| Term |
Definition |
| X.21 |
Recommendations developed by CCITT that define a protocol for communication between user devices and a circuit-switched network. |
| X.25 data protocol |
A packet switching standard developed in the mid-1970s for transmission of data over twisted pair copper wire. |
| X.400 |
International standard for a store-and-forward message handling system in a multivendor environment. |
| XA |
eXtended Architecture. |
| XCA |
eXternal Communication Adapter. |
| xDSL |
Digital subscriber lines that allow simultaneous two-way communication of voice and data over regular phone lines. |
| XMT |
TransMiT. |
| X-on X-off Protocol |
An extension of the TTY protocol which uses additional Transmission On (X-on) and Transmission Off (X-off) characters to accommodate differences in speed between the transmitter and the receiver. |
| XPC |
A set of protocols developed by Tymnet to allow asynchronous terminals to connect to and use their X.25 packet switched network. |
| XTAL |
crysTAL. |
| XTC |
Fiber optic connector developed by OFTI |
