V.26
V.26 is an older ITU standard for synchronous transmission for modems with 2,400 bit/s full duplex over leased four- wire lines.
With V.26, two bits are combined to form the dibits 00, 01, 11, 10 and transmitted in differential phase shift ke ying( DPSK) with quaternary signals that differ in phase differences of 45°, 135°, 225° and 315°.