📄 001_19.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=im0><IMG height=64 src="1_19.files/right.jpg" width=563 
border=0></DIV><!-- text starts here --><SPAN 
class=ps0><NOBR>需要快速参考指南来记住每个位的位置和它们的含义。在本节中，将说明一些主要协议的</NOBR></SPAN> <SPAN 
class=ps1><NOBR>包格式，它们是用<SPAN class=em0>HP</SPAN>因特网监视器（<SPAN 
class=em0>Hewlett-Packard Internet Advisor</SPAN>，<SPAN 
class=em0>HP-IA</SPAN>）捕获的实</NOBR></SPAN> <SPAN class=ps2><NOBR>时跟踪记录。有关<SPAN 
class=em0>HP-IA</SPAN>的说明以及信令呼叫建立的相关记录，可以在本书的配套光盘中</NOBR></SPAN> <SPAN 
class=ps3><NOBR>找到。</NOBR></SPAN> <SPAN class=ps4><NOBR>图<SPAN 
class=em0>1-7</SPAN>中是在<SPAN class=em0>HP-IA</SPAN>上捕获的一个典型的<SPAN 
class=em0>S/MGCP</SPAN>包。这个包显示了一个<SPAN class=em0>RQNT</SPAN>命令从<SPAN 
class=em0>IPv4</SPAN></NOBR></SPAN> <SPAN class=ps5><NOBR>源地址<SPAN 
class=em0>10.01.1.65<SPAN class=em1>，</SPAN></SPAN>经由以太网传输，最后到达<SPAN 
class=em0>IPv4</SPAN>目的地址<SPAN class=em0>10.1.70.16</SPAN>的<SPAN 
class=em0>aaln/2</SPAN>端点。如果端</NOBR></SPAN> <SPAN 
class=ps6><NOBR>点处于脱机工作状态，该命令要求发送通知，同时设置或者请求端点震铃。</NOBR></SPAN> <SPAN 
class=ps7><NOBR>出现在以太网链路上的包是一个包含<SPAN class=em0>UDP</SPAN>有效载荷的标准<SPAN 
class=em0>IPv4</SPAN>包，<SPAN class=em0>UDP</SPAN>有效载荷是实际</NOBR></SPAN> <SPAN 
class=ps8><NOBR>的<SPAN class=em0>S/MGCP</SPAN>命令数据。在<SPAN 
class=em0>UDP</SPAN>包头的校验和字段之后，紧跟着是包的<SPAN 
class=em0>S/MGCP</SPAN>部分。注意，</NOBR></SPAN> <SPAN class=ps9><NOBR><SPAN 
class=ft1>S/MGCP<SPAN class=em1>段中的每个文本行之间都通过</SPAN>x' 0A'<SPAN 
class=em1>（</SPAN><SPAN 
class=em1>换行）字符分隔开来。当检测到被请求的事件</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps10><NOBR>时，为这个<SPAN class=em0>RQNT</SPAN>命令发送到<SPAN 
class=em0>MG</SPAN>的请求标识值会在<SPAN class=em0>NTFY</SPAN>信息中返回，从<SPAN 
class=em0>MG</SPAN>发送给<SPAN class=em0>MGC</SPAN>。</NOBR></SPAN> <SPAN 
class=ps11><NOBR>同时要注意，<SPAN class=em0>MGC</SPAN>已经把端点名解析为一个<SPAN 
class=em0>IP</SPAN>地址，用<SPAN 
class=em0>aaln/2@10.1.70.16</SPAN>语法来表示。按照</NOBR></SPAN> <SPAN 
class=ps12><NOBR>约定，端点名<SPAN class=em0>aaln/2</SPAN>映射为介质网关的端口<SPAN 
class=em0>2</SPAN>。</NOBR></SPAN> <SPAN class=ps13><NOBR>有些实现可能同时提供了<SPAN 
class=em0>\r<SPAN class=em1>（</SPAN></SPAN>回车）字符和<SPAN class=em0>\n<SPAN 
class=em1>（</SPAN></SPAN>换行）字符作为文本行的分隔符。在讨论</NOBR></SPAN> <SPAN 
class=ps14><NOBR>更为复杂的信令实例之前，首先简要地介绍一下会话描述协议（&nbsp;<SPAN class=em0>Session 
Description Protocol</SPAN>，</NOBR></SPAN> <SPAN class=ps15><NOBR><SPAN 
class=ft1>SDP<SPAN class=em1>）</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR>。</NOBR></SPAN> <SPAN class=ps17><NOBR><SPAN class=ft2>1.2.1 
