002_39.htm

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border=0></DIV><!-- text starts here --><SPAN 
class=ps0><NOBR>在第二个问题中，接收者的持久请求<SPAN 
class=em1>Q1</SPAN>可能会在某个中间节点失败，这样会阻止接收来</NOBR></SPAN> <SPAN 
class=ps1><NOBR>自不同接收者的更小请求<SPAN class=em1>Q0</SPAN>。而如果<SPAN 
class=em1>Q0</SPAN>不和<SPAN class=em1>Q1</SPAN>混合则会被接受。可以通过阻塞状态处理这个</NOBR></SPAN> 
<SPAN class=ps2><NOBR>问题。在<SPAN 
class=em1>RESVERR</SPAN>消息通过的每个路由器中建立起有效的阻塞状态，以防止在合并过程中加</NOBR></SPAN> <SPAN 
class=ps3><NOBR>入失败的请求，这样就可以允许接纳控制考虑更小的请求并接受它。关于阻塞状态的创建和</NOBR></SPAN> <SPAN 
class=ps4><NOBR>使用，在<SPAN class=em1>RFC 2205</SPAN>中有深入的建议。</NOBR></SPAN> <SPAN 
class=ps5><NOBR><SPAN class=ft1>5. <SPAN class=em3>跨越非</SPAN>RSVP<SPAN 
class=em3>网络</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps6><NOBR><SPAN 
class=ft1>PATH<SPAN class=em0>消息跨越一个非</SPAN>RSVP<SPAN 
class=em0>网络的原则是，在进入非</SPAN>RSVP<SPAN 
class=em0>云之前，携带它们遇到的最后</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps7><NOBR>一个<SPAN class=em1>RSVP</SPAN>路由器的<SPAN 
class=em1>IP</SPAN>地址。实际上，这样将使得非<SPAN class=em1>RSVP</SPAN>云对于网络中的<SPAN 
class=em1>RSVP</SPAN>部分是不可见</NOBR></SPAN> <SPAN 
class=ps8><NOBR>的。有一个严重警告，那就是在通过非<SPAN class=em1>RSVP</SPAN>中继站时，要求的<SPAN 
class=em1>QoS</SPAN>可能不一致，而且没有办</NOBR></SPAN> <SPAN 
class=ps9><NOBR>法把它发送到路由器。所以，当存在非<SPAN class=em1>RSVP</SPAN>路由器，广告说明形成并通过<SPAN 
class=em1>OPWA</SPAN>发送到接</NOBR></SPAN> <SPAN class=ps10><NOBR>收者时，一个“非<SPAN 
class=em1>RSVP<SPAN class=em0>”</SPAN></SPAN>标志被送到本地通信量控制，以使它们能在<SPAN 
class=em1>QoS</SPAN>请求上做出选择。</NOBR></SPAN> <SPAN 
class=ps11><NOBR>很自然，呼叫路径中非<SPAN class=em1>RSVP</SPAN>网段的存在会导致端到端<SPAN 
class=em1>QoS</SPAN>的不确定性。但是，如果两个</NOBR></SPAN> <SPAN class=ps12><NOBR><SPAN 
class=ft1>RSVP<SPAN class=em0>网段分别在</SPAN>PSTN<SPAN 
class=em0>的两端连接，例如不直接在</SPAN>IP<SPAN class=em0>上连接的两个载波</SPAN>VoIP<SPAN 
class=em0>网络，那么得到</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps13><NOBR>的质量将是最大可能值。</NOBR></SPAN> <SPAN class=ps14><NOBR><SPAN 
class=ft2>2.3 &nbsp;&nbsp;AAL2<SPAN class=em4>用于语音传输</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps15><NOBR>从本地载波获得高带宽提供给家庭和小型办公室的竞争非常活跃。</NOBR></SPAN> <SPAN 
class=ps16><NOBR><SPAN class=ft1>ISDN <SPAN 
class=em0>基本速率接口</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps17><NOBR>（<SPAN 
class=em1>Basic Rate Interface</SPAN>，<SPAN 
class=em1>BRI</SPAN>）没有获得它的支持者所预期的成功，部分原因在于<SPAN 
class=em1>IP</SPAN>技术正在解</NOBR></SPAN> <SPAN class=ps18><NOBR>决一些费用和升级问题。<SPAN 
class=em1>ISDN</SPAN>基本上是电路交换服务，本地载波费用巨大。升级<SPAN class=em1>ISDN 
BRI</SPAN>意</NOBR></SPAN> <SPAN class=ps19><NOBR>味着在<SPAN 
class=em1>5</SPAN>类系统中购买越来越多的电路容量，这样产生的费用结构已经使<SPAN 
class=em1>BRI</SPAN>超出了很多用户</NOBR></SPAN> <SPAN 
class=ps20><NOBR>的价格承受范围。数字用户环路（<SPAN class=em1>Digital Subscriber 
