The process of setting up a data transfer over a PSTN line by an operator at one data station dialing the telephone number of a line access unit at another data station and switching from voice to data when an answer tone is received.; 一個(gè)數(shù)據(jù)站的操作員通過(guò)撥另一個(gè)數(shù)據(jù)站線路訪問(wèn)單元的電話號(hào)碼,當(dāng)收到應(yīng)答單 音時(shí)從語(yǔ)音變換為數(shù)據(jù),在公用交換電話網(wǎng)(PSTN)線路上建立數(shù)據(jù)傳送的過(guò)程。