16068675
Experimental investigation on VoIP performance and the resource utilization in 802.11b WLANs
In a shared medium network like the 802.11b WLAN, predicting the quality of VoIP calls from the resource usage of the wireless medium is highly desirable. Analyzing the bandwidth usage at the L2/MAC layer may be especially useful for potential QoS provisioning and call admission schemes. This paper experimentally investigates the relationship between resource utilization in WLANs and the quality of VoIP calls transmitted over wireless medium. Specifically we evaluate how the amount of free bandwidth influences transmission impairments (i.e. delay, loss, and jitter) and thus call quality. Resource utilization (under the MAC bandwidth components framework) is calculated by a WLAN resource monitoring application that passively "sniffs" packets at the L2/MAC layer and analyses their headers and temporal characteristics. The quality of VoIP calls is predicted using an extended version of the ITU-T E-model, which estimates user satisfaction from time varying transmission impairments
12221702
The capability of the EDCA mechanism to support voice traffic in a mixed voice/data transmission over 802.11e WLANs - an experimental investigation
In this paper we experimentally evaluate the capability of the EDCA mechanism to support voice traffic in a mixed voice/data transmission over 802.1 le WLANs. In particular we investigate how real-time voice transmission can be supported by tuning four EDCA parameters, namely AIFSN, CWmin , CWmax, and TXOP and how this impacts on background data transmission. The experimental set-up involves fifteen VoIP terminals sending bi-directional traffic between wired and wireless subnets and another station injecting various types of heavy background loads to the wireless subnet. End-to-end voice transmission quality is predicted from time-varying transmission impairments with the use of the latest appendix to the ITU-T E-model. Our experimental results show that the AIFSN parameter more effectively protects voice calls against background data traffic than CWmin. We also demonstrate that tuning of the TXOP parameter does not improve the quality of voice transmission. To the best of our knowledge, this is the first experimental investigation regarding tuning of MAC layer EDCA parameters in a real 802.1 le WLAN network from the perspective of end-to-end voice transmission quality and end user satisfaction.
6738991
An Experimental Investigation of IEEE 802.11e TXOP Facility for Real-Time Video Streaming
Real-time multimedia streaming applications require a strict bounded end-to-end delay and are considered to be bursty as each video frame is typically transmitted as a burst of packets. In this paper we show how the distribution of video frame sizes can be used to efficiently dimension the IEEE 802.lie TXOP limit parameter to efficiently deal with this burstiness in order to enhance the transmission of real-time video streaming services. Through experimental investigation, we show that by using the mean video frame size to dimension the TXOP limit parameter, the transmission delay for the video frame is reduced by 67% under heavily loaded conditions. Other techniques investigated in this paper include applying the TXOP facility separately to each of the constituent I, P, and B video frame types.
571649
An Evaluation of a Conservative Transmit Power Control Mechanism on an Indoor 802.11 Wireless Mesh Testbed
Power control techniques for IEEE 802.11 wireless networks have already gained considerable attention. Such techniques are particularly attractive because they can improve various aspects of wireless network operation such as interference mitigation, spatial reuse in dense wireless deployments, topology control, and link quality enhancement. In this paper we propose a novel delivery ratio based Conservative Transmit Power Control (ConTPC) mechanism. Our implementation is conservative when it comes to deciding if the transmit power should be reduced for a given link. This is because we do not want poor quality wireless links to further reduce their quality and be overwhelmed by other links transmitting at maximum power. We have experimentally evaluated the benefit of the proposed power control scheme when compared with fixed power level systems. We show that our ConTPC mechanism can increase the throughput, however the magnitude of this enhancement largely depends on the topology of the wireless network.
