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"X" Networking Definitions & Concepts...

X.21 .. to .. XON/XOFF

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X.21 is the physical layer interface standand for the X.25 protocol. Another version of the standard, X.21 bis, is nearly identical with EIA-232-D and is more commonly used.


X.25 is a set of recommendations defined by the CCITT (Consultative Committee for International Telegraphy and Telephony) for transmitting data over a packet-switched network. It provides a CCITT-standard interface to packet-switched networks and has become the most widely used interface for wide-area networks (WANs).

This interface encompasses the three lower layers in the OSI Reference Model. At the physical layer, the X.25 standard assumes an X.21 interface, but can also support V.35 and EIA RS232-D interfaces. At the data-link layer, X.25 assumes LAPB (Link Access Protocol, Balanced) is being used but also supports other protocols, such as the older LAP (Link Access Procedure (or Protocol)) and IBM's Bisync (BSC) protocol. At the network layer, X.25 uses PLP (Packet-Level Protocol).

X.25 is suitable for data (but not voice) transmissions. It defines procedures for exchanging data between a DTE (Data Terminal Equipment), such as a computer, and the network. The connection to this network is represented by a DTE, which may be a modem, multiplexer, or PAD (packet assembler/disassembler). Asynchronous devices (such as a PC) can be connected to the X.25 network through the use of a PAD.

X.25 uses LCNs (logical channel numbers) to distinguish the connections between DTEs at either end of a communication. These LCNs make it possible to send a packet into a packet-switched network at one end (with no control over the packet's journey) and then to pick the packet out at the receiving end.

The interface supports transmission speeds of up to 64 kilobits per second (kbps). The 1992 revision of the X.25 recommendations has increased the throughput to 2 megabits per second (Mbps), but this faster X.25 is not yet widely used. X.25 also has a relatively high overhead for error checking and packet sequencing.

X.25 does not specify how a packet should be shipped across the network. In fact, X.25 has nothing at all to say about the details of the network transmissions. The WAN itself is represented as a network "cloud" (an assumed connection). X.25 is responsible for getting packets into that cloud at one end and for retrieving them at the other end.

Basically, X.25 is a standard, well-tested, and often revised protocol that has been a workhorse packet-switching service since 1976. It is suitable for light loads and was commonly used to provide remote terminal connections to mainframe systems. X.25 packet-switched networks are not suitable for most LAN-to-LAN traffic because they are slow and require a large protion of the bandwidth to handle error checking. This error checking was important in the days of low-quality analog telephone lines, but is not needed today. Frame Relay provides a better choice.

X.25 was the first protocol issued defining packet switching. Access speeds range upto 56kbps. Trunks between network nodes are limited to 56/64 kbps (with the capability for fractional and full T1 speeds under proprietary implementations). X.25 contains the error detection and correction and flowcontrol needed for the older analog transport networks of the 1980s. But much of this overhead and the processor-intensive operations are not needed in today's fiber-optic networks environment. X.25 Packet switching is solely a connectionless service using connection-oriented virtual circuits, and is good for time-insensitive data transmission but poor for connection-oriented and time-sensitive voice and video. Packet switches pass data through the network, node to node, employing a queuing scheme for buffering and transmitting data. Data is received and passed if bandwidth is available. If it is not, data is stored in the queue until bandwidth is available (and to the extent of the memory buffer). The end nodes are responsible for error detection and correction and to initiate error recovery. The newer fast packet services do not perform this queuing, rather they drop the extra traffic that cannot be transmitted over a congested network and force the appliction to do the error checking at higher layer. Congestion is one parameter that has not yet been effectively solved in these new services.

X.25 allows numerous virtual circuits on the same physical path, and can transport packet sizes up the 4,096 bytes. Both Permanent Virtual Circuits (PVCs) and Switched Virtual Circuits (SVCs) are support in X.25, and the addressing scheme allows any user to send or recieve data from any other user. Traffic can also be prioritized.

X.25 Packet Switching Compared to Circuit Switching:

Packet switching is much different from circuit switching. It allows multiple users to share data-network facilities and bandwidth, rather than providing specific amounts of dedicated bandwidth to each user. The traffic passed by packet-switched networks is "bursty" in nature, and therefore can be aggregated statistically to maximize the use of on-demand bandwidth resources. While there is much more overhead associated with packet switching as compared to circuit-switching, X.25 error checking and correcting overhead guarantees error-free delivery by the use of addressed packets that transit the network. Due to the connectionless characteristic of packet switching in contradistinction to connection-oriented circuit switching, the intelligence of the network nodes will route packets around failed links, whereas in circuit switching the entire circuit would need to be switched, leading to service interruption.


X.400 is a message handling standard defined by the CCITT (Consultative Committee for International Telegraphy and Telephony). X.400 has been through two major versions and a revision:

  • The original 1984 draft, referred to as X.400/84, provides the basic definitions and model. This version has been implemented for years. Unfortunately, the model has major shortcomings.
  • A 1988 version, referred to as X.400/88, addresses most of the major flaws in the 1984 draft, but is not yet widely implemented.

A round of revisions in 1992 addressed additional flaws and ambiguities, and also defined two new types of message contents:

  • EDI (Electronic Data Interchange) messages for use in business transactions and record keeping,
  • and voice messages.

