有线用户接入网的宽带化

有线用户接入网的宽带化是当前电信网建设中的热门话题之一。本文首先介绍什么是宽带业务，随后着重讨论了几种主要的有线用户接入网技术，如ＨＤＳＬ、ＡＤＳＬ、ＦＩＴＬ和ＨＦＣ，最后简单介绍了Ｖ５接口。     

宽带接入网技术简介

目前宽带接入技术主要有ｘＤＳＬ技术、Ｃａｂｌｅ Ｍｏｄｅｍ技术、以ＡＴＭ为基础的无源光网络（ＡＰＯＮ）、宽带固定无线接入、宽带以太网接入技术等。ｘＤＳＬ和ＨＦＣ是目前应用较广、技术较成熟的过渡技术；无线接入技术是一种新型的接入网技术，具有很大发展前景；以太网技术由于其在局域网上的巨大成功，大有前途。但接入网的最终目标是光纤到用户（ＦＴＴＨ）。下面着重介绍ｘＤＳＬ、ＨＦＣ技术、宽带固定无线接入技术和宽带以太网接入技术。一、接入网技术 １． ｘＤＳＬ技术 ＤＳＬ（Ｄｉｇｉｔａｌ Ｓｕｂｓｃｒｉｂｅｒ Ｌｉ…     

HDSL ADSL FTTC HFC有线接入网技术（下）

介绍了高比特率数字用户环路（ＨＤＳＬ）的系统结构、原理及应用；不对称数字用户环路（ＡＤＳＬ）的技术特点及应用；光纤到路边（ＦＴＴＣ）的网络结构、应用及网络的升级；光纤─同轴混合网（ＨＦＣ）的网络结构以及上述接入网技术经济比，并讨论了ＨＦＣ技术与当今通信系统数字化的关系。     

面向21世纪的接入网技术

本首先介绍新形势对接入网宽带化和ＩＰ化的,然后重点讨论几种接入网技术的最新发展态势,主要有ＵＤＳＬ、ＶＤＳＬ、ＳＤＨ、ＨＦＣ、ＡＰＯＮ、ＬＭＤＳ、ＥＤＳＬ技术,最后简要介绍我国接入网发展的现状和问题。     

ADSL和HFC接入网技术的分析及前景

介绍和分析限ＡＤＳＬ和ＨＦＣ两种接入网技术及其特点,并对这两种接入网方案在性能及成本上作了一些比较。最后指出,ＨＦＣ更适合于现有的ＣＡＴＶ网,而电信运营商则应将ＡＤＳＬ方案作为首选。     

接入网发展的思考

本介绍了接入网的现实需求及的问题的接入网带宽的几种方案,如ｘＤＳＬ、ＨＦＣ、无线传输的技术。     

接入网发展的思考

本文介绍了接入网发展的现实需求及面临的问题，解决接入网带宽的几种方案，如ｘＤＳＬ、ＨＦＣ、无线传输技术。     

宽带接入网技术与HFC

介绍了基于现有钢缆技术的宽带接入网高比特率数字用户环路（ＨＤＳＬ）与不对称数字用户环路（ＡＤＳＬ）,基于光缆技术的光纤同轴混合网络（ＨＦＣ）等几种典型的宽带用户接入网技术,特别是对光纤同轴混合网络（ＨＦＣ）接入技术的网络结构及其关键技术进行了详细分析。     

世纪之交的接入网技术与发展展望

目前,接入网已成为全网带宽的瓶颈,宽带化和ＩＰ化是接入网大势所趋。本文讨论了近来发展势头较好的接入网技术：ＡＤＳＬ（ＵＤＳＬ）和ＶＤＳＬ、ＨＦＣ、ＰＯＮ和ＡＰＯＮ、ＳＤＨ接入、ＬＭＤＳ、不可见光纤无线系统以及以太网等;同时指出要构筑公共的接入平台来综合多元经的接入技术。另外,中国接入网也正开始从窄带向宽带方式转变,但制造业在宽带接入技术研发方面滞后于人。     

