通信工程软件项目开发及风险研究

文章主要介绍了通信工程软件开发研究实例,并根据该实例论述了软件项目开发的各种风险和风险影响因素,研究了风险怎样识别和预测以及风险的缓解、监控和管理方法。     

A survey of proposals for an alternative group communication service

As expectations for the Internet to support multimedia applications grow, new services need to be deployed. One of them is the group communication service for one-to-many or many-to-many data delivery. After more than a decade of important research and development efforts, the deployment of multicast routing in the Internet is far behind expectations. Therefore, a first motivation for an alternative group communication service is to bypass the lack of native IP multicast routing. Although less efficient and scalable than native multicast routing, such alternative services are generally suitable for the purpose. A second possible motivation is to go beyond the limitations of classic multicast routing for very specific working environments. We identify, classify, and discuss some of these alternative approaches.     

量子保密通信:一种无条件安全的通信方式

量子通信是经典通信和量子力学相结合的一门新兴交叉学科,它是目前科学界公认的唯一能实现无条件安全的通信方式。本文分析了现有密码系统存在的安全问题,介绍了量子密码术概念,深入剖析了最典型的量子密钥分发协议—BB84协议的全过程,并分析了它的安全性。     

Information theory and communication networks: an unconsummatedunion

Information theory has not yet had a direct impact on networking, although there are similarities in concepts and methodologies that have consistently attracted the attention of researchers from both fields. In this paper, we review several topics that are related to communication networks and that have an information-theoretic flavor, including multiaccess protocols, timing channels, effective bandwidth of bursty data sources, deterministic constraints on datastreams, queuing theory, and switching networks     

Complex-wall effect on propagation characteristics and MIMO capacities for an indoor wireless communication environment

The effects of complex wall structures on the characteristics of fading and the capacity of multi-input multi-output (MIMO) wireless communication systems for some typical indoor propagation environments are investigated. Two cases of wall structures are examined in this paper. In the first case, the walls are considered to be homogenous solid slabs, while, in the second case, the walls are assumed to be of complex structures. A two-dimensional finite difference time domain method is employed to calculate the electric field distributions, and then, the local mean power, the Rician K factor, and the MIMO capacity are calculated and analyzed. It is found that the patterns of the local mean power distributions are different for the two wall-structure cases. As for the small-scale fading, it is shown that the Rician K factors for the two cases may be different by 5 dB. The resulting values of MIMO capacities are also quite different and are less than the ideal cases, where the elements of the transfer (H) matrix are assumed to be zero-mean Gaussians with unit variance. We also investigate the cases where complex walls are replaced by effective slab walls. It is found that complex walls cannot be appropriately characterized by simple effective slab walls as considerable difference exists between the two cases.     

Testing communication and computer networks: an overview

Many activities in today's organizations are becoming ever more dependent on communications and computer networks, and network managers are faced with the enormous challenge of increasing the availability and efficiency of their infrastructures, which grow both in size and complexity. This makes it crucial to plan network operation systematically, to define and implement appropriate procedures for regular monitoring and performance assessment, and to set up proper tools for maintenance and troubleshooting purposes. Furthermore, proactive network testing must be pursued; it is vital to gather some ideas of normal network operation documented in order to have a comparison term when problems occur. The article has dealt with the measurement instruments and procedures currently adopted for network testing and advances in the field for the I&M research community.     

MF-TDMA卫星通信系统技术体制研究

MF-TDMA卫星通信体制组网灵活,支持综合业务接入,能够实现大小终端同网工作,并且可以针对不同的应用需求进行灵活设计,目前已成为国内外卫星应用、发展及研究的热点。从透明转发和星上处理转发2个方面介绍了目前国内外提出的一些MF-TDMA技术体制以及各自特点。结合MF-TDMA技术体制介绍了几种安全机制和抗衰落技术,以适应某些特殊应用的设计要求。     

