solution requires minimal changes in the access point firmware, but no change in the client device is necessary. Simulation results show that the proposed mechanism efficiently and effectively solves the uplink/downlink throughput imbalance problem in an infrastructure WLAN.

New stimulation pattern design to improve P300-based matrix speller performance at high flash rate.

OBJECTIVE We propose a new stimulation pattern design for the P300-based matrix speller aimed at increasing the minimum target-to-target interval (TTI). APPROACH Inspired by the simplicity and strong performance of the conventional row-column (RC) stimulation, the proposed stimulation is obtained by modifying the RC stimulation through alternating row and column flashes which are selected based on the proposed design rules. The second flash of the double-flash components is then delayed for a number of flashing instants to increase the minimum TTI. The trade-off inherited in this approach is the reduced randomness within the stimulation pattern. MAIN RESULTS We test the proposed stimulation pattern and compare its performance in terms of selection accuracy, raw and practical bit rates with the conventional RC flashing paradigm over several flash rates. By increasing the minimum TTI within the stimulation sequence, the proposed stimulation has more event-related potentials that can be identified compared to that of the conventional RC stimulations, as the flash rate increases. This leads to significant performance improvement in terms of the letter selection accuracy, the raw and practical bit rates over the conventional RC stimulation. SIGNIFICANCE These studies demonstrate that significant performance improvement over the RC stimulation is obtained without additional testing or training samples to compensate for low P300 amplitude at high flash rate. We show that our proposed stimulation is more robust to reduced signal strength due to the increased flash rate than the RC stimulation.

A novel smart-DCF scheme for high-speed WLANs

In this paper, a novel Smart-DCF scheme is proposed for high-speed WLANs. The proposed Smart-DCF scheme may be viewed as a merger of the IEEE 802.11 DCF and PCF schemes. It can achieve a virtually collision-free WLAN environment and efficiently support a desirable quality-of-service requirement as in PCF while maintaining simplicity of medium access with the CSMA mechanism as in DCF. The Smart-DCF scheme can almost completely eliminate the signal collision problem, which is the major factor limiting performance of the standard IEEE 802.11 DCF scheme. In Smart-DCF, each client station relies on smart backoff timer information from a master station for use during the backoff procedure of each signal transmission. The master station schedules the backoff timer for each client station such that no collision would occur, and can effectively support a quality-of-service requirement by smartly scheduling the backoff timers for each station based on the traffic or stations privilege such that a higher-priority station can get a chance to transmit sooner or more frequently. Each client station acquires the smart backoff timer information through an acknowledgement frame from the master station for its previous frame transmission. If the client station has not yet acquired the smart backoff timer information from the master station for any reason, it employs a random backoff value to initiate the current backoff procedure. The backoff slot structure is smartly designed so that the random backoff stations can easily determine their allowable backoff slots. Since the signal collision problem is almost completely eliminated, the proposed Smart-DCF scheme is expected to considerably outperform the standard IEEE 802.11 DCF scheme. The numerical results show that the throughput improvement can be more than 40 percents in many cases.

A Privacy-Preserving Framework for Location-Based Service: A Review of Structural Design and Analysis

ABSTRACT Traditional location tracking and positioning systems rely on the implicit assumption that users are willing to reveal their credentials or private location to an untrustworthy service provider; if a situation suddenly deteriorates, personal information can be intercepted and abused by adversary attacks. Addressing the need for location privacy is essential to preserve users’ credentials when they need to access location information from their service providers. Therefore, this review paper will address the phases of authentication and the querying process in location-based services (LBS). The first contribution is to rigorously investigate the mechanisms of location-based authentication and location-based personal identification in order to determine the level of trust for deciding whether traditional methods have provided a strong enough security structure to preserve privacy. The second contribution is to resolve the above privacy problems in the existing LBS systems. The idea of early intervention has brought a conceptual and practical framework into the field of preserving location-privacy-based anonymizer solutions. The framework has been designed based on a simple, centralized architecture, and integrated with two important techniques: location anonymity and encryption technology. In this paper, one-way hash function is applied as a means to create a fingerprint of a message that can be used for subscription or authentication services, and thus data integrity can be preserved. Location blurring (also known as the k-anonymity model) has also been proposed as an important factor in disguising user location when users need to interact with untrusted service providers. In the querying process, time-fuzzy logic is used to examine the degree of confidence whether the user is requesting the service under the right privileges. Finally, performance analysis regarding the derived framework of preserving location privacy is presented.

