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10/5/2009
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D
A plural noun representing facts which can be processed to result in a decision - by a person, a computing device, or both. The telecommunications and computing definition (often by convention singular) covers digital information that is transferred over data communications networks.
In telecommunications "data" is often used to mean "not voice" but even that is blurring with real-time voice (and video) also being sent over data networks.
Data is also the foundation of a tiered hierarchy of concepts building on data. These are: information, knowledge and intelligence, all of which have relevance to networks and ICT technologies.
A data processing file or files organised in a way that multiple users can quickly make use of and/or update the data contained within and where a specific user sees data relevant to them and which appears to be independent of the file structure. Management of the data in a large database is carried out by a database management system (DBMS) very often controlled by a person known as a database administrator. Databases can be distributed over different servers in different locations, in some case having different data linked by a common identifier and in others being the same data with local variations both of which which provide many challenges to large database design.
Many books are dedicated to this topic. It is generally taken to mean the links between digital devices that pass information between them. This can range from the link between two computers using infra-red, or simple modems over a pair of wires, to the vast international networks based on packet switching, including the Internet.
Data encryption standard (DES and 3DES)
A cryptography method that uses a 56-bit key. DES is based on an IBM algorithm from 1977 and became a standard adopted by the National Institute of Standards and Technology (NIST) in the USA. There are several DES encryption modes.
Triple DES increases security by extending the key to 112 or 168 bits and encoding the data three times taking more time. Variants use three keys to encrypt three times, to encrypt, decrypt and encrypt again and using only two keys with the first and third operations using the same key.
DES and 3DES have been superseded by the Advanced Encryption Standard (AES).
The official unit of information exchanged between IP protocol entities at the network level in the OSI protocol stack and commonly referred to as a packet. A datagram is also exchanged at the transport layer when UDP rather than TCP is being employed.
Using a range of techniques to draw inferences which would not otherwise be obvious about information drawn from large amounts of data. The mining analogy comes from the similarities to finding hidden high value, such as gold nuggets or diamonds, in proportionately vast amounts of unwanted material.
A term which refers to the speed at which circuits or devices operate, measured in bits per second. The data signalling rate is a measure of how quickly data is transmitted and the data transfer rate is a measure of how much information is transferred excluding signal elements needed for housekeeping (bits for start and finish, synchronisation etc). See symbol, bit and Baud.
A wide-ranging term with many interpretations. Generally associated with the way an enterprise stores its data in a way that provides easy manipulation of it with the aim of business performance advantage.
Basic-rate ISDN has two 64 kbit/s communication channels called the B-channels and a 16 kbit/s channel for signalling which is also available for access to a packet-switched service. This 16 kbit/s channel is called the D-channel. The B channels were so called because it was the next letter after A for analogue! The signalling channel was considered a small increment on the communications channels and called the delta channel, shortened to D-channel.
Decision feedback equaliser (DFE)
An adaptive non-linear equalisation technique used in high-speed digital communications systems to minimise errors. It can be employed in a wide variety of systems and components such as transmission systems, cellular systems, backplanes and lasers. Equalisation is used to reduce inter-symbol interference (ISI) to improve the detection of bits. DFE is one form of equalisation which uses decisions based on the symbol stream to deal with distorted sequences. It assumes that the deduced ISI from a series of symbols can be applied to later symbols through a decision device and in a way that improves the signal.
Delay in normal English implies lateness. In telecommunications and electronics it can have many meanings and many are a feature of the medium in which the systems operate or of the design itself. It is therefore better defined as the time interval between two events. The time interval can be long, such as in store and forward message switching, or very short, such as milliseconds, microseconds or nanoseconds in transmission systems. In digital systems delay may be referred to as latency as described below. Delayed signals that are fed back to the origination are called echoes.
It is in transmission systems where the most common use of the term delay is to be found. This stems from the limiting condition of the speed of light (approximately 300,000,000 metres/second) which also applies to electromagnetic waves travelling through space. A signal being transmitted over a long distance will therefore experience delay, called the propagation delay. The simplest example is of a radio signal being sent up to, and back from, a geostationary satellite approximately 36,000km above earth. The signal will therefore experience a delay related to 72,000 km at 300,000km/sec which is about one quarter of a second.
Extra delays, compared to the speed of light, occur in the transmission of signals over cable systems. These vary by type of cable, the related transmission systems and whether the signals are analogue or digital. For example, analogue systems will also be affected by filters that will also introduce delay in the transmission path. Further examples are that pair-type cables will introduce more delay than air-spaced co-axial cables, whereas in optical fibre cables any extra delay will be minimal.
The propagation delay of metallic cables is related to the dielectric (the insulator between conductors) which determines a velocity ratio - which is the factor by which the speed of light is reduced in that medium. The transmission delay will include the cable propagation delay but include other delays due to the equipment employed.
