Encyclopaedic Dictionary of Information Technology and Systems
`
`Page 1 of 2
`
`Cawkell, A.E.
`Encyclopaedic Dictionary of Information Technology and Systems
`DE GRUYTER SAUR
`
`eBook (PDF)
`Reprint 2012
`Publication Date:
`October 2013
`Copyright year:
`1993
`ISBN
`978-3-11-096853-8
`
`See all formats and pricing
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`Overview
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`https://www.degruyter.com/view/product/161405?print
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`Encyclopaedic Dictionary of Information Technology and Systems
`
`Page 2 of 2
`
`Details
`
`23.0 x 15.5 cm
`
`vi, 339 pages
`
`Num. figs.
`
`DE GRUYTER SAUR
`
`Language:
`
`English
`
`Type of Publication:
`
`Reference Work
`
`Keyword(s):
`
`Subjects
`
`Library and Information Science, Book Studies > Libraries and Library Science
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`MARC record for eBook
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`Pacific and PTAT Atlantic cables, to offer a 64
`Kbps ISDN-like service, aimed at multi-
`nationals. One stop billing and integration
`with private networks will be included, and
`users will appear to be using their own inter-
`national network.
`Many businesses already have extensive
`private networks. The requirement of a Vir-
`tual Private Network (VPN) is to get at least
`the facilities currently obtained by using
`leased lines but at a lower cost. A VPN
`includes dynamic bandwidth allocation pur-
`chased on a "pay as you use" basis as an
`alternative to paying for dedicated
`lines
`which are only occasionally used at full capac-
`ity.
`
`NETWORKS - WIDE AREA (WANs)
`A WAN is any network covering a wide area -
`for instance a private leased line network.
`In the commonest type of telecommunica-
`tion network - such as the telephone network
`- any station is connected to any other by a
`switch. A network does not have to be
`switched. Large organizations may own a net-
`work of leased lines to carry heavy traffic be-
`tween
`sites,
`for
`instance
`carrying
`LAN-originated traffic.
`Two examples of WANs are shown on the
`next two pages. The first shows the British
`Library Network, reproduced here with their
`permission, as it is now before consolidation
`within their new centre. It consists of a num-
`ber of LANs in London and Yorkshire, each
`with numerous PCs connected to it, with the
`LANs
`interconnected with bridges and
`64Kbps private lines.
`The second Figure, by courtesy of Spider
`Systems, illustrates the use of Spider equip-
`ment for interconnecting several types of net-
`work.
`
`NEURAL NETWORKS
`See COMPUTERS - NEURAL NETWORKS.
`
`NLM
`
`National Library of Medicine.
`
`NLQ
`
`Near Letter Quality.
`
`NMOS
`η-type Metal Oxide Semiconductor.
`
`229
`
`NOISE
`
`NNI
`Network Node Interface. See under SONET.
`
`NODE
`A computer or switching device situated at
`connection points on a communication net-
`work to monitor, switch, or attach communi-
`cation channels.
`NOISE
`Unwanted signal. Interference caused by ran-
`dom electron motion, or by the intrusion of
`signals from external electrical equipment.
`Electrical noise is manifested as a hiss in
`audio equipment. It produces errors in data
`transmission where noise signals may be mis-
`taken for data signals. Electrical noise is of
`four kinds - thermal noise, flicker noise, shot
`noise and popcorn noise. Thermal noise is
`generated by the random motion of electrons
`in resistors; flicker noise is associated with
`emission from a hot surface such as a heated
`cathode; shot noise is generated by charges
`moving within semiconductors, and popcorn
`noise is a popping noise which is probably
`due to impurities within semiconductors.
`Noise value is usually expressed as the sum
`of the RMS values of the different noise
`sources in volts. Much noise is of the "white
`noise" type which means that it is generated
`over the entire frequency range of the equip-
`ment; accordingly the wider the bandwidth
`the greater the noise level.
`"Thermal", "Gaussian", or "White" noise
`is a fundamental property of all electrical cir-
`cuits. This type of noise manifests itself in
`audio-frequency circuits as a steady back-
`ground hiss which may be almost inaud-ible,
`but often becomes obvious on long distance
`circuits. The noise voltages generated are nor-
`mally small compared with "signal" voltages
`- that is those conveying information - and do
`not cause trouble.
`The telephone network is a complex of
`cables, switching devices and other equip-
`ment some old, some new. Bad contacts at
`switching points cause noise, so do the
`impulses caused by dialling which can break
`through by inductive coupling from one line
`to another.
`Non-telephone electrical equipment and
`electrical storms can also cause impulsive
`noise. It is this kind of noise, heard as clicks
`and crashes which may go on intermittently
`
`3
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