Polite sender: A resource-saving spam email countermeasure based on sender responsibilities and recipient justifications

Currently, most of the existing spam countermeasures are deployed on the email recipient side. However, they cannot diminish the amount of wasteful traffic sent from the SMTP server and the wasteful data storage in the receivers inbox incurred by spam emails. This paper presents an alternative approach on the sender side in order to overcome these problems and create a bandwidth-saving reduced-storage email system. Additional functions are added to the SMTP server on the sender side to examine whether should allow the particular email sender. If a proper authorization from the recipient has not been granted, the sending SMTP server will not forward the full email message. Instead, it sends the email header together with some additional inquiries for the recipient to authorize this particular sender. Once the authorization is granted, each pair of a given sender and receiver will be kept in a whitelist at the sending SMTP server. The proposed approach can be easily deployed without modifying the existing SMTP protocol stack. The experiment results based on a prototype and data analysis from real email servers demonstrate that the proposed scheme could drastically reduce the amount of wasteful traffic and storage associated with the annoying spam messages.

Managing Mobile Device Security in Critical Infrastructure Sectors

Information security in critical infrastructure sectors has become a crucial issue in recent years. A drastic increase of mobile device usage for accessing to corporate networks could expose the critical systems to major security risks. The paper provides an overview of research and practice trends in mobile device security focusing on critical infrastructure. This includes access control, next-generation firewall, BYOD control mechanism, and BYOD management and policy. Confidentiality, integrity, and availability issues related on the research and practice trends are discussed, followed by the conclusion.

Adaptive bandwidth allocation for per-station fairness on wireless access router

According to the nature of IEEE 802.11 wireless LAN, the bandwidth is shared among wireless devices. However, the bandwidth could not be fairly shared among stations, because some users may use greedy applications which prevent other applications from connecting to the Internet. These applications are for example, video streaming applications, download accelerators and P2P applications. Moreover, Distributed Coordination Function (DCF) mechanism provided in the 802.11 MAC protocol originates the per-flow unfairness problem between uplink and downlink flows, as uplink flows usually dominate downlink flows and leads to unfair bandwidth allocation among wireless stations. In this paper, we propose an adaptive bandwidth allocation mechanism to provide per-station fairness. The proposed scheme can control the wireless traffic for fairness bandwidth allocation among stations. The bandwidth of each station can be adjusted to minimize the remaining bandwidth. The proposed scheme also covers a method to prevent domination of uplink flow which affects per-station fairness. Furthermore, the proposed scheme can be implemented on wireless access router. Hence, in order to support legacy wireless clients, our solution can be efficiently used.

Analysis of a Multiple-Token Contention Scheme for Broadband Wireless Access Networks

In this paper, we present a mathematical analysis to evaluate the contention phase of a wireless system in which each contending station transmits multiple copies of the bandwidth reservation/request packet to increase the probability of successful transmissions using multiple slots randomly selected from the pool of available slots at the given moment. Here, we define this type of systems as a multi-token reservation scheme with uniform random backoff (MT-UNI). The proposed MT-UNI mechanism can be used in various broadband wireless access systems including IEEE 802.16, PRMA, C-TDMA and others. The simulation results show that the proposed performance formulae are very accurate, and the MT-UNI mechanism can improve the performance of the system at low load conditions. In addition, when each station uses 2 reservation tokens, the MT- UNI scheme seems to produce the near-optimal performance.

On Backoff Timer Scheduling for Smart-DCF and Performance Evaluation

In this work, we present a generic backoff-timer scheduling algorithm for the Smart-DCF scheme. In Smart-DCF, the master station schedules the backoff timer value for each client station, and the backoff timer information is sent to the client station via an acknowledgment frame. The proposed backoff-timer scheduling mechanism enables the Smart-DCF access point to provide differentiated services to different classes of wireless stations or traffic. The simulation results demonstrate that the proposed scheduling algorithm is effective. The results also show that the Smart-DCF scheme noticeably outperforms the existing DCF scheme. In addition, the throughput performance of Smart-DCF starts to reach the saturated level when each data burst is longer than 5 frames.