In analogue systems the transmission delay can be dependent on frequency, mainly because of the effects of filters and, for example, a loaded cable looking like a filter. As a result, not all frequency components of a transmitted signal will arrive at the receiving end at the same time. This is called group delay and is a form of phase distortion that can be overcome by group delay equalisation (making the fastest frequencies slow down to the slowest frequencies).
Digital signals experience similar transmission delays, for example shorter pulses have higher frequency components which in turn may experience higher delays causing distortion of the pulse and eventually intersymbol interference (ISI).
Delay in digital systems is often referred to as latency that, as well as the transmission delays described above, also takes into account for example, length of packet and storage and packet inspection delays in routers. End-to-end latency in digital systems may also include application related delays such as server and hard disc access times.
See also quality of service for other parameters that affect the user's perception of network service.
The opposite of availability in computer systems. It generally means that someone is prevented from accessing their computing services through a deliberate action, usually malicious and often resulting from a virus, trojan horse or similar. There a number of ways of achieving DoS, of which the most common is flooding a network with more traffic than it can handle so that wanted traffic is suppressed or seriously degraded. Other methods can include physical damage, corrupted routing resulting in traffic redirection and technical exploitations of shortcomings in, for example, the transmission control protocol (TCP) or domain name server (DNS). The effect can be multiplied many-fold if multiple sources, such as the use of otherwise benign trojan horses called into play on demand, can be used to generate flooding traffic and this is referred to as a distributed denial of service (DDoS).
Dense Wavelength Division Multiplexing (DWDM) - see WDM
Design forms the bridge between requirement and implementation by representing the essential structures, logic, and data of products and their components. Design follows the conceptual and feasibility stages of a project management process, and may comprise a two-stage process of high-level design (HLD) sometimes known as solution design and component design. The control of design activities is realised by the use of change control procedures, design reviews, and designated design authorities.
A design authority, usually a chief designer, owns the design of a product throughout its lifecycle and is responsible for reviewing and authorising any subsequent design changes. The design authority ensures that such changes are reflected in all dependent documentation and will participate as necessary in design reviews and in the testing processes.
In any project there will always be one or more points where the project team should pause and ask whether they are going in the right direction, and if decisions made up to that point are still valid and correct. These reviews are usually held at pre-determined times depending on the project, so that future progress and direction may be dependent on the review outcome. The senior-engineer who will sign-off the high-level design documentation will usually set up a major review of a large project before the commencement of development.
Differentiated services (Diffserv) is a protocol for specifying and controlling network traffic by class in an IP network so that certain types of traffic get precedence - for example voice traffic which requires a relatively uninterrupted flow of data might get precedence over other kinds of traffic. Differentiated Services is the most advanced method for managing traffic in terms of class of service.
Digital audio broadcasting (DAB)
DAB was developed in Europe from the Eureka 147 project. It was then adopted by the ITU-R as a world-wide standard for terrestrial radio broadcasting delivering digital CD quality stereo audio to fixed, mobile and portable receivers. As well as offering better sound quality, compared to traditional analogue AM and FM broadcast systems, DAB allows broadcasters to transmit text or data relating to the programme so that listeners can read related information on a small LCD screen. A DAB broadcast signal has an overall capacity of 1.536 Mbit/s (called a "Multiplex") that can carry several digital services simultaneously - such as six stereo or 12 mono services or any combination in between. DAB can be extended beyond audio to use some of the "Multiplex" capacity to transmit data that is not related to audio programming, such as the broadcast of financial data and even digital video (which requires the full channel) - but see also Digital Video Broadcasting (DVB).
A European Directive says that an electronic signature should be considered equivalent to a handwritten signature (including, for example, its use as evidence in legal proceedings) if it meets a certain number of conditions. It must be an "advanced electronic signature", meaning that it:
- is uniquely linked to the signatory;
- is capable of identifying the signatory ;
- is created using means that the signatory can maintain under his sole control; and
- is linked to the data to which it relates in such a manner that any subsequent change of the data is detectable, i
- must be based on a qualified certificate; and it must be created by a secure signature creation device.
These requirements are generally interpreted as requiring a PKI with smart cards used as signing devices.
Digital enhanced cordless telephony (DECT)
DECT is a European wireless access standard developed by ETSI but, like GSM, now used in other parts of the world as well. Its preferred operation is in the 1880 - 1900 MHz band but provision is made for operation in other frequency bands. It is widely, and mostly, used for cordless telephony handsets by residential consumers but this only makes use of a small part of its capability which was included in the ITU IMT-2000 family and embraces data applications as well. Other uses include ISDN access, wireless PABX, GSM access, LAN access and wireless local loop (WLL). DECT uses multi-carrier, time division multiple access, time division duplex (MC/TDMA/TDD) techniques. Dynamic channel selection and allocation allow high capacity systems to operate in hostile radio environments to provide a high quality of service without frequency planning. Some additional information can be found in the tetherless section.
