The SIP Trunk provides a virtual SIP relay which is similar to the traditional PSTN relay to enable the direct interconnection between the internal IP-PBX of enterprises and the external INTERNET IP phone operators .

SIP Trunk提供了一种类似于传统PSTN中继的虚拟SIP 中继来使企业IP-PBX与外部的INTERNET电话运营商之间通过IP进行直接互连。

Therefore a good way to calculate the intermodulation noise in practical transmission systems and to analyze effects of the transmission characteristics in trunk on the FM multiplex telephony signal of radio relay systems are provided .

从而为应用该理论公式，计算传输 线路的串噪声值和分析 通道一般传输特性对调频多路电话信号的影响找到了一条有效的途径。