<p>信令消息有三种类型的单元,即MSU Message Signaling Unit消息信令单元,FISU Filled In Signaling Unit填充信令单元,LSSU Link Status Signaling Unit链路状态信令单元。在MSU里有SIF Signaling Information Field字段,这个字段里有路由标记用作信令网络路由。该标记由SLS Signaling Link Selector,DPC Destination Point Code和OPC Origination Point Code组成。其中SLS为4 BITS长度,DPC和OPC国内一般用24 BITS长度,国际一般用14 BITS长度。SLS的字节长度决定了两个信令点之间的信令链路不能超过2^4 = 16条。如果超过16条,可以采用虚拟多信令点或采用2M HSL (High Signaling Link)的方式满足业务增长的需求。关于64KBPS和2MBPS信令链路,在ITU-T G.703,G.704里有详细说明。I really agree with the opinion by 18th floor in some extent. If we just consider the signaling volume(throughput),2Mbps equals to 32 64kbps LSL (Low Signaling Link). But coz it's high speed signaling link, so in order to ensure error control, a redundancy of 50% is usually adopted. That's to say, 16X64kbps LSL.</p>