002_3.htm

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HTM
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<DIV class=pg></DIV><!-- bitmap and vector images are written here -->
<DIV class=im0><IMG height=107 src="002_3.files/002_3_1.jpg" width=429 
border=0></DIV>
<DIV class=im1><IMG height=154 src="002_3.files/002_3_2.jpg" width=558 
border=0></DIV>
<DIV class=im2><IMG height=64 src="002_3.files/right.jpg" width=563 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN 
class=ft0>图<SPAN class=em4>2-1 
&nbsp;&nbsp;</SPAN>计算介质传输的带宽需要</SPAN></NOBR></SPAN> <SPAN class=ps1><NOBR><SPAN 
class=ft1>表<SPAN class=em5>2-1 
&nbsp;&nbsp;RTP</SPAN>上的语音的名义带宽计算，不考虑链路层开销的影响</SPAN></NOBR></SPAN> <SPAN 
class=ps2><NOBR><SPAN class=ft2>有效载荷</SPAN></NOBR></SPAN> <SPAN 
class=ps3><NOBR><SPAN class=ft2>所需带宽，</SPAN></NOBR></SPAN> <SPAN 
class=ps4><NOBR><SPAN class=ft2>所需带宽，</SPAN></NOBR></SPAN> <SPAN 
class=ps5><NOBR><SPAN class=ft2>有效载荷</SPAN></NOBR></SPAN> <SPAN 
class=ps6><NOBR><SPAN class=ft2>名义速率</SPAN></NOBR></SPAN> <SPAN 
class=ps7><NOBR><SPAN class=ft2>打包速率</SPAN></NOBR></SPAN> <SPAN 
class=ps8><NOBR><SPAN class=ft2>大小</SPAN></NOBR></SPAN> <SPAN 
class=ps9><NOBR><SPAN class=ft2>非压缩报头</SPAN></NOBR></SPAN> <SPAN 
class=ps10><NOBR><SPAN class=ft2>压缩报头</SPAN></NOBR></SPAN> <SPAN 
class=ps11><NOBR><SPAN class=ft2>格</SPAN></NOBR></SPAN> <SPAN 
class=ps12><NOBR><SPAN class=ft2>式</SPAN></NOBR></SPAN> <SPAN 
class=ps13><NOBR><SPAN class=ft2>（<SPAN 
class=em0>ms</SPAN>）</SPAN></NOBR></SPAN> <SPAN class=ps14><NOBR><SPAN 
class=ft2>（字节）</SPAN></NOBR></SPAN> <SPAN class=ps15><NOBR><SPAN 
class=ft2>（<SPAN class=em0>Kbps</SPAN>）</SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR><SPAN class=ft2>（<SPAN 
class=em0>Kbps</SPAN>）</SPAN></NOBR></SPAN> <SPAN class=ps17><NOBR><SPAN 
class=ft3>G.711</SPAN></NOBR></SPAN> <SPAN class=ps18><NOBR><SPAN class=ft3>64 
Kbps</SPAN></NOBR></SPAN> <SPAN class=ps19><NOBR><SPAN 
class=ft3>20</SPAN></NOBR></SPAN> <SPAN class=ps20><NOBR><SPAN 
class=ft3>160</SPAN></NOBR></SPAN> <SPAN class=ps21><NOBR><SPAN 
class=ft3>80</SPAN></NOBR></SPAN> <SPAN class=ps22><NOBR><SPAN 
class=ft3>64.8</SPAN></NOBR></SPAN> <SPAN class=ps23><NOBR><SPAN 
class=ft3>G.711</SPAN></NOBR></SPAN> <SPAN class=ps24><NOBR><SPAN 
class=ft3>10</SPAN></NOBR></SPAN> <SPAN class=ps25><NOBR><SPAN 
class=ft3>80</SPAN></NOBR></SPAN> <SPAN class=ps26><NOBR><SPAN 
class=ft3>96</SPAN></NOBR></SPAN> <SPAN class=ps27><NOBR><SPAN 
