001_43.htm
来自「通讯类的标准。对要开发SS7的朋友有很大帮助的。（通讯协议）」· HTM 代码 · 共 269 行
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<DIV class=im3><IMG height=9 src="001_43.files/001_43_4.jpg" width=8
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN
class=ft0>14. <SPAN class=em1>关于</SPAN>RAS<SPAN
class=em1>计时器</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps1><NOBR>除了<SPAN
class=em0>GRQ</SPAN>和<SPAN class=em0>LRQ</SPAN>消息之外，<SPAN
class=em0>RAS</SPAN>为每条消息规定的一般的超时值是<SPAN class=em0>3</SPAN>秒。<SPAN
class=em0>GRQ</SPAN>和<SPAN class=em0>LRQ</SPAN>的超</NOBR></SPAN> <SPAN
class=ps2><NOBR>时值建议设为<SPAN class=em0>5</SPAN>秒。除了<SPAN
class=em0>URQ</SPAN>和<SPAN class=em0>IRQ</SPAN>重试计数为<SPAN
class=em0>1</SPAN>秒以外，其他每条消息都是<SPAN class=em0>2</SPAN>秒。</NOBR></SPAN> <SPAN
class=ps3><NOBR>大的超时值和高的重试计数会影响呼叫建立的性能，妨碍关守处理大量端点的能力，在</NOBR></SPAN> <SPAN
class=ps4><NOBR>分布式实现中尤其如此。<SPAN
class=em0>RIP</SPAN>允许端点灵活地通知关守所期望的产生要求应答的时间。当得到</NOBR></SPAN> <SPAN
class=ps5><NOBR>请求的应答时，端点马上把它发送到<SPAN class=em0>GK</SPAN>。通过这种类型的延迟应答，<SPAN
class=em0>GK</SPAN>可以知道在区域内</NOBR></SPAN> <SPAN
class=ps6><NOBR>它要处理的消息执行延迟的量级。</NOBR></SPAN> <SPAN
class=ps7><NOBR>在设计实现中，协议超时和重试不应该是程序上的主流信令操作的一部分。也就是说，</NOBR></SPAN> <SPAN
class=ps8><NOBR>对于正常操作，预期的端点、关守和网关之间的信令过程不应该包括超时和重试，但是它应</NOBR></SPAN> <SPAN
class=ps9><NOBR>该被网络管理员作为特例看待，以找出系统性能下降的可能原因。</NOBR></SPAN> <SPAN
class=ps10><NOBR><SPAN class=ft2>1.4 <SPAN
class=em2>会话初始化协议的特性</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps11><NOBR>会话初始化协议（<SPAN class=em0>Session Initiation
Protocol</SPAN>，<SPAN class=em0>SIP</SPAN>，<SPAN class=em0>RFC
2543</SPAN>）是基于文本的信令协议，</NOBR></SPAN> <SPAN
class=ps12><NOBR>用来创建和控制两方或者多方参加的多媒体会话。它是三个主要的相互竞争的、用于</NOBR></SPAN> <SPAN
class=ps13><NOBR><SPAN class=ft0>VoIP<SPAN
class=em3>服</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps14><NOBR>务的信令协议 之一，已经在实现中被采用。用简单的术语就是：</NOBR></SPAN> <SPAN
class=ps15><NOBR><SPAN class=ft0>SIP<SPAN class=em1>是在</SPAN>TCP<SPAN
class=em1>或者</SPAN>UDP<SPAN class=em1>上传输的客户</SPAN>/<SPAN
class=em1>服务器协议，但是最常用的实现是在</SPAN>UDP<SPAN
class=em1>上</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps16><NOBR><SPAN
class=ft3>使用<SPAN class=em4>SIP<SPAN
class=em1>，</SPAN></SPAN>以保证简单性和速度。</SPAN></NOBR></SPAN> <SPAN
class=ps17><NOBR><SPAN class=ft0>SIP<SPAN
class=em3>会话包括多媒体会议或者简单的电话呼叫，可以是单播，也可以是组播。会话可以在</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps18><NOBR>企业网的用户之间建立，也可以跨越多个网段，只要能够达到<SPAN
