包含两篇关于FXO、FXS的文档，详细介绍了他们的区别和定义，是开发VOIP设备的很好的参考材料。这种材料可是很难找哦。-contains two of FXO, FXS document, detailing their differences and definitions, the development of VoIP equipment is a good reference material. But this material is difficult to find, oh.