Modem speed is rated in bps (bits per second). So a 56k modem transmits at 56,000 bits per second in theory. While 8 bits make a byte, and 1024 bits equal one kilobits, a 56k modem will transmit a maximum of approximately 7 kilobytes of information every second. A modem will never connect higher than what it is rated for.
1200bps ,ran at 4 bits per baud for a total of 2400bps
This allowed the modem to send and receive data at the same time, although it could send data only at 75bps. This standard was developed to lower the cost of modems. A 1200 bps modem was very expensive at the time.
A half-duplex standard, meaning one-way. It works at 9600bps. The standard is not laid out well for modems, therefore isn't much used. The protocol is most often used for fax machines
K56Flex - for 56k communication, created by Lucent and Rockwell.
X2 - for 56k communication, created by US Robotics (3com).
V.90 - This is the International Standard for 56k modems.
V.92 - This is an ITU (international Telecommunication Union) standard for 56K modems, but three new features have been added -- faster connections (on 50%), better uploading, and data and voice support: