Computing and Internet Terminology

Everything you always wanted to know about computing and Internet terminology and didn't know who to ask, when you did you wish you hadn't!!

ADSL (Asymmetric Digital Subscriber Line). A new technology that allows more data to be sent over existing copper telephone lines (POTS). ADSL supports data rates of from 1.5 to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate).
ADSL requires a special ADSL modem. It is not currently available to the general public except in trial areas, but many believe that it will be one of the more popular choices for Internet access over the next few years.

Analogue
A signal which can vary continuously, taking any value between certain limits. The human voice, for which the public telephone network is designed, is an analogue signal varying in frequency and volume.

ANSI
(American National Standards Institute) The national clearing house and co-ordinating organisation for voluntary standards in the USA. ANSI does not directly develop standards but accredits other groups to do so.

ASCII
(American Standard Code of Information Interchange). Pronounced "asky". The US version of the ISO 7-bit data code, usually transmitted in 8-bit characters, the additional bit often being an (odd or even) parity bit.

Asynchronous
A communications channel capable of transmitting data but not timing. Uses start and stop bits for synchronisation.

AT commands
A protocol between comms equipment (typically a modem), and a terminal to permit testing/ line-connection etc.

ATM
(Asynchronous Transfer Mode). Strictly speaking this refers to a generic mode of data transfer in which units of data are not time related to each other. Information is handled in fixed length cells of 53 octets.

Back-Bone Network
A central network that links smaller or lower speed networks, e.g. a 100Mbps. FDDI backbone for linking a number of smaller work group subnetworks.

Bandwidth
The range of frequencies available for signaling in a communications channel. The capacity is measured in cycles per second, or Hertz (Hz) expressed as the difference between the highest and lowest limits of the frequency range which can be used. Often specified as bits per second.

Baseband
A transmission technique whereby digital signal is applied directly to the transmission media. Suitable for use when there is a large bandwidth and the data only needs to travel short distances.

Baud
One signaling element per second. A measure of the signaling rate on the communications line itself.

Bit
Binary digit which can be in a state of either "0" or "1"..

Bit/s
Basic unit of measure for serial data transmission capacity. Shown as kbits/s; Mbits/s; Gbits/s etc.
See bps.

BNC
A bayonet locking connector for use on Thinnet coaxial cables for connecting to Network Interface Cards, transceivers and other Ethernet network devices.

Bps
(Bits Per Second). Alternative denotation for bit/s.
See bit/s.

BRI
(Basic Rate Interface). The basic ISDN 2B + D service worldwide. Each B channel is 64 kbps and the D for signaling is 16 kbps.
BRI-Nylon.... mainstay of the authors wardrobe.

Bridge
Used to connect one or more Local Area Networks to form a large LAN. They can improve LAN performance and improve security by filtering network traffic. Local bridges are used to connect LAN's on one site, while a remote bridge connects LAN's over a Wide Area Network Link (typically Kilostream or Megastream).

Broadband
Broadband is a term applied to coaxial cable LAN systems which employ multiple radio frequency channels. Nowadays, broadband is being used to describe the generation of high-speed digital services beyond the currently planned ISDN.

Broadband ISDN
Refers to services that require higher-speed channels than primary rate ISDN. ATM is the first two layers of the Broadband ISDN model.

Byte
A digital unit of information comprising eight binary digits (bits). In computer technology usually used to encode an alphanumeric character.
Quite a mouthful!

Carrier
A continuous frequency capable of being modulated or impressed with a second data carrying signal.

Channel
A physical or logical path allowing the transmission of information.

Character
Standard bit representation of a symbol, letter, number, or punctuation mark. Generally means the same as byte.
Character: interesting person, with a life worth discussing. Not prevalent in the datacomms industry.

Checksum
The mathematical sum of a block of data. Used for checking purposes (what else).

Circuit-switching
A type of network whereby a circuit is established and maintained between the communicating parties for the duration of the "call".

Clock
A name commonly used for any of the sources of timing signals used in synchronous data transmission.

Codec
Short for coder-decoder. A device for digitising analogue voice or video signals.

Data compression
Various techniques for increasing data throughput on a communications link, primarily the use of abbreviated codes for the transmission of repetitive data. Transparent to the user.

DHCP
(Dynamic Host Configuration Protocol). An extension of BOOTP, which can provide just about any information required for network configuration. Although not yet widely prevalent, DHCP could eventually be a service found on all corporate networks, and also provided by ISPs in preference to IPCP.

Dial-up
A communications link that connects a terminal and a computer via the Public Switched Telephone Network (PSTN).

Digital
Referring to communications procedures, techniques and equipment where information is encoded as either a binary "0" or "1"

DPNSS
(Digital Private Network Signaling System): Developed in the early1980's, a protocol to support connection between telecom equipment from different vendors digital equipment. Presented as G.703, with a 64k timeslot per voice connection, and a separate 64k timeslot handling the signaling Based on pre-ISDN technology, in no way diminishes its usefulness and importance.

