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All communications between devices require that the devices agree on the format of the data. The set of rules defining a format is called a protocol. At the very least, a communications protocol must define the following: In addition, protocols can include sophisticated techniques for detecting and recovering from transmission errors and for encoding and decoding data. The table lists the most commonly used protocols for communications via modems. These protocols are almost always implemented in the hardware; that is, they are built into modems. In addition to the standard protocols listed in the table, there are a number of protocols that complement these standards by adding additional functions such as file transfer capability, error detection and recovery, and data compression. The best-known are Xmodem, Kermit, MNP, and CCITT V.42 . These protocols can be implemented either in hardware or software.
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