The notion of a Message Handling System (MHS) figures prominently in both versions, but the details of an MHS are somewhat different. Similarly, both versions include a Message Transfer Service (MTS) as an MHS component, but the contents of this MTS differ.

The X.400/84 version dealt only with MHS interfaces for end users. In X.400/88, an MHS is an object that has interfaces for communicating with end users, with other CCITT and special services, and possibly with other networks.

The X.400 recommendations series addresses the contents and workings of the MHS and the manner in which the MHS communicates with outside entities. The documents say nothing about how to implement these recommendations.

xB/tB Encoding:

xB/tB encoding is a general label for any of several data-translation schemes that can serve as a preliminary to signal encoding in telecommunications or networking contexts.

In xB/tB, every group of x bits is represented as a y-bit symbol. This symbol is associated with a bit pattern that is then encoded using a standard signal encoding method (usually NRZI).

The following are commonly used translation schemes of this sort:

  • 4B/5B, used in FDDI networks,
  • 5B/6B, used in the 100BaseVG fast Ethernet standard proposed by Hewlett-Packard,
  • 8B/10B, used in SNA (Systems Network Administration) networks.
XML (Extensible Markup Language):

XML is a markup language for documents containing structured information.

Structured information contains both content (words, pictures, etc.) and some indication of what role that content plays (for example, content in a section heading has a different meaning from content in a footnote, which means something different than content in a figure caption or content in a database table, etc.). Almost all documents have some structure.

A markup language is a mechanism to identify structures in a document. The XML specification defines a standard way to add markup to documents.

What is XML?

  • XML stands for EXtensible Markup Language
  • XML is a markup language much like HTML
  • XML was designed to describe data
  • XML tags are not predefined. You must define your own tags
  • XML uses a Document Type Definition (DTD) or an XML Schema to describe the data
  • XML with a DTD or XML Schema is designed to be self-descriptive
  • XML is a W3C Recommendation

XML is a W3C Recommendation

The Extensible Markup Language (XML) became a W3C Recommendation 10. February 1998.

The Main Difference Between XML and HTML

XML was designed to carry data.

  • XML is not a replacement for HTML.
  • XML and HTML were designed with different goals:
  • XML was designed to describe data and to focus on what data is.
  • HTML was designed to display data and to focus on how data looks.
  • HTML is about displaying information, while XML is about describing information.
XML Schema:

XML Schema is an XML-based alternative to DTDs.

An XML Schema describes the structure of an XML document.

The XML Schema language is also referred to as XML Schema Definition (XSD).

In this tutorial you will learn how to read and create XML Schemas, why XML Schemas are more powerful than DTDs, and how to use the XML Schema language in your application.

What is an XML Schema?

The purpose of an XML Schema is to define the legal building blocks of an XML document, just like a DTD.

An XML Schema:

  • defines elements that can appear in a document
  • defines attributes that can appear in a document
  • defines which elements are child elements
  • defines the order of child elements
  • defines the number of child elements
  • defines whether an element is empty or can include text
  • defines data types for elements and attributes
  • defines default and fixed values for elements and attributes

Xmodem is a popular file transfer protocol available in many off-the-self and shareware communications packages, as well as on many bulletin board systems (BBSs).

Xmodem divides the data for the transmission into blocks. Each block consists of the start-of-header character, a block number, 128 bytes of data, and a checksum.

An extension to Xmodem, called Xmodem-CRC, adds a more stringent error-checking method by using a cyclical redundancy check (CRC) to detect transmission errors.


In asynchronous communications, characters used to control the flow of data. The XOFF (ASCII 19, or Ctrl-S) tells the sender to stop transmitting until further notice; the XON (ASCII 17, or Ctrl-Q) tells the sender to resume transmission after an XOFF.


Short for Extensible Style Language Transformation (XSLT), the language used in XSL style sheets to transform XML documents into other XML documents. It is one of two parts of the XSL specification and is a language for transforming XML documents (actually the transformation part, T stands for transformation).

XSLT is a XML transformation language, which transforms documents in XML format. To transform in this context means to take all data or part of it (Query of a selection with XPath) and create another XML document or a document in a format which can directly be used for displaying or printing (e.g. an HTML, RTF or TeX document). In particular the transformations involve:

  • adding constant text like HTML document type and header information
  • moving text
  • sorting text

An XML document is a tree on which the transformations are applied. The language is declarative, i.e. a program consist of a collection of several rules which transformations should be performed. The rules are applied recursively.

An XSL processor reads the XML document and follows the instructions in the XSL style sheet, then it outputs a new XML document or XML-document fragment.

The XSLT processor checks which rules can be applied and executes the associated transformations based on a sequence of priorities.

You can use XSLT in combination with CSS to produce HTML documents.

This is extremely useful in e-commerce, where the same data need to be converted into different representations of XML. Not all companies use the exact same programs, applications and computer systems.

An XSLT program is an XML document as the following template shows

 <?xml version="1.0" ?>
 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">


XSLT Recommendation was written and developed by the XSL Working Group and became ratified by the W3C on November 16, 1999 as XSLT 1.0.

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