📄 001_39.htm

📁 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=im5><IMG height=9 src="001_39.files/001_39_6.jpg" width=8 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR>在<SPAN 
class=em0>fastStart</SPAN>后的任何时候都可以激活<SPAN 
class=em0>H.245</SPAN>信令过程，但是一旦激活以后，它们会一直起</NOBR></SPAN> <SPAN 
class=ps1><NOBR>作用，直到呼叫终止为止。使用<SPAN class=em0>fastStart</SPAN>的呼叫终止是用一条<SPAN 
class=em0>RELEASE COMPLETE</SPAN>消息完成</NOBR></SPAN> <SPAN 
class=ps2><NOBR>的。如果<SPAN class=em0>H.245</SPAN>过程已经激活，那么将使用标准的阶段<SPAN 
class=em0>E</SPAN>完成过程。<SPAN class=em0>fastStart</SPAN>方法将在下一节的</NOBR></SPAN> 
<SPAN class=ps3><NOBR>混合网络信令中讨论。</NOBR></SPAN> <SPAN class=ps4><NOBR><SPAN 
class=ft1>11. <SPAN class=em4>在混合网络拓扑中的</SPAN>H.323<SPAN 
class=em4>呼叫建立</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps5><NOBR>对于起源和终止于<SPAN 
class=em0>H.323</SPAN>区域中的呼叫，<SPAN class=em0>H.225.0</SPAN>足以用来初始化呼叫建立过程，而<SPAN 
class=em0>H.245</SPAN></NOBR></SPAN> <SPAN 
class=ps6><NOBR>作为所有参加呼叫的端点之间的容量协商载体。但是，生活并非总是如此简单，大多数情况</NOBR></SPAN> <SPAN 
class=ps7><NOBR>下，为了能够拨叫不能用<SPAN class=em0>H.323</SPAN>的用户，或者通过<SPAN 
class=em0>PSTN</SPAN>拨叫旧的普通电话机时，执行<SPAN class=em0>H.323</SPAN>信</NOBR></SPAN> 
<SPAN class=ps8><NOBR>令的端点必须进行协议映射。</NOBR></SPAN> <SPAN 
class=ps9><NOBR>关守的可能实现方法引入了更大的复杂度。在关守和介质网关控制器（&nbsp;<SPAN class=em0>Media 
Gateway</SPAN></NOBR></SPAN> <SPAN class=ps10><NOBR><SPAN 
class=ft1>Controller<SPAN class=em1>，</SPAN>MGC<SPAN class=em1>）</SPAN><SPAN 
class=em1>之间存在明显的结构上的相似性，但是在某些情况下，很自然地就会把</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps11><NOBR><SPAN class=ft1>H.323 <SPAN 
class=em1>关守的功能和介质网关控制器的功能合并起来，这种合并可能是基于</SPAN>IETF MEGACO<SPAN 
class=em1>协</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps12><NOBR>议或者<SPAN 
class=em0>S/MGCP</SPAN>协议。其结果可能和图<SPAN 
class=em0>1-23</SPAN>中的参考图类似。</NOBR></SPAN> <SPAN class=ps13><NOBR><SPAN 
class=ft1>MGC<SPAN class=em1>支持所有必须的信令协议，从而允许能用</SPAN>H.323<SPAN 
class=em1>的端点与</SPAN>S/MGCP<SPAN class=em1>端点和</SPAN>POTS<SPAN 
class=em1>电话</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps14><NOBR>建立呼叫。这种拓扑使用的呼叫流在用来支持基于<SPAN 
class=em0>IP</SPAN>的电话技术细节方面是最一般的和最广</NOBR></SPAN> <SPAN 
class=ps15><NOBR>泛的。存在一些性能问题与这种拓扑以及用于传输信令和载体信道的主干网类型相关，这正</NOBR></SPAN> <SPAN 
class=ps16><NOBR>是我们在本章中所讨论的。但是，首先我们将讨论<SPAN class=em0>H.323</SPAN>、<SPAN 
class=em0>H.255.0</SPAN>和<SPAN class=em0>H.245</SPAN>的一些特性，以及</NOBR></SPAN> 
<SPAN class=ps17><NOBR>如何使它们跨越主干网和一个异构区域中的其他信令协议协作。</NOBR></SPAN> <SPAN 
class=ps18><NOBR>在图<SPAN class=em0>1-24</SPAN>中创建了一个利用<SPAN class=em0>MGC<SPAN 
class=em1>，</SPAN></SPAN>在能用<SPAN class=em0>H.323</SPAN>的端点和通过<SPAN 
class=em0>S/MGCP</SPAN>发送信令的端点之</NOBR></SPAN> <SPAN 
class=ps19><NOBR>间进行的呼叫流。假定一个<SPAN class=em0>MGC</SPAN>连接介质网关和信令平面上的<SPAN 
class=em0>H.323</SPAN>网关</NOBR></SPAN> <SPAN 
class=ps20><NOBR>，并且支持两种信</NOBR></SPAN> <SPAN 
class=ps21><NOBR>令协议。协议栈之间所必须的映射工作对任何类型的端点和端点都是透明的。</NOBR></SPAN> <SPAN 
class=ps22><NOBR>在这种情况下，介质网关的用户希望呼叫在能用<SPAN 
