Search results for "Computer network"

Significance of channel failures on network performance in CRNs with reserved spectrum

2016

It is well understood that in wireless networks, channel failures, which are typically caused by equipment or power failures as well as intrinsic features in radio transmissions, such as fading and shadowing, can easily result in network performance degradation. Therefore, fast recovery from channel failures is an important measure that should be incorporated with those networks. Consequently, in a cognitive radio network (CRN), channel failures can cause significant performance degradation in both primary and secondary networks. Instead, retainability, i.e., the capability of providing continuous connection for users must be guaranteed even if a significant network element is disrupted. In…

An analytical study of mixed backoff schemes for QoS differentiation in wireless LAN

2009

In this paper, we propose an analytical model to evaluate performances of a wireless network composed by two groups of nodes using two different backoff schemes for channel access. The two different backoff schemes are used for QoS differentiation in term of priority. Our analysis results show that the priority group can have its performances enhanced, in term of saturation throughput and delay, without significant degradation of normal group nodes performances. This enhancement is also noticed for throughput efficiency at system level.

Enabling Cognitive-Radio Paradigm on Commercial Off-The-Shelf 802.11 Hardware

2013

Cognitive Radio paradigm (CR) has been recognized as key enabler for next generation wireless networking: the pos- sibility to access the limited radio spectrum in an oppor- tunistic manner allows secondary users to boost their trans- mission performance without interfering with existing pri- mary networks. Full testing and experimenting with this paradigm, however, is still a tough task, given either the i) limited capabilities above the PHY layer of cheap SDR so- lutions, or the ii) heavy investment required for setting up multi-node testbeds powered by FPGAs. In this demo we show how we leveraged our Wireless MAC Processor archi- tecture to tackle the two issues at the same time, providi…

Making WiFi work in multi-hop topologies: Automatic negotiation and allocation of airtime

2015

We propose a solution for mitigating the performance impairments of CSMA/CA protocols in multi-hop topologies based on the dynamic adaptation of the contention process experienced by nodes in a wireless network. A distributed protocol is used to negotiate the channel airtime for a node as a function of the traffic requirements of its neighbourhood, taking into account bandwidth reserved for the control operations. A mechanism is provided for a node to tune its contention window depending on its allocated airtime. Different from previous schemes, a node's contention window is fixed in size unless the traffic requirements of its neighbourhood change. The scheme is implemented on legacy commer…

Power consumption analysis for mobile stations in hybrid relay-assisted wireless networks

2010

While Internet access using mobile or wireless technologies has become ubiquitous these days, the energy consumption aspect of such connections has not been studied in-depth yet. In this paper, a hybrid wireless network, which consists of a cellular component and a relay-assisted ad hoc component, is studied focusing on energy consumption by mobile stations with respect to the amount of data communicated and achieved battery lifetime. Four alternative paths are considered, including both pure cellular and hybrid ad hoc/cellular links for uplink and downlink traffic. The effects of each alternative connection on energy consumption of the involved mobile stations are analyzed in terms of the …

Mobile information systems - executives' view

1997

The concepts of travelling executive and executives' mobile information system are first defined in this paper. The main findings are in the form of collected data and opinions concluded from our personal discussions with 49 executives in the United Kingdom, France, Italy and Finland concerning the nature of the work of executives and the usage of information technology (IT) to support their work today. The near future expectations of the executives are also analysed, especially concerning the mobile use of IT services in order to construct executives' holistic view of mobile computing. The use of IT services was found to be very widespread. Big differences were found: some of the executive…

Guest Editorial

2011

You’ve Got Photos! The design and evaluation of a location-based media-sharing application

2008

PhotoJournal is a novel location-based media sharing appli- cation that enables users to build interactive journals that associate multimedia files with locations on maps and share this information with other users. Its underlying informa- tion discovery and sharing mechanism is 7DS that runs in either pure peer-to-peer or centralized server-to-client mode, depending on the availability of a server and/or an infras- tructure. 7DS-enabled devices act as miniature caches, shar- ing information with each other. When access to an informa- tion server (e.g., web server) is not available, the local 7DS instance running on the device enables the device to search and access information from other p…

Chemical Bath Deposition as a Simple Way to Grow Isolated and Coalesced ZnO Nanorods for Light-Emitting Diodes Fabrication

2018

A way to grow and characterize isolated and coalesced ZnO nanorods on $p$ -GaN/sapphire structure is presented. Chemical bath deposition can be used to grow ZnO nanorods of device-quality, simply controlling the duration time of the growth process and the concentration of the nutrient solution in the bath. Increasing the duration of the process, as well as the concentration of the solution, leads to compact and sound layers instead of separated nanorods. However, too high concentrations stop the growth process. Light-emitting diodes fabricated on these ZnO-p-GaN heterostructure have a peak of electroluminescence at 400 nm and exhibit interesting electrical and optical properties. Optical po…

Fast Secure Routing for Highly Mobile Large-Scale Ad-Hoc Vehicular Networks

2009

In highly mobile large-scale vehicular ad-hoc networks, routing protocols must be fast and efficient in terms of low routing discovery delay and communication overhead to support the applications such as car talk. Such requirement becomes more difficult to achieve when inducing security enhancement to tackle the authentication of routing messages. In this paper, we propose a lightweight (fast) secure routing protocol, Secure Proactive Tree-based Routing (SPTR) to hit these two points. SPTR makes use of the characteristics of VANET networks, that is, the traffic originates from or terminates at the gateway or road side unit. SPRT also takes advantages of certificate-less ID-based cryptograph…