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<DIV class=im1><IMG height=387 src="001_2_4.files/001_2_4_2.jpg" width=542
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN
class=ft0>图<SPAN class=em3>1-2 VoIP</SPAN>参考图</SPAN></NOBR></SPAN>
<SPAN class=ps1><NOBR>在<SPAN class=em0>PSTN</SPAN>上支持电话时，必须在网络的某个地方提供<SPAN
class=em0>PSTN</SPAN>网关，它可以接收由<SPAN class=em0>MGC</SPAN>发送</NOBR></SPAN> <SPAN
class=ps2><NOBR>的信号。这种设备通常在<SPAN class=em0>CO</SPAN>中出现，虽然<SPAN
class=em0>PSTN</SPAN>网关可能不会和<SPAN class=em0>MGC</SPAN>在同一个地方。</NOBR></SPAN>
<SPAN class=ps3><NOBR>现在可以开始讨论<SPAN
class=em0>VoIP</SPAN>信令了。我们将先讨论基本需要，然后再讨论信令协议。</NOBR></SPAN> <SPAN
class=ps4><NOBR><SPAN class=ft2>1.1 IP<SPAN
class=em4>电话基本原理</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps5><NOBR>我们先来看看有关I P</NOBR></SPAN> <SPAN
class=ps6><NOBR>网络上的语言及音频带电话的信令和呼叫控制的概念和基本原理。</NOBR></SPAN> <SPAN
class=ps7><NOBR><SPAN class=ft3>IP<SPAN
class=em1>上的普通语音电话和音频带电话之间存在着差别如对传真机和调制解调器的支持。这里讨</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps8><NOBR>论的<SPAN
class=em0>VoIP</SPAN>信令协议定义了一种使用两种电话应用的方法，只是程度和复杂性不同。</NOBR></SPAN> <SPAN
class=ps9><NOBR>我们从<SPAN
class=em0>S/MGCP</SPAN>协议开始，先讨论其基本构造、消息格式、包的定义，并简单介绍它在控</NOBR></SPAN> <SPAN
class=ps10><NOBR>制连接中的使用。接着将讨论<SPAN class=em0>ITU-T</SPAN>的<SPAN
class=em0>H.323</SPAN>协议，最后是会话初始协议（<SPAN class=em0>Session
Initiation</SPAN></NOBR></SPAN> <SPAN class=ps11><NOBR><SPAN
class=ft3>Protocol<SPAN class=em1>，</SPAN>SIP<SPAN
class=em1>）</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps12><NOBR>。<SPAN
class=em0>S/MGCP</SPAN>和<SPAN
class=em0>SIP</SPAN>来自于提出了基于文本的协议的思想。文本协议的一个最大优</NOBR></SPAN> <SPAN
class=ps13><NOBR>点是它消除了对不同机器平台的依赖性。再也不用考虑不同平台之间的兼容性了。开发和调</NOBR></SPAN> <SPAN
class=ps14><NOBR>试协议软件也更加简单和经济。<SPAN class=em0>SGCP</SPAN>不再有人问津，而<SPAN
class=em0>MGCP</SPAN>却被电缆电话广泛接受，并且</NOBR></SPAN> <SPAN class=ps15><NOBR>成为<SPAN
class=em0>NCS</SPAN>规范的主要部分。另一方面，用于<SPAN class=em0>VoIP</SPAN>控制的<SPAN
class=em0>H.323</SPAN>是个二元协议。后面章节将讨论</NOBR></SPAN> <SPAN
class=ps16><NOBR>和比较这两种协议的优点。</NOBR></SPAN> <SPAN
class=ps17><NOBR>在本书付印的时候，<SPAN class=em0>IETF</SPAN>和<SPAN
class=em0>ITU</SPAN>正在合作制订一个通用的<SPAN
class=em0>VoIP</SPAN>信令协议，这样可以联合</NOBR></SPAN> <SPAN
class=ps18><NOBR>双方的力量达到一种实现下一代<SPAN class=em0>VoIP</SPAN>网络的通用方法。有消息表明，<SPAN
class=em0>IETF</SPAN>和<SPAN class=em0>ITU</SPAN>将接纳一个</NOBR></SPAN> <SPAN
class=ps19><NOBR>简单的用于<SPAN class=em0>IP</SPAN>上的语音和语音带宽电话的呼叫控制和信令标准。这是<SPAN
class=em0>IETF</SPAN>的<SPAN class=em0>MEGACO</SPAN>工作组</NOBR></SPAN> <SPAN
class=ps20><NOBR>和<SPAN class=em0>ITU</SPAN>的<SPAN
class=em0>H.GCP</SPAN>工作组之间的议事日程。</NOBR></SPAN> <SPAN class=ps21><NOBR><SPAN
class=ft4>4 <SPAN class=em2>IP </SPAN><SPAN
class=em5>电话技术：稳定的</SPAN><SPAN class=em2>VoIP</SPAN><SPAN
class=em5>服务集成</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps22><NOBR><SPAN
class=ft5>文档</SPAN></NOBR></SPAN> <SPAN class=ps23><NOBR><SPAN
class=ft6>介质网关控制器呼叫代理</SPAN></NOBR></SPAN> <SPAN class=ps24><NOBR><SPAN
class=ft7>SS7<SPAN class=em6>网关</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps25><NOBR><SPAN class=ft6>信令</SPAN></NOBR></SPAN> <SPAN
class=ps26><NOBR><SPAN class=ft6>信令</SPAN></NOBR></SPAN> <SPAN
class=ps27><NOBR><SPAN class=ft7>PSTN<SPAN
class=em6>网关</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps28><NOBR><SPAN
class=ft7>RTP<SPAN class=em6>介质流</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps29><NOBR><SPAN class=ft6>介质网关</SPAN></NOBR></SPAN> <SPAN
class=ps30><NOBR><SPAN class=ft6>参考点<SPAN
class=em3>B</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN
class=ft6>参考点<SPAN class=em3>A</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps32><NOBR><SPAN class=ft6>调制解调器</SPAN></NOBR></SPAN> <SPAN
class=ps33><NOBR><SPAN class=ft6>电话</SPAN></NOBR></SPAN> <SPAN
class=ps34><NOBR><SPAN class=ft6>传真</SPAN></NOBR></SPAN> <SPAN
class=ps35><NOBR><SPAN class=ft6>交换机/路由器</SPAN></NOBR></SPAN> <SPAN
class=ps36><NOBR><SPAN class=ft6>集中器</SPAN></NOBR></SPAN> </BODY></HTML>
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