Glossary - B
- Bipolar 8 Zero Substitution (physical layer, communications)
- Back Door (security)
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.
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].
- Backbone (network)
A primary transmission medium in a hierarchically distributed system;
designed to interconnect and integrate a variety of voice, data and image
networks with a diverse array of systems, equipment and applications.
- French acronym for mailbox.
An impedance-matching device that connects a balanced line (such as a twisted-pair line)
with an unbalanced line (such as a coaxial cable).
- Bandwidth (network)
Measure of transmission capacity of a communications medium; measured differently for
analog and digital transmissions. The greater the bandwidth, the more information that can
be transmitted in a specified amount of time.
- Bandwidth on Demand
The assignment of bandwidth to a trunk or circuit link as needed.
The frequency band occupied by a single or composite signal in its original or unmodulated
Buffer Allocation Size (ATM)
A 1-byte field in the CPCS-PDU
header to indicate to the receiving end the buffer space that needs to be reserved for
reassembling the CPCS-PDU.
- Batch (programming)
A term describing a system that takes a set (a "batch") of commands or jobs, executes them
and returns the results, all without human intervention. This contrasts with an interactive
system where the user's commands and the computer's responses are interleaved during a
A batch system typically takes its commands from a disk file (or a set of punched cards in
the old days) and returns the results to a file (or prints them). Often there is a queue of
jobs which the system processes as resources become available.
Hackers use this somewhat more loosely than the traditional technical definitions justify;
in particular, switches on a normally interactive program that prepare it to receive
non-interactive command input are often referred to as "batch mode" switches. A "batch file"
is a series of instructions written to be handed to an interactive program running in batch mode.
- Baud (unit, network)
A unit of signaling speed equal to the number of signal events per second;
not necessarily the same as bits per second.
- Bulletin Board System (network)
Binary Coded Decimal (network)
6-bit code that allows 64 combinations to code alphanumeric caracters. Has been replaced
- Broadband Connection Oriented Bearer (ATM)
- Broadband Connection Oriented Bearer Class A (ATM)
- Broadband Connection Oriented Bearer Class C (ATM)
- Broadband Connection Oriented Bearer Class X (ATM)
Backward Error Correction (network)
An error-correction scheme where the sender retransmits any data to be found in error,
based on the feedback from the receiver.
Backward Explicit Congestion Notification (network)
A bit in the frame relay header. The
bit is set by a congested network node in any frame which is traveling in the reverse
direction of the congestion (in frame relay, a node can be congested in one direction of
frame flow but not in the other).
Bell Communications Research (network)
The research arm of the seven Bell Operating Companies which were originally part of
AT&T. Bellcore was originally part of Bell Laboratories.
- Bit error Rate (network)
Basic Encoding Rules (ASN.1) (network)
Standard rules for encoding data units described in the Abstract Syntax Notation One
(ASN.1). Sometimes incorrectly lumped under the term
ASN.1, which properly refers only to the abstract syntax description language, not the
- Busty Errored Seconds (ATM)
- Best Effort (ATM)
A QoS class in which no specific traffic
parameters and no absolute guarantees are provided. Best effort includes
UBR and ABR. See also
- Border Gateway Protocol (routing, network)
- BICI - B-ICI
Broadband Inter-Carrier Interface (ATM)
A carrier-to-carrier interface similar to PNNI
(private network-to-network interface) but lacking some of the detailed information offered
by the latter. The difference arises because carriers are less likely to let their switches
share routing information or detailed network maps with their competitors' gear. BICI now
supports only PVCs (permanent virtual circuits) between
carrier networks; the ATM Forum's BICI working group
is currently addressing SVCs (switched virtual circuits).
An interface that supports service connections (such as CRS,
FR) across public ATM networks and/or carriers.
- Bit Interleaved Parity (physical layer, communications)
Broadband aspects of Integrated Services Digital Network (network)
An ITU-T-introduced protocol platform to support the
integrated, high-speed transmission of data, audio and video in a seamless fashion. ATM
emerged as a suitable transport standard.
- Broadband Inter-Switching System Interface (network)
Broadband ISDN User's Part (network)
A SS7 protocol which defines the
signaling messages to control connections and services.
It is used to establish, maintainand release broadband switched network connections accross
an SS7/ATM network.
Binary digIT (general)
The smallest unit of information (data) and the basic unit in data communications.
- Building Integrated Timing Supply (physical layer)
- Bit Error (network)
The value of an encoded bit can be changed due to a transmission
problem (e.g., noise on the line) and then interpreted incorrectly by the receiver.
- Bit Rate
The number of bits of data transmitted over a communications line
Bayonet-Neill-Concelman (hardware, network)
A bayonet-locking connector used to terminate coaxial cables.
A BNC connector has a bayonet-type shell with two small knobs on the female connector which
lock into spiral slots in the male connector when it is twisted on.
BNC is also referred to as Bayonet Network Connector, Bayonet Navy Connector, British Naval
Connector, Bayonet Nut Connection.
- Bit/s - Bps
Bits per second (unit, network)
A measure of data transmission speeds. Notations: Bit/s, bps. See also
- Body (communications,email, news, HTML)
In a message, name given to the part that contains the real message (what people want to
say). It is different from the header that contains
service and control information (sender, date, subject...) used to deliver or handle the
IN HTML, it is the name of the tag that delimits the data that is to be displayed.
- BOF (communications)
Birds Of a Feather (network,standardization)
A Birds Of a Feather (flocking together) is an informal discussion group. It is formed,
often ad hoc, to consider a specific issue and, therefore, has a narrow focus.
