📄 002_42.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_42.files/left.jpg" width=564 
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<DIV class=im1><IMG height=335 src="002_42.files/002_42_2.jpg" width=505 
border=0></DIV><!-- text starts here --><SPAN 
class=ps0><NOBR>于网络和高层多路复用。</NOBR></SPAN> <SPAN class=ps1><NOBR>图<SPAN 
class=em0>2-23</SPAN>中是<SPAN class=em0>CPS</SPAN>子层支持的包格式。</NOBR></SPAN> <SPAN 
class=ps2><NOBR>有效载荷信息的每个贡献源被分配给一个信道编号（&nbsp;<SPAN class=em0>Channel 
ID</SPAN>）。长度指示单元是一个</NOBR></SPAN> <SPAN class=ps3><NOBR>基于<SPAN 
class=em0>0</SPAN>的包长度，它要么最多<SPAN 
class=em0>45</SPAN>字节，要么可以在各信道基础上动态设置，最多可达<SPAN 
class=em0>64</SPAN>个字节。</NOBR></SPAN> <SPAN 
class=ps4><NOBR>共享一个虚拟电路的信道不需要分配相同的包长度。但是，一旦系统的信令过程做出了分配，</NOBR></SPAN> <SPAN 
class=ps5><NOBR><SPAN class=ft1>VC<SPAN class=em2>内的信道会保持它分配到的长度。</SPAN>CID<SPAN 
class=em2>数值是双向的，可以静态或动态分配。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps6><NOBR>用户到用户信息域携带只对信道端点有意义的编码，而<SPAN 
class=em0>HEC</SPAN>域是简单报头误码控制。</NOBR></SPAN> <SPAN class=ps7><NOBR><SPAN 
class=ft1>PDU<SPAN class=em2>以开始域（<SPAN class=em0>Start Field</SPAN>，<SPAN 
class=em0>STF</SPAN>）开始，包括偏移域、顺序域和奇偶位。偏移域只是说</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps8><NOBR>明<SPAN class=em0>ATM</SPAN>中的下一个<SPAN 
class=em0>PDU</SPAN>从哪里开始。要提供协议执行的非一致包长度多路复用，必须用到这</NOBR></SPAN> <SPAN 
class=ps9><NOBR>个域。<SPAN class=em0>SN</SPAN>域是简单的顺序编码，只在<SPAN 
class=em0>0</SPAN>和<SPAN class=em0>1</SPAN>之间转换。<SPAN 
class=em3>P</SPAN>是一个奇偶位，保持整个<SPAN class=em0>8</SPAN>比特<SPAN 
class=em0>STF</SPAN>奇</NOBR></SPAN> <SPAN class=ps10><NOBR>对称。</NOBR></SPAN> 
<SPAN class=ps11><NOBR>通常情况下，不同长度的包会在一个<SPAN 
class=em0>VC</SPAN>中多路复用。即使对于简单语音传输也是如此，</NOBR></SPAN> <SPAN 
class=ps12><NOBR>因为不同的压缩方案会产生不同的包长度。图<SPAN 
class=em0>2-24</SPAN>中显示了一个任意的例子，其中来自<SPAN class=em0>3</SPAN>个不</NOBR></SPAN> 
<SPAN class=ps13><NOBR>同贡献源的<SPAN class=em0>3</SPAN>个<SPAN 
class=em0>PDU</SPAN>在一个<SPAN class=em0>VC</SPAN>上多路复用。</NOBR></SPAN> <SPAN 
class=ps14><NOBR>在图<SPAN class=em0>2-24</SPAN>中，在一个信源中多路复用<SPAN class=em0>PDU 
A</SPAN>和<SPAN class=em0>B</SPAN>是不可行的，我们将停止跨越信源边界。</NOBR></SPAN> <SPAN 
class=ps15><NOBR>第一个信元的开始域紧挨着放在<SPAN class=em0>STF</SPAN>之后，指向包的报头。在第二个信元中，<SPAN 
class=em0>PDU B</SPAN>的最后<SPAN class=em0>12</SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR>字节紧挨着放在<SPAN class=em0>STF</SPAN>之后，但是<SPAN 
class=em0>STF</SPAN>指向<SPAN class=em0>PDU 
C</SPAN>的包报头的第一个字节，它也不在同一个信元</NOBR></SPAN> <SPAN 
class=ps17><NOBR>中。这样，我们将再次跨越信元边界。</NOBR></SPAN> <SPAN class=ps18><NOBR><SPAN 
class=ft2>图<SPAN class=em4>2-24 &nbsp;&nbsp;AAL2</SPAN>中<SPAN 
class=em4>PDU</SPAN>多路复用的一般情况</SPAN></NOBR></SPAN> <SPAN 
class=ps19><NOBR>在最后的<SPAN class=em0>PDU</SPAN>中，<SPAN 
class=em0>STF</SPAN>指向填充字节（<SPAN class=em0>10</SPAN>）的开始处，<SPAN class=em0>PDU 
C</SPAN>剩下的<SPAN class=em0>11</SPAN>字节放在<SPAN 
class=em0>PDU</SPAN>开始</NOBR></SPAN> <SPAN class=ps20><NOBR>之后。</NOBR></SPAN> 
<SPAN class=ps21><NOBR><SPAN class=ft3>98&nbsp;&nbsp;<SPAN class=em1>IP 
</SPAN><SPAN class=em5>电话技术：稳定的</SPAN><SPAN class=em1>VoIP</SPAN><SPAN 
class=em5>服务集成</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps22><NOBR><SPAN 
class=ft4>文档</SPAN></NOBR></SPAN> <SPAN class=ps23><NOBR><SPAN class=ft5>包<SPAN 
class=em4>A</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps24><NOBR><SPAN 
class=ft5>包<SPAN class=em4>B</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps25><NOBR><SPAN class=ft5>包<SPAN class=em4>C</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps26><NOBR><SPAN class=ft6>44<SPAN 
class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps27><NOBR><SPAN 
class=ft6>20<SPAN class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps28><NOBR><SPAN class=ft6>8<SPAN class=em6>字节</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps29><NOBR><SPAN class=ft6>PDU(48<SPAN 
class=em6>字节</SPAN>)</SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN 
class=ft6>12<SPAN class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps31><NOBR><SPAN class=ft6>33<SPAN 
class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps32><NOBR><SPAN 
class=ft6>11<SPAN class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps33><NOBR><SPAN class=ft6>PAD 36<SPAN 
class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps34><NOBR><SPAN 
class=ft6>35<SPAN class=em6>字节</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps35><NOBR><SPAN class=ft6>35<SPAN 
class=em6>字节</SPAN></SPAN></NOBR></SPAN> </BODY></HTML>
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