📄 001_55.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=im1><IMG height=453 src="001_55.files/001_45_55_2.jpg" width=453 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR>析器捕获到的会话参数的<SPAN 
class=em1>SDP</SPAN>说明。</NOBR></SPAN> <SPAN class=ps1><NOBR><SPAN 
class=ft1>图<SPAN class=em2>1-31 &nbsp;&nbsp;SIP</SPAN>典型的<SPAN 
class=em2>INVITE</SPAN>消息</SPAN></NOBR></SPAN> <SPAN 
class=ps2><NOBR>在这个例子中，呼叫方要求在<SPAN class=em1>RTP</SPAN>端口号<SPAN 
class=em1>20134</SPAN>上建立音频连接，并如<SPAN class=em1>Media</SPAN>域中所示，</NOBR></SPAN> 
<SPAN class=ps3><NOBR>在三种介质流编码中选择一种。在<SPAN class=em1>Attributes</SPAN>中描述了<SPAN 
class=em1>rtp</SPAN>映射下的有效载荷类型。有效载荷</NOBR></SPAN> <SPAN class=ps4><NOBR>类型在<SPAN 
class=em1>RFC1890<SPAN class=em0>—</SPAN></SPAN>具有最少控制的音频视频会议的<SPAN 
class=em1>RTP</SPAN>配置中定义。我们将在介绍<SPAN class=em1>RTP</SPAN>时，</NOBR></SPAN> <SPAN 
class=ps5><NOBR>与音频<SPAN 
class=em1>/</SPAN>视频传输标准组中正在进行的工作一起讨论它。在这个例子中，提供给被叫方的选</NOBR></SPAN> <SPAN 
class=ps6><NOBR>择有：</NOBR></SPAN> <SPAN class=ps7><NOBR><SPAN class=ft2>&#8226; 5<SPAN 
class=em0>类有效载荷，编码为</SPAN>DVI4<SPAN class=em0>，</SPAN><SPAN 
class=em0>基于样本的编码，每个样本</SPAN>4<SPAN class=em0>个比特，<SPAN 
class=em1>8Khz</SPAN>时钟频率。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps8><NOBR><SPAN class=ft2>&#8226; 0<SPAN class=em0>类有效载荷，基本的</SPAN>8Khz PCM 
Ulaw<SPAN class=em0>编码。</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps9><NOBR><SPAN 
class=ft3>&#8226;&nbsp; &nbsp;7<SPAN class=em0>类有效载荷，线性预测编码（&nbsp;<SPAN 
class=em1>Linear Prediction Coding</SPAN>，<SPAN 
class=em1>LPC</SPAN>），时钟频率也是</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps10><NOBR><SPAN class=ft2>8Khz<SPAN 
class=em0>。</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps11><NOBR>注意，<SPAN 
class=em1>rtp</SPAN>映射是用来映射这个例子中的所有有效载荷类型，即使有些有效载荷类型是标</NOBR></SPAN> <SPAN 
class=ps12><NOBR>准的，不需要使用<SPAN class=em1>rtp</SPAN>映射。</NOBR></SPAN> <SPAN 
class=ps13><NOBR><SPAN class=ft2>10. <SPAN 
class=em3>总结</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps14><NOBR>我们已经讨论了<SPAN 
class=em1>VoIP</SPAN>电话信令的三个主要竞争者。每个协议优化的变量都是当时工业界</NOBR></SPAN> <SPAN 
class=ps15><NOBR>优先性能的反映，但是它们之间存在着重大的和明显的不同。<SPAN 
class=em1>SIP</SPAN>通过精巧的简单性同时做到</NOBR></SPAN> <SPAN class=ps16><NOBR><SPAN 
class=ft4>第<SPAN class=em4>1</SPAN>章<SPAN 
class=em4>IP</SPAN>语音和交互应用的协议</SPAN></NOBR></SPAN> <SPAN class=ps17><NOBR><SPAN 
class=ft5>55</SPAN></NOBR></SPAN> <SPAN class=ps18><NOBR><SPAN 
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