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📁 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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<BODY>
<DIV class=pg></DIV><!-- bitmap and vector images are written here -->
<DIV class=im0><IMG height=64 src="002_29.files/right.jpg" width=563 
border=0></DIV>
<DIV class=im1><IMG height=379 src="002_29.files/002_29_2.jpg" width=468 
border=0></DIV><!-- text starts here --><SPAN 
class=ps0><NOBR>只需计算连续有效包之间的一阶差值，并为每个信源把该数值保存到会话场景空间中。</NOBR></SPAN> <SPAN 
class=ps1><NOBR><SPAN class=ft1>图<SPAN class=em3>2-14 &nbsp;&nbsp;</SPAN>合成<SPAN 
class=em3>RTP/UDP/IP</SPAN>报头</SPAN></NOBR></SPAN> <SPAN 
class=ps2><NOBR>一旦发现完整的、非压缩的包报头，解压缩器可以把剩余的域为该信源保存到会话场景</NOBR></SPAN> <SPAN 
class=ps3><NOBR>中。所以，必须以某种方法在压缩器和解压缩器之间协商场景会话<SPAN class=em1>ID<SPAN 
class=em0>（</SPAN>Context Session ID<SPAN 
class=em0>，</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps4><NOBR><SPAN 
class=ft2>CID<SPAN class=em0>）</SPAN><SPAN class=em0>参数以标识包。</SPAN>CID<SPAN 
class=em0>要么是</SPAN>16<SPAN class=em0>比特数值，允许标识</SPAN>65536<SPAN 
class=em0>个场景，要么是</SPAN>8<SPAN class=em0>比特数值，允</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps5><NOBR>许标识<SPAN class=em1>256</SPAN>个场景。前者产生最小<SPAN 
class=em1>2</SPAN>字节的压缩报头，而后者只有<SPAN class=em1>2</SPAN>字节长。</NOBR></SPAN> <SPAN 
class=ps6><NOBR>剩下的问题是在压缩包中表示<SPAN class=em1>RTP</SPAN>顺序编号。到<SPAN 
class=em1>4</SPAN>比特的合理压缩可以紧急恢复中等程度</NOBR></SPAN> <SPAN 
class=ps7><NOBR>的丢失包突发。否则，将需要一个完全解压缩的报头交换来重新同步压缩器和解压缩器。</NOBR></SPAN> <SPAN 
class=ps8><NOBR>不幸的是，不仅端点需要解压缩器，网络自身也都需要解压缩器，在网络损耗情况下，</NOBR></SPAN> <SPAN 
class=ps9><NOBR>如适度阻塞期间，这会导致压缩方案的性能和健壮性考虑。也就是说，要规定实现报头压缩</NOBR></SPAN> <SPAN 
class=ps10><NOBR>的<SPAN class=em1>RTP</SPAN>路由器持久的性能，以便网络工程师能够通过它们可靠地分配一般<SPAN 
class=em1>RTP</SPAN>通信量，这一点</NOBR></SPAN> <SPAN 
class=ps11><NOBR>有时候说起来容易做起来难。</NOBR></SPAN> <SPAN 
class=ps12><NOBR>还有一个警告。协议如何传送这个信息：一个<SPAN 
class=em1>RTP</SPAN>包报头是完整的还是部分压缩的？对于</NOBR></SPAN> <SPAN 
class=ps13><NOBR>只要传送需要刷新解压缩器的特定报头元素的情况，部分报头压缩很理想。这个方案依赖于</NOBR></SPAN> <SPAN 
class=ps14><NOBR>链路层协议来指示四种不同的压缩<SPAN class=em1>RTP</SPAN>包格式。<SPAN 
class=em1>ATM</SPAN>并没有实现它，因为它不适合于这样的</NOBR></SPAN> <SPAN 
class=ps15><NOBR>调节。一种可能是使用<SPAN class=em1>PPP</SPAN>协议传输压缩<SPAN 
class=em1>RTP<SPAN class=em0>，</SPAN></SPAN>在传输编码语音数据的第一个比特之前，新模</NOBR></SPAN> 
<SPAN class=ps16><NOBR>型可能是<SPAN class=em1>PPP/IP/UDP/RTP</SPAN>。要协调<SPAN 
class=em1>RTP</SPAN>报头压缩，还需要带外信令或者链路层协议。</NOBR></SPAN> <SPAN 
