📄 003_5.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=im5><IMG height=2 src="003_5.files/003_5_6.jpg" width=134 
border=0></DIV><!-- text starts here --><SPAN class=ps0><NOBR><SPAN 
class=ft0>104&nbsp;&nbsp;<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>机连接到被叫方，那么它就是终端交换机，信令终止。一旦终端交换机通知端点（被叫方电</NOBR></SPAN> <SPAN 
class=ps3><NOBR>话振铃），而且有人接听电话，呼叫便建立起来了。如果紧邻的交换机不是<SPAN class=em2>TE<SPAN 
class=em1>，</SPAN></SPAN>信令就一直在</NOBR></SPAN> <SPAN 
class=ps4><NOBR>多个交换机中进行下去，直到到达<SPAN class=em2>TE<SPAN 
class=em1>，</SPAN></SPAN>即被叫方的宿主交换机为止。即便如此，当到达<SPAN 
class=em2>TE</SPAN>以</NOBR></SPAN> <SPAN 
class=ps5><NOBR>后，呼叫信令也可能没有结束，例如，被叫方使用了某种当电话打进来时的呼叫转移功能。</NOBR></SPAN> <SPAN 
class=ps6><NOBR>这种情况下，信令还要一直找到当前的宿主<SPAN class=em2>TE</SPAN>。</NOBR></SPAN> 
<SPAN class=ps7><NOBR>由于呼叫信令在网络节点之间进行，每一个中间交换机都需要选择通向其相邻交换机的</NOBR></SPAN> <SPAN 
class=ps8><NOBR>出口干线，并把进来的语音线路从入口干线切换到出口干线。对于本地呼叫，这个过程十分</NOBR></SPAN> <SPAN 
class=ps9><NOBR>简单，只有在需要经过同一个区域中的多个本地交换机时，才会牵涉到<SPAN 
class=em2>STP</SPAN>。如果一个交换机</NOBR></SPAN> <SPAN 
class=ps10><NOBR>对某个呼叫既是本地交换机，又是终端交换机，那么就不必使用<SPAN 
class=em2>SS7</SPAN>信令，直接由<SPAN class=em2>SSP</SPAN>通过其</NOBR></SPAN> <SPAN 
class=ps11><NOBR>控制的语音交换机连接两个用户。</NOBR></SPAN> <SPAN class=ps12><NOBR>在支持<SPAN 
class=em2>800</SPAN>免费号码或<SPAN class=em2>900</SPAN>收费号码拨号时，<SPAN 
class=em2>SCP</SPAN>的作用和重要性显而易见。免费和<SPAN class=em2>900</SPAN>号</NOBR></SPAN> 
<SPAN class=ps13><NOBR>码不是真正的电话号码，也就是说，它们不符合标准北美号码分配方案，并且主管这些号码</NOBR></SPAN> 
<SPAN class=ps14><NOBR>的交换机也不可能通过简单查表而获得。当拨叫了这样的号码后，&nbsp;&nbsp;<SPAN 
class=em2>SSP</SPAN>通过其本地<SPAN class=em2>STP</SPAN>用</NOBR></SPAN> <SPAN 
class=ps15><NOBR><SPAN class=ft3>TCAP<SPAN class=em1>访问某个</SPAN>SCP<SPAN 
class=em1>，</SPAN><SPAN class=em1>要求进行号码翻译。</SPAN>SCP<SPAN 
class=em1>能访问这样的数据库</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps16><NOBR>，一旦号码被翻译以后，</NOBR></SPAN> <SPAN 
class=ps17><NOBR>新的路由号码则返回到<SPAN class=em2>SSP<SPAN 
class=em1>，</SPAN></SPAN>同时传送的还有计费信息以及其他完成呼叫所必须的信息。这时</NOBR></SPAN> <SPAN 
class=ps18><NOBR>本地交换机就可以像处理一个通常的电话号码一样了。免费和收费<SPAN 
class=em2>900</SPAN>号码通常由供应商发布，</NOBR></SPAN> <SPAN 
class=ps19><NOBR>他们拥有这些号码。有必要调整使得所有供应商都能访问到公用网中分配的所有<SPAN 
class=em2>800</SPAN>号码，而</NOBR></SPAN> <SPAN 
class=ps20><NOBR>不管谁拥有它们。</NOBR></SPAN> <SPAN class=ps21><NOBR><SPAN 
class=ft3>SCP<SPAN 
class=em1>的另外一个用途与转移到别的电话公司的号码有关。当拨叫一个号码以后，起始交换</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps22><NOBR>机要确定号码是否已被转移，这样它才能决定如何路由。它回初始化一条<SPAN 
class=em2>LNP</SPAN>查询，通过<SPAN class=em2>STP</SPAN></NOBR></SPAN> <SPAN 
class=ps23><NOBR>到达<SPAN class=em2>SCP</SPAN>以访问数据库&nbsp;。<SPAN 
class=em2>SCP</SPAN>返回一个<SPAN class=em2>LRN<SPAN 
class=em1>，</SPAN></SPAN>说明该号码是否已被转移。当信令开始到达被</NOBR></SPAN> <SPAN 
class=ps24><NOBR>叫方时，<SPAN class=em2>LRN</SPAN>就被用来替代被叫方号码。<SPAN 
