A simulation study of service differentiation for the bandwidth request scheme in fixed IEEE 802.16 network

Various flexible mechanisms related to quality of service (QoS) provisioning have been specified for uplink traffic at the medium access control (MAC) layer in the IEEE 802.16 standards. Among the mechanisms, contention based bandwidth request scheme can be used to indicate bandwidth demands to the base station for the non-real-time polling and best-effort services. These two services are used for most applications with unknown traffic characteristics. Due to the diverse QoS requirements of those applications, service differentiation (SD) is anticipated over the contention based bandwidth request scheme. In this paper we investigate the SD with the bandwidth request scheme by means of assigning different channel access parameters and bandwidth allocation priorities at different packets arrival probability. The effectiveness of the differentiation schemes is evaluated by simulations. It is observed that the initial backoff window can be efficient in SD, and if combined with the bandwidth allocation priority, the SD performances will be better. Foundation item: the National Basic Research Program (973) of China (No. 2005C13321804)     

In-home power line communication media access control protocol based on collision resolution

Most existing media access control (MAC) protocols in power line communication (PLC) networks just discard the colliding data packets when collision occurs. The collision deteriorates throughput and delay performance of system under high traffic conditions. This article presents a novel media access scheme with fast collision resolution for in-home power line networks. It works by first recognizing the colliding stations through detecting the inserted unique ID sequence ahead of data packets, then the source nodes retransmitting their packets immediately after the collision slot. The proposed protocol maintains the benefits of ALOHA systems. It needs no scheduling overhead and is suitable for bursty sources, such as multimedia data packets. Computer simulations have demonstrated that this approach can achieve high throughput due to its ability of resolving collisions.     

珠江黄埔大桥南汊悬索桥跨缆吊机静载试验研究

钢箱梁吊装施工是悬索桥重要的施工阶段,跨缆吊机是钢箱梁提升安装的专用设备,跨缆吊机的承载能力和运行效率直接关系到施工质量、进度和安全.该文首先对跨缆吊机重要受力构件进行了结构细部分析和验算,并得出最不利受力位置,根据细部分析确定静载试验方案,拟定加载顺序和测试位置.通过跨缆吊机行走、静载试验,跨缆吊机整体的可靠性、安全性、操作方便性等得到充分验证.     

柱式桥墩台盖梁的深梁计算与设计

柱式桥墩是公路桥梁设计中普遍采用的结构形式,由于桥梁跨径、斜度、桥宽的变化,很难完全套用现行标准图和通用图,尤其是盖梁标准化程度底,往往需要设计者对盖梁进行内力分析与设计,<公路钢筋混凝土及预应力混凝土桥涵设计规范>仅对钢筋混凝土盖梁跨高比L/h＞5和2＜L/h≤5的情况进行了抗弯、抗剪承载力规定,而跨高比L/h≤2即盖梁作为深梁的承载力未作规定.文中参照建筑部门规范,论述了盖梁作为深梁时的承载力计算方法和步骤,以及设计的构造要求,并列举了计算实例.     

混凝土徐变模型在桥梁预拱度设置中的分析比较

通过MIDAS有限元软件建立了悬臂法施工连续刚构桥梁计算模型,并考虑不同的混凝土徐变模型计算了桥梁的预拱度值,并与实际工程做对比,得出选择徐变模型的一些原则.     

南京市三山桥设计研究

以南京市三山桥的设计为研究对象,阐述自锚上承式拱桥的结构构思,研究预应力筋和矢跨比2个关键因素对自锚上承式拱桥拱脚推力的影响,介绍自锚上承式拱桥的预应力筋的优化设计方法.实桥测试结果表明,该设计是可行的.     

高速公路扩建工程新增上跨桥施工技术

沪宁高速公路扩建工程采取"两侧拼接加宽为主、局部分离加宽"的办法,将主线扩建为双向8车道并增设上跨桥,新增上跨桥施工时,要兼顾主线扩建施工并确保高速公路安全畅通,对交通组织和施工工艺提出了较高要求.现就高速公路扩建工程中新增加的简支梁、连续箱梁、钢箱梁等上跨桥施工工艺和受安全影响最大的中墩施工方法和安全技术进行总结.     

浅议跨高速公路现浇箱梁施工期间交通安全维护

杭浦高速公路(杭州段)大井枢纽工程的主线1号桥及G匝道9号上跨杭州绕城北高速公路,由于绕城高速公路连接着进人杭州市的所有高速公路,车流量大,这给两座桥的上构施工带来了诸多困难.重点介绍了1号和9号桥跨杭州绕城北高速公路路段上构施工时门洞型钢平台搭设、拆除期间交通安全维护措施等.     

高墩大跨连续刚构桥合龙方案研究

基于高墩大跨连续刚构桥的特点和野三河大桥施工时需要调整合龙方案的实际情况,运用有限元分析软件,计算比较了不同合龙方案的结构受力情况,探讨了不同的合龙温度对结构产生的影响.根据计算结果,提出了大桥合龙方案,为顺利合龙提供了指导.     

黄土残塬区高墩大跨桥梁布设方案研究

以黄土残塬区的地形地貌为基础,以成旬项目为依托,从桥位及桥型选择、桥孔布置、综合排水等角度出发,对高墩大跨桥梁布设方案进行了深入研究.