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<DIV class=im1><IMG height=252 src="003_11.files/003_11_2.jpg" width=428
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN
class=ft0>110 <SPAN class=em0>IP </SPAN><SPAN
class=em3>电话技术：稳定的</SPAN><SPAN class=em0>VoIP</SPAN><SPAN
class=em3>服务集成</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps1><NOBR><SPAN
class=ft1>文档</SPAN></NOBR></SPAN> <SPAN class=ps2><NOBR><SPAN class=ft2>MTP<SPAN
class=em2>使用以</SPAN>128<SPAN class=em2>为模的前向和后向顺序编号（<SPAN class=em1>Forward
Sequence Number</SPAN>，<SPAN
class=em1>FSN</SPAN>和</SPAN>Back-</SPAN></NOBR></SPAN> <SPAN
class=ps3><NOBR><SPAN class=ft2>ward Sequence Number<SPAN
class=em2>，</SPAN>BSN<SPAN class=em2>）</SPAN><SPAN class=em2>来实现</SPAN>MSU<SPAN
class=em2>中的错误控制。这是一个滑动窗机制，类似于其</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps4><NOBR><SPAN class=ft3>他包交换协议中的机制。</SPAN></NOBR></SPAN> <SPAN
class=ps5><NOBR><SPAN class=ft2>FSN<SPAN class=em2>被设置为当前正在被传送的</SPAN>MSU<SPAN
class=em2>的顺序编号。接收者的</SPAN>BSN<SPAN
class=em2>表示上一个正确接收的</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps6><NOBR><SPAN
class=ft2>MSU<SPAN class=em2>。图</SPAN>3-9<SPAN class=em2>中的流图可以用</SPAN>STP<SPAN
class=em2>之间的交换帮助解释基本的错误方法。</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps7><NOBR><SPAN class=ft3>当消息被正确接收时，<SPAN
class=em1>BIB</SPAN>的值不会改变，保持在上一个消息发回到发送者的消息中设置</SPAN></NOBR></SPAN> <SPAN
class=ps8><NOBR><SPAN class=ft3>的值。同样地，对于新的<SPAN class=em1>MSU<SPAN
class=em2>，</SPAN>FIB</SPAN>的值也不会改变。</SPAN></NOBR></SPAN> <SPAN
class=ps9><NOBR><SPAN class=ft3>图<SPAN class=em1>3-10</SPAN>中是当接收到错误的或者失序的<SPAN
class=em1>MSU</SPAN>时的基本恢复方法。前向<SPAN class=em1>MSU</SPAN>编号<SPAN
class=em1>104</SPAN>出现错</SPAN></NOBR></SPAN> <SPAN class=ps10><NOBR><SPAN
class=ft3>误。发送<SPAN class=em1>STP</SPAN>继续发送<SPAN class=em1>MSU<SPAN
class=em2>，</SPAN></SPAN>并做最好的打算。但是当接收者得到它时，它返回的<SPAN class=em1>BIB=1<SPAN
class=em2>（</SPAN></SPAN>位</SPAN></NOBR></SPAN> <SPAN class=ps11><NOBR><SPAN
class=ft3>发生切换），<SPAN class=em1>BSN=103</SPAN>。这表示<SPAN
class=em1>103</SPAN>之后的所有<SPAN
class=em1>MSU</SPAN>都被丢弃，或者没有被远端收到。这时发</SPAN></NOBR></SPAN> <SPAN
class=ps12><NOBR><SPAN class=ft3>送者重传<SPAN class=em1>MSU 104</SPAN>和<SPAN
class=em1>105<SPAN class=em2>，</SPAN>FIB</SPAN>被设置为<SPAN
class=em1>1</SPAN>以表示重传。</SPAN></NOBR></SPAN> <SPAN class=ps13><NOBR><SPAN
class=ft2>FIB<SPAN class=em4>和</SPAN>BIB<SPAN
class=em4>的值保持在它们的新值上，直到另一个错误发生为止。</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps14><NOBR><SPAN class=ft3>当<SPAN class=em1>STP</SPAN>间的单向延迟很大，也就是<SPAN
class=em1>15~20ms</SPAN>甚至更大的时候，必须使用<SPAN
class=em1>PCR</SPAN>方法。其思想</SPAN></NOBR></SPAN> <SPAN class=ps15><NOBR><SPAN
class=ft3>是使信令尽可能快地进行，因为它会影响呼叫建立事件。当没有新的<SPAN class=em1>MSU</SPAN>或<SPAN
class=em1>LSSU</SPAN>可以发送时，</SPAN></NOBR></SPAN> <SPAN class=ps16><NOBR><SPAN
class=ft3>所有未确认的<SPAN
class=em1>MSU</SPAN>被循环重传。它认为，如果发送所有未确认的消息，占用否则将是空闲的</SPAN></NOBR></SPAN> <SPAN
class=ps17><NOBR><SPAN class=ft3>链路，那么出错的消息在第二次就可以被正确收到。在这种方法中没有使用<SPAN
class=em1>FIB</SPAN>和<SPAN class=em1>BIB</SPAN>位的切</SPAN></NOBR></SPAN> <SPAN
class=ps18><NOBR><SPAN class=ft3>换，它们都被设置为<SPAN class=em1>1</SPAN>。只有正的<SPAN
class=em1>ACK</SPAN>才会被返回，这样将导致另一个潜在的问题。如果一个</SPAN></NOBR></SPAN> <SPAN
class=ps19><NOBR><SPAN class=ft2>MSU<SPAN
