Asynchronous Channels Research Articles

An Asynchronous Channel Hopping Sequence for Rendezvous Establishment in Self Organized Cognitive Radio Networks

A cognitive radio performs spectrum sensing, so that the device is able to self-organize appropriate networking functions. Rendezvous is the process by which the nodes explore the available channels to establish a link when they want to communicate with each other. Although using a dedicated control channel can be a solution to find the common control channel, it may result in a bottleneck and create a single point of failure. In this paper, we propose an asynchronous channel hopping sequences to establish a link in self organizing cognitive radio networks without the assumption of global clock synchronization. Our scheme provides successful communication rendezvous within a bounded time interval. Theoretical analysis and simulation results showed that our scheme outperforms the existing asynchronous schemes used for similar purpose in terms of many performance criteria.

State Synchronization Approaches in Web-based Applications

Abstract The main objective of the article is to provide insight into technologies and approaches available to maintain consistent state on both client and server sides. The article describes basic RIA application state persistence difficulties and offers approaches to overcoming such problems using asynchronous data transmission synchronization channels and other user-available browser abilities.

On asynchronous eventful session semantics

Event-driven programming is one of the major paradigms in concurrent and communication-based programming, where events are typically detected as the arrival of messages on asynchronous channels. Unfortunately, the flexibility and performance of traditional event-driven programming come at the cost of more complex programs: low-level APIs and the obfuscation of event-driven control flow make programs difficult to read, write and verify.This paper introduces a π-calculus with session types that modelsevent-driven session programming(called ESP) and studies its behavioural theory. The main characteristics of the ESP model are asynchronous, order-preserving message passing, non-blocking detection of event/message arrivals and dynamic inspection of session types. Session types offer formal safety guarantees, such as communication and event handling safety, and programmatic benefits that overcome problems with existing event-driven programming languages and techniques. The new typed bisimulation theory developed for the ESP model is distinct from standard synchronous or asynchronous bisimulation, capturing the semantic nature of eventful session-based processes. The bisimilarity coincides with reduction-closed barbed congruence.We demonstrate the features and benefits of ESP and the behavioural theory through two key use cases. First, we examine an encoding and the semantic behaviour of the event selector, a central component of general event-driven systems, providing core results for verifying type-safe event-driven applications. Second, we examine the Lauer–Needham duality, building on the selector encoding and bisimulation theory to prove that a systematic transformation from multithreaded to event-driven session processes is type- and semantics-preserving.

Asynchronous-Channels Within Petri Net-Based GALS Distributed Embedded Systems Modeling

Model-based development approaches can provide a major contribution in the development of globally asynchronous locally synchronous distributed embedded systems (GALS-DES) if supported by suited modeling formalisms and design automation tools. The use of Petri nets (either low-level or high-level classes) extended with asynchronous-channels (ACs), time domains, priorities, inputs, and outputs is proposed in this paper to model GALS-DES (composed by deterministic components), ensuring that the created GALS model is locally deterministic, distributable, network-independent, and platform-independent. The proposed ACs, with high level of abstraction, specify the components interaction through Petri net objects with specific attributes that unambiguously identify this interaction within the GALS model. Two algorithms are proposed to translate and decompose the GALS model into Petri net models without ACs, which can be used as inputs in model-checking tools and automatic code generators supporting GALS-DES verification and implementation. The specification of a small goods lift distributed controller illustrates the use of the proposed ACs.

Teaching the disembodied: Othering and activity systems in a blended synchronous learning situation

<p>This study examines what happens when online and campus students participate in real time in the same campus classroom. Before this study, postgraduate students studying online in a course intended primarily as professional development for language educators were taking the course through reading the course literature including assigned articles, writing reflective texts in the asynchronous forum and doing the course assignments. They had a very different experience than the campus students who met weekly for discussion of the reading. Some online students were not active enough in the course, and showed low levels of engagement. The online students were invited to participate in scheduled campus classes via Skype on iPads. After some hesitation, four of the six online students took up this real-time participation option. Initial difficulties with the technology were addressed after seeking input from campus and online students. A series of adjustments were made and evaluated, including a move to a model in which three online students in different locations participated in a single Skype group video call on a laptop in the campus classroom rather than on multiple individual Skype calls on iPads. After the course, the online and campus students were asked to evaluate the experience of having physical and virtual participants sharing a physical space and to relate this experience to the asynchronous channels previously available to the participants. The comments of both groups of participants were interpreted in the light of previous work on social presence and of activity theory. It appears that student beliefs and student expectations lead to hidden challenges associated with mixing these groups of students, and the study concludes that unless teaching assistance is available, it is not easy to afford online students the same right to speak as campus students.</p>

Erratum to “A Rendezvous Scheme for Self-Organizing Cognitive Radio Sensor Networks”

