Xerox Network Services:-

Abbreviated XNS. A multilayer communications protocol, first developed by Xerox, and later used by Novell and other network software suppliers. XNS also supports a distributed file system that lets users access other computer files and printers as if they were local.

Xmodem:-

A popular file transfer protocol available in many off-the-shelf and shareware communications packages, as well as on many bulletin board systems. Xmodem was originally developed by Ward Christiansen for early PCs using the CP/M operating system.

Xmodem divides the data for a transmission into blocks. Each block consists of the start-of-header character, a block number, 128 bytes of data, and a check-sum. An. acknowledgement byte is returned to the sender if the checksum calculation is identical to the sender's checksum; however, this requirement to acknowledge every transmitted block can lead to poor performance.

An extension to Xmodem, called Xmodem-CRC, adds a more stringent error-checking method by using a cyclical redundancy check (CRC) to detect transmission errors rather then Xmodem's simple additive checksum. Another variation' is Xmodem-IK, which transfers data in 1,024 byte blocks.

XON/XOPF:-

In asynchronous transmissions between two PCs, a simple method of flow control. The receiving PC sends an XOFF control character (ASCII 19, Ctrl-®) to  pause the transmission of data when the receive buffer is full, and then sends an XON character (ASCII 17, Ctrl-Q) when it is  ready to continue the transmission.