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X.

ITU (CCITT) standarder

X.21

Circuit-switching dedicated line.
Protokol til kommunikation i et kredsløbkoblet netværk. = Datex.

 

X.25

(= Datapak) hardware protocol suite for synchronous transmission.
International standard for packet switching network.
56kbps or multiplexed for higher data rates. NT may use X.25 directly with a Smart Card - or inderectly with a PAD (Peripheral Access Device).[NWNT.373]

The X.25 protocol, adopted as a standard by the CCITT, is a commonly-used network protocol. The X.25 protocol allows computers on different public networks (such as CompuServe, Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network layer level. X.25's protocols correspond closely to the data-link and physical-layer protocols defined in the Open Systems Interconnection (OSI) communication model.

X.25 is a service provided by the phone company to accept and route individual data packets. To make it work, the phone company must have the kind of computer equipment that US antitrust regulators would not allow AT& T to offer. A German company delivers a packet of data to the German phone company. The national phone companies now hand it off through France, Spain, and Portugal. Finally, it is delivered to the other computer. The phone companies charge by the packet.
The link between a computer and the phone company in X.25 is also based on HDLC. However, it is a somewhat more complex version of the option set called LAP-B (Link Access Protocol - Balanced). The term "Balanced" contrasts to "Normal Response Mode" and indicates that the two ends are free to send data at any time. Errors will still occur, and they have to be corrected. The process is slightly more complicated when both ends are transmitting at the same time

> frame relay, ISDN, QLLC

X.400

Global e-post standard. Bruges i DK især af offentlige institutioner. Benytter message-switching.
>SMTP
 
X.400 is the messaging (notably e-mail) standard specified by the ITU-TS. It's an alternative to the more prevalent e-mail protocol, SMTP. X.400 is common in Europe and Canada. It's actually a set of standards, each in the 400-number range.
Because X.400 stipulates a number of possible address characteristics that SMTP does not, an X.400 address can be long and cumbersome. On the other hand, X.400 adherents note that it is an official standard whereas SMTP is a "de facto" standard. Thus, products with X.400 implementations can be tested more rigorously than products with SMTP implementations can. X.400 offers more capabilities than SMTP does. However, many of these capabilities are seldom used.
An SMTP e-mail address that looks like this hypothetical address:

    georg.hansen@delab.sintef.no

might look like this in an X.400 e-mail message:

    G=Georg; S=Hansen; O=sintef; OU=delab; PRMD=uninett; ADMD=uninett; C=no

X.400 is a complex standard that is difficult to summarize here. The arguments in favor of and against each of the opposing e-mail standards are also complicated.
> VANS

 

X.500 Directory Service

Fortegnelse over brugere på X.400.
 
X.500 Directory Service is a standard way to develop an electronic directory of people in an organization so that it can be part of a global directory available to anyone in the world with Internet access. Such a directory is sometimes called a global White Pages directory. The idea is to be able to look up people in a user-friendly way by name, department, or organization. Many enterprises and institutions have created an X.500 directory. Because these directories are organized as part of a single global directory, you can search for hundreds of thousands of people from a single place on the World Wide Web.
The X.500 directory is organized under a common "root" directory in a "tree" hierarchy of: country, organization, organizational unit, and person. An entry at each of these levels must have certain attributes; some can have optional ones established locally. Each organization can implement a directory in its own way as long as it adheres to the basic schema or plan. The distributed global directory works through a registration process and one or more central places that manage many directories.
Providing an X.500 directory allows an organization to make itself and selected members known on the Internet. Two of the largest directory service providers are InterNIC, the organization that supervises domain name registration in the U.S., and ESnet, which maintains X.500 data for all the U.S. national laboratories. ESNet and similar providers also provide access to looking up names in the global directory, using a number of different user interfaces including designated Web sites, whois, and finger. These organizations also provide assistance to organizations that are creating their own Directory Information Tree (DIT).
In X.500, each local directory is called a Directory System Agent (DSA). A DSA can represent one organization or a group of organizations. The DSAs are interconnected from the Directory Information Tree (DIT). The user interface program for access to one or more DSAs is a Directory User Agent (DUA). DUAs include whois, finger, and programs that offer a graphical user interface. X.500 is implemented as part of the Distributed Computing Environment (DCE) in its Global Directory Service (GDS). The University of Michigan is one of a number of universities that use X.500 as a way to route e-mail as well as to provide name lookup, using the Lightweight Directory Access Protocol (LDAP).

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X-Web

> Broadway, X-Windows

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X-Windows

The X Window System is a graphics system primarily used on Unix systems (and, less commonly on MS-Windows systems) that provides an inherently client/server oriented base for displaying windowed graphics. It provides a public protocol by which client programs can query and update information on X servers.

> Broadway

The X Window System

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xDSL

> DSL

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XENIX

A UNIX-derived OS developed by Microsoft for the PC AT

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XHTML

[w3.org]: The Extensible HyperText Markup Language (XHTML™) is a family of current and future document types and modules that reproduce, subset, and extend HTML, reformulated in XML. XHTML Family document types are all XML-based, and ultimately are designed to work in conjunction with XML-based user agents. XHTML is the successor of HTML, and a series of specifications has been developed for XHTML

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XML = eXtensible Markup Language

a subset of SGML, that allows for custom tags to be processed

 

> HTML, SVG, XHTML

www.w3.org/XML/
www.ucc.ie/xml
bombay.npac.syr.edu/fms/xml/index.htm

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Xmodem

protokol for filoverførelse. 128-byte blok + 1-byte checksum

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XMS = eXtended Memory Specification

Standard for brugen af extended memory under DOS
> EMS, HMA

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XNS = Xerox Network System

Xerox protocol suite. Layers:
Appl.: MS-DOS Redirector server
Pres.: SMB
Sess.: Courier, Netbios
Tran.: SPP
Netw.: IDP, PEP

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