class=ps17><NOBR>如图<SPAN class=em1>2-15</SPAN>所示，<SPAN 
class=em1>FULL_HEADER</SPAN>包携带完整的未压缩合成报头，加上<SPAN 
class=em1>CID</SPAN>和压缩报头的<SPAN class=em1>4</SPAN></NOBR></SPAN> <SPAN 
class=ps18><NOBR>比特顺序编号。两个额外的域分别填在<SPAN class=em1>IP</SPAN>和<SPAN 
class=em1>UDP</SPAN>报头长度域中，用于一个<SPAN class=em1>8</SPAN>比特<SPAN 
class=em1>CID<SPAN class=em0>，</SPAN></SPAN>或者相</NOBR></SPAN> <SPAN 
class=ps19><NOBR>反地，用于<SPAN class=em1>16</SPAN>比特<SPAN class=em1>CIP<SPAN 
class=em0>，</SPAN></SPAN>以避免扩大标准<SPAN class=em1>IP/UDP</SPAN>报头大小。</NOBR></SPAN> 
<SPAN class=ps20><NOBR><SPAN class=ft3>第<SPAN 
class=em4>2</SPAN>章&nbsp;包网络的介质传输</SPAN></NOBR></SPAN> <SPAN 
class=ps21><NOBR><SPAN class=ft4>85</SPAN></NOBR></SPAN> <SPAN 
class=ps22><NOBR><SPAN class=ft5>文档</SPAN></NOBR></SPAN> <SPAN 
class=ps23><NOBR><SPAN class=ft6>版本</SPAN></NOBR></SPAN> <SPAN 
class=ps24><NOBR><SPAN class=ft6>协议号</SPAN></NOBR></SPAN> <SPAN 
class=ps25><NOBR><SPAN class=ft6>标志</SPAN></NOBR></SPAN> <SPAN 
class=ps26><NOBR><SPAN class=ft6>UDP长度</SPAN></NOBR></SPAN> <SPAN 
class=ps27><NOBR><SPAN class=ft6>源端口号</SPAN></NOBR></SPAN> <SPAN 
class=ps28><NOBR><SPAN class=ft7>32<SPAN class=em2>字节的目标</SPAN>IP<SPAN 
class=em2>地址</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps29><NOBR><SPAN 
class=ft6>偏移量</SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN 
class=ft7>32<SPAN class=em2>字节的源</SPAN>IP<SPAN 
class=em2>地址</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN 
class=ft6>标识</SPAN></NOBR></SPAN> <SPAN class=ps32><NOBR><SPAN 
class=ft6>报头校验</SPAN></NOBR></SPAN> <SPAN class=ps33><NOBR><SPAN 
class=ft6>总长度<SPAN class=em5>(</SPAN>字节<SPAN 
class=em5>)</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps34><NOBR><SPAN 
class=ft6>顺序编号</SPAN></NOBR></SPAN> <SPAN class=ps35><NOBR><SPAN 
class=ft6>时间戳</SPAN></NOBR></SPAN> <SPAN class=ps36><NOBR><SPAN 
class=ft7>UDP<SPAN class=em2>校验，如果未使用则为</SPAN>0</SPAN></NOBR></SPAN> <SPAN 
class=ps37><NOBR><SPAN class=ft6>同步信源标识符<SPAN class=em5>(<SPAN 
class=em2>SSRC&nbsp;&nbsp;</SPAN>)<SPAN 
class=em2>，</SPAN></SPAN>对每个信源是固定的值</SPAN></NOBR></SPAN> <SPAN 
class=ps38><NOBR><SPAN class=ft6>贡献信源标识符(CSRC)(从混合器得到)</SPAN></NOBR></SPAN> 
<SPAN class=ps39><NOBR><SPAN class=ft6>从包到包会发生变化</SPAN></NOBR></SPAN> <SPAN 
class=ps40><NOBR><SPAN class=ft6>携带冗余信息</SPAN></NOBR></SPAN> <SPAN 
class=ps41><NOBR><SPAN class=ft6>目标端口号</SPAN></NOBR></SPAN> </BODY></HTML>

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