18478829
Gauging VoIP call quality from 802.11 WLAN resource usage
In this paper we experimentally study the relationship between resource utilization in the wireless LAN and the quality of VoIP calls transmitted over the wireless medium. Specifically we evaluate how its overall capacity is shared between three basic MAC bandwidth components (load, access, and free) as the number of VoIP calls increases and how it influences transmission impairments (delay, loss, and jitter) and thus call quality. Resource utilization (under the MAC bandwidth components framework) is calculated by a WLAN probe application that passively "sniffs" packets at the L2/MAC layer of the wireless medium and analyses their headers and temporal characteristics. The quality of VoIP calls is predicted using an extended version of the ITU-T E-model, which estimates user satisfaction from time varying transmission impairments. Through experimentation with various codecs and packetization schemes we found that as the load (number of calls) reaches the available capacity level, packet delays and jitter increase dramatically resulting in the call quality becoming degraded. We show how these MAC bandwidth components maybe used to assess the VoIP call quality on 802.11 WLANs
553221
An Experimental Investigation of Parallel Multimedia Streams Over IEEE 802.11e WLAN Networks Using TXOP
In this paper we perform an experimental investigation of the IEEE 802.11e TXOP facility to enhance the transmission of parallel multimedia streaming sessions through efficient bandwidth reservation and explicitly consider both the audio and video streams. The delay constraints associated with the audio and video streams that comprise a multimedia session pose the greatest challenge since real-time multimedia is particularly sensitive to delay as the packets require a strict bounded end-to-end delay. We show how the TXOPLimit parameter can be efficiently dimensioned to reduce the transmission delay for the video frames. Due to its frame-based nature, video applications are considered to be bursty as each video frame is typically transmitted as a burst of packets. The size of the burst is related to the size of the video frame and the number of packets required to transmit the video frame. The TXOP facility is particularly suited to efficiently deal with this burstiness since it can be used to reserve bandwidth for the duration of the packet burst. Through experimental investigation, we show that there is a significant performance improvement for the video streams by using the TXOPLimit parameter however there is no such improvement for the audio streams. We show that over-dimensioning the TXOPLimit parameter can cause the video stream to seize too much bandwidth which results in a deterioration in performance for the other competing traffic streams. This deterioration becomes more prominent as the number of parallel multimedia streams increases. We show that there is a performance improvement to all traffic streams by providing differentiated service to the constituent I, P, and B video frame types in conjunction with the TXOP facility.
5625582
Experimental evaluation of the Hewlett-Parkard exemplar file system
This article presents results from an experimental evaluation study of the HP Exemplar file system. Our experiments consist of simple micro-benchmarks that study the impact of various factors on the file system performance. These factors include I/O request/buffer sizes, vectored/non-vectored access patterns, read-ahead policies, multi-threaded (temporally irregular) requests, and architectural issues (cache parameters, NUMA behavior, etc.). Experimental results indicate that the Exemplar file system provides high I/O bandwidth, both for single- and multi-threaded applications. The buffer cache, with prioritized buffer management and large buffer sizes, is effective in exploiting temporal and spatial access localities. The performance of non-contiguous accesses can be improved by either using vectored I/O interfaces or tuning the read-ahead facilities. The file system performance depends on the relative locations of the computing threads and the file system, and also on various Exemplar design parameters such as the NUMA architecture, TLB/data cache management and paging policies.
36665753
Extracting Concealed Data from BIOS Chips
The practice of digital forensics demands thorough, meticulous examinations of all data storage media seized in investigations. However, BIOS chips and other firmware are largely overlooked in forensic investigations. No forensically sound procedures exist for imaging BIOS chips and no tools are available specifically for analyzing BIOS image files. Yet, significant amounts of data may be stored on BIOS chips without hindering machine performance.
10980387
Classical optimizations in Ada
Ada contains several features which have posed problems for optimizers when included in other languages. These features are subprograms, exception handling, and multi-tasking. They are defined in Ada in such a way that classical optimizations, such as common sub-expression elimination and code motion, are minimally affected. This paper discusses the impact of Ada's semantics on classical optimizations and the information needed for the correct application of these transformations.
31126198
Detecting Hidden Data in Ext2/Ext3 File Systems
The use of digital forensic tools by law enforcement agencies has made it difficult for malicious individuals to hide potentially incriminating evidence. To combat this situation, the hacker community has developed anti-forensic tools that remove or hide electronic evidence for the specific purpose of undermining forensic investigations. This paper examines the latest techniques for hiding data in the popular Ext2 and Ext3 file systems. It also describes techniques for detecting hidden data in the reserved portions of these file systems.
6966708
Countering Hostile Forensic Techniques
Digital forensic investigations can be subverted by hostile forensic techniques and tools. This paper examines various hostile forensic techniques, including the exploitation of vulnerabilities in standard forensic procedures and denial of service attacks on forensic tools during imaging and analysis. Several techniques for concealing evidence within file systems and external to file systems are highlighted. In addition, strategies for countering hostile forensic techniques and tools are discussed.