In the paper entitled “A rendezvous scheme for self-organizing cognitive radio sensor networks,” the corresponding author’s information was not correct and it is corrected here. We proposed an asynchronous channel hopping mechanism, called a hopping sequence guaranteeing rendezvous within a single period (HS-GRSP); cognitive radio (CR) nodes can rendezvous in a single period and reliably exchange control messages with each other. Equation (1) gives the length of guard sequence and its subsequences. However, it does not guarantee the appearance of a center channel in every subsequence in all cases. To make our scheme more robust, we changed (1) as follows:

Source–Channel Communication Over Phase-Incoherent Multiuser Channels

We study the transmission of two correlated and memoryless sources (U 1 , U 2 ) over several multiuser phase asynchronous channels. Namely, we consider a multiple access relay channel (MARC) with causal, and a MARC with non-causal unidirectional cooperation between encoders, referred to as phase-incoherent causal (respectively, non-causal) cognitive MARCs. We also consider phase-incoherent interference channel models with and without relay, in the same context. In all cases, the input signals are assumed to undergo non-ergodic phase shifts, which are unknown to the transmitters and known to the receivers as a realistic assumption. Both necessary and sufficient conditions in order to reliably send the correlated sources to the destinations are derived. In particular, for all of the channel models, by using a key lemma, we first derive an outer bound for reliable communication. Then, using separate source and channel coding and under specific gain conditions, we establish the same region as the inner bound. We thus conclude that without the knowledge of the phase shifts at transmitters, and under specific gain conditions, separation is optimal.

Low‐complexity ad‐hoc non‐linearities for blind multiuser detection of long‐code code‐division multiple access signals and asymptotic performance evaluation

In this study, monomials with odd integer powers are proposed for blind multiuser detection of code-division multiple access signals with long spreading codes, in the near-far situation. The performance of the new non-linear detectors are substantiated asymptotically and confirmed via simulations. It is demonstrated that the proposed non-linear detectors significantly outperform the matched filter detector in the block fading and Ricean fading channels, at the cost of a small increase in computational complexity. The optimum integer power value for the block fading channel with a limited interferer level, the Ricean channel with a given K factor and asynchronous channel is determined. The proposed multiuser detectors are blind in the sense that they require neither training nor the spreading code of the interferers. The detectors also do not require long convergence time for decision making in contrast to conventional blind multiuser detectors.

A stepwise approach to developing staged applications

The staged event-driven architecture (SEDA) can be seen as a milestone as regards integration of threads and events in a single model. By decomposing applications into sets of multi-threaded stages connected by event queues, SEDA allows for the use of each concurrency model where most appropriate. Inside each SEDA stage, the number and scheduling policy of threads can be adjusted to enhance performance. SEDA lends itself to parallelization on multi-cores and is well suited for many high-volume data stream processing systems and highly concurrent event processing systems. In this paper, we propose an extension to the staged model that decouples application design from specific execution environments, encouraging a stepwise approach for designing concurrent applications, similar to Foster's PCAM methodology. We also present Leda, a platform that implements this extended model. In Leda, stages are defined purely by their role in application logic, with no concern for locality of execution, and are bound together through asynchronous communication channels, called connectors, to form a directed graph representing the flow of events inside the application. Decisions about the configuration of the application at execution time are delayed to later phases of the implementation process. Stages in the application graph can then be grouped to form clusters, and each cluster is mapped to an exclusive OS process, running on an arbitrary host. Finally, we discuss two example applications which we developed to evaluate the Leda platform.

On the Efficiency of Berger Codes Against Error Injection Attacks on Parallel Asynchronous Communication Channels

ABSTRACTThe paper deals with the efficiency of Berger codes under error injection attacks. Berger codes are unordered codes; that is, they are insensitive to propagation delays of individual bits in a codeword. Berger codes are commonly used (in concatenation with linear codes) to provide reliable communication over parallel asynchronous channels. The paper examines the robustness of Berger codes under error injection attacks on the communication channels. It is shown that if the number of information bits is not equal to , where is the number of redundancy bits, then the Berger code is robust; that is, any injected error can be detected.