A name model for nested group communication

Group communication permits a single sender to communicate with multiple receivers. A nested group permits one or more of the receivers to be itself a group. Nested groups are particularly useful for reducing communication traffic on internetwork links and supporting subnetwork autonomy. A name graph model is used to characterize nested groups and formalize the problems of loops and duplications. The authors design and analyze a spanning shadow tree algorithm that detects potential loops and duplication. The algorithm is considered static because loops and duplicates are detected at the time of group membership modification rather than at the time a message is sent. The algorithm changes the system-level representation of the name graph in a transparent manner to avoid infinite loops and to suppress duplicated messages. The worst-case message complexity of name graph update operations is on the order of O(|N|²), where |N| is the number of groups in the system. The complexity of updates can be justified, since run-time overhead for actual message communication is reduced     

Scalable secure group communication over IP multicast

We introduce and analyze a scalable rekeying scheme for implementing secure group communications Internet protocol multicast. We show that our scheme incurs constant processing, message, and storage overhead for a rekey operation when a single member joins or leaves the group, and logarithmic overhead for bulk simultaneous changes to the group membership. These bounds hold even when group dynamics are not known a priori. Our rekeying algorithm requires a particular clustering of the members of the secure multicast group. We describe a protocol to achieve such clustering and show that it is feasible to efficiently cluster members over realistic Internet-like topologies. We evaluate the overhead of our own rekeying scheme and also of previously published schemes via simulation over an Internet topology map containing over 280 000 routers. Through analysis and detailed simulations, we show that this rekeying scheme performs better than previous schemes for a single change to group membership. Further, for bulk group changes, our algorithm outperforms all previously known schemes by several orders of magnitude in terms of actual bandwidth usage, processing costs, and storage requirements.     

Secure group communication in wireless mesh networks

Wireless mesh networks (WMNs) have emerged as a promising technology that offers low-cost community wireless services. The community-oriented nature of WMNs facilitates group applications, such as webcast, distance learning, online gaming, video conferencing, and multimedia broadcasting. Security is critical for the deployment of these services. Previous work focused primarily on MAC and routing protocol security, while application-level security has received relatively little attention. In this paper we focus on providing data confidentiality for group communication in WMNs. Compared to other network environments, WMNs present new challenges and opportunities in designing such protocols. We propose a new protocol framework, Secure Group Overlay Multicast (SeGrOM), that employs decentralized group membership, promotes localized communication, and leverages the wireless broadcast nature to achieve efficient and secure group communication. We analyze the performance and discuss the security properties of our protocols. We demonstrate through simulations that our protocols provide good performance and incur a significantly smaller overhead than a baseline centralized protocol optimized for WMNs.     

Pilot: probabilistic lightweight group communication system for ad hoc networks

Providing reliable group communication is an ever recurring topic in distributed settings. In mobile ad hoc networks, this problem is even more significant since all nodes act as peers, while it becomes more challenging due to highly dynamic and unpredictable topology changes. In order to overcome these difficulties, we deviate from the conventional point of view, i.e., we "fight fire with fire," by exploiting the nondeterministic nature of ad hoc networks. Inspired by the principles of gossip mechanisms and probabilistic quorum systems, we present in this paper PILOT (probabilistic lightweight group communication system) for ad hoc networks, a two-layer system consisting of a set of protocols for reliable multicasting and data sharing in mobile ad hoc networks. The performance of PILOT is predictable and controllable in terms of both reliability (fault tolerance) and efficiency (overhead). We present an analysis of PILOT's performance, which is used to fine-tune protocol parameters to obtain the desired trade off between reliability and efficiency. We confirm the predictability and tunability of PILOT through simulations with ns-2.     