社区宽带网络技术研究

社区宽带网络覆盖了接入网和用户驻地网的范围,向用户提供足以实现交互视频等应用的高速接入,它的技术实现手段有ＨＦＣ,ｘＤＳＬ,ＦＴＴｘ。     

RBAC模型在医疗系统中的研究与应用

基于角色的访问控制是目前应用在系统控制用户访问中比较主流的一门技术。在此针对医疗系统的特点,在基于角色的访问控制模型的基础上,分析医疗系统中的访问主体和客体,引入角色,将权限和角色相关联,重点研究不同用户对记录的访问控制,提出一个访问控制算法,通过分配用户适当的角色,然后授予用户适当的访问权限,使用户和访问权限逻辑分离,从而提高了在医疗系统中权限分配和访问控制的灵活性与安全性。     

一种增强的GEO卫星CDMA通信系统随机接入方式

本文提出了一种适用于GEO卫星CDMA移动通信系统的随机接入方式。该方式参考了S-UMTS中的随机接入方式设计,扩展了其接入帧和接入时隙的长度,并提出了一种新颖的用户随机接入请求的发送策略。通过接入时隙长度的扩展,增大了接入前缀码的个数和前缀的长度,提高了接入消息的扩频增益,从而提高了用户接入的首次捕获概率和接入消息的正确解码概率,降低用户的接入功率,并有效削弱功率攀升问题给系统容量造成的影响。增加了保护间隔的长度,以应对GEO卫星环境下用户之间接入的大时延差问题。本文提出的用户随机接入请求发送策略,有效改善了用户的接入时间性能。     

基于属性证书的跨域访问技术设计与实现

首先,在基于传统访问控制策略的基础上,针对当前互联网环境中存在的跨域访问安全问题,提出一种基于属性证书的访问控制策略。然后,给出属性证书的结构模型和管理方式,并对采用属性证书访问的优势进行分析。指出不同于传统的访问控制,基于属性证书的访问控制其本质是基于属性的访问控制。最后,给出属性证书在跨域访问控制中的应用。在单应用域或跨域访问时均可采用属性证书的验证方式,其能够针对不同应用域中用户的访问需求,动态实施资源管理和访问控制。     

A Channel Access Structure for Wideband ISDN

To be fully integrated, the Integrated Service Digital Network (ISDN) arena is preparing an evolution from its current narrowband structure (at basic and primary access rates) to a futuristic wideband structure where access rates will be capable of supporting services beyond 100 Mbits/s. A wideband channel structure for ISDN, supplementing the narrowband channel standards, is thus needed. This paper presents a channel structure in which the access bandwidth is organized in a hierarchy of channels of increasing speeds. Channels at a given speed level accommodate services with specific bandwidth and holding time requirements. However, to improve performance, the access interface uses a control strategy whereby a service request at a given level can overflow to a lower level for access at lower bit rate. The performance, in terms of access delay and access throughput, of this access scheme is analyzed. Numerical results are provided for the special case of an interface with two levels of channels. These results show that a significant improvement in access performance can be obtained with the overflow mechanism of the access control discipline used.     

Sheetlike Waveguide for 2.4 GHz and 5 GHz Bands

We present a useful design for a free access mat which supports two frequency bands of 2.4 GHz and 5 GHz. The free access mat is a sheet‐shaped waveguide which consists of a tightly coupled double‐layered microstrip resonator array. It provides easy access for devices in short‐range wireless communications. Interference is a common problem with conventional applications which use free space transmission. Our proposed wireless access system uses a subsidiary waveguide, the free access mat. Wireless devices are proximately coupled to the free access mat through which the coupled electromagnetic (EM) wave transmits. The arrival domain of the EM wave of an application is therefore limited to an area close to the free access mat. Wireless devices can be coupled to the free access mat at an arbitrary position without contact. We previously presented a free access mat for a single frequency band. This paper presents a free access mat for the two frequency bands of 2.4 GHz and 5 GHz. The free access mat uses a ring patch resonator array which is easily excited by typical antennas and is resistant to interference. These characteristics are demonstrated by numerical simulation and confirmed by experiment.     

