📄 002_9.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=pg></DIV><!-- bitmap and vector images are written here -->
<DIV class=im0><IMG height=7 src="002_9.files/002_9_1.jpg" width=7 
border=0></DIV>
<DIV class=im1><IMG height=64 src="002_9.files/right.jpg" width=563 
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<DIV class=im4><IMG height=446 src="002_9.files/002_9_5.jpg" width=403 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN 
class=ft0>当<SPAN class=em1>RTCP</SPAN>包被加密时，填充可能成为必不可少的部分。</SPAN></NOBR></SPAN> 
<SPAN class=ps1><NOBR><SPAN class=ft1>图<SPAN class=em2>2-4 
&nbsp;&nbsp;RTCP</SPAN>发送者报告包的格式</SPAN></NOBR></SPAN> <SPAN 
class=ps2><NOBR><SPAN class=ft2>SR<SPAN class=em0>的发送信源在</SPAN>SSRC<SPAN 
class=em0>中标识。</SPAN>NTP<SPAN class=em0>时间戳域就是</SPAN>SR<SPAN 
class=em0>发送时时钟的时间，前提是发送系</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps3><NOBR><SPAN class=ft0>统能够获得这个时间读数。如果不能获得，那么可能会把该域设置为<SPAN 
class=em1>0</SPAN>。<SPAN class=em1>RTP</SPAN>时间戳以它的单</SPAN></NOBR></SPAN> <SPAN 
class=ps4><NOBR><SPAN class=ft0>元来表示，和<SPAN 
class=em1>NTP</SPAN>时间戳相关，而不是特定介质包的时间戳。这意味着，在<SPAN 
class=em1>SR</SPAN>的产生过程中，</SPAN></NOBR></SPAN> <SPAN class=ps5><NOBR><SPAN 
class=ft0>由软件查询<SPAN class=em1>NTP</SPAN>时间，并产生相应的<SPAN 
class=em1>RTP</SPAN>时间戳。只要这个发送者一直在包中使用<SPAN 
class=em1>SSRC</SPAN>标识，</SPAN></NOBR></SPAN> <SPAN class=ps6><NOBR><SPAN 
class=ft0>发送者的包和字节总数就是会话期间发送的<SPAN 
class=em1>RTP</SPAN>包和字节的总和。如果因为冲突或者环路而</SPAN></NOBR></SPAN> <SPAN 
class=ps7><NOBR><SPAN class=ft0>导致<SPAN 
class=em1>SSRC</SPAN>发生改变，这个总数一定会改变。</SPAN></NOBR></SPAN> <SPAN 
class=ps8><NOBR><SPAN class=ft2>rc<SPAN class=em0>域标识了包中包含其统计数据的</SPAN>SSRC<SPAN 
class=em0>数量，最大值是</SPAN>32<SPAN class=em0>，</SPAN><SPAN 
class=em0>并为每个信源返回一个报告</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps9><NOBR><SPAN 
class=ft0>块。对于每个<SPAN class=em1>SSRC<SPAN class=em0>，</SPAN></SPAN>从发送最后一个<SPAN 
class=em1>SR</SPAN>起丢失的包的比例会放在这个数据块第<SPAN class=em1>2</SPAN>个字的<SPAN 
class=em1>FL</SPAN>域</SPAN></NOBR></SPAN> <SPAN class=ps10><NOBR><SPAN 
class=ft0>中&nbsp;。<SPAN class=em1>FL</SPAN>数值被编码为一个整数，它表示丢失包的比例乘以<SPAN 
class=em1>256<SPAN class=em0>，</SPAN></SPAN>而且必须是非负的。选择这</SPAN></NOBR></SPAN> 
<SPAN class=ps11><NOBR><SPAN class=ft0>个数值时必须小心。</SPAN></NOBR></SPAN> <SPAN 
class=ps12><NOBR><SPAN class=ft0>如果丢失的包的数目为<SPAN class=em1>0<SPAN 
class=em0>，</SPAN></SPAN>但是存在包复制使得该数值为负，那么必须输入数值<SPAN 
class=em1>0</SPAN>。但是，</SPAN></NOBR></SPAN> <SPAN class=ps13><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=ps14><NOBR><SPAN class=ft4>65</SPAN></NOBR></SPAN> <SPAN 
class=ps15><NOBR><SPAN class=ft5>文档</SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR><SPAN class=ft6>作者注意到通信业界出现了在计算位数、字节数时使用自然数的趋势，也就是说，从<SPAN 
class=em1>1</SPAN>开始。但是在本书中，</SPAN></NOBR></SPAN> <SPAN class=ps17><NOBR><SPAN 
class=ft6>仍然将使用普通的旧式数字来计数，即从<SPAN class=em1>0</SPAN>开始。</SPAN></NOBR></SPAN> 
<SPAN class=ps18><NOBR>发送者的<SPAN class=em1>SSRC</SPAN></NOBR></SPAN> <SPAN 
class=ps19><NOBR>长度</NOBR></SPAN> <SPAN class=ps20><NOBR><SPAN 
class=ft8>NTP<SPAN class=em0>时间戳，<SPAN 
class=em1>MSW</SPAN></SPAN></SPAN></NOBR></SPAN> <SPAN class=ps21><NOBR><SPAN 
class=ft8>NTP<SPAN class=em0>时间戳，<SPAN 
class=em1>LSW</SPAN></SPAN></SPAN></NOBR></SPAN> <SPAN class=ps22><NOBR><SPAN 
class=ft8>RTP<SPAN class=em0>时间戳</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps23><NOBR>发送者的包数</NOBR></SPAN> <SPAN class=ps24><NOBR>发送者的</NOBR></SPAN> 
<SPAN class=ps25><NOBR>丢失比例</NOBR></SPAN> <SPAN 
class=ps26><NOBR>到达间抖动</NOBR></SPAN> <SPAN class=ps27><NOBR>从最后一个<SPAN 
class=em1>SR</SPAN>以来的延迟（<SPAN class=em1>DLSR)</SPAN></NOBR></SPAN> <SPAN 
class=ps28><NOBR>最后一个<SPAN class=em1>SR<SPAN class=em0>（</SPAN>LSR<SPAN 
class=em0>）</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps29><NOBR>与配置有关的扩展</NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN 
class=ft8>SSRC_2<SPAN class=em0>（</SPAN><SPAN class=em0>第</SPAN>2<SPAN 
class=em0>个信源的</SPAN>SSRC<SPAN class=em0>）</SPAN><SPAN class=em0>第</SPAN>2<SPAN 
class=em0>块开始</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN 
class=ft8>SSRC_1(<SPAN class=em0>第</SPAN>1<SPAN class=em0>个信源的</SPAN>SSRC)<SPAN 
class=em0>第</SPAN>1<SPAN class=em0>块</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps32><NOBR>发送者的包数</NOBR></SPAN> <SPAN 
class=ps33><NOBR>包丢失的总数</NOBR></SPAN> <SPAN 
class=ps34><NOBR>收到的最高的扩展顺序编号</NOBR></SPAN> </BODY></HTML>
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