&nbsp;&nbsp;<SPAN class=em2>会话描述协议概要</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps18><NOBR><SPAN class=ft1>1. <SPAN 
class=em3>背景</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps19><NOBR>在一个域中，不管使用的是何种信令协议，都需要为点对点或多点呼叫的每个参与用户</NOBR></SPAN> <SPAN 
class=ps20><NOBR>指定会话参数。呼叫管理所必须的参数包括介质有效载荷类型，后者又可能参与端点之间的</NOBR></SPAN> <SPAN 
class=ps21><NOBR>编码解码方案协商。<SPAN class=em0>SDP</SPAN>具有专门的机制和相应的语法来帮助参数在<SPAN 
class=em0>MGC</SPAN>和它所控制的端点</NOBR></SPAN> <SPAN class=ps22><NOBR>之间传递。<SPAN 
class=em0>SDP</SPAN>语法简单易懂，已经被接受作为基于文本的<SPAN 
class=em0>IP</SPAN>信令协议中呼叫参数协商的编</NOBR></SPAN> <SPAN 
class=ps23><NOBR>码方法。</NOBR></SPAN> <SPAN class=ps24><NOBR><SPAN class=ft1>2. 
SDP<SPAN class=em3>协议基础</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps25><NOBR>会话启动协议是在<SPAN class=em0>RFC 2327</SPAN>中定义。<SPAN 
class=em0>RFC</SPAN>摘要中写到：“<SPAN class=em0>SDP</SPAN>是为了会话通告、会话邀请</NOBR></SPAN> 
<SPAN class=ps26><NOBR>和其他形式的多媒体会话启动而描述多媒体会话过程。”</NOBR></SPAN> <SPAN 
class=ps27><NOBR><SPAN 
class=ft3>一次多媒体会话的定义为多媒体发送者、接收者和从发送者到接收者的数据流</SPAN></NOBR></SPAN> <SPAN 
class=ps28><NOBR><SPAN 
class=ft3>的集合。视频电话会议呼叫就是多媒体会话的一个典型例子。</SPAN></NOBR></SPAN> <SPAN 
class=ps29><NOBR><SPAN class=ft1>3. <SPAN 
class=em3>消息格式</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR>在<SPAN 
class=em0>S/MGCP</SPAN>协议和它派生出来的协议中，<SPAN 
class=em0>SDP</SPAN>标记用来携带会话描述，而不需要使用整个</NOBR></SPAN> <SPAN 
class=ps31><NOBR><SPAN class=ft1>SDP<SPAN class=em1>协议。在</SPAN>S/MGCP<SPAN 
class=em1>有效载荷中只使用</SPAN>SDP<SPAN class=em1>语法。正像人们经常称呼的那样，&nbsp;<SPAN 
class=em0>SDPs</SPAN>在呼叫</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps32><NOBR>启动阶段出现在<SPAN 
class=em0>S/MGCP</SPAN>消息中，或者当需要修改会话描述，以协商各方都能接受的参数时</NOBR></SPAN> <SPAN 
class=ps33><NOBR>出现。</NOBR></SPAN> <SPAN 
class=ps34><NOBR>会话描述是用下面的缩略消息格式编码成<SPAN 
class=em0>ASCII</SPAN>字符串。在发送多个描述时，“<SPAN class=em0>v=</SPAN>”行用</NOBR></SPAN> 
<SPAN class=ps35><NOBR>来分隔会话描述。所有行都通过换行符或者回车符分隔开来。</NOBR></SPAN> <SPAN 
class=ps36><NOBR><SPAN class=ft1>v=<SPAN 
class=em1>协议版本</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps37><NOBR><SPAN 
class=ft1>o=<SPAN class=em1>拥有者／创建者和会话标识符</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps38><NOBR><SPAN class=ft1>s=<SPAN 
class=em1>会话名称</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft3>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=ps40><NOBR><SPAN 
class=ft4>19</SPAN></NOBR></SPAN> <SPAN class=ps41><NOBR><SPAN 
class=ft5>文档</SPAN></NOBR></SPAN> </BODY></HTML>
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