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X
X Modem
The X
modem protocol is a data communications file transfer protocol developed
for transmitting files between PCs.
X Series Recommendations
The X
Series Recommendations are the ITU-T standards for the DTE to DCE
interface. Examples of the X Series are X.25, X.75, X.28 and X.29.
X.25
An X.25 network,
whether public or private, is typically built largely upon the facilities
of the public telephone networks, in other words, leased lines. In the United States,
X.25 is offered by most carriers and VARs. These include AT&T, Sprint,
CompuServe, Ameritech, Pacific Bell and others.
Several
concepts are important for understanding X.25 protocols and networks. These
include:
- physical circuit: the physical hardware and
cabling used to connect devices together
- virtual circuit: the complete path between
two communicating devices
- logical channel: the logical connection
between the user node and the network
X.25
has been a long-time standard for packet switching. The X.25 interface lies
at OSI layer 3, rather than at layer 1. X.25 defines a protocol stack
having three layers.
X.500
X.500 is an OSI
(Open System Information) based directory services protocol designed by the
CCITT (International Telegraph and Telephone Consultative Committee). X.500
provides distributed directory services to network users. The X.500
directory specifies a model for connecting directory services to form one
distributed global directory. Each directory service holds a part of the
global database and the directory information is made available via a
server (called a Directory System Agent — DSA). The database is
maintained locally. From the user point of view, the entire directory is
accessible from the local server.
X.75
X.75 is a
standard that defines the interface between two distinct X.25 networks and
is nearly identical to X.25.
X-ON/X-OFF
X-ON/X-OFF is a
flow control mechanism used for asynchronous communication. It is used when
the receiving device is not capable of receiving data as fast as the
sending device can send it. When this occurs the X-OFF signal is sent to
notify the sender to stop transmitting data until the receiver can
"catch up." When the receiver does catch up the X-ON signal is sent.
This technique is also called pacing. Most implementations use the ASCII
DC1 code for X-ON and the ASCII DC3 code for X-OFF.
X Window
The X Window
System uses the X Window protocol on TCP to draw windows on a workstation's
bitmap display. X Window is much more than a utility for drawing windows;
it is entire philosophy for designing a user interface.
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