ITU-T U.203-1993
在国际用户电报业务的终端与PSPDN上的、或经PSTN的数据终端设备之间提供实时双向通信
Technical Requirements to be Met When Providing Real-Time Bothway Communications between Terminals of the International Telex Service and Data Terminal Equipments on a PSPDN or Via the PSTN - Telegraph Switching International Telex Service Study Group IX;