📄 001_36.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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border=0></DIV><!-- text starts here --><SPAN 
class=ps0><NOBR>在新一代包信令协议中，一件难以决定的事情就是端到端连接用户所感受到的语音质量。</NOBR></SPAN> <SPAN 
class=ps1><NOBR>如果呼叫跨越了区域和域，或者和<SPAN 
class=em0>PSTN</SPAN>相连，那么在域的边界处很可能进行语音转换编码，</NOBR></SPAN> <SPAN 
class=ps2><NOBR>这会降低语音质量。在跨越<SPAN 
class=em0>PSTN</SPAN>的两个包域之间的呼叫情况下，会出现两次转换编码。由于</NOBR></SPAN> <SPAN 
class=ps3><NOBR>边界处不同域为消除抖动需要进行缓冲，因此会出现严重的延迟。所以，在通常情况下，即</NOBR></SPAN> <SPAN 
class=ps4><NOBR>使一个适度的<SPAN class=em0>60ms</SPAN>的端到端延迟请求也有可能无法达到。<SPAN 
class=em0>VoIP</SPAN>容易受到过多延迟的影响而导</NOBR></SPAN> <SPAN 
class=ps5><NOBR>致质量降低。即使在适度大小的网络中，延迟也会很快地积累起来。</NOBR></SPAN> <SPAN 
class=ps6><NOBR><SPAN class=ft1>H.245<SPAN 
class=em2>提供了一种确定呼叫中端点状态的方法。端点可以发送一条</SPAN>H.245 
roundTripDelay</SPAN></NOBR></SPAN> <SPAN class=ps7><NOBR><SPAN 
class=ft1>Request<SPAN 
class=em2>消息，来确定该连接是否还有效。远程端点会发送具有相同序列号的</SPAN>roundTripDelay</SPAN></NOBR></SPAN> 
<SPAN class=ps8><NOBR><SPAN class=ft1>Response<SPAN 
class=em2>消息，从中可以大概估计出信道的往返行程时间。除了直接路由的信令模型之外，</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps9><NOBR>这个值和介质往返行程延迟并没有关系，它包括排队和运算时间延迟。该延迟的范围在十几</NOBR></SPAN> <SPAN 
class=ps10><NOBR><SPAN class=ft1>ms<SPAN 
class=em2>的量级。因此，在使用它来估计介质流的往返行程延迟时，可能会得到错误的结论。通过</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps11><NOBR><SPAN class=ft1>mediaLoop<SPAN class=em2>命令，<SPAN 
class=em0>H.323</SPAN>也可以在回环模式下为诊断目的设置逻辑信道。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps12><NOBR>在适当大小的域内，直接信令呼叫模型能够充分升级，但是<SPAN class=em0>GK</SPAN>路由模型必须和<SPAN 
class=em0>GK</SPAN>本身</NOBR></SPAN> <SPAN 
class=ps13><NOBR>一起升级。无论什么时候实现集中式信令，都会出现升级的问题。对于中等的和较大的<SPAN 
class=em0>H.323</SPAN></NOBR></SPAN> <SPAN 
class=ps14><NOBR>信令拓扑，为了处理端点总数升级，必须实现<SPAN class=em0>GK</SPAN>的分布式网络。<SPAN 
class=em0>GK</SPAN>的分布式实现包括<SPAN class=em0>GK</SPAN>计</NOBR></SPAN> <SPAN 
class=ps15><NOBR>算能力的模块化扩展，以处理性能升级，同时保证呼叫信令延迟不变。这一点对于支持基于</NOBR></SPAN> <SPAN 
class=ps16><NOBR><SPAN class=ft1>MC<SPAN 
class=em2>的会议非常重要，因为它的特性需要集中式的呼叫信令。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps17><NOBR>为了在服务人数增加时以不变的<SPAN class=em0>QoS</SPAN>支持介质传输，<SPAN 
class=em0>H.323</SPAN>遇到了<SPAN class=em0>RSVP</SPAN>的升级缺点。但</NOBR></SPAN> <SPAN 
class=ps18><NOBR>是，如前所述，这一问题并不是</NOBR></SPAN> <SPAN class=ps19><NOBR><SPAN 
class=ft1>H&nbsp;.&nbsp;3&nbsp;2&nbsp;3<SPAN 
class=em2>所特有的。当讨论完会话初始化协议（&nbsp;<SPAN 
class=em1>S&nbsp;e&nbsp;s&nbsp;s&nbsp;i&nbsp;o&nbsp;n</SPAN></SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps20><NOBR><SPAN class=ft1>Initiation Protocol<SPAN 
class=em2>，</SPAN>SIP<SPAN class=em2>）</SPAN><SPAN 
class=em2>以后将会看到，三种主要的竞争信令协议中，在基于文本的呼叫信</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps21><NOBR>令和二进制呼叫信令之间存在一些有趣的功能差别。</NOBR></SPAN> <SPAN 
