Communications and Networking Glossary

Asymmetric Digital Subscriber Line. A technology used for multiplexing voice and data, and transmitting on a copper telephone line. Commonly used for home Internet access.

Amplitude Modulation. A signal encoding technique that takes ananlog data and combines it with an analog carrier signal to produce an analog signal for transmission. For example, AM radio.

Alternate Mark Inversion. Technique for encoding digital data as digital signals.

American National Standard Institute. Produces standards on many topics in the US. www.ansi.org

Address Resolution Protocol. Protocol for mapping IP addresses to MAC/hardware addresses inside a LAN.

Automatic Repeat Request. A set of mechanisms for retransmitting packets to overcome errors. Examples include: Stop-and-Wait, Go-Back-N and Selective-Reject. The figure shows an example of Stop-and-Wait ARQ.

American Standard Code for Information Interchange. Scheme for encoding English characters (and other computer/keyboard characters) into binary values.

Amplitude-Shift Keying. Technique for encoding digital data as an analog signal. The amplitude of an analog signal is varied according to the digital data.

Asynchronous Transfer Mode. Technology that uses virtual circuit packet switching to create WANs.

Bit Error Rate. The fraction of all bits transmitted that arrive at the destination in error. For example, a BER of 0.01 means on average, 1 bit in every 100 is in error.

Border Gateway Protocol. A protocol for exchanging routing information between Autonomous Systems (e.g. ISPs).

Collision Avoidance. A mechanism used in LANs, especially wireless LANs, for stations to avoid transmitting frames such that collisions occur. Used for example in IEEE 802.11 wireless LAN MAC protocol, CSMA/CA.

Constant Bit Rate. A sending rate which is constant, not changing (as opposed to a variable bit rate). For example, if an application is always sending at 64 kb/s then that is considered CBR. Whereas, an application (like a web browser) sends occasionally, and then sends nothing, and so on, is not CBR.

Collision Detection. A mechanism used in LANs for stations to detect that a collision of frames is in progress, and therefore stopping their transmission. Used in old IEEE 802.3 Ethernet protocols when a bus or hub-based network was used.

Cyclic Redundancy Check. An algorithm used for error detection. For example, given some data, the CRC-32 algorithm is applied on that data to produce an error detection code. Someone receiving the data and code can check whether the received data has an errors.

Carrier Sense Multiple Access. An algorithm used in LANs to allow multiple stations to access a shared medium.

Federal Communications Commission. A regulatory authority in the US that controls and licenses, among other things, the spectrum available for transmission. http://www.fcc.gov/

Frame Check Sequence. Used for error detection in protocols. Given some data to be transmitted in a frame, an error detection algorithm (e.g. CRC) is applied to determine an error detecting code, the Frame Check Sequence. This FCS is attached to the data and transmitted in a frame. The receiver of the frame uses the FCS to determine if the received frame contains any errors.

Frequency-Division Multiplexing. The procedure of combining signals from multiple sources and transmitting them as a single signal across a link at the same time, but using different frequencies for the individual source data. An alternative is Time Division Multiplexing (TDM).

Frequency Modulation. A signal encoding technique that takes ananlog data and combines it with an analog carrier signal to produce an analog signal for transmission. The output analog signal varies in frequency according to the input analog data. Alternatives are Amplitude Modulation (AM) and Phase Modulation (PM). For example, FM radio.

Frequency-Shift Keying. The procedure of encoding digital data as an analog signal by changing the frequency of the signal according to the input digital data. For example, to transmit bit 1, transmit a low frequency signal for some portion of time; to transmit bit 0, transmit a high frequency signals for the same portion of time. Alternatives inlcude Amplitude-Shift Keying (ASK) and Phase-Shift Keying (PSK).

File Transfer Protocol. Protocol used by applications to transfer files between computers.

Global Positioning System. Network of satellites provide location and time information to devices on the ground.

High-Level Data Link Control. Data link layer protocol mainly used for point-to-point links.

In the context of IP, a host is a device that is either an original source (creator) of IP datagrams or final destination (consumer) of IP datagrams. A host does not forward IP datagrams.

Hypertext Markup Language. Language for creating web pages.

Hypertext Transfer Protocol. Web browsers send requests to web server for content; the server sends back the content, usually a HTML web page. The figure shows a simple example of a request and response.

Internet Control Message Protocol. Protocol for error reporting and diagnostics in the Internet. Part of network layer, uses IP. Example application that uses ICMP is ping.

Institute of Electrical and Electronics Engineers. Produces standards in electrical and electronic engineering, including 802 series of LAN, WAN and MAN protocols such as IEEE 802.3 Ethernet and IEEE 802.11 wireless LANs. http://www.ieee.org/

Internet Engineering Task Force. Produces and maintains standards related to Internet technologies, including IP, TCP, SIP, FTP. Documents produced are called Internet Drafts and Requests For Comments (RFC). http://www.ietf.org/

Informal document produced by the IETF that reports on Internet protocols and technologies. Considered work-in-progress and not necessarily approved by the IETF. http://www.ietf.org/id-info/

Internet Protocol. Core protocol in the Internet. Provides connectionless, datagram packet switching at the network layer. Implemented in hosts and routers attached to Internet. The figure shows the structure of an IP datagram.

International Reference Alphabet. Character set encoding that defines mapping from English characters to binary values. US based instance is ASCII.

Integrated Services Digital Network. Technology used for homes and businesses to connect to Internet over copped lines. Gained some support in Europe, but now mainly surpassed by DSL and fibre optic technologies.

