📄 002_7.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="002_7.files/right.jpg" width=563 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN 
class=ft0>SSRC<SPAN class=em0>（</SPAN>32<SPAN class=em0>比特）：<SPAN 
class=em1>SSRC</SPAN>标识同步信源。同步信源就是发送者，它可以是一台机器、一个</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps1><NOBR>广播服务器或者一个人。<SPAN 
class=em1>SSRC</SPAN>的数值是随机选取的，因此在同一个<SPAN 
class=em1>RTP</SPAN>段内没有两个发送者</NOBR></SPAN> <SPAN class=ps2><NOBR>具有相同的<SPAN 
class=em1>SSRC</SPAN>标识。当呼叫建立起来时，可能会出现冲突，也就是发送者具有相同标识。</NOBR></SPAN> <SPAN 
class=ps3><NOBR>当检测到冲突时，<SPAN class=em1>RTP</SPAN>中的一个简单的机制被指定用来解决它。当只有一个<SPAN 
class=em1>SSRC</SPAN>时，<SPAN class=em1>CSRC</SPAN></NOBR></SPAN> <SPAN 
class=ps4><NOBR>数值域被设置为<SPAN class=em1>0</SPAN>。</NOBR></SPAN> <SPAN 
class=ps5><NOBR>在<SPAN class=em1>RTP</SPAN>中解决<SPAN 
class=em1>SSRC</SPAN>冲突很简单。首先，每个<SPAN class=em1>RTP</SPAN>包的接收者需要记住与每个<SPAN 
class=em1>SSRC</SPAN>相关的</NOBR></SPAN> <SPAN 
class=ps6><NOBR>信源传输地址。如果从一个新的信源传输地址接收到一个已经存在并且有效的<SPAN class=em1>SSRC<SPAN 
class=em0>——</SPAN></SPAN>不是</NOBR></SPAN> <SPAN 
class=ps7><NOBR>接收者自身，接收者忽略这个包，并可选地更新<SPAN 
class=em1>SSRC</SPAN>检测到的冲突的统计数据。如果发现接</NOBR></SPAN> <SPAN 
class=ps8><NOBR>收者自己的<SPAN 
class=em1>SSRC</SPAN>形成了环，也就是说，从一个不同的信源传输地址出现，或者接收者自己被</NOBR></SPAN> <SPAN 
class=ps9><NOBR>确定为包的始发站，那么将在环状的<SPAN class=em1>SSRC</SPAN>中发送一个<SPAN 
class=em1>RTCP BYE</SPAN>包，选择一个新的数值。</NOBR></SPAN> <SPAN class=ps10><NOBR><SPAN 
class=ft0>BYE<SPAN class=em0>包会有效地把参与者从当前会话中挂断。</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps11><NOBR>如果一个信源改变它的信源传输地址，它还必须选择一个新的<SPAN 
class=em1>SSRC</SPAN>标识，以免被解释为</NOBR></SPAN> <SPAN 
class=ps12><NOBR>环路信源。这是一个非常有趣的问题，当网络中存在混合器和转换器时会出现这种问题。混</NOBR></SPAN> <SPAN 
class=ps13><NOBR>合器插入自己的<SPAN class=em1>SSRC<SPAN 
class=em0>，</SPAN></SPAN>而转换器原封不动地转发包，但是它把自己的信源传输地址（&nbsp;<SPAN 
class=em1>IP</SPAN>地址</NOBR></SPAN> <SPAN class=ps14><NOBR><SPAN 
class=ft0>+UPD<SPAN class=em0>端口号）放到包中。当一个端点从两个不同的源传输地址接收到具有相同</SPAN>SSRC<SPAN 
class=em0>数值的</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps15><NOBR><SPAN 
class=ft0>RTP<SPAN class=em0>包时，环路已经闭合。当一个信源发现与它自身相同的</SPAN>SSRC<SPAN 
class=em0>包时，也会形成闭环，但是</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR>最初选择中的冲突也会有相同的效果。</NOBR></SPAN> <SPAN class=ps17><NOBR><SPAN 
class=ft0>CSRC<SPAN class=em0>列表（<SPAN class=em1>0</SPAN>～<SPAN 
class=em1>15</SPAN>项，每项</SPAN>32<SPAN class=em0>比特）：<SPAN 
class=em1>CSRC</SPAN>列表标识了提供这个包中包含的有效载荷的</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps18><NOBR>所有信源。这只对混合过的包有意义。标识符的数量是由<SPAN class=em1>CC</SPAN>域确定的。<SPAN 
class=em1>CSRC</SPAN>标识符是包信</NOBR></SPAN> <SPAN class=ps19><NOBR>源最初的<SPAN 
class=em1>SSRC<SPAN class=em0>，</SPAN></SPAN>由混合器插入。<SPAN 