class=em0>H.323</SPAN>的端点上的用户。呼叫代理程序已</NOBR></SPAN> <SPAN 
class=ps23><NOBR>经设置了<SPAN class=em0>RGW</SPAN>终点查询<SPAN class=em0>Off-Hook<SPAN 
class=em1>（</SPAN>RQNT<SPAN class=em1>（</SPAN>1<SPAN 
class=em1>）），</SPAN></SPAN>当用户拿起话机时，数字被收集起来（（<SPAN 
class=em0>3</SPAN>）</NOBR></SPAN> <SPAN class=ps24><NOBR>到（<SPAN 
class=em0>6</SPAN>）），与发起端点建立半双工连接。在（<SPAN class=em0>9</SPAN>）到（<SPAN 
class=em0>12a</SPAN>）步中，呼叫代理程序打开与远程端</NOBR></SPAN> <SPAN 
class=ps25><NOBR>点的<SPAN class=em0>TCP</SPAN>连接</NOBR></SPAN> <SPAN 
class=ps26><NOBR>，使用直接呼叫信令选项执行<SPAN class=em0>RAS</SPAN>信令接受请求（<SPAN 
class=em0>Admission Request</SPAN>，<SPAN class=em0>ARQ</SPAN>），</NOBR></SPAN> 
<SPAN class=ps27><NOBR>并得到接受确认（<SPAN class=em0>AdmissionConfirm </SPAN>，<SPAN 
class=em0>ACF</SPAN>）。两个端点都使用直接呼叫信令选项完成呼叫建</NOBR></SPAN> <SPAN 
class=ps28><NOBR>立。<SPAN class=em0>GK</SPAN>的<SPAN 
class=em0>GW</SPAN>功能部分作为呼叫的端点，为<SPAN 
class=em0>RGW</SPAN>的用户代理和解释信令。</NOBR></SPAN> <SPAN 
class=ps29><NOBR>这里应该注意到，在<SPAN class=em0>GK</SPAN>之间可以交换其他的<SPAN 
class=em0>RAS</SPAN>消息，例如带宽请求（&nbsp;<SPAN 
class=em0>Bandwidth</SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN 
class=ft1>Request<SPAN class=em1>，</SPAN>BRQ<SPAN class=em1>），</SPAN><SPAN 
class=em1>这时</SPAN>GK<SPAN class=em1>可以用带宽确认（<SPAN class=em0>Bandwidth 
Confirm</SPAN>，<SPAN class=em0>BCF</SPAN>）应答。</SPAN>ARQ<SPAN 
class=em1>的参数很</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps31><NOBR>多，最重要的有呼叫类型、呼叫模型、请求的连接带宽，以及传输服务质量。呼叫类型是指</NOBR></SPAN> <SPAN 
class=ps32><NOBR>点对点或多点。呼叫模型或者是两个端点之间的直接路由，或者是通过<SPAN 
class=em0>GK</SPAN>的路由，带宽是指</NOBR></SPAN> <SPAN 
class=ps33><NOBR>最开始时请求的带宽，服务质量是指端点为提供确定的服务质量而保留资源的能力。</NOBR></SPAN> <SPAN 
class=ps34><NOBR>然后网关直接发送一条<SPAN class=em0>H.225.0 
SETUP</SPAN>消息到端点，其中在用户到用户信息单元中包含</NOBR></SPAN> <SPAN class=ps35><NOBR><SPAN 
class=ft1>fastStart<SPAN class=em1>选项。</SPAN>H.225.0<SPAN 
class=em1>呼叫</SPAN>SETUP<SPAN class=em1>种定义了使用</SPAN>UUIE<SPAN 
class=em1>信息单元（<SPAN class=em0>Information Element</SPAN>，<SPAN 
class=em0>IE</SPAN>）</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps36><NOBR>来完成<SPAN 
class=em0>H.323</SPAN>协议所特有的信令操作。使用<SPAN 
class=em0>fastStart</SPAN>的信令已经本节前面讨论过了。</NOBR></SPAN> <SPAN 
class=ps37><NOBR><SPAN class=ft2>第<SPAN class=em5>1</SPAN>章<SPAN 
class=em5>IP</SPAN>语音和交互应用的协议</SPAN></NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN 
class=ft3>39</SPAN></NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft4>文档</SPAN></NOBR></SPAN> <SPAN class=ps40><NOBR><SPAN 
class=ft5>在集中式设计和分布式设计中，网关功能都可以在<SPAN 
class=em3>MGC</SPAN>设备内部实现。</SPAN></NOBR></SPAN> <SPAN class=ps41><NOBR><SPAN 
class=ft5>如果一个简单的盒子被用做<SPAN class=em3>MGC<SPAN class=em2>，</SPAN></SPAN>而<SPAN 
class=em3>GW</SPAN>服务器作用的域也是一个<SPAN 
class=em3>H.323</SPAN>区域，那么这个连接可能在同一个</SPAN></NOBR></SPAN> <SPAN 
class=ps42><NOBR><SPAN class=ft5>服务器的内部。在分布式实现中，<SPAN 
class=em3>TCP</SPAN>连接位于两个服务器之间，也就是说，在外部。</SPAN></NOBR></SPAN> </BODY></HTML>
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