Drop and Insert
A technique used with three or more TDM multiplexers whereby data is routed from one to another via an intermediate multiplexer.

DSL (Digital Subscriber Line). A technology using sophisticated modulation schemes to pack data onto copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations.

DTE
Data Terminal Equipment. CCITT terminology for the data terminal devices themselves. Examples: printer, pc/dumb-terminal.

DTMF
(Dial Tone Multi Frequency). Analogue signal consisting of a mix of different frequencies, optimised for use on analogue circuits. Used extensively throughout telecomms.

E1 Frame
Name for the digital carrier used for connecting 30 voice channels together with associated signaling Each PCM channel takes 64Kbps. The signaling for all 30 channels takes a further 64Kbps. Because the E1 framing rate is also 64Kbps, the operating rate of the carrier is 2.048Mbps.

E1 Service
The common name for the (almost) worldwide digital 2.048 Mbps service (as per E1 Frame above). NB: USA use T-1 not E-1!

E & M signaling
A telecomms signaling method employing separate dc lines for 'ear' and 'mouth indication. Usually associated with pulse dialing. Simple and cheap. Semaphore is better!

EBCDIC
(Extended Binary Coded Decimal Interchange Code). 8-bit character code set developed by IBM.

EPS8
4 wire Baseband Circuit available from BT for connection of sites within the same exchange area. Can be used with line-drivers for high-speed data transmission between geographically close locations.

EPS9
2 wire Baseband Circuit available from BT for connection of sites within the same exchange area. Can be used with line-drivers for high-speed data transmission between geographically close locations.

Error-correction
A technique that restores data integrity in received data, either by changing the received data or by requesting retransmission from the source

Ethernet
A type of local area network based upon CSMA/CD technology - originally developed by Xerox.

Fast Ethernet
A 100Mbps technology based on the 10base-T Ethernet CSMA/CD network access method.

Fast packet switching
A term used to describe a generic mode of packet switching in hardware at layers 1 or 2 of the OSI seven layer model, in order to achieve high speed and low delay across a network.

FDDI
A high-speed LAN standard employing 100 Mbps fibre optic rings.

Flow control
A technique whereby control signals or characters are used to enable or disable the flow of data to DCE equipment.
Hardware flow control ('out-of-band') using RTS/CTS lines etc. or
Software flow control ('in-band') using XON/XOFF characters etc.

Frame
A group of bits exchanged between data link layer entities that contains its own control information for addressing and error checking in addition to user data. As used in HDLC. Also the repeating sequence of timeslots in a time division multiplex.

Frame relay
A standardised form of fast packet switching, in which packets are switched in hardware at the data link layer.

Full duplex
A channel capable of transmitting data in both directions at the same time.

Gateway
Devices which operate at the highest layer of the OSI model, performing protocol conversion, thus enabling them to interconnect dissimilar networks.

Half duplex
A channel capable of transmitting data in either direction, but only one direction at any one time.

Handshaking
A predefined exchange of signals or control characters between two devices or nodes that sets up the conditions for data transfer or transmission.

HDLC
(High-level Data Link Control). A standard for the structure of a frame of bits at the data link layer of the OSI seven-layer model.

HDSL (High Speed Digital Subscriber Line). A form of Digital Subscriber Line, providing T1 or E1 connections over two or three twisted-pair copper lines respectively. Unlike most other forms of DSL, HDSL is not a typical consumer service, it's used mostly to replace traditional T1/E1 connections, such as connecting PBXes to telco offices. The advantage of HDSL over the technology traditionally used on T1/E1 lines, is that it requires much lower bandwidth to carry the same traffic.

Host
A general term for a computer system which executes applications for connected users and devices.

Hub
The centre of a star topology network or cabling system.

IEEE
(US Institute of Electrical and Electronic Engineers). An accredited Standards Committee of ANSI active in standards making, mainly in LANs.

Internet
A global information system that is logically linked together by a unique address based on IP, and which is able to support communications using the TCP/IP suite. It provides, uses or makes accessible, high level services layered on a related communications infrastructure.

IP
(Internet Protocol). An industry standard packet switching protocol used as the network portion of the TCP/IP protocol suite. Uses TCP for transport. The working tool that makes the Internet possible.

IPX
(Internetwork Packet Exchange) A networking protocol from Novell that interconnects networks that use Novell's NetWare clients and servers. IPX is a packet protocol. IPX works at the network layer of communication protocols and is connectionless (it doesn't require that a connection is set up before packets are sent to a destination as, for example, a regular voice phone call does).

ISDN
(Integrated Services Digital Network). An internationally agreed public network for dial-up digital services, comprising both voice and data.