A wireless local loop (WLL) adaptation of DECT, called CorDECT, originated in India where it is used to provide telephony and Internet services in low-telecommunications density areas over a 10km radius (25km with repeater). CorDECT is used in more than 10 other countries.
In general the digital home is taken to embrace communications and entertainment and also house control systems. Its embodiment relies on the availability of consumer goods and services which may or may not be able to be integrated, at least initially. A digital home might therefore be able to share printers, scanners or other peripherals between computer; use a single broadband Internet connection simultaneously for many different devices and services; share the output of devices such as DVD players or web-cams with other computers and devices; consume on-line content in the most convenient form and in the most convenient location; and be able to control heating and lighting remotely when away from home.
A discussion on whether or not the digital home is only for the wealthy is in this paper on Digital homes - For richer or poorer, who are they for? BTTJ Vol 20, No 2.
See also home networks.
A means of providing new exchange-lines to customers some distance from the telephone exchange (central office) without taking up an individual pair for each. Typically it will use several digital transmission links at 1.5 Mbit/s (US standard) or 2 Mbit/s (European standard) to serve a remotely located rack of equipment which has line cards and then serves customers over short copper-pair cables. The exchange equipment will terminate the digital lines and may have multiplexing equipment and then interface to the exchange at voice level or omit the multiplex and interface at digital level (e.g. DASS2 UK or TR303 US). The remote rack will usually be installed in a small building, apartment basement or some such. Sometimes it may be installed in a controlled environment vault. DLC is a pair-gain system as it provides a discreet channel all the way back over the digital line and does not employ concentration. It is widely used in North America where average local-loop lengths are long. In Europe average local-loops are shorter permitting wider coverage by direct copper-pairs but there is also widespread use of remote concentrators hosted on the telephone exchange and making use of the central processor for call-handling.
Digital rights management (DRM)
This covers a broad range of issues associated with the management of digital content. An aim is usually to have an enforcable commercial contract to support the raising of revenue to support a business model covering all the stages of a value chain. To take a music example, it can apply to a CD, to a digital radio broadcast over terrestrial radio or cable TV, or to an Internet download.
There is no universally agreed definition although one is “the end-to-end (i.e. Content Provider to Consumer) management and enforcement of digital media usage policies, as specified by the rights owner or their agent, and the ability to account for usage”. In practice, many aspects must be considered some of which are listed below:
- at least some of the following: the end-to-end process including the content itself; hardware; software; a variety of underlying technologies; services and business processes; confining content to authorised use (access protection) and protection from copying.
- content can include: music; video; image; speech; text; data (including software) or anything else that can be digitised, and combinations of them.
- technologies include: the means of securing the rights through access protection and copy protection involving authentication, encryption, digital certificates etc. Commercial processes include: licensing; intellectual property rights including copyright; usage rules and subscription or payment by use - including for periods of time, defined numbers of usage or a defined replay platform.
- the value chain of content providers will vary depending on whether or not there is a physical representation (e.g. CD or DVD) and includes: writers; artists; artistes; aggregators; distributors; service providers; network operators; wholesalers/retailers; end-users.
Digital signal processing (DSP)
A fundamental building-block for much of telecommunications but often largely taken for granted by, and invisible to, the network systems engineer. It is about processing digital signals that were once analogue and relies on DSP chips - semiconductor silicon chips that are usually designed for a specific purpose and called digital signal processors - also known as DSP. DSP involves mathematically based computation and is a science in its own right with large representation at academic institutions accompanied by a large body of published work.
Digital subscriber line (DSL/xDSL)
A technology that uses electronic systems working over copper pairs in the access network to transmit data at high speeds. The data rate that can pass over the pair of wires reduces with distance. Planning rules usually determine a distance from the exchange at which tests must be performed to prove the viability of the chosen service level.
There are a number of variants. Asymmetric DSL (ADSL) is probably the most common as it delivers high-speed data (typically 500 kbit/s into the customer with a much lower return bit-rate from the customer - hence the term asymmetric) together with continued voice telephony service. A splitter is used to separate out the lower frequencies used for the telephony voice channel from the range of much higher frequencies used for the data channel. Very high bit-rate DSL (VSL) is still developing and allows a much higher data rate but predominantly from the cabinet to the customer rather than all the way from the exchange. Typically around 14 - 16.5 Mbit/s is apportioned between the two directions of transmission (10/4 or 14/2.5 according to band plan) but over very much shorter lines (<1km). Symmetric DSL (SDSL), such as Symmetric high bit-rate DSL (SHDSL), has the same data speed in both directions.
