backbone network

backbone cabal


A group of large-site administrators who pushed through the Great Renaming and reined in the chaos of Usenet during most of the 1980s. The cabal mailing list disbanded in late 1988 after a bitter internal cat-fight.

[Jargon File]

Last updated: 1994-11-28

backbone network


In a hierarchical network, a top-level network that carries network traffic between the mid-level networks and stub networks that connect to it.

The largest backbone network is the Internet backbone.

Last updated: 2017-12-02

backbone site

<networking, history>

A key Usenet, electronic mail and/or Internet site that processed a large amount of third-party traffic, especially home sites of any of the regional coordinators for the Usenet maps. Notable backbone sites as of early 1993 included uunet and the mail machines at Rutgers University, UC Berkeley, DEC's Western Research Laboratories, Ohio State University and the University of Texas.

Compare rib site, leaf site.

[Jargon File]

Last updated: 1994-11-28

back door


(Or "trap door", "wormhole"). A hole in the security of a system deliberately left in place by designers or maintainers. The motivation for such holes is not always sinister; some operating systems, for example, come out of the box with privileged accounts intended for use by field service technicians or the vendor's maintenance programmers. See also iron box, cracker, worm, logic bomb.

Historically, back doors have often lurked in systems longer than anyone expected or planned, and a few have become widely known. The infamous RTM worm of late 1988, for example, used a back door in the BSD Unix "sendmail(8)" utility.

Ken Thompson's 1983 Turing Award lecture to the ACM revealed the existence of a back door in early Unix versions that may have qualified as the most fiendishly clever security hack of all time. The C compiler contained code that would recognise when the "login" command was being recompiled and insert some code recognizing a password chosen by Thompson, giving him entry to the system whether or not an account had been created for him.

Normally such a back door could be removed by removing it from the source code for the compiler and recompiling the compiler. But to recompile the compiler, you have to *use* the compiler - so Thompson also arranged that the compiler would *recognise when it was compiling a version of itself*, and insert into the recompiled compiler the code to insert into the recompiled "login" the code to allow Thompson entry - and, of course, the code to recognise itself and do the whole thing again the next time around! And having done this once, he was then able to recompile the compiler from the original sources; the hack perpetuated itself invisibly, leaving the back door in place and active but with no trace in the sources.

The talk that revealed this truly moby hack was published as ["Reflections on Trusting Trust", "Communications of the ACM 27", 8 (August 1984), pp. 761--763].

[Jargon File]

Last updated: 1995-04-25



Any software performing either the final stage in a process, or a task not apparent to the user. A common usage is in a compiler. A compiler's back-end generates machine language and performs optimisations specific to the machine's architecture.

The term can also be used in the context of network applications. E.g. "The back-end of the system handles socket protocols".

Contrast front end.

Last updated: 1996-04-09

Back End Generator


(BEG) A code generator developed by H. Emmelmann et al at GMD, University Karlsruhe, Germany. Its input language is Back End Generator Language (BEGL).

["BEG - A Generator for Efficient Back Ends", H. Emmelmann et al, SIGPLAN Notices 24(7):227-237 (Jul 1989)].

["BEG - A Back End Generator - User Manual", H. Emmelmann, GMD, U Karlsruhe, 1990].


Last updated: 2000-12-16

Back End Generator Language

Back End Generator


<operating system>

1. A task running in the background (a background task) is detached from the terminal where it was started (and often running at a lower priority); opposite of foreground. This means that the task's input and output must be from/to files (or other processes).

Nowadays this term is primarily associated with Unix, but it appears to have been first used in this sense on OS/360.

Compare amp off, batch, slopsucker.


2. For a human to do a task "in the background" is to do it whenever foreground matters are not claiming your undivided attention, and "to background" something means to relegate it to a lower priority. "For now, we'll just print a list of nodes and links; I'm working on the graph-printing problem in the background." Note that this implies ongoing activity but at a reduced level or in spare time, in contrast to mainstream "back burner" (which connotes benign neglect until some future resumption of activity). Some people prefer to use the term for processing that they have queued up for their unconscious minds (often a fruitful tack to take upon encountering an obstacle in creative work).

Last updated: 1996-05-28

backing store


1. Computer memory, usually magnetic disks, storing data and programs. Sections of this information can then be copied into the main memory (RAM) for processing. Backing store is cheaper but RAM is faster. Such a hierarchy of memory devices allows a trade-off between performance and cost.


2. Character storage in memory or on disk, as opposed to displayed or printed characters. This distinction is important where the visual ordering of characters differs from the order in which they are stored, e.g. bidirectional or non-spacing layout.

In a Unicode encoding, text is stored in sequential order in the backing store. Logical or backing store order corresponds to the order in which text is typed on the keyboard (after corrections such as insertions, deletions, and overtyping). A text rendering process converts Unicode text in the backing store to readable text.

["The Unicode Standard: Worldwide Character Encoding", Version 1.0, Vol. 1. Addison-Wesley, 1991].

Last updated: 2001-02-25

back link


A link in one direction implied by the existence of an explicit link in the other direction.

