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X2
- X2 is a transmission
scheme developed by UsRobotics (3COM) that is capable of achieving line
speeds up to 56 Kbps. This is done by schemes such as V.42 bis, to
further increase throughput. See our modem
standard page for additional information about X2 and other
technologies.
X.25
- A CCITT X series
standard for packet-switching protocol on local-area networks (LANs). It
is used by all types of computers from mainframes to minicomputers.
X.400
- A CCITT X series
standard and the universal E-mail protocol, which translates E-mail
messages, allowing them to be transmitted between different systems
regardless of which program was used to create and / or send the E-mail
message. Unfortunately does not include the sender's name in the message.
X.500
- A CCITT X series
standard that builds upon the standardization of the X.400, extending the
protocol to include address formats.
X-axis
- The horizontal
reference line found on a plane (which can be a grid, chart, or graph) is
designed to measure the placement of a point in a relation to the X-axis,
Y-axis (vertical plane), and Z-axis (third dimensional plane).
X-height
- The height of the
lowercase letter x. The x-height represents only the height of the body
of a lowercase letter; it doesn't include descends (such as the
"tail" on the letter y), or ascenders (such as the upper
"stem" on the letter h). Two fonts can be the same point size,
yet appear to be different in size because they have different X-heights.
In desktop publishing this can be one factor to consider when choosing a
font.
X-Windows
- Windowing and
graphics system developed at the Massachusetts Institute of Technology
for Unix workstations. In the X-Windows system, graphical user interfaces
are created independent of computer hardware. To get into X-Windows you
must type startx.
XENIX
- Microsoft Corp's version of the Unix operation system that runs on PCs.
Xerox
network services - Now
obsolete communications protocol used in early Ethernet networks that has
been replaced with Transport Control Protocol / Interface Program
(TCP/IP).
Xerox
PARC - Refer to Xerox
Palo Alto Research Center.
Xerox
Palo Alto Research Center - The
site of vital computer-related research center in California beginning in
the 1970s and continuing today.
XFCN
- Refer to external
function.
XGA
- Refer to Extended
Graphics Array.
.XLS
file - File extension
usually associated with Microsoft Excel.
Xmodem
- A popular
file-transfer protocol developed in 1977 by Ward Christansen. The
protocol works by sending blocks of data in 128-byte blocks from PC to
PC. Included with this data is an error-detection system called a
checksum. When the data is received, the error detection system ensures
that the entire message reached its destination. If not, the receiving
computer sends a request for retransmission of the data.
Xmodem
1k - More powerful
version of Xmodem that allows larger files to be sent greater distances
with the same reliable error checking. Information is transmitted in 1KB
blocks as opposed to Xmodem, which transmits information in 128-byte
blocks.
Xmodem-CRC
- The Xmodem protocol
with an additional cycle redundancy check (CRC).
XMS
- Refer to Extended
Memory Specification.
XMT
- Refer to transmit.
XON/XOFF
- XON and XOFF are
control characters used in data transmission. XOFF is used by a receiving
PC to tell the transmitting PC to stop transmitting. When it is all right
for the PC to recommence, the XON character is sent.
XOR
operator - A Boolean
operation operation, often used in database searches, that returns a
value of true only if its operands have different values (one is true and
one is false). Also called the exclusive OR.
XT - Refer to crosstalk, extended technology.
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