The V.23 standard is an interface standard for modems that operates at different speeds in the outward and return channels.
V.23 is a relatively old standard. It originates from the 60s and was developed at that time for the retrieval of online services. V.23 was used at T-Online; the transmission from the Datex-J computer to the user was at 1,200 bit/s, in the reverse direction at 75 bit/s. V.23 uses frequency shift ke ying (FSK) as the modulation method.