Digital video broadcasting (DVB)
A specific term also having a generic meaning. In the early 1990s a European working group into digital television resulted in the formation of the Digital Video Broadcasting (DVB) project. This group develops specifications for digital television systems that are turned into standards by international standards bodies such as ETSI and CENELEC. DVB's main open digital television standards are DVB-C for cable, DVB-H for handheld, DVB-S for satellite and DVB-T for terrestrial. They are designed with a maximum amount of commonality and based on using MPEG-2 packets as "data containers" together with the service information surrounding and identifying those packets. See also Digital Audio Broadcasting (DAB).
A directory organises and maintains data about objects that exist within a network in a structured way, making the information easier to understand and access. Directories also have the property that they are queried far more than they are updated.
Where close control is needed of the software on workstations/PCs on a network it is possible regularly to rewrite the disc with a clean copy of the desired contents of the drive (e.g. overnight). This is particularly useful where there is uncontrolled access to a terminal, such as with public workstations in libraries, Internet cafes, schools, universities etc. Such terminals are prone to the downloading and installation of unapproved software or even to software such as key loggers to collect passwords for subsequent illegal purposes. A number of products will automatically restore the authorised image to many terminals on a network provided that the server and network capability exists. The master copy of the correct image can be updated with new authorised software which is subsequently distributed to the terminals. The master image also provides a back-up capability as a simple means of restoring a software build after problems.
A colloquial term for a microwave antenna shaped broadly like a dish. More formally it is an antenna where the element is at the focus of a reflector in the shape of a paraboloid of revolution. In the same way that a searchlight provides a sharply defined beam of light from a source of light, so the dish can be thought of as a transmitting antenna for a radio signal with a narrow beam. It can also be thought of in reverse as a receiving antenna gathering in a very low level signal from a distant point-source transmission to deliver to a radio receiver. It is highly directional and whilst used for terrestrial point-to-point links it is extensively used for satellite communications, including for the reception of domestic television services.
Dishes vary in size, complexity and cost. A dish for domestic television will be mass-produced to a price with tolerances to match and typically be 60cm in diameter. A commercial satellite ground-station dish for transmitting and receiving may be XX metres in diameter and will be engineered to extraordinarily high tolerances across many parameters to maximise the ability to receive very weak signals. A dish is not the only form of microwave antenna and other forms of parabola (offset, truncated and cylindrical), horns and variations on slots in waveguides have all been used at some time.
In the context of telecommunications, a disruptive technology is a new product or innovation, very often drawn from another market, that sneaks into an established business because the 'threat' it poses to an established product is not recognised. Disruptive technologies generally display three characteristics: initially they underperform established products; they present new benefits that enable new applications for new customers; and their performance improves rapidly.
A distributed computing system is an arrangement of networked computers proving data through a client/server architecture.
Distributed denial of service - see denial of service.
A point at, or towards the end of, the access network where a multi-pair cable is terminated and the final connection is made for each customer line and is typically mounted on a telegraph pole. A DP can also be a wall-mounted cabinet in a multi-dwelling unit (apartments). A similar wall-mounted cabinet serving the same purpose in a wiring scheme within a customers premises (such as a office building) may or may not be known as a DP.
This refers to the use of systems for minimising the consequences of signal fading in radio transmissions. (In, in telecom companies as in others, it also relates to human diversity by minimising any discrimination due to ethnicity, religion, gender, age and sexual orientation). Diversity receiving systems take two forms - space diversity and frequency diversity. The former takes advantage of the fact that signals received on antennas at different locations do not fade together and where the optimum spacing between two antennas to overcome the problem depends on the frequency in use. Two or more antennas go to a separate receiver that feed into a common output where the strongest signal is selected electronically using a variation of an automatic gain control (AGC) system.
A system with two antenna/receivers is known as dual-diversity reception and with three as triple-diversity reception. Frequency diversity takes account of the fact that signals of slightly different frequencies do not fade synchronously. This was used in wireless telegraphy systems where the keyed carrier wave was also modulated at an audio frequency providing some discrimination between the carrier and the sidebands.
Domains/domain names/domain name system (DNS)/domain name server (DNS)
DNS is name resolution software that enables users to locate computers on the Internet or other IP network through domain names. Under the system, the DNS server keeps a database of host-or domain-names and their corresponding Internet Protocol (IP) addresses. This is now one of the most crucial components of the Internet. The article An Introduction to IP networks, BTTJ, Vol 18, No 3 provides more information on the domain name system and its realisation.
Dual diversity - see diversity
A system of operation in which transmission between two terminals can take place simultaneously in both directions. Full duplex means that the transmission media and the terminals can handle duplex working. Half duplex means that the transmission media can handle it but the terminals handle the transmission one way at a time - although some definitions of half duplex include what is more properly known as Simplex.
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