The ISDN network layer is defined in ITU-T Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These standards specify user-to-user, circuit-switched, and packet-switched connections.是什么意思_The ISDN network layer is defined in ITU-T Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These standards specify user-to-user, circuit-switched, and packet-switched connections.在线翻译_英语_读音_用法_例句