V.27 is a transmission standard for modems. V.27ter works with a transmission speed of 4,800 bit/s on telephone leased lines with manually adjustable equalizer. Three bits are combined to form the tribits 001, 000, 010, 011, 111, 110, 100 and 101 and are rescanned and transmitted as 8- phase Differential PhaseShift Keying( DPSK) with octonary signals in the phases 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°.
Like V. 22, the V. 27ter method works with phase modulation and with doubled step rate, which increases the transmission rate to max. 9.6 kbit/s. In contrast to V.22, V.27ter can only operate in half- duplex mode, since the entire bandwidth is occupied with V.27ter. V.27ter is used in telefax, where the receiver only briefly acknowledges the received transmission. If the high transmission rate cannot be maintained due to interference, the modem switches to a lower transmission rate of 7.2 kbit/s or 4.8 kbit/s via Automatic Fallback( AFB).