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<DIV class=im0><IMG height=64 src="003_24.files/right.jpg" width=563
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN
class=ft0>AIN<SPAN
class=em1>基本呼叫模型的优点在于它节省了大量成本,以及新设备供应的及时性。语音电话的</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps1><NOBR>公用包网络可以从<SPAN class=em0>AIN</SPAN>呼叫模型中受益,因为在任何保持<SPAN
class=em0>SS7</SPAN>信令能力的拓扑中,<SPAN class=em0>PSTN</SPAN>中创</NOBR></SPAN> <SPAN
class=ps2><NOBR>建服务的设备都可以重新使用,至少在<SPAN
class=em0>TCAP</SPAN>层中可以。所以服务的设计和应用就变得快捷和</NOBR></SPAN> <SPAN
class=ps3><NOBR>物有所值,主要的精力可以放在细调包网络的新元件上,例如呼叫处理信令和介质传输机制</NOBR></SPAN> <SPAN
class=ps4><NOBR>和拓扑。</NOBR></SPAN> <SPAN class=ps5><NOBR><SPAN class=ft2>3.2.2
IP<SPAN class=em2>电话中的</SPAN>SS7<SPAN class=em2>和对</SPAN>PSTN<SPAN
class=em2>信令最后的思考</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps6><NOBR>即使是不经意地看一眼<SPAN class=em0>TCAP/SCCP</SPAN>的说明文档,也足以让我们相信在<SPAN
class=em0>PSTN</SPAN>电话中加入</NOBR></SPAN> <SPAN class=ps7><NOBR><SPAN
class=ft0>TCAP<SPAN
class=em1>的价值非常巨大,这是任何新的包网络应用都不能低估的。在企业私有网络或者因特网</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps8><NOBR>上的<SPAN
class=em0>IP</SPAN>电话的情况下,不需要执行全局标题翻译、信用卡数据查找和<SPAN
class=em0>LNP</SPAN>查询等,但是一旦</NOBR></SPAN> <SPAN class=ps9><NOBR>这样的网络希望和<SPAN
class=em0>PSTN</SPAN>交互,情况立即就会发生改变。</NOBR></SPAN> <SPAN class=ps10><NOBR><SPAN
class=ft0>TCAP<SPAN
class=em1>函数的功能非常有望出现在新的语音电话网络中,至少在短期内是这样,这意味着</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps11><NOBR><SPAN class=ft0>TCAP<SPAN class=em1>要么在一些实现中使用</SPAN>SCCP<SPAN
class=em1>,</SPAN><SPAN class=em1>要么将</SPAN>SCCP<SPAN class=em1>放在</SPAN>IP<SPAN
class=em1>协议栈的第</SPAN>4<SPAN class=em1>层,通过</SPAN>TCP<SPAN
class=em1>或其他可</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps12><NOBR>靠的传输机制来进行端到端传输。</NOBR></SPAN> <SPAN
class=ps13><NOBR>另外需要注意的是,在最初导致在<SPAN
class=em0>PSTN</SPAN>中设计带外信令的原因中,有两个是健壮性和可</NOBR></SPAN> <SPAN
class=ps14><NOBR>靠性。任何将<SPAN class=em0>SS7</SPAN>协议转变为基于<SPAN
class=em0>IP</SPAN>的配置的尝试都必须同等地考虑服务的可用性。所以,这</NOBR></SPAN> <SPAN
class=ps15><NOBR>样的努力会造成要么减慢从<SPAN class=em0>SS7</SPAN>链路到基于<SPAN
class=em0>IP</SPAN>链路的转化,要么同时实现一种并行的、带外的、</NOBR></SPAN> <SPAN
class=ps16><NOBR>基于<SPAN class=em0>IP</SPAN>的、载波之间的信令网络。</NOBR></SPAN> <SPAN
class=ps17><NOBR>解决这个问题的方向涉及到标准组的更多工作,因此现在<SPAN
class=em0>TCAP/SCCP/MTP</SPAN>协议层会继续</NOBR></SPAN> <SPAN