Capacity Region of the Asynchronous Gaussian Vector Multiple-Access Channel

In this paper, we derive explicit expressions for the capacity region of the two-user symbol-asynchronous Gaussian vector multiple-access channel. Verdu considered the case where each user linearly modulates a fixed waveform in each symbol period, where the symbol periods for the users are not perfectly aligned at the receiver. He derived explicit capacity region expressions for the case where the transmitters have knowledge of the mutual offset and also for the case where the transmitters have no knowledge of the mutual offset. In this paper, we extend Verdu's results to allow each user to linearly modulate a set of orthonormal waveforms, instead of a single waveform, in each symbol period and with no restrictions imposed on the waveforms. We consider group power constraints, which include individual sum power constraints, as orthonormal waveforms assigned to each user may come from different frequency bands with different power constraints. Similar to the case where each user is allowed to linearly modulate only a single waveform, our results hold regardless of whether or not the transmitters are frame synchronous. In addition, we present some results that are necessary to numerically compute the capacity region expressions with general purpose convex optimization algorithms. Next, we simplify the capacity region expression when there are only individual sum power constraints and when the transmitters know the mutual offset. We also prove a sufficient condition for a similar simplification to hold when the transmitters have no knowledge of the mutual offset. Finally, we consider a specialized algorithm to numerically compute the simplified capacity region expression when the transmitters know the mutual offset.

Maximizing Rendezvous Diversity in Rendezvous Protocols for Decentralized Cognitive Radio Networks

In decentralized cognitive radio (CR) networks, establishing a link between a pair of communicating nodes requires that the radios rendezvous” in a common channel-such a channel is called a rendezvous channel-to exchange control information. When unlicensed (secondary) users opportunistically share spectrum with licensed (primary or incumbent) users, a given rendezvous channel may become unavailable due to the appearance of licensed user signals. Ideally, every node pair should be able to rendezvous in every available channel (i.e., maximize the rendezvous diversity) so that the possibility of rendezvous failures is minimized. Channel hopping (CH) protocols have been proposed previously for establishing pairwise rendezvous. Some of them enable pairwise rendezvous over all channels but require global clock synchronization, which may be very difficult to achieve in decentralized networks. Maximizing the pairwise rendezvous diversity in decentralized CR networks is a very challenging problem. In this paper, we present a systematic approach for designing CH protocols that maximize the rendezvous diversity of any node pair in decentralized CR networks. The resulting protocols are resistant to rendezvous failures caused by the appearance of primary user (PU) signals and do not require clock synchronization. The proposed approach, called asynchronous channel hopping (ACH), has two noteworthy features: 1) any pair of CH nodes are able to rendezvous on every channel so that the rendezvous process is robust to disruptions caused by the appearance of PU signals; and 2) an upper bounded time-to-rendezvous (TTR) is guaranteed between the two nodes even if their clocks are asynchronous. We propose two optimal ACH designs that maximize the rendezvous diversity between any pair of nodes and show their rendezvous performance via analytical and simulation results.

Organizational Communication in Emergencies: Using Multiple Channels and Sources to Combat Noise and Capture Attention

This study relies on information theory, social presence, and source credibility to uncover what best helps people grasp the urgency of an emergency. We surveyed a random sample of 1,318 organizational members who received multiple notifications about a large-scale emergency. We found that people who received 3 redundant messages coming through at least 1 synchronous communication source—for example, phone or face-to-face communication—perceived the urgency of the situation the most quickly, whereas those receiving official messages through asynchronous channels—most often text message—had the lowest sense of urgency. Our findings suggest that by understanding people's reactions to various kinds of redundant communication, organizations can design more strategic emergency messages that capture attention.

SYSTEM AND METHOD FOR MEDICAL IMAGING WITH ROBUST MODE SWITCHING VIA SERIAL CHANNEL

In one embodiment of the invention, there is an ultrasound processing system that communicates images over a single asynchronous serial channel according to a scheme that does not require an isochronous serial channel and that switches among ultrasound imaging modes robustly. For example, the system is configured to packetize ultrasound image data of at least one ultrasound imaging mode into a stream of data frames and to convey the stream of data frames via the asynchronous channel. Each data frame includes indication of the ultrasound imaging mode and includes ultrasound-imaging-mode-specific imaging parameters. Other embodiments exist.

APPLYING COMBINATORIAL OPTIMIZATION HEURISTICS FOR ENHANCING THE PERFORMANCE OF TH-PPM UWB SYSTEMS: CHAOTIC VERSUS CLASSICAL CODE SEQUENCES

In this article, we propose a process to minimize the Multi-User Interference (MUI) caused by Multiple Access (MA) in a Time Hopping Ultra Wide Band (TH-UWB) system with No Line-Of-Sight (NLOS) multipath channel in a residential environment IEEE 802.15.4a Channel Model 2 (CM2). We discuss both synchronous and asynchronous channels with an Additive White Gaussian Noise (AWGN). The choice of codes allowing multiple access is crucial for minimizing the MUI and to improve the system performance. Recently much works has been done in order to optimize performance by code selection [Bravo et al., 1999; Moreno & Marić, 2000; Guvenc & Arslan, 2004]. This paper is a contribution to this research axis. The minimization of the MUI is formulated as a Combinatorial Optimization Problem (COP) and solved by means of an efficient generic optimization method called Guided Local Search (GLS) [Voudouris, 1997]. In this context, we exploit the richness of chaotic sequences with respect to topologic properties. Simulation results based on the Binary Error Rate (BER) show that optimized chaotic sequences outperforms the classical code sequences.