class=ps22><NOBR><SPAN class=ft1>10. <SPAN 
class=em5>快速连接过程</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps23><NOBR>到现在为止，读者一定相信<SPAN 
class=em0>H.323</SPAN>呼叫信令非常复杂，复杂的信令需要计算能力，这就需</NOBR></SPAN> <SPAN 
class=ps24><NOBR>要端点或者它的服务器耗费时间来执行。其结果是连简单的<SPAN 
class=em0>H.323</SPAN>呼叫都需要很长的拨号后延</NOBR></SPAN> <SPAN class=ps25><NOBR>迟。<SPAN 
class=em0>ITU</SPAN>已经意识到这一点，在协议的版本<SPAN 
class=em0>2</SPAN>中，出现了一种更快的呼叫建立的方法，叫做</NOBR></SPAN> <SPAN 
class=ps26><NOBR><SPAN class=ft1>fastStart<SPAN 
class=em2>。它对于我们迄今为止讨论过的标准呼叫建立方法进行了重大改进。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps27><NOBR>在基本的<SPAN class=em0>H.225.0 
TCP</SPAN>连接建立之后，快速启动过程将呼叫建立降低到一个往返行程的时</NOBR></SPAN> <SPAN 
class=ps28><NOBR>间。其思想是在<SPAN class=em0>SETUP</SPAN>命令的<SPAN 
class=em0>UUIE</SPAN>中包含一串<SPAN 
class=em0>openLogicalChannel</SPAN>结构，以及一步打开的逻</NOBR></SPAN> <SPAN 
class=ps29><NOBR>辑信道所需要的所有参数，一步完成。逻辑信道对被叫方而言是“建议”，被叫方可以通过它</NOBR></SPAN> <SPAN 
class=ps30><NOBR>们选择最适合自己的那些信道。如果远程端点支持</NOBR></SPAN> <SPAN 
class=ps31><NOBR><SPAN class=ft1>fastStart<SPAN class=em2>，</SPAN><SPAN 
class=em2>它也用</SPAN>CONNECT<SPAN class=em2>消息返回</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps32><NOBR><SPAN class=ft1>fastStart<SPAN class=em2>，</SPAN><SPAN 
class=em2>否则该端点恢复到以前讨论过的标准</SPAN>H.245<SPAN 
class=em2>逻辑信道信令过程。如果远程端点决定</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps33><NOBR>使用<SPAN class=em0>fastStart<SPAN 
class=em2>，</SPAN></SPAN>它将返回在<SPAN class=em0>ALERTING</SPAN>、<SPAN 
class=em0>CALL PROCEEDING</SPAN>或者更可能是<SPAN 
class=em0>CONNECT</SPAN>消息中</NOBR></SPAN> <SPAN 
class=ps34><NOBR>要用到的逻辑信道参数。除非它们从呼叫端点接收到了<SPAN class=em0>fastStart<SPAN 
class=em2>，</SPAN></SPAN>否则被叫端点不应该启动</NOBR></SPAN> <SPAN class=ps35><NOBR><SPAN 
class=ft1>fastStart<SPAN class=em2>。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps36><NOBR>呼叫端点一旦发出<SPAN class=em0>fastStart<SPAN 
class=em2>，</SPAN></SPAN>就必须准备接受介质流，以免被叫端点接受了该命令。注意</NOBR></SPAN> <SPAN 
class=ps37><NOBR>到这一点很重要，因为呼叫者必须在它向远程端点提出的任何逻辑信道上准备接收。因此，</NOBR></SPAN> <SPAN 
class=ps38><NOBR>必须安装和端点中逻辑信道的传输地址相关的软件，并且在发送<SPAN 
class=em0>fastStart</SPAN>之前准备好。</NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft2>36&nbsp;&nbsp;<SPAN class=em3>IP </SPAN><SPAN 
class=em6>电话技术：稳定的</SPAN><SPAN class=em3>VoIP</SPAN><SPAN 
class=em6>服务集成</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps40><NOBR><SPAN 
class=ft3>文档</SPAN></NOBR></SPAN> <SPAN class=ps41><NOBR><SPAN 
class=ft4>在本书写作的时候，<SPAN class=em4>H.323</SPAN>协议的版本<SPAN 
class=em4>3</SPAN>还没有完成。<SPAN 
class=em4>H.323</SPAN>正在向着更简化的操作的方向发展。再考虑到它</SPAN></NOBR></SPAN> <SPAN 
class=ps42><NOBR><SPAN class=ft4>丰富的功能和庞大的安装数量，其他协议将很难与之竞争。</SPAN></NOBR></SPAN> 
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