V.26bis

V.26bis is comparable to V.26 as a transmission standard. The difference is that V.26 operates over four- wire leased lines, whereas V.26bis transmits over two-wire lines and the public telephone network. V.26bis modems operate at 2,400 bit/s and are equipped with a fallback to 1,200 bit/s. V.26bis uses phase difference modulation( PDM) as the transmission method.

Since V.26bis is relatively slow, it is hardly used any more.