一种改进的安全访问控制矩阵模型

操作系统安全保密的核心是访问控制。基本访问控制矩阵作为一种数学模型,在许多实际系统的访问控制中得到了应用,但是它不能满足对安全性有特殊要求的操作系统。本文在基本访问控制矩阵的基础上加上了访问条件,提出了一种三维访问控制矩阵;并且就主体和客体按访问条件的分类对这种矩阵的简化提出了一些看法。作为上述观点的实际例子,本文还介绍了一种非接触式IC卡的访问控制机制。     

LTE-Advanced时隙接入中动态帧长选择算法

通过分析LTE-Advanced系统中基于时隙接人信道控制协议(SCACP)的随机接入模型,提出一种时隙接入方法中接入帧长的优化算法.利用网络实时接入负载实现时隙接人中动态帧长的选择,使得优化后的接入帧长更适合当前接入负载,保障了较高的接入成功率,消除了不必要的接入时延,还在一定程度上减少了前导重传次数.最后通过仿真验证了所提动态帧长选择算法的有效性和实用性.     

一种基于CoMP的随机接入方案

提出了一种新的基于协作多点传输和接收（CoMP）技术的多发随机接入流程,并根据 随机接入的目的和业务类型设计了接入优先级,在此基础上设计了一种新的随机接入方案, 对 不同类型的随机接入请求提供不同的接入机制,以保证边缘用户的接入性能和满足不同业务 的QoS需求。仿真结果表明,与LTE标准随机接入方案相比,该方案对系统整体接入性能略有 提高,且大大提高了边缘高优先级用户的接入性能。     

Achieving distributed user access control in sensor networks

User access control in sensor networks defines a process of granting user an access right to the stored information. It is essential for future real sensor network deployment in which sensors may provide users with different services in terms of data and resource accesses. A centralized access control mechanism requires the base station to be involved whenever a user requests to get authenticated and access the information stored in the sensor node, which is inefficient, not scalable, and is exposed to many potential attacks along long communication paths. In this paper, we propose a distributed user access control under a realistic adversary model in which sensors can be compromised and user may collude. We split the access control into local authentication conducted by a group of sensors physically close to a user, and a light remote authentication based on the endorsement of the local sensors. We implement the access control protocols on a testbed of TelosB motes. Our analysis and experimental results show that our schemes are feasible for real access control requirements.     

MCCA: a high-throughput MAC strategy for next-generation WLANs [medium access control protocols for wireless LANs]

Wireless LAN technology has been shown to he a revolutionary development during the last decade. Recently popularized IEEE 802.11a/g-based products can support up to 54 Mb/s physical layer rate and provide wireless access to the Internet. However, in order to deal robustly with the unreliable wireless nature, the 802.11 medium access control protocol has a relatively large overhead and hence, the throughput performance is much worse than the underlying physical layer rate. Moreover, along with many emerging applications and services over WLANs, such as voice over WLAN and audio/video streaming, the demand lor faster and higher- capacity WLANs has been growing recently. In this article, we propose a new medium access control protocol for the next-generation high-speed WLANs. The proposed medium access control, called multi-user polling controlled channel access, is composed of two components: multi-layer frame aggregation, which performs aggregation at both the medium access control and the physical layers; and multi-user polling, used to reduce the contention overhead and in turn, achieve higher network utilization. Multi-user polling controlled channel access is compared with the 802.11e-enhanced distributed channel access medium access control. Highly enhanced medium access control efficiency can be achieved by applying multi-user polling controlled channel access. We show the improved medium access control performance in terms of the aggregate throughput of non-QoS Hows with relevant QoS requirements.