Loop</SPAN>，&nbsp;<SPAN class=em1>DSL</SPAN>），通常也叫做<SPAN class=em1>xDSL<SPAN 
class=em0>，</SPAN></SPAN>开始</NOBR></SPAN> <SPAN 
class=ps21><NOBR>出现，它有很多特点和性能。只要满足来自本地交换机载波局的一定的距离限制，&nbsp;<SPAN 
class=em1>DSL</SPAN>技术</NOBR></SPAN> <SPAN 
class=ps22><NOBR>就可以调制“最后一英里”的铜线，用数字方法传送高比特率。<SPAN 
class=em1>DSL</SPAN>的对称版本，如<SPAN class=em1>HDSL</SPAN>和</NOBR></SPAN> <SPAN 
class=ps23><NOBR><SPAN class=ft1>SDSL<SPAN class=em0>，</SPAN><SPAN 
class=em0>为家庭和小型办公室提供了和</SPAN>T1<SPAN 
class=em0>相当的比特率的希望，但是非对称版本，或者说</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps24><NOBR><SPAN class=ft1>ADSL<SPAN class=em0>，</SPAN><SPAN 
class=em0>才是对家庭广泛应用的最大希望。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps25><NOBR><SPAN class=ft1>ADSL<SPAN class=em0>能够支持下游方向的</SPAN>6<SPAN 
class=em0>～</SPAN>8Mbps<SPAN class=em0>带宽，上游</SPAN>600Mbps<SPAN 
class=em0>。这对于接入</SPAN>Internet<SPAN class=em0>和文档大</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps26><NOBR>文件是一个理想的解决方案，但是，如果需要将文档的文件发送到其他地方，而集成多媒体</NOBR></SPAN> 
<SPAN class=ps27><NOBR>应用也同时有效，那么在速度上会有明显的不同。尽管如此，&nbsp;<SPAN 
class=em1>ADSL</SPAN>还是比<SPAN class=em1>ISDN BRI</SPAN>更有希</NOBR></SPAN> <SPAN 
class=ps28><NOBR>望。工业界希望在以后的几年中，它能够作为服务普遍存在。</NOBR></SPAN> <SPAN 
class=ps29><NOBR>乍看起来，<SPAN 
class=em1>ADSL</SPAN>的先期计划和早期服务应用已经表现出了很好的性能，价格合理，提供</NOBR></SPAN> <SPAN 
class=ps30><NOBR>的带宽比<SPAN class=em1>ISDN BRI</SPAN>服务现在可行的带宽高一个数量级。当我们考虑<SPAN 
class=em1>PSTN</SPAN>用户正在<SPAN class=em1>SS7</SPAN>网络中</NOBR></SPAN> <SPAN 
class=ps31><NOBR>使用的呼叫特性以及高速数据接入<SPAN 
class=em1>Internet</SPAN>的愿望时，接入带宽毕竟还不是太大。对于一、两</NOBR></SPAN> <SPAN 
class=ps32><NOBR>个用户的住宅解决方案，带宽已经足够了，但是随着新用户和新特性的加入，包括将来的智</NOBR></SPAN> <SPAN 
class=ps33><NOBR>能网特性，还是必须仔细考虑如何使用带宽。</NOBR></SPAN> <SPAN 
class=ps34><NOBR>很自然的想法是用压缩语音代替简单的<SPAN class=em1>G.711 &micro;Law</SPAN>或<SPAN 
class=em1>Alaw</SPAN>。这个解决方法有它自身的</NOBR></SPAN> <SPAN 
class=ps35><NOBR>优点和缺点。尽管它可以提高带宽利用率，但是它提出了用户感知的语音</NOBR></SPAN> <SPAN 
class=ps36><NOBR><SPAN class=ft1>QoS<SPAN 
class=em0>问题。如前所</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps37><NOBR>述，<SPAN 
class=em1>RTP/UDP/IP/ATM</SPAN>之上的简单<SPAN 
class=em1>G.711</SPAN>令人头痛，而且效率低下。它为打包周期提供可接受</NOBR></SPAN> <SPAN 
class=ps38><NOBR><SPAN class=ft3>第<SPAN 
class=em5>2</SPAN>章&nbsp;包网络的介质传输</SPAN></NOBR></SPAN> <SPAN 
class=ps39><NOBR><SPAN class=ft4>95</SPAN></NOBR></SPAN> <SPAN 
class=ps40><NOBR><SPAN class=ft5>文档</SPAN></NOBR></SPAN> <SPAN 
class=ps41><NOBR><SPAN class=ft6>广告单通（<SPAN class=em2>One Pass With 
Advertising</SPAN>）。</SPAN></NOBR></SPAN> <SPAN class=ps42><NOBR><SPAN 
class=ft7>&micro;Law</SPAN></NOBR></SPAN> <SPAN class=ps43><NOBR><SPAN 
class=ft8>。</SPAN></NOBR></SPAN> </BODY></HTML>

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