International Organization for Standardization. Creates standards in many areas, including communications and network, e.g. OSI reference model. http://www.iso.org/

International Telecommunications Union. Creates standards for communications, including protocols (OSI reference model joint with ISO) and codecs (e.g. H.264). http://www.itu.int/

Local Area Network. A network normally connecting end-user devices across a home, office or building

Logical Link Control. Part of data link layer, responsible for setting up and control data transfer across a link

Medium Access Control. Part of data link layer, responsible for allowing multiple stations access a shared medium without interfering with each other

Metropolitan Area Network. A network normally connecting buildings, offices and other networks across a city

Multi-Purpose Internet Mail Extension. Allows non-text attachments to be added to emails

Multiprotocol Label Switching. WAN technology often used inside ISP or telecom provider networks across cities and countries

Nonreturn to Zero, Inverted. Signal encoding technique that takes digital data as input and produces a digital signal as output. Alternatives include NRZL, Bipolar AMI, Manchester encoding

Nonreturn to Zero, Level. Signal encoding technique that takes digital data as input and produces a digital signal as output. Alternatives include NRZI, Bipolar AMI, Manchester encoding

Open Systems Interconnection. Set of protocols and mechanisms for allowing computer networks from different organisations to connect with each other. The OSI 7-layer reference model separates the functionality needed into layers. Protocols part of OSI are no longer commonly used, but the concepts, in particular the layers, are still referred to. An alternative 5-layer model is often used.

Open Shortest Path First. Link state routing protocol used within autonomous systems

Pulse Code Modulation. Technique for sampling analog data to produce digital data (which then can be transmitting as analog or digital signal). The figure shows examples of sampling and reconstructing an analog signal.

Protocol Data Unit. General name for the information sent by a protocol. Normally contains headers, data (payload) and sometimes trailers. More commonly referred to as a packet.

Phase Modulation. A signal encoding technique that takes ananlog data and combines it with an analog carrier signal to produce an analog signal for transmission. The output analog signal varies in phase according to the input analog data. Alternatives are Amplitude Modulation (AM) and Frequency Modulation (FM).

Phase-Shift Keying. The procedure of encoding digital data as an analog signal by changing the phase of the signal according to the input digital data. For example, to transmit bit 1, transmit a signal with aome phase for some portion of time; to transmit bit 0, transmit a signal with a different phase for the same portion of time. Alternatives inlcude Amplitude-Shift Keying (ASK) and Frequency-Shift Keying (FSK).

Public Switched Telephone Network. The traditional telephone network connecting homes and businesses across the world. E.g. phone calls from your home telephone or mobile phone most likely go via the PSTN. Whereas phone calls from your computer using VoIP software (e.g. Skype) go via the Internet.

Quadrature Amplitude Modulation. A signal encoding technique that combines Amplitude-Shift Keying (ASK) with Phase-Shift Keying (PSK). Alternatively used to refer to combination of Amplitude Modulation (AM) with Phase Modulation (PM).

Quality of Service. Techniques to provide priority and/or guaranteed delivery of information in a network. E.g. packets from voice calls get higher priority than web browsing packets

Quadrature Phase Shift Keying. Phase-shift keying (PSK) with four different phases. Two bits of digital data are mapped to a signal with one of four phases.

Radio Frequency. Portion of electromagnetic spectrum containing frequencies from 3 kHz to 300 GHz

Request For Comment. Name of an official, approved document released by the Internet Engineering Task Force (IETF). Usually describes a protocol, standard or recommendations for using Internet technologies. Alternative is an Internet Draft, which is an informal document produced by IETF. http://www.ietf.org/rfc.html

In the contact of IP, a router is a device that forwards IP datagrams, i.e. it receives an IP datagram from one device and then sends the same datagram to another device. Occasionally, a router may also act as an original source or final destination of datagrams (e.g. when someone is managing/configuring the router).

Synchronous Digital Hierarchy. WAN technology often used in point-to-point or ring networks to connect across cities and countries. Uses optical fibre and provides speeds in excess of 10 Gb/s. SONET is the US-based variant

Session Initiation Protocol. Protocol for setting up a voice (or video) call in the Internet. Performs similar role in the Internet and VoIP, as SS7 in a PSTN.

Simple Mail Transfer Protocol. Protocol for delivering email between email clients and servers and between servers

Simple Network Management Protocol. Protocol for managing devices in a networks. For example, information can be automatically collected from a set of switches and routers in a network and reported back to a central server.

Synchronous Optical Network. WAN technology similar to Synchronous Digital Hierarchy (SDH), the International variant.

Signalling System Number 7. Protocols for setting up a telephone call in a public switched telephone network (PSTN).

Shielded Twisted Pair. Cable containing pairs of copper wires, twisted around each other. Shielding provides extra protection from interference giving better performance than Unshielded Twisted Pair (UTP), but making the cable more rigid and harder to install.

Transmission Control Protocol. Transport protocol that provides connection-oriented data delivery between application processes, ensuring reliability of the data. Also provides flow and congestion control. The figure shows the structure of a TCP segment.

User Datagram Protocol. Simple transport protocol for connection-less data delivery between application processes. Provides no reliability: just sends the data. The figure shows the structure of a UDP datagram.

Unshielded Twisted Pair. Cable containing pairs of copper wires, twisted around each other. Without shielding, interference can be a problem leading to lower performance compared with Shielded Twisted Pair (STP). However it is much easier to use and install and hence much more common than STP. Most LAN cables and telephone wiring use UTP.

Virtual LAN. Technology that allows one physical LAN (i.e. switches, hosts and cabling) to support multiple virtual networks, i.e. from the perspective of the DLL on two computers they are on two different LANs, but actually are connected to the same physical switch.

Voice over Internet Protocol. Voice calls made using an IP network (e.g. the Internet), as opposed to a Public Switched Telephone Network (PSTN)

World Wide Web. Refers to web servers offering web pages to clients on the Internet. Lowercase www also commonly used at start of domain names (although not always required).