class=ft3>65.6</SPAN></NOBR></SPAN> <SPAN class=ps28><NOBR><SPAN 
class=ft3>G.729</SPAN></NOBR></SPAN> <SPAN class=ps29><NOBR><SPAN class=ft3>8 
Kbps</SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR><SPAN 
class=ft3>20</SPAN></NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN 
class=ft3>20</SPAN></NOBR></SPAN> <SPAN class=ps32><NOBR><SPAN 
class=ft3>24</SPAN></NOBR></SPAN> <SPAN class=ps33><NOBR><SPAN 
class=ft3>8.8</SPAN></NOBR></SPAN> <SPAN class=ps34><NOBR><SPAN 
class=ft3>G.729</SPAN></NOBR></SPAN> <SPAN class=ps35><NOBR><SPAN 
class=ft3>10</SPAN></NOBR></SPAN> <SPAN class=ps36><NOBR><SPAN 
class=ft3>10</SPAN></NOBR></SPAN> <SPAN class=ps37><NOBR><SPAN 
class=ft3>40</SPAN></NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN 
class=ft3>9.6</SPAN></NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft4>在每个包产生的瞬间，需要传送的字节数是：</SPAN></NOBR></SPAN> <SPAN class=ps40><NOBR><SPAN 
class=ft4>每个介质样本字节数＝<SPAN class=em1>40+N+</SPAN>链路层开销</SPAN></NOBR></SPAN> <SPAN 
class=ps41><NOBR><SPAN class=ft5>N<SPAN 
class=em6>的数值能够由介质编码，也就是应用中使用的语音编码器类型，很容易地计算出来。</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps42><NOBR><SPAN class=ft4>显然，链路层协议会产生带宽需求。如果使用<SPAN 
class=em1>ATM</SPAN>作为链路层协议，那么必须计算信元</SPAN></NOBR></SPAN> <SPAN 
class=ps43><NOBR><SPAN class=ft4>负担，以及需要多信元传输一个包的可能性。使用<SPAN 
class=em1>AAL5</SPAN>做链路传输时，信元会增加<SPAN 
class=em1>64</SPAN>比特的</SPAN></NOBR></SPAN> <SPAN class=ps44><NOBR><SPAN 
class=ft4>报尾开销，用于控制信息和<SPAN class=em1>CRC</SPAN>。在这个例子中，<SPAN 
class=em1>20ms</SPAN>打包速率的简单<SPAN 
class=em1>G.711&micro;Law</SPAN>语音编码，</SPAN></NOBR></SPAN> <SPAN 
class=ps45><NOBR><SPAN class=ft4>至少会占用<SPAN class=em1>5</SPAN>个信元。第<SPAN 
class=em1>1</SPAN>个信元用于<SPAN class=em1>RTP/UDP/IP</SPAN>和封装开销，而剩下的<SPAN 
class=em1>4</SPAN>个信元将携带<SPAN class=em1>160</SPAN>字</SPAN></NOBR></SPAN> <SPAN 
class=ps46><NOBR><SPAN class=ft4>节有效载荷，加上<SPAN class=em1>8</SPAN>字节的报尾。最后的<SPAN 
class=em1>AAL5</SPAN>信元被部分地用空白字节填充，但是在计算中</SPAN></NOBR></SPAN> <SPAN 
class=ps47><NOBR><SPAN class=ft4>仍然作为一个完整的信元考虑。</SPAN></NOBR></SPAN> <SPAN 
class=ps48><NOBR><SPAN class=ft4>在这种简单情况下，每个样本产生的总比特数是：</SPAN></NOBR></SPAN> 
<SPAN class=ps49><NOBR><SPAN class=ft5>RTP/UDP/IP/Encaps/AAL5<SPAN 
class=em7>之上的</SPAN>G.711<SPAN class=em7>每</SPAN>20 ms<SPAN 
class=em7>打包间隔的比特数＝<SPAN class=em2>2120</SPAN></SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps50><NOBR><SPAN class=ft4>这个结果要求<SPAN class=em1>106Kbps</SPAN>、<SPAN 