class=em0>IP</SPAN>上的网段就可以。各方可以</NOBR></SPAN> <SPAN
class=ps19><NOBR>请求创建新的会话或者加入已经存在的会话中。会话可以用其他方法通告，如会话通告协议</NOBR></SPAN> <SPAN
class=ps20><NOBR>（<SPAN class=em0>Session Announcement Protocol</SPAN>，<SPAN
class=em0>SAP</SPAN>）。<SPAN class=em0>SIP</SPAN>协议比<SPAN
class=em0>H.323</SPAN>还要简单，但是它至少是实用的。</NOBR></SPAN> <SPAN
class=ps21><NOBR>它来自于较早的文本协议，如<SPAN class=em0>HTTP</SPAN>。</NOBR></SPAN> <SPAN
class=ps22><NOBR>使用<SPAN class=em0>SIP</SPAN>信令的网络拓扑和<SPAN
class=em0>S/MGCP</SPAN>和<SPAN class=em0>H.323</SPAN>非常相似。图<SPAN
class=em0>1-26</SPAN>中的网络显示了通过路由云</NOBR></SPAN> <SPAN
class=ps23><NOBR>连接起来的两个域，每个域都执行<SPAN
class=em0>SIP</SPAN>信令。我们将讨论这幅图中的基本呼叫模型，它和</NOBR></SPAN> <SPAN
class=ps24><NOBR><SPAN class=ft0>H.323<SPAN
class=em3>的讨论中的基本呼叫模型非常相似。</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps25><NOBR>和它的两个竞争者一样，<SPAN
class=em0>SIP</SPAN>能够和网关协作，在跨越不同的网段时提供信令协议和介质</NOBR></SPAN> <SPAN
class=ps26><NOBR>转换。这些网段可以是使用<SPAN class=em0>SS7/ISUP</SPAN>信令和<SPAN
class=em0>TDM</SPAN>主干网的<SPAN class=em0>PSTN</SPAN>、<SPAN
class=em0>S/MGCP</SPAN>域或者是<SPAN class=em0>H.323</SPAN>区。</NOBR></SPAN> <SPAN
class=ps27><NOBR>在过去的几年中，信令协议的协作可能以这种或那种形式出现了各种可能的变化，但是在网</NOBR></SPAN> <SPAN
class=ps28><NOBR>络和服务实现中仍然有很多值得学习的东西。由于在<SPAN
class=em0>Internet</SPAN>电话技术方面的成功，<SPAN class=em0>H.323</SPAN>到</NOBR></SPAN>
<SPAN class=ps29><NOBR>目前为止获得了最广泛的采用，但是在公用网的电话服务实现方面，它并不一定是竞争中的</NOBR></SPAN>
<SPAN class=ps30><NOBR>领先者。<SPAN
class=em0>SIP</SPAN>有重要的性能、灵活性和升级优点，被看做主要信令应用的可行竞争者，包括公</NOBR></SPAN> <SPAN
class=ps31><NOBR>用网中载波质量<SPAN class=em0>VoIP</SPAN>电话技术中的呼叫控制。<SPAN
class=em0>S/MGCP</SPAN>藉着应用于由电缆提供的电话服务方面</NOBR></SPAN> <SPAN
class=ps32><NOBR>的<SPAN class=em0>NCS</SPAN>标准，在<SPAN
class=em0>VoIP</SPAN>公用网中也是一个主要竞争者。</NOBR></SPAN> <SPAN class=ps33><NOBR><SPAN
class=ft0>SIP<SPAN class=em3>支持高级智能网（<SPAN class=em0>Advanced Intelligent
Network</SPAN>，<SPAN
class=em0>AIN</SPAN>）的特性，如名称映射、呼叫</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps34><NOBR>发送和呼叫重定向。如果<SPAN
class=em0>SIP</SPAN>被接受为公用网中的信令协议，这一点非常有用，因为在公用网</NOBR></SPAN> <SPAN
class=ps35><NOBR><SPAN class=ft3>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=ps36><NOBR><SPAN
class=ft4>43</SPAN></NOBR></SPAN> <SPAN class=ps37><NOBR><SPAN
class=ft5>文档</SPAN></NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN
class=ft6>其他的是<SPAN class=em6>S/MGCP</SPAN>和<SPAN
class=em6>H.323</SPAN>。</SPAN></NOBR></SPAN> </BODY></HTML>
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