Deterministic end-to-end delay analysis in an avionics network

Avionics Full DupleX Switched Ethernet is a deterministic communication protocol for distributed embedded real-time applications over asynchronous channels. It is a promising technique that can replace the existing avionics data buses, such as ARINC429 and MIL-STD-1553B. One of the key challenges for deploying and maintaining Avionics Full DupleX Switched Ethernet is to determine the end-to-end transmission delay in such a network. This article aims at handling this challenge effectively applying a completely theoretical analysis. An analytical method based on the network calculus theory is presented in detail to calculate the end-to-end transmission delay in an Avionics Full DupleX Switched Ethernet network, which consists of many interconnect nodes with different scheduling disciplines. Further, this approach is improved by taking into account the effects of source node initial jitter and first in, first out optimization. Additionally, a Matlab/TrueTime simulation platform is constructed to verify the effectiveness of the method. Simulation results show that switches with advanced scheduling algorithms, i.e. static priority scheduling, can significantly improve the delay performance in a multi-hop network.

The complexity of asynchronous model based testing

In model based testing (MBT), testing is based on a model M that typically is expressed using a state-based language such as an input output transition system (IOTS). Most approaches to MBT assume that communications between the system under test (SUT) and its environment are synchronous. However, many systems interact with their environment through asynchronous channels and the presence of such channels changes the nature of testing. In this paper we investigate the situation in which the SUT interacts with its environment through asynchronous channels and the problems of producing test cases to reach a state, execute a transition, or to distinguish two states. In addition, we investigate the Oracle Problem. All four problems are explored for both FIFO and non-FIFO channels. It is known that the Oracle Problem can be solved in polynomial time for FIFO channels but we also show that the three test case generation problems can also be solved in polynomial time in the case where the IOTS is observable but the general test generation problems are EXPTIME-hard. For non-FIFO channels we prove that all of the test case generation problems are EXPTIME-hard and the Oracle Problem in NP-hard, even if we restrict attention to deterministic IOTSs.

Intercultural Relationship, Prejudice and Ethnocentrism in a Computer-Mediated Communication (CMC): A Time-Series Experiment

This paper investigated the effects of relationship types (intracultural versus intercultural conditions) on relational development in computer-mediated communications (CMC) and face-to-face settings by controlling for the effect of prejudice and ethnocentrism. A total of 140 participants were involved in the experiment, and they were randomly assigned to the face-to-face, synchronous and asynchronous computer-mediated channels. Within each group, they were assigned to intracultural versus intercultural conditions and were required to interact in pairs with their zero-history partners. All participants underwent four experimental sessions.. The results exhibited a significant main effect of relationship types on relational progression in face-to-face and asynchronous CMC groups. No significant main effect of relationship types was observed in the synchronous CMC group. The results showed no significant effects of prejudice and ethnocentrism on relational development in all groups. The findings of the study provide partial support for the “equalization” view of CMC.

Distributed power control with multiuser detection for asynchronous DS-CDMA networks subject to time-delays

If transmission power is increased in a direct-sequence code-division multiple-access network, interference increases, thus reducing network capacity. In this paper, we address these opposite goals of reducing transmission power and increased network capacity by jointly considering power control and multiuser detection. Since propagation delay occurs inevitably during data communication, we present an analysis by considering also that the interference measurement is not available at the transmitter instantaneously but with some time-delay, something that is generally neglected in the literature. This framework is used to investigate the design of a distributed power control strategy enhanced with linear multiuser receivers in an asynchronous uplink channel, subject to multipath and quality of service constraints. Thus, a cross-layer solution that combines a Linear-Quadratic-Gaussian power control scheme with a linear multiuser receiver is proposed to compensate for the round-trip delay and the time-varying channel characteristics in the communication link. Simulation results are used to show the advantages of such scheme in terms of saving energy, increasing network capacity and robustness against propagation delays.

Completely Irrepressible Sequences for the Asynchronous Collision Channel Without Feedback

A channel is asynchronous if all users can start to transmit at an arbitrary point in time. Protocol sequences are used for multiple access in the collision channel without feedback. In this paper, we consider protocol sequence sets with the property that each user is able to successfully send at least one packet in each sequence period for the asynchronous channel. Such sequence sets are said to be completely irrepressible (CI). We analyze the class of CI sequences with the minimum number of ones in each period and derive lower bound on the minimum period. Moreover, if the sequence structure satisfies some technical conditions, which are called equi-difference, we improve the lower bound and present a construction that asymptotically meets this lower bound. We also show that the deterministic sequences proposed in this paper yield better performance in terms of average delay than the random approach.