class=em2>LNP</SPAN>并不意味着被叫方一定处在不同的物理位置。</NOBR></SPAN> <SPAN 
class=ps25><NOBR><SPAN class=ft3>1. SS7<SPAN 
class=em4>协议栈</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps26><NOBR><SPAN 
class=ft3>SS7<SPAN class=em1>的定义早于</SPAN>OSI<SPAN 
class=em1>模型，但是在</SPAN>SS7<SPAN class=em1>协议栈和</SPAN>OSI<SPAN 
class=em1>模型之间有大致的对应关系（见图</SPAN>3-3<SPAN class=em1>）</SPAN></SPAN></NOBR></SPAN> 
<SPAN class=ps27><NOBR>。</NOBR></SPAN> <SPAN class=ps28><NOBR>消息传输部件（<SPAN 
class=em2>Message Transfer Part</SPAN>，<SPAN 
class=em2>MTP</SPAN>）层实际上由三个子层组成，下两层对应于<SPAN class=em2>OSI</SPAN></NOBR></SPAN> 
<SPAN class=ps29><NOBR>模型的<SPAN class=em2>1</SPAN>、<SPAN 
class=em2>2</SPAN>层。<SPAN class=em2>MTP1</SPAN>是物理层信令数据链路（<SPAN 
class=em2>Signaling Data Link</SPAN>，<SPAN class=em2>SDL</SPAN>），最初是对<SPAN 
class=em2>64Kbps</SPAN></NOBR></SPAN> <SPAN class=ps30><NOBR>和<SPAN 
class=em2>56Kbps</SPAN>的数字电路进行了优化。现在，网络拓扑已经有<SPAN 
class=em2>SS7</SPAN>信令专用的全<SPAN class=em2>T1<SPAN 
class=em1>（</SPAN>1.536Mbps<SPAN class=em1>）</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps31><NOBR>设备实现，并且信令也可以通过卫星甚至<SPAN class=em2>ATM</SPAN>干线路由。</NOBR></SPAN> 
<SPAN class=ps32><NOBR>第<SPAN class=em2>2</SPAN>层是<SPAN 
class=em2>MTP2</SPAN>层，即信令链路（<SPAN class=em2>Signaling Link</SPAN>，<SPAN 
class=em2>SL</SPAN>）层，这层负责把消息无错、依次发</NOBR></SPAN> <SPAN 
class=ps33><NOBR>送到相邻的<SPAN class=em2>STP<SPAN 
class=em1>，</SPAN></SPAN>实现流量控制、消息重发、在发生故障时选择备用链路。纠错是很有必要的，</NOBR></SPAN> <SPAN 
class=ps34><NOBR>因为在<SPAN 
class=em2>SL</SPAN>上允许的错误消息比指定比特率的数字设备所能允许的要少得多。有多少物理链路</NOBR></SPAN> <SPAN 
class=ps35><NOBR>连接到<SPAN class=em2>STP</SPAN>上就有多少<SPAN 
class=em2>MTP2</SPAN>层。<SPAN class=em2>STP</SPAN>之间交换的消息叫做信令单元（<SPAN 
class=em2>signaling unit</SPAN>，<SPAN class=em2>SU</SPAN>）。有</NOBR></SPAN> <SPAN 
class=ps36><NOBR>三种类型的<SPAN class=em2>SU<SPAN class=em1>，</SPAN></SPAN>它们都由<SPAN 
class=em2>HDLC</SPAN>标志（<SPAN class=em2>X ’ 7 E&nbsp;’</SPAN></NOBR></SPAN> 
<SPAN class=ps37><NOBR>）分隔，并使用填充比特：</NOBR></SPAN> <SPAN class=ps38><NOBR><SPAN 
class=ft3>1) <SPAN class=em1>消息信令单元（<SPAN class=em2>Message Signaling 
Unit</SPAN>，<SPAN class=em2>MSU</SPAN>），包含</SPAN>SSP<SPAN 
class=em1>产生的信令消息。</SPAN></SPAN></NOBR></SPAN> <SPAN class=ps39><NOBR><SPAN 
class=ft3>2) <SPAN class=em1>链路状态信令单元（<SPAN class=em2>Link Status Signaling 
Unit</SPAN>，<SPAN class=em2>LSSU</SPAN>），由</SPAN>MTP2<SPAN 
class=em1>产生，并提供与</SPAN>STP</SPAN></NOBR></SPAN> <SPAN 
class=ps40><NOBR>相连的链路的健康及状态的实时信息。</NOBR></SPAN> <SPAN class=ps41><NOBR><SPAN 
class=ft4>SCP<SPAN class=em5>经常和数据库同义。</SPAN></SPAN></NOBR></SPAN> <SPAN 
class=ps42><NOBR><SPAN class=ft5>数据库在<SPAN class=em6>SCP</SPAN>或者集成的<SPAN 
class=em6>STP</SPAN>中。如果<SPAN class=em6>STP</SPAN>可以访问这些信息，那么就不需要访问<SPAN 
class=em6>SCP</SPAN>。</SPAN></NOBR></SPAN> </BODY></HTML>
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