(上接第 2 1期 )2　ＷＡＮ2 .1　ＰＳＴＮＰＳＴＮ(公用电话网 )是公用通信网络中的基础网 ,通信区域覆盖全国 ,利用电话网进行远程通信是投资少、见效快、实现大范围数字通信最便捷的方法。利用现有的电话网 ,只要在ＰＣ或终端设备进入电话网的前端装入Ｍｄ(Ｍｏｄｅｍ ,调制 (上上 第 21 期) 2 WAN2 .1 PSTNPSTN (public switched telephone network) is the basic network in the public communications network, the communication area covering the whole country, the use of telephone network for long-distance communications is less investment, quick, to achieve a wide range of digital communications The most convenient way. Use of existing telephone network, as long as the PC or terminal equipment into the front of the telephone network into Md (Modem, modulation