class=em1>CSRC</SPAN>域用来当端点发出有效载荷时对信源标识进行更正。</NOBR></SPAN> <SPAN 
class=ps20><NOBR>让我们看看两个已解码的<SPAN class=em1>RTP</SPAN>包，以说明报头域的使用。</NOBR></SPAN> 
<SPAN class=ps21><NOBR>图<SPAN class=em1>2-3a</SPAN>中是从一个信源来的典型的<SPAN 
class=em1>VoIP &micro;Law</SPAN>包。图<SPAN class=em1>2-3b</SPAN>中是在帧边界处的<SPAN 
class=em1>H.261 RTP</SPAN>视</NOBR></SPAN> <SPAN 
class=ps22><NOBR>频包，并对基本的报头做了修改以包含特定信息。在有效载荷开始出现之前，在基本协议报</NOBR></SPAN> <SPAN 
class=ps23><NOBR>头之后的数据包含<SPAN class=em1>H.261</SPAN>控制信息。这种类型的报头扩展是得到<SPAN 
class=em1>ITU-T H.323</SPAN>要求的规定许可</NOBR></SPAN> <SPAN 
class=ps24><NOBR>的（参看参考文献<SPAN class=em1>[1]</SPAN>中，使用<SPAN 
class=em1>H.263+</SPAN>的<SPAN 
class=em1>H.323</SPAN>视频流的有效载荷报头格式的实例）。</NOBR></SPAN> <SPAN 
class=ps25><NOBR><SPAN class=ft2>2.1.2 &nbsp;&nbsp;<SPAN 
class=em2>简单介绍</SPAN>RTCP</SPAN></NOBR></SPAN> <SPAN class=ps26><NOBR>在<SPAN 
class=em1>RFC1889</SPAN>中说明的和<SPAN class=em1>RTP</SPAN>相配的控制协议是<SPAN 
class=em1>RTCP</SPAN>。<SPAN class=em1>RTCP</SPAN>经常被误认为是信令协议，但</NOBR></SPAN> 
<SPAN class=ps27><NOBR>是它不是。<SPAN 
class=em1>RTCP</SPAN>包把关于会话质量的端到端信息传送给每个参与者。像包延迟、抖动、收到</NOBR></SPAN> <SPAN 
class=ps28><NOBR>和包丢失等数值对网络而言非常有价值，可以用来实时估计网络自身的状态。<SPAN 
class=em1>RTCP</SPAN>作为核心</NOBR></SPAN> <SPAN 
class=ps29><NOBR>可以用来监视数据传输是否发生，端点也可以用它来确定在网络故障的情况下，相应的</NOBR></SPAN> <SPAN 
class=ps30><NOBR><SPAN class=ft0>RTP</SPAN></NOBR></SPAN> <SPAN 
class=ps31><NOBR>流部分是否被丢失。<SPAN class=em1>RTP</SPAN>和<SPAN 
class=em1>RTCP</SPAN>并不需要在相同的端到端路径上路由，但是如果能够实现，</NOBR></SPAN> <SPAN 
class=ps32><NOBR>这样做有重大意义。如果使用了服务质量路由，网络设计考虑可以规定<SPAN 
class=em1>RTCP</SPAN>包不能经过相同</NOBR></SPAN> <SPAN 
class=ps33><NOBR>的路由器集合。</NOBR></SPAN> <SPAN class=ps34><NOBR>有<SPAN 
class=em1>5</SPAN>种类型的<SPAN class=em1>RTCP</SPAN>包：</NOBR></SPAN> <SPAN 
class=ps35><NOBR><SPAN class=ft0>&#8226; SR<SPAN class=em0>：</SPAN>Sender Report<SPAN 
class=em0>，</SPAN><SPAN class=em0>发送者报告</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps36><NOBR><SPAN class=ft0>&#8226; RR<SPAN class=em0>：</SPAN>Receiver 
Report<SPAN class=em0>，</SPAN><SPAN class=em0>接收者报告</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps37><NOBR><SPAN class=ft0>&#8226; SDES&nbsp;<SPAN 
class=em0>：</SPAN>Source DEScription<SPAN class=em0>，</SPAN><SPAN 
class=em0>信源说明</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN 
class=ft0>&#8226; BYE&nbsp;<SPAN class=em0>：</SPAN>Hang up from a session<SPAN 
class=em0>，</SPAN><SPAN class=em0>挂断会话</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps39><NOBR><SPAN class=ft3>第<SPAN 
class=em3>2</SPAN>章&nbsp;包网络的介质传输</SPAN></NOBR></SPAN> <SPAN 
class=ps40><NOBR><SPAN class=ft4>63</SPAN></NOBR></SPAN> <SPAN 
class=ps41><NOBR><SPAN class=ft5>文档</SPAN></NOBR></SPAN> </BODY></HTML>
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