ISO
(International Standardisation Organisation). The primary Worldwide force in developing network architecture, working closely with CCITT in the development of standards.

IT
(Information Technology). A catchall term used to describe the techniques used for the automation of information handling and retrieval, including computing, telecommunications, and office systems.

Kilostream
The registered trademark for British Telecom's point-to-point digital network service. Typically 19.2k; 64k; 128k; or 256k.

LAN
(Local Area Network). A system that interconnects a variety of computers and terminals in the same general area.

Layer
In the OSI reference model, this refers to a collection of related functions within a communications system, with a defined path to layers above and below.

Leased line
A telephone connection leased from a PTT giving a permanent point-to-point service.

Local loop
The name for the local analogue circuit (usually copper wires) which connects a subscriber to a local telephone exchange. Also test mode for datacomms equipment to check local equipment is functioning.

MAC
(Media Access Control). The lower sublayer of the data link layer of the OSI model. There are different MAC protocols for the different IEEE 802 LANs.

Megabyte
1,048,576 bytes, equal to 1,024 kbytes. Basic unit of measurement of mass storage, also used in describing data transfer rates as a function of time (i.e. Mbyte/s)

Megastream
The registered trademark for British Telecom's 2.048 Mbps digital network service.

Modem
A contraction of the term "modulator-demodulator". A modem converts serial digital data from a transmitting terminal into a form suitable for transmission over an analogue line. A second unit reconverts this signal to serial digital data for acceptance by the receiving terminal.

Multiplexing
The function of transporting more than one stream of data over a single circuit (real or virtual) apparently simultaneously. Multiplexing can be either in the frequency domain (FDM) or the time domain (TDM)

Network
The equipment and transmission facilities for communication between computer systems.

Network layer
Layer three of the OSI model, the Network Layer ensures that in a multi-node network the blocks of data arrive at their correct destinations by use of the appropriate call set up and routing procedures.

Node
A point where one or more functional units interconnect transmission lines.

Octet
A digital unit of information comprising eight binary digits (bits). A more generic description than byte.

OSPF
(Open Shortest Path First). A routing protocol used within larger networks in preference to RIP. A host that detects a change in the network immediately sends the information to all other hosts in the network so that all will have the same routing information. Unlike the RIP, only the part that has changed is sent, and only when the network has changed.
Since many networks using RIP are already in use, router manufacturers tend to include RIP support within a router designed primarily for OSPF.
See RIP.

PABX
(Private Automatic Branch Exchange). Simply a term for an automatic telephone switching system within a private enterprise. Originally, such systems - called private branch exchanges (PBXs) - required the use of a live operator. Since almost all-private branch exchanges today are automatic, the abbreviation "PBX" usually implies a "PABX."

Packet switching
Technique of switching data in a network whereby individual blocks or "packets" of controlled size and format are accepted by the network and routed to their destination. The sequence is maintained and the destination is determined by exchange of control packets between the originating terminal and the network before transfer starts.

Parity checking
A technique of error detection in which one bit is added to each data character so that the number of logic one bits per character is always even (or always odd; or always a mark condition; or always a space condition)

PBX
(Private Branch Exchange). A telephone system within an enterprise that switches calls between users on local lines, while allowing all users to share a certain number of external phone lines. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company's central office

Physical layer
The lowest layer of the OSI reference model, defines the physical and electrical characteristics of the interface to the network and provides a transparent transmission path for the information across the chosen medium.

PING
(Packet Internet Groper). A utility used determine whether a specific IP address is accessible. It works by sending a packet the specified address and waiting for a reply. PING is used primarily to troubleshoot Internet connections. Very useful!!

Point-to-point
Transmission of data directly between two points.

POTS (Plain Old Telephone Service). This is the standard telephone service that most homes use. In contrast, telephone services based on high-speed, digital communications lines, such as ISDN and FDDI, are not POTS. The main distinctions between POTS and non-POTS services are speed and bandwidth. POTS is generally restricted to about 52 Kbps (52,000 bits per second).
The POTS network is also called the public switched telephone network (PSTN).

Presentation layer
Layer six in the reference model, the Presentation Layer fulfils the requirement for identifying and agreeing the common syntax (grammar) to be used between two applications.

PRI
(Primary Rate Interface). The Primary Rate Interface in ISDN is 32B + D, for the North American standard (T-1), and 30B + D for the European standard (E-1)

Protocol
Set of rules governing the format, timing, sequencing and error control of messages exchanged between communication entities in a network.

PSS
(Packet SwitchStream Service). The British telecom X.25 public packet switching network.

PSTN
(Public Switched Telephone Network). A series of public dial-up analogue telephone circuits available for voice calls, and data transmission via a modem.

PTT
A national organisation responsible for the provision of telecommunications services within that country.