Project performance and the liability of group harmony

There have been numerous debates about the proper makeup and behavior of effective project teams, and the literature is contradictory with regard to this issue. In the present study, 44 members of 14 project teams who worked on the same complex computer-simulated project were observed. A survey administered four times during the life of the project revealed that initial ratings of group attributes were good predictors of later success. Teams which ultimately performed well began the project with lower opinions of their respective groups than did teams which ultimately did not perform well. Peer ratings of individual members' contributions were generally lowest in the high-performing teams. Additionally, technical expertise appears to have been valued more highly in the high-performing groups than in low-performing groups. These results suggest that project teams which begin with harmonious interpersonal relations may not perform as well as those which experience early disharmonics. The managerial implications of these results are discussed in relation to the literature on groups, with special attention to `groupthink' theories     

The communication characteristics of virtual teams: a case study

Organizations are encountering novel external environments requiring flexible structures. A number of organizations have used virtual teams to provide the customer responsiveness, human resource flexibility, and speed in project completion these environments demand. Virtual teams create significant communication challenges for its leaders and members. This research analyzed the communication technologies that the Customer Support Virtual Team (CST) of International Consulting Systems (ICS), the pseudonym for a Fortune 500 organization, uses to support team interaction, the degree to which ICS systems and culture supported CST, and finally, the CST members' mindset toward communication and the methods its leader used to create the trust required for effective team interaction. Interviews revealed that ICS mission, strategy, tasks, reward systems, and attitudes toward technology supported virtual team structure. CST members were provided a suite of robust technologies to facilitate interaction; however, they relied heavily on voice mail and a large number of team, project, and organizational databases supported by Lotus Notes to generate a common language that facilitated task completion     

短波在无线电通信中的作用及特点

短波的频率范围1．5MHz-30MHz，传播途径分为地波和天波两种。其中地波因为受地面吸收和地面电气特性的影响而衰减的程度较大，只适用于短距离通信。因此短波通信的主要传播路径是天波。天波的传播是利用电离层的反射来实现的，尤其是多次反射后可以实现全球通信。地波传播受天气的影响较小，比较稳定，信道参数基本上不随时间的变化而变化，是恒参信道。     

An evaluation of flow control in group communication

Group communication services have been successfully used to construct applications with high availability, dependability, and real-time responsiveness requirements. Flow control techniques enable group members to manage their local buffers, which they use to temporarily store multicast updates. Despite buffer overflow being one of the main causes of process failures, flow control has not been studied much in the literature. We study different flow control techniques used in some of the group communication services and present two generic flow control techniques: a conservative and an optimistic technique. All existing flow control techniques for group communication can be classified as either conservative or optimistic. We then present discrete event simulation results that compare the effect of these two generic flow control techniques on the performance of two, different atomic multicast protocols, a positive acknowledgment protocol and a negative acknowledgment protocol, under several different operating conditions. Based on the study of differed existing flow control techniques for group communication and the results obtained from the simulation experiment, we provide some design guidelines for the design and implementation of a suitable flow control technique for a given group communication service     

A new group demodulator for mobile communication systems

This paper proposes a group demodulator having a dual-parallel structure that employs multisymbol chirp Fourier transform to demodulate band-limited and bit asynchronous signals; the demodulator is sufficiently feasible for practical use unlike conventional group demodulators based on the chirp Fourier transform. Moreover, a new symbol timing offset detection scheme is also proposed to realize the aforementioned group demodulator, and the power consumption of the proposed group demodulator including the symbol timing offset detector is also evaluated. Computer simulation results show that the overall bit error rate (BER) degradation of the proposed group demodulator employing six-symbol chirp Fourier transform is less than 0.5 dB at BER=10^-3 even when a root Nyquist (α=0.6) filter is used as the transmission band-limiting filter and symbol timing offset between the desired channel and the chirp sweep exists. The power consumption is lower than that of conventional demodulators when more than 120 carriers are handled     

Experimental investigation of a pipeline for optical communication

Methods of measuring the parameters of a pipeline for light transmission are described, and results are presented. The pipeline is 100m long and metallised by aluminium on the inside. The cross-section of the pipeline is about 20mm, and it was constructed inside a building.     

Experimental investigation of the input-impedance characteristics of an antenna in a rectangular waveguide

The input impedance of a monopole antenna in a rectangular waveguide is investigated experimentally. It is found that theoretical results based on the assumption of a sinusoidal current distribution are in good agreement for monopole lengths up to about 0.6 of the guide height and about 1/4 ?0. For longer monopoles, a more accurate theory should be sought.