Last updated: 1996-05-28



A host which has experienced a collision on a network waits for a amount of time before attempting to retransmit. A random backoff minimises the probability that the same nodes will collide again, even if they are using the same backoff algorithm. Increasing the backoff period after each collision also helps to prevent repeated collisions, especially when the network is heavily loaded.

An example algorithm is binary exponential backoff.

Last updated: 1996-05-28



A suite of network server software from Microsoft that includes Windows NT Server, BackOffice Server (for the integrated development, deployment, and management of BackOffice applications in departments, branch offices, and medium sized businesses); Exchange Server; Proxy Server; Site Server for intranet publishing, management, and search; Site Server Commerce Edition For comprehensive Internet commerce transactions; Small Business Server for business operations, resource management, and customer relations; SNA Server for the integration of existing and new systems and data; SQL Server for scalable, reliable database and data-warehousing; Systems Management Server (SMS) for centralised change- and configuration-management.

Last updated: 2000-12-16


<hardware, electronics>

A printed circuit board with slots into which other cards are plugged.

A backplane,is typically just a connector and does not usually have many active components on it. This contrasts with a motherboard.

Designing a backplane.

Last updated: 2002-09-08



To make a feature from a later version of a piece of software available in an earlier version. Backporting of features enables users of the older version to benefit from a feature without upgrading fully.

Last updated: 2003-12-18


<artificial intelligence>

(Or "backpropagation") A learning algorithm that modifies a feed-forward neural network so as to minimise a continuous "error function" or "objective function." Back-propagation is a "gradient descent" method of training in that it uses gradient information to modify the network weights to decrease the output error. Other gradient based methods from numerical analysis can be used to train networks more efficiently.

Back-propagation uses a mathematical trick when the network is simulated on a digital computer, yielding, in just two traversals of the network (once forward, and once back), both the difference between the desired and actual output, and the derivatives of this difference with respect to the connection weights.

Last updated: 2020-05-17

back quote


"`" ASCII code 96. Common names: left quote; left single quote; open quote; ITU-T: grave accent; grave. Rare: backprime; INTERCAL: backspark; unapostrophe; birk; blugle; back tick; back glitch; push; ITU-T: opening single quotation mark; quasiquote.

Back quote is used in Unix shells to invoke command substitution.

Last updated: 1996-11-26



(Backward acronym) A word which has been turned into an acronym by inventing an expansion, rather than the other way around. E.g. "ping".

Last updated: 2005-06-22

backside cache

<hardware, processor>

An implementation of secondary cache memory that allows it to be directly accessed by the CPU.

Backside cache is used by Apple Computers, Inc. in their PowerPC G3 processor. Previous PowerPC processors used the system bus to access both secondary cache and main memory. In the PowerPC G3 a dedicated bus handles only CPU/cache transactions. This bus can operate faster than the system bus thus improving the overall performance of the processor.

The term apparently derives from the relocation of the secondary cache from the motherboard to the processor card itself, i.e. on the backside of the processor card.

Last updated: 1998-09-10



"\" ASCII code 92. Common names: escape (from C/Unix); reverse slash; slosh; backslant; backwhack. Rare: bash; ITU-T: reverse slant; reversed virgule; INTERCAL: backslat.

Backslash is used to separate components in MS-DOS pathnames, and to introduce special character sequence in C and Unix strings, e.g. "\n" for newline.

Last updated: 2000-02-21



(BS) ASCII code 8, Control-H. The control character that should cause most output devices to move their current output position back to the previous character so that the next character output will replace (or overprint) it. Inputting a backspace (typically by pressing the backspace key) causes many systems to delete the character before the input cursor, though others use delete for this.

See twirling baton for an imaginitive use of backspace.

Last updated: 2003-10-25


back quote



A scheme for solving a series of sub-problems each of which may have multiple possible solutions and where the solution chosen for one sub-problem may affect the possible solutions of later sub-problems.

To solve the overall problem, we find a solution to the first sub-problem and then attempt to recursively solve the other sub-problems based on this first solution. If we cannot, or we want all possible solutions, we backtrack and try the next possible solution to the first sub-problem and so on. Backtracking terminates when there are no more solutions to the first sub-problem.

This is the algorithm used by logic programming languages such as Prolog to find all possible ways of proving a goal. An optimisation known as "intelligent backtracking" keeps track of the dependencies between sub-problems and only re-solves those which depend on an earlier solution which has changed.

Backtracking is one algorithm which can be used to implement nondeterminism. It is effectively a depth-first search of a problem space.

Last updated: 1995-04-13


<operating system>

("back up" when used as a verb) A spare copy of a file, file system, or other resource for use in the event of failure or loss of the original.

The term commonly refers to a copy of the files on a computer's disks, made periodically and kept on magnetic tape or other removable medium (also called a "dump").

This essential precaution is neglected by most new computer users until the first time they experience a disk crash or accidentally delete the only copy of the file they have been working on for the last six months. Ideally the backup copies should be kept at a different site or in a fire safe since, though your hardware may be insured against fire, the data on it is almost certainly neither insured nor easily replaced.

See also backup software, differential backup, incremental backup, full backup. Compare archive, source code management.