See also: Working Group.
Beginning of Message (network)
A PDU that constitutes the beginning of a message.
- Broadband Optical Network using ATM PON Access facilities in Realistic Telecommunications Environments (ATM)
- Boucle Locale
- French equivalent for Local Loop.
- Bridge Protocol Data Unit (Datalink layer)
Bits per second (unit, network)
A measure of data transmission speeds. Notations: Bit/s, bps. See also
- BiPolar Violation (Physical layer)
Basic Rate Interface (network)
A kind of Integrated Services Digital Network channel
consisting of two 64 Kbit per second "bearer" (B) channels for user-data transfer plus a 16
kbps "delta" (D) channel for control and signalling information. A BRI provides a total
data rate of 144 kilobits per second. The B channels are used for voice or data, and the D
channel is used for signalling and/or X.25 packet networking. BRI is the kind of ISDN
interface most likely to be found in residential service.
- Bridge (network)
A LAN interconnection device which forwards traffic between network segments based on
datalink layer information. These segments would have a common network layer address.
Bridges are used extensively in LAN systems to extend their physical dimensions or modify
their performance. See also gateway,
- Bridging (network)
Techniques for interconnecting two LAN segments that utilize
the same LLC procedures but may use the same or different
- Broadband (communications, network)
Originally described the frequency bandwidth of analog circuits.
Now used informally to describe any networking technology that employs high-speed digital
circuits, generally over 1.544 Mbps.
The term applied to networks having bandwidths significantly greater (more than 64 kilobits
per second) than that found in telephony networks. Broadband systems are capable of carrying
a large number of moving images or a vast quantity of data simultaneously. Broadband
techniques usually depend on coaxial or optical cable for transmissions. They utilize
multiplexing to permit the simultaneous operation of multiple channels or services on a
single cable. Frequency division multiplexing or cell relay techniques can both be used in
In LAN terminology, it refers to a system in which multiple
channels access a medium, for example co-axial cable, that has a large bandwidth using Radio
Frequency (RF) modems. This may allow the co-axial cable to carry multiple separate LANs
whose transmission is being modulated at different frequencies. In cable television (CATV),
broadband describes the ability to carry 30 or more TV channels and is synonymous with
wideband or wide band.
- Broadcast (network)
A message forwarded to all network destinations(multiple, unspecified recipients). On
Ethernet, a broadcast packet is a special type of multicast packet which all nodes on the
network are always willing to receive.
In local area networks, a device that combines the dynamic
routing capability of an internetwork router with the
ability of a bridge to interconnect LANs.
- Browser (software)
- A program that is used to look through a set of data.
Web Browser (www, Internet)
The client software that enables you to browse various kinds of Internet resources.
Browsers can be both text-based or graphic. Netscape Navigator, Microsoft's Internet
Explorer and derivatives of Mosaic represent about 90 percent of the graphic browsers on
Binary Synchronous Communications (network)
datalink layer synchronous protocol introduced by IBM. It is replaced by SDLC.
British Standards Institute (standard, body)
British member of the ISO (International Standard Organisation).
Burst Tolerance (ATM)
Proportional to the MBS, burst tolerance is used as a measure
(leaky bucketparameter) for conformance checking of
Begin Tag (ATM)
8-bit field of the CPCS header in
AAL3/4. This field is used to number CPCS modulo 256
and must be identical to the BTAG field.
Broadband Terminal Equipment (ATM)
An equipement category for B-ISDN wich includes terminal adapters an
Area in a device for temporary storage of data in transit; can accommodate differences in
processing speeds between devices by storing data blocks until they are ready to be
processed by a slower device.
- Burstiness (ATM)
A source traffic characteristic that is defined as the ratio of the peak cell rate
(PCR) to the average
cell rate. It is a measure of the intercell spacing. See also
- Bursty - Batchy
Communications characterized by high volumes of data transmitted intermitantly, as opposed
to steady-stream data.
Broadcast and Unknown Server (ATM)
A server that forwards multicast,broadcast and unknown-destination address traffic to the
ATM Forum-defined specifications in support of
LAN-to-LAN connectivity, called
The BUS is designed for carrying broadcast data, such as TCP/IP
address resolution broadcasts or Novell Service Advertising Protocol (SAP) messages. It also
handles all multicast traffic. Finally, it broadcasts the initial unicast frames sent by the
LEC while the LES works in
tandem to provide the appropriate ATM address for
establishing a direct link.
- Bus Topology
LAN topology with a single transmission path shared by every
user. All nodes listen to all transmissions and select specific messages based on
address identification; requires a contention-control mechanism for accessing
the transmission medium.
- French (from Quebec) synonym of browser.
- Bypass (communications)
Facilities Bypass: The use of facilities other than those of the local telephone
company (LEC) (Facilities Bypass), using
satellites and microwave systems for instance.
The service bypass is the use of operating telephone company private lines to
connect a customer premise to a Point of Presence (POP) or another customer premise.
- Byte (unit, network)
A sequence of adjacent binary data bits treated as a unit; 8 bits equal
a byte; a byte is usually the smallest addressable unit of information in memory.
A byte may be 9 bits on 36 bit computers. Some older architectures used "byte" for
quantities of 6 or 7 bits, and the PDP-10 and IBM 7030 supported "bytes" that were actually
bit fields of 1 to 36 (or 64) bits! These usages are now obsolete, and even 9 bit bytes
have become rare in the general trend toward power-of-2 word sizes. See also