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📁 通讯类的标准。对要开发SS7的朋友有很大帮助的。（通讯协议）
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<DIV class=pg></DIV><!-- bitmap and vector images are written here -->
<DIV class=im0><IMG height=7 src="002_35.files/002_35_1.jpg" width=7 
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<DIV class=im3><IMG height=2 src="002_35.files/002_35_4.jpg" width=134 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN class=ft0>&#8226; 
<SPAN class=em0>它为每个保留提供动态的</SPAN>QoS<SPAN 
class=em0>。接收者能够修改端点间的路径上已经存在的</SPAN>QoS<SPAN 
class=em0>。如果</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps1><NOBR>使用<SPAN 
class=em1>ATM</SPAN>作为传输网络，这种灵活性会导致<SPAN 
class=em1>ATM</SPAN>信息的其他问题。</NOBR></SPAN> <SPAN class=ps2><NOBR><SPAN 
class=ft0>&#8226; <SPAN 
class=em0>它支持不同应用的不同需要的多保留方式，例如音频和视频。在通常不需要明确的发送</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps3><NOBR>者标识情况下，这对于在网络中保留资源很重要。</NOBR></SPAN> <SPAN 
class=ps4><NOBR><SPAN class=ft0>&#8226; <SPAN 
class=em0>它是一个单工协议。这意味着，在一个双向呼叫中，两个接收者都需要独立地建立和固</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps5><NOBR>定它们的路径。</NOBR></SPAN> <SPAN class=ps6><NOBR><SPAN 
class=ft0>&#8226; <SPAN class=em0>该协议为每个</SPAN>RSVP<SPAN 
class=em0>路由器内的每个保留保持一个“软”状态。这个状态需要发送者和</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps7><NOBR>接收者用适当的消息周期性地刷新。如果得不到刷新，这个软状态会终止。</NOBR></SPAN> <SPAN 
class=ps8><NOBR><SPAN class=ft0>RSVP<SPAN 
class=em0>对发送者和接收者的处理是分开的，即使在单一应用同时是发送者和接受者的情况</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps9><NOBR>下也是如此，这一点可能是<SPAN class=em1>MGC</SPAN>中电话服务的情况。协议要求发送者请求特定的<SPAN 
class=em1>QoS</SPAN>。这意</NOBR></SPAN> <SPAN 
class=ps10><NOBR>味着，对于基于包的电话技术，如果要将<SPAN class=em1>RSVP</SPAN>用于<SPAN 
class=em1>QoS</SPAN>目的，发送者和接收者不得不连续请</NOBR></SPAN> <SPAN 
class=ps11><NOBR>求该<SPAN class=em1>QoS<SPAN 
class=em0>，</SPAN></SPAN>这会影响呼叫建立时的性能。由于它们从数据传输起点沿相反路径传输，&nbsp;<SPAN 
class=em1>QoS</SPAN>请求</NOBR></SPAN> <SPAN 
class=ps12><NOBR>会连续执行。所以，路径选择从呼叫方开始，到被叫方结束。被叫方在相反方向开始保留过</NOBR></SPAN> <SPAN 
class=ps13><NOBR>程，同时试图为自己的传输建立一条向呼叫方的路径。最后，呼叫方保留向被叫方的路径。</NOBR></SPAN> <SPAN 
class=ps14><NOBR>如果一切进行顺利的话，一个连接就建立起来了。</NOBR></SPAN> <SPAN 
class=ps15><NOBR><SPAN class=ft0>QoS<SPAN 
class=em0>是通过合称为“通信量控制”的机制来为特定数据流实现的。图</SPAN>2-20<SPAN 
class=em0>中是这些机制的</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR>简化框图，它包括：<SPAN class=em1>(1)</SPAN>包分类器；<SPAN 
class=em1>(2)</SPAN>策略和接纳控制；<SPAN 
class=em1>(3)</SPAN>包调度程序和其他一些依赖于链路</NOBR></SPAN> <SPAN 
class=ps17><NOBR>层的机制，以确定何时转发特定包。包分类器为每个包确定<SPAN 