Last updated: 2004-03-16

Backup Domain Controller


(BDC) A server in a network of Microsoft Windows computers that maintains a copy of the SAM database and handles access requests that the Primary Domain Controller (PDC) doesn't respond to. There may be zero or more BDCs in a network. They increase reliability and reduce load on the PDC.

Last updated: 2006-09-18

backup pumpkin


backup rotation

<operating system>

Any system for re-using backup media, e.g. magnetic tape. One extreme would be to use the same media for every backup (e.g. copy disk A to disk B), the other extreme would be to use new media every time. The trade-off is between the cost of buying and storing media and the ability to restore any version of any file. One example is the Grandfather, Father, Son (GFS) scheme.

Last updated: 2004-10-08

backup software

<tool, software>

Software for doing a backup, often included as part of the operating system.

Backup software should provide ways to specify what files get backed up and to where. It may include its own scheduling function to automate the procedure or, preferably, work with generic scheduling facilities. It may include facilities for managing the backup media (e.g. maintaining an index of tapes) and for restoring files from backups.

Examples are Unix's dump command and Windows's ntbackup.

Last updated: 2004-03-16

Backus-Naur Form

<language, grammar>

(BNF, originally "Backus Normal Form") A formal metasyntax used to express context-free grammars. Backus Normal Form was renamed Backus-Naur Form at the suggestion of Donald Knuth.

BNF is one of the most commonly used metasyntactic notations for specifying the syntax of programming languages, command sets, and the like. It is widely used for language descriptions but seldom documented anywhere (how do you document a metasyntax?), so that it must usually be learned by osmosis (but see RFC 2234).

Consider this BNF for a US postal address:

 <postal-address> ::= <name-part> <street-address> <zip-part>

 <personal-part> ::= <name> | <initial> "."

 <name-part> ::= <personal-part> <last-name> [<jr-part>] <EOL>
        | <personal-part> <name-part>

 <street-address> ::= [<apt>] <house-num> <street-name> <EOL>

 <zip-part> ::= <town-name> "," <state-code> <ZIP-code> <EOL>

This translates into English as: "A postal-address consists of a name-part, followed by a street-address part, followed by a zip-code part. A personal-part consists of either a first name or an initial followed by a dot. A name-part consists of either: a personal-part followed by a last name followed by an optional "jr-part" (Jr., Sr., or dynastic number) and end-of-line, or a personal part followed by a name part (this rule illustrates the use of recursion in BNFs, covering the case of people who use multiple first and middle names and/or initials). A street address consists of an optional apartment specifier, followed by a street number, followed by a street name. A zip-part consists of a town-name, followed by a comma, followed by a state code, followed by a ZIP-code followed by an end-of-line."

Note that many things (such as the format of a personal-part, apartment specifier, or ZIP-code) are left unspecified. These lexical details are presumed to be obvious from context or specified somewhere nearby.

There are many variants and extensions of BNF, possibly containing some or all of the regexp wild cards such as "*" or "+". EBNF is a common one. In fact the example above isn't the pure form invented for the ALGOL 60 report. "[]" was introduced a few years later in IBM's PL/I definition but is now universally recognised. ABNF is another extension.

Last updated: 1997-11-23

Backus Normal Form

Backus-Naur Form

backward analysis


An analysis to determine properties of the inputs of a program from properties or context of the outputs. E.g. if the output of this function is needed then this argument is needed.

Compare forward analysis.

Last updated: 1997-11-23

backward chaining


An algorithm for proving a goal by recursively breaking it down into sub-goals and trying to prove these until facts are reached. Facts are goals with no sub-goals which are therefore always true. Backward training is the program execution mechanism used by most logic programming language like Prolog.

Opposite: forward chaining.

Last updated: 2004-01-26

backward combatability


/bak'w*d k*m-bat'*-bil'*-tee/ (Play on "backward compatibility") A property of hardware or software revisions in which previous protocols, formats, layouts, etc. are irrevocably discarded in favour of "new and improved" protocols, formats and layouts, leaving the previous ones not merely deprecated but actively defeated. (Too often, the old and new versions cannot definitively be distinguished, such that lingering instances of the previous ones yield crashes or other infelicitous effects, as opposed to a simple "version mismatch" message.) A backward compatible change, on the other hand, allows old versions to coexist without crashes or error messages, but too many major changes incorporating elaborate backward compatibility processing can lead to extreme software bloat.

See also flag day.

[Jargon File]

Last updated: 2003-06-23

backward compatibility


Able to share data or commands with older versions of itself, or sometimes other older systems, particularly systems it intends to supplant. Sometimes backward compatibility is limited to being able to read old data but does not extend to being able to write data in a format that can be read by old versions.

For example, WordPerfect 6.0 can read WordPerfect 5.1 files, so it is backward compatible. It can be said that Perl is backward compatible with awk, because Perl was (among other things) intended to replace awk, and can, with a converter, run awk programs.

See also: backward combatability.

Compare: forward compatible.

Last updated: 2003-06-23

backward compatible

backward compatibility

backwards compatibility

backward compatibility

backwards compatible

backward compatibility

Nearby terms:

Bachman Information Systemsbackbonebackbone cabalbackbone network

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