class=ps18><NOBR>存在于<SPAN class=em0>SS7</SPAN>的实现中,继续为来自各种类型的<SPAN
class=em0>IXC</SPAN>和<SPAN class=em0>LEC</SPAN>的<SPAN
class=em0>IP</SPAN>电话提供初始转出服务。</NOBR></SPAN> <SPAN class=ps19><NOBR><SPAN
class=ft3>3.3 <SPAN class=em3>包网络中的呼叫控制</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps20><NOBR>本节中将讨论基于<SPAN
class=em0>IP</SPAN>的基本呼叫模型的基本原理。首先将讨论两方呼叫的呼叫流,以</NOBR></SPAN> <SPAN
class=ps21><NOBR>及在各自端点上的活动。基本呼叫流是</NOBR></SPAN> <SPAN class=ps22><NOBR><SPAN
class=ft0>MGC<SPAN class=em1>和介质网关上的端点之间交换的信令集,以</SPAN></SPAN></NOBR></SPAN>
<SPAN class=ps23><NOBR>建立和终止呼叫,还有收集收费信息、性能和其他重要的统计数据。当引入最常用的用</NOBR></SPAN>
<SPAN class=ps24><NOBR>户电话特性以后,或者一方或两方连接到介质网关另一端的</NOBR></SPAN> <SPAN
class=ps25><NOBR><SPAN class=ft0>PBX<SPAN
class=em1>上时,呼叫流会变得非</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps26><NOBR>常复杂。</NOBR></SPAN> <SPAN
class=ps27><NOBR>基本呼叫建立包括在两个端点之间创建一个连接,以及选择介质类型。在每个网络实现</NOBR></SPAN> <SPAN
class=ps28><NOBR>中,路径选择和路由问题可能是,也可能不是呼叫处理的一部分。从信令和介质传输的角度</NOBR></SPAN> <SPAN
class=ps29><NOBR>来看,有两种呼叫处理的情况必须引起注意:端点在同类网络中的呼叫,以及端点在异类网</NOBR></SPAN> <SPAN
class=ps30><NOBR>络中的呼叫。</NOBR></SPAN> <SPAN class=ps31><NOBR><SPAN class=ft0>1.
<SPAN class=em4>同类网络</SPAN></SPAN></NOBR></SPAN> <SPAN
class=ps32><NOBR>在这种情况下,端点位于使用相同的信令协议进行呼叫建立的管理域中,不需要跨越管</NOBR></SPAN> <SPAN
class=ps33><NOBR>理边界或网络边界进行信令交换的协作转换。介质根据相同的传输协议端到端地发送。注意,</NOBR></SPAN> <SPAN
class=ps34><NOBR>并不需要在同类网络内支持相同的介质格式。也就是说,介质转换编码是可能的,它依赖于</NOBR></SPAN> <SPAN
class=ps35><NOBR>连接中涉及到的端点的容量。</NOBR></SPAN> <SPAN class=ps36><NOBR>在听到<SPAN
class=em0>PSTN</SPAN>自身并不是一个同类信令网络时,读者不应该感到奇怪。例如,用户信令</NOBR></SPAN> <SPAN
class=ps37><NOBR>使用不同的硬件和软件协议集(<SPAN class=em0>Loop start</SPAN>、<SPAN
class=em0>Ground start</SPAN>、<SPAN class=em0>PBX</SPAN>上的<SPAN
class=em0>PRI</SPAN>或<SPAN class=em0>CAS</SPAN>等),干线信</NOBR></SPAN> <SPAN
class=ps38><NOBR>令可以基于<SPAN class=em0>SS7<SPAN class=em1>,</SPAN></SPAN>使用<SPAN
class=em0>ISUP</SPAN>消息,或者使用多频(<SPAN class=em0>multifrequency</SPAN>,<SPAN
class=em0>MF</SPAN>)信令(紧急<SPAN class=em0>911</SPAN>服务</NOBR></SPAN> <SPAN
class=ps39><NOBR><SPAN class=ft4>第<SPAN
class=em5>3</SPAN>章 语音电话的基本呼叫控制</SPAN></NOBR></SPAN> <SPAN
class=ps40><NOBR><SPAN class=ft5>123</SPAN></NOBR></SPAN> <SPAN
class=ps41><NOBR><SPAN class=ft6>文档</SPAN></NOBR></SPAN> </BODY></HTML>
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