class=em2>发生错误，而且由于发送者繁忙而在相当长的时间内没有被检测出来，那么呼叫建立会</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps20><NOBR><SPAN class=ft3>失败，或者长得不可接收。因此，发送者必须使用定时器，或者<SPAN
class=em1>MSU</SPAN>发送缓冲阈值来停止所</SPAN></NOBR></SPAN> <SPAN class=ps21><NOBR><SPAN
class=ft3>有新的<SPAN class=em1>MSU</SPAN>活动，重新周期性地发送未确认的<SPAN
class=em1>MSU</SPAN>。这就叫做被迫重传过程。</SPAN></NOBR></SPAN> <SPAN
class=ps22><NOBR><SPAN class=ft2>4. <SPAN
class=em4>信令链路失败时的路由方法</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps23><NOBR><SPAN
class=ft3>链路拥塞比硬链路故障更容易导致信令消息的重新路由。拥塞也比简单的链路故障更容</SPAN></NOBR></SPAN> <SPAN
class=ps24><NOBR><SPAN class=ft3>易导致信令网络的暂时不稳定。当某个事件发生而导致链路不能使用时， <SPAN
class=em1>MTP3</SPAN>的信令网络管</SPAN></NOBR></SPAN> <SPAN class=ps25><NOBR><SPAN
class=ft3>理（<SPAN class=em1>Signaling Network Management</SPAN>，<SPAN
class=em1>SNM</SPAN>）功能会采取行动，在拓扑中寻找备份路由。如图</SPAN></NOBR></SPAN> <SPAN
class=ps26><NOBR><SPAN class=ft2>3-11<SPAN class=em2>所示，消息以</SPAN>MSU<SPAN
class=em2>的服务信息字节中的</SPAN>SI=0<SPAN
class=em2>开始，以标识信令网络管理操作。</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps27><NOBR><SPAN class=ft4>图<SPAN class=em5>3-11
</SPAN>信令链路控制和路由管理的<SPAN class=em5>SNM</SPAN>消息</SPAN></NOBR></SPAN>
<SPAN class=ps28><NOBR>7<SPAN class=em6>比特</SPAN></NOBR></SPAN> <SPAN
class=ps29><NOBR>7<SPAN class=em6>比特</SPAN></NOBR></SPAN> <SPAN
class=ps30><NOBR>6 LSB</NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN
class=ft6>(1<SPAN class=em7>个字节</SPAN>)</SPAN></NOBR></SPAN> <SPAN
class=ps32><NOBR><SPAN class=ft7>(1<SPAN class=em7>个字节</SPAN><SPAN
class=em8>)</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps33><NOBR>SNM<SPAN
class=em6>消息注释</SPAN></NOBR></SPAN> <SPAN class=ps34><NOBR>1. <SPAN
class=em6>改变命令</SPAN></NOBR></SPAN> <SPAN class=ps35><NOBR>2. <SPAN
class=em6>改变确认</SPAN></NOBR></SPAN> <SPAN class=ps36><NOBR>3. <SPAN
class=em6>改变回声明</SPAN></NOBR></SPAN> <SPAN class=ps37><NOBR>4. <SPAN
class=em6>改变回确认</SPAN></NOBR></SPAN> <SPAN class=ps38><NOBR>5. <SPAN
class=em6>紧急改变</SPAN></NOBR></SPAN> <SPAN class=ps39><NOBR>6. <SPAN
class=em6>紧急改变确认</SPAN></NOBR></SPAN> <SPAN class=ps40><NOBR>7. <SPAN
class=em6>传输受控</SPAN></NOBR></SPAN> <SPAN class=ps41><NOBR>8. <SPAN
class=em6>传输禁止</SPAN></NOBR></SPAN> <SPAN class=ps42><NOBR>9. <SPAN
class=em6>传输允许</SPAN></NOBR></SPAN> <SPAN class=ps43><NOBR>10. <SPAN
class=em6>传输受限</SPAN></NOBR></SPAN> <SPAN class=ps44><NOBR>11. <SPAN
class=em6>信令路由集测试</SPAN></NOBR></SPAN> <SPAN class=ps45><NOBR>12. <SPAN
class=em6>信令路由集拥塞测试</SPAN></NOBR></SPAN> <SPAN class=ps46><NOBR>13. <SPAN
class=em6>传输簇禁止</SPAN></NOBR></SPAN> <SPAN class=ps47><NOBR>14. <SPAN
class=em6>传输簇允许</SPAN></NOBR></SPAN> <SPAN class=ps48><NOBR>15. <SPAN
class=em6>传输簇受限</SPAN></NOBR></SPAN> <SPAN class=ps49><NOBR>16. <SPAN
class=em6>簇路由集测试</SPAN></NOBR></SPAN> <SPAN class=ps50><NOBR>SF=<SPAN
class=em6>国家网络</SPAN></NOBR></SPAN> <SPAN
class=ps51><NOBR>SI=SNM(X'O')</NOBR></SPAN> <SPAN class=ps52><NOBR>SNM<SPAN
class=em6>消息</SPAN></NOBR></SPAN> <SPAN class=ps53><NOBR>SNM<SPAN
class=em6>消息</SPAN></NOBR></SPAN> <SPAN class=ps54><NOBR><SPAN
class=ft8>参数</SPAN></NOBR></SPAN> <SPAN class=ps55><NOBR><SPAN class=ft6>(7<SPAN
class=em7>个字节</SPAN>)</SPAN></NOBR></SPAN> <SPAN class=ps56><NOBR><SPAN
class=ft6>2<SPAN class=em7>个字节</SPAN></SPAN></NOBR></SPAN> </BODY></HTML>
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