class=em1>20 ms</SPAN>打包速率、每个包<SPAN class=em1>5</SPAN>个信元，或者<SPAN 
class=em1>127.2Kbps</SPAN>、<SPAN class=em1>10 ms</SPAN>的打包</SPAN></NOBR></SPAN> 
<SPAN class=ps51><NOBR><SPAN class=ft4>速率、每个包<SPAN 
class=em1>2</SPAN>个信元。类似的计算会得出，传送<SPAN class=em1>20 ms</SPAN>打包速率的<SPAN 
class=em1>G.729</SPAN>样本需要<SPAN class=em1>42.4Kbps</SPAN>的</SPAN></NOBR></SPAN> 
<SPAN class=ps52><NOBR><SPAN class=ft4>带宽，而在<SPAN class=em1>20 
ms</SPAN>打包速率下，相同编码会产生恰好两倍的带宽需求。</SPAN></NOBR></SPAN> <SPAN 
class=ps53><NOBR><SPAN class=ft4>尽管<SPAN 
class=em1>PSTN</SPAN>可能有很多缺点，但是并不包括主干网上的带宽分配错乱。还应该考虑到我们</SPAN></NOBR></SPAN> 
<SPAN class=ps54><NOBR><SPAN 
class=ft4>使用的两个例子十分简单，不涉及包多路复用。</SPAN></NOBR></SPAN> <SPAN 
class=ps55><NOBR><SPAN class=ft4>显然，使用简单的非压缩<SPAN 
class=em1>RTP/UDP/IP</SPAN>报头会严重损害语音流低比特率编码所节约的带宽。</SPAN></NOBR></SPAN> <SPAN 
class=ps56><NOBR><SPAN 
class=ft4>统计多路复用获益在这样高的带宽数字下也毫无意义，因为它们的可靠性不足以进入确定性</SPAN></NOBR></SPAN> <SPAN 
class=ps57><NOBR><SPAN class=ft5>CAC<SPAN 
class=em6>算法。因此，这就开始产生矛盾：为了达到节约带宽的目的，应该对语音使用基于包的</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps58><NOBR><SPAN class=ft4>传输，否则，带宽将在<SPAN 
class=em1>PSTN</SPAN>中被浪费掉。这个问题会被两个伴随问题进一步恶化。由于端点</SPAN></NOBR></SPAN> <SPAN 
class=ps59><NOBR><SPAN 
class=ft4>处编码、打包和去抖动缓冲，以及介质传输在非一致拓扑中需要中继，所以可能需要转换编</SPAN></NOBR></SPAN> <SPAN 
class=ps60><NOBR><SPAN class=ft6>第<SPAN 
class=em8>2</SPAN>章&nbsp;包网络的介质传输</SPAN></NOBR></SPAN> <SPAN 
class=ps61><NOBR><SPAN class=ft7>59</SPAN></NOBR></SPAN> <SPAN 
class=ps62><NOBR><SPAN class=ft8>20<SPAN 
class=em9>字节</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps63><NOBR><SPAN 
class=ft9>未压缩的<SPAN class=em3>RTP/UDP/IP</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps64><NOBR><SPAN class=ft9>报头＝<SPAN 
class=em3>40</SPAN>字节</SPAN></NOBR></SPAN> <SPAN class=ps65><NOBR><SPAN 
class=ft8>8<SPAN class=em9>字节</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps66><NOBR><SPAN class=ft8>12<SPAN 
class=em9>字节</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps67><NOBR><SPAN 
class=ft9>有效载荷</SPAN></NOBR></SPAN> <SPAN class=ps68><NOBR><SPAN 
class=ft9>每个<SPAN class=em3>pkt</SPAN>间隔出现一次，</SPAN></NOBR></SPAN> <SPAN 
class=ps69><NOBR><SPAN class=ft9>每个<SPAN class=em3>pkt</SPAN>对应<SPAN 
class=em3>N</SPAN>个字节</SPAN></NOBR></SPAN> <SPAN class=ps70><NOBR><SPAN 
class=ft10>文档</SPAN></NOBR></SPAN> </BODY></HTML>
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