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X.21
X.21 is an ITU (International Telegraphic Union) standard governing the interface between DTE and DCE for synchronous
operation on public data networks.
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X.25
XX.25 is an ITU (International Telegraphic Union) standard governing the interface between DTE and DCE for terminals
operating in the packet mode on public data networks.
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X.25 PAD
X.25 PAD (Packet Assembler-Disassembler) is a device that permits communication between non-X.25 devices and the
devices in an X.25 network.
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xDSL
xDSL is a term that encompasses a broad range of DSL (Digital Subscriber Line) services. These DSL-based services,
which operate over existing phone lines, provide users with speeds significantly faster than those of 56-kbps analog
modems. A major draw for customers is that they pay only for the bandwidth they use. Telephone companies like DSL
because they don't have to invest in costly rewiring work to offer broadband services.
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XML (Extensible Markup Language)
XML follows HTML,providing more flexibility and ease
of use.
XML is the `Extensible Markup Language' (extensible because it is not a fixed format like HTML). It is designed
to enable the use of SGML on the World Wide Web. SGML is the Standard Generalized Markup Language (ISO 8879), the
international standard for defining descriptions of the structure and content of different types of electronic
document.
XML is not a single, predefined markup language: it's a metalanguage -- a language for describing other languages
-- which lets you design your own markup. (A predefined markup language like HTML defines a way to describe information
in one specific class of documents only: XML lets you define your own customized markup languages for limitless
different classes of document, see the question on creating your own DTD.) It can do this because it's written
in SGML, the international standard metalanguage for text markup systems.
XML is designed ‘to make it easy and straightforward to use SGML on the Web: easy to define document types, easy
to author and manage SGML-defined documents, and easy to transmit and share them across the Web.’
It defines ‘an extremely simple dialect of SGML which is completely described in the XML Specification. The goal
is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with
HTML.’ For this reason, XML has been designed for ease of implementation, and for interoperability with both SGML
and HTML.
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X-ON/X-OFF (Transmitter On/Transmitter Off)
X-ON/X-OFF are control characters used for flow control that instructs a terminal to start transmission (X-ON)
and end transmission (X-OFF).
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