The numerous Modems offered on the market differ mainly in their transmission modes (asynchronous and synchronous) and transmission rates. Asynchronous transmission uses characters as the basic data unit. Each transmitted character is assigned a supplementary start bit and one or two stop bits. In synchronous transmissions all of the data bits are sent in a continuous sequential stream. Synchronisation is achieved via separate clocking lines.
However, given that synchronous Modems require the corresponding synchronous connections with the appropriate protocols, asynchronous Modems are normally used. When large amounts of data are to be transmitted, a modem can never be too fast. The higher cost of buying a high-performance modem is quickly returned thanks to the lower operating costs.

The V.34+ transfer standard, which specifies a gross transfer rate of 33,600 bit/s, still has not reached the upper capacity limits of traditional telephone wires though experts had previously predicted this would happen with 28,800 bit/s. Meanwhile, the new 56000 bit/s speed category has been standardised as V.90.
Employing V.90 allows a download transfer rate of up to 56 kbit/s. A prerequisite for this purpose is a so-called 56k host. The higher transfer rates are achieved by dispensing with the analogue conversion of the signal between the 56k host and the digital exchange. Only the user's connection continues to be analogue in nature. The V.90 standard includes components of its precursors X2 (US Robotics/3Com) and K56flex (Rockwell/Lu cent/Conexant) and is forward compatible to these proprietary technologies.
Older, manufacturer-specific protocols such as PEP, HST, V.32terbo and V.Fast are no longer important, since the transfer rate of the V.34+ standard surpasses those of them all.

The unit employed to define a device's transmission rate is the number of bits per second (bit/s). The baud unit indicates the signalling rate over the transmission route. As more recent transmission methods encode information not only in terms of frequency but also using amplitude and phase, several bits can be transferred per baud. The ITU-TSS (previously CCITT) V guidelines have acquired the status of an international standard. The Bell standards used in the US are no longer relevant in Europe. Please refer to the table for a summary of the standards.