QSIG
ISDN based protocol for signaling between nodes of a Private Integrated Services Network, allows products from different vendors to work together. A protocol based closely on internationally agreed Standards for ISDN. Both QSIG and DPNSS support similar sets of features.
See DPNSS.

RARP
(Reverse Address Resolution Protocol). A link layer mechanism used by many diskless workstations (Sun) to find IP addresses and download hosts. This protocol is only suitable for LANs.

Repeater
A device used to extend transmission distances by restoring signals to their original size and shape. Typically used within Ethernet networks to extend segment lengths.

RIP
(Routing Information Protocol). Widely used protocol for managing interconnected groups of LANs. A router sends its entire routing table to its closest neighbour every 30 seconds. The neighbour will pass this to the next, and so on until all have the same knowledge, known as network convergence.
RIP uses a hop count to determine network distance. Considered an effective solution for small networks, but for complex networks an alternative is the Open Shortest Path First Protocol (OSPF)
See OSPF.

SDLC
(Synchronous Data Link Control) A transmission protocol developed by IBM in the 1970s as a replacement for its binary synchronous (BSC) protocol. SDLC is equivalent to layer 2 of the OSI model.

SDSL (Symmetric Digital Subscriber Line). A new technology that allows more data to be sent over existing copper telephone lines (POTS), supporting data rates up to 3 Mbps. This works by sending digital pulses in the high-frequency area of telephone wires, not used by normal voice communications, so can operate simultaneously with voice connections over the same wires.
SDSL requires a special SDSL modem. SDSL is called symmetric because it supports the same data rates for upstream and downstream traffic. A similar technology that supports different data rates for upstream and downstream data is called asymmetric digital subscriber line (ADSL). ADSL is more popular in North America, whereas SDSL is being developed primarily in Europe.

Session layer
Layer five of the OSI Reference Model, the Session Layer provides the synchronisation and management of the dialogue between applications.

Seven layer model
See OSI Reference Model

Signaling
In telephony, signaling refers to the process and the standards involved in passing control information between terminal equipment on the network. e.g. between a PABX and a local exchange, or between networked PABXs.

SNA
(Systems Network Architecture). A series of synchronous communications protocols developed in the 1970's by IBM, for synchronous communications.

SNMP
(Simple Network Management Protocol). A network management protocol for use with TCP/IP which is becoming increasingly popular as a precept to NW/OSI.

SPX
(Sequenced Packet Exchange). A transport layer protocol (layer 4 of the OSI Model) used in Novell Netware networks. The SPX layer sits on top of the IPX layer (layer 3) and provides connection-oriented services between two nodes on the network. SPX is used primarily by client/server applications.
Whereas the IPX protocol is similar to IP, SPX is similar to TCP. Together, therefore, IPX/SPX provides connection services similar to TCP/IP.

Synchronous data channel
A communications channel capable of transmitting timing information in addition to data.

T1 Service
The common name for the mainly American digital 1.544 Mbps service.

TCP/IP
(Transmission Control Protocol/Internet Protocol). The name applied to a popular protocol suite developed by the Defence Advanced Research Projects Agency (DARPA). TCP is the transport layer protocol in the suite and IP covers layer three.

Telnet
An application program with an underlying TCP/IP protocol, providing terminal emulation for accessing remote computers, or remote devices, in dumb-terminal mode.

TDM
(Time Division Multiplexer). A multiplexer which operates by dividing the shared high-speed data stream into a sequence of time units. One or more units is then allocated to the data channels being combined on the high-speed line.

Terminal Server
A device generally used for connecting a number of async devices over an ISDN link.

Token passing
A media access control method. In a token passing arrangement, the right to send data passes from one device to another in an ordered sequence determined by means of a token. A token is a special message which represents permission to send data.

Token Ring
A LAN technology specified in IEEE 802.5, primarily supported by IBM. Employs a system of token passing for media access.

Transport layer
Fourth layer of the Reference Model, the Transport Layer supplies the upper layers with a Network Service which has improved quality of service features.

UTP
Unshielded Twisted Pair Cable.

Virtual circuit
In networks, the feature which gives to the user the appearance of an end-to-end circuit without an actual physical connection being set up.

WAN
(Wide Area Network). To be distinguished from the LAN, a WAN interconnects geographically remote sites.

X.25
The CCITT recommendation defining interfaces to packet-switched networks, as used by many public and private networks.

xDSL (x Digital Subscriber Line). This refers collectively to all types of digital subscriber lines, the two main categories being ADSL and SDSL. Two other types of xDSL technologies are High-data-rate DSL (HDSL) and Single-line DSL (SDSL).
xDSL is similar to ISDN inasmuch as both operate over existing copper telephone lines (POTS) and both require the short runs to a central telephone office (usually less than 20,000 feet). However, xDSL offers much higher speeds - up to 32 Mbps for downstream traffic, and from 32 Kbps to over 1 Mbps for upstream traffic.