class=em1>QoS</SPAN>类型（也可能有路由）。对于</NOBR></SPAN> <SPAN 
class=ps18><NOBR>每个向外的接口，包调度程序或其他依赖于链路层的机制实现承诺的</NOBR></SPAN> <SPAN 
class=ps19><NOBR><SPAN class=ft0>QoS<SPAN 
class=em0>。通过这些功能，</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps20><NOBR>通信量控制可以实现要求的<SPAN class=em1>QoS</SPAN>性能。</NOBR></SPAN> <SPAN 
class=ps21><NOBR>在保留建立期间，一条<SPAN class=em1>RSVP 
QoS</SPAN>请求传送到本地决定实体&nbsp;、策略和接纳控制。接纳控</NOBR></SPAN> <SPAN 
class=ps22><NOBR>制决定该节点是否具有足够资源提供请求的<SPAN 
class=em1>QoS</SPAN>。策略控制决定用户是否具有做出所请求的</NOBR></SPAN> <SPAN 
class=ps23><NOBR>保留的管理权限。如果两个检查都成功，在包分类器和链路层接口（如包调度程序）上设置</NOBR></SPAN> <SPAN 
class=ps24><NOBR>参数，以得到要求的<SPAN class=em1>QoS</SPAN>。如果任何一个检查失败，<SPAN 
class=em1>RSVP</SPAN>程序向产生请求的应用过程返回一</NOBR></SPAN> <SPAN 
class=ps25><NOBR>个错误通知。</NOBR></SPAN> <SPAN class=ps26><NOBR><SPAN 
class=ft0>RSVP<SPAN 
class=em0>协议机制提供了通用功能，可以用来创建和保持分布式保留状态，穿过组播或单播</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps27><NOBR>传送路径的网格。<SPAN class=em1>RSVP</SPAN>自己将<SPAN 
class=em1>QoS</SPAN>和策略控制参数作为不透明数据来传输和操作，将它们传</NOBR></SPAN> <SPAN 
class=ps28><NOBR>送到合适的通信量控制和策略控制模块来进行解释。在集成服务工作组开发的规范中记录了</NOBR></SPAN> <SPAN 
class=ps29><NOBR><SPAN class=ft0>QoS<SPAN 
class=em0>参数的结构和内容，参见</SPAN>RFC2210<SPAN 
class=em0>。策略参数的结构和内容仍在开发之中。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps30><NOBR>由于一个大规模的组播组的加入，得到的组播树拓扑可能会随时间改变，&nbsp;<SPAN 
class=em1>RSVP</SPAN>设计假定</NOBR></SPAN> <SPAN class=ps31><NOBR>在路由器和主机中，<SPAN 
class=em1>RSVP</SPAN>状态和通信量控制状态会递增地被建立和破坏。为了达到这个目的，</NOBR></SPAN> <SPAN 
class=ps32><NOBR><SPAN class=ft0>RSVP<SPAN class=em0>建立了“软”状态，也就是说，<SPAN 
class=em1>RSVP</SPAN>发送周期性的刷新消息来沿着保留的路径保持该</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps33><NOBR>状态。没有刷新消息时，这些状态自动超时并被删除掉。</NOBR></SPAN> <SPAN 
class=ps34><NOBR>最后，读者应该记住，一个呼叫路径上的所有路由器都必须支持<SPAN class=em1>RSVP<SPAN 
class=em0>，</SPAN></SPAN>这样介质包的传</NOBR></SPAN> <SPAN 
class=ps35><NOBR>输才是可预测的。这个细节有望要么为<SPAN 
class=em1>RSVP</SPAN>创建一个巨大的市场，这样所有的供应商都提供</NOBR></SPAN> <SPAN 
class=ps36><NOBR>这种类型的路由，要么工业界将继续研究，找出最适合的<SPAN 
class=em1>QoS</SPAN>路由协议，把简单电话的需要</NOBR></SPAN> <SPAN 
class=ps37><NOBR>和新的包网络承诺的服务集成起来。</NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN 
class=ft2>第<SPAN class=em2>2</SPAN>章&nbsp;包网络的介质传输</SPAN></NOBR></SPAN> <SPAN 
class=ps39><NOBR><SPAN class=ft3>91</SPAN></NOBR></SPAN> <SPAN 
class=ps40><NOBR><SPAN class=ft4>文档</SPAN></NOBR></SPAN> <SPAN 
class=ps41><NOBR><SPAN class=ft5>策略和接纳控制可以作为不同的实体实现。</SPAN></NOBR></SPAN> 
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