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10/5/2009
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Half-duplex - see Duplex
The means of continuing a call when a mobile crosses from one cell to another in a cellular wireless system. Handoff generally refers to analogue systems and handover to digital systems. Different processes apply in a CDMA system.
A term generally used to refer to a mobile phone, a DECT phone, or to the handheld part of a fixed telephone containing the earpiece and microphone.
Fourier discovered that any periodic waveform (i.e. one that is cyclical and repeats after a fixed time interval) can be expressed as the sum of a series of harmonically related sinusoids (a waveform with the shape of a sine wave). Even a square wave can be reduced to its component sinusoids, so the underlying principles extend from the analogue world into the digital world. Harmonically related sinusoids are sinusoids whose frequencies are integer multiples of some basic frequency. For example, taking a group of sinusoids of 10kHz, 20kHz, 30 kHz, 40 kHz and 50 kHz the 10 kHz signal is called the fundamental frequency with the others respectively called the second, third, fourth and fifth harmonics. Unwanted harmonics can be removed by filters.
The unit of measurement of the frequency of a signal in cycles per second. One Hertz represents one cycle per second. Named after Heinrich Hertz, a scientist contributing to the discovery of electromagnetic waves.
Two heterochronous digital signals are isochronous, asynchronous digital signals whose respective timings have the same frequency values only nominally and can vary within a given tolerance range. See also asynchronous, anisochronous, isochronous, mesochronous, plesiochronous and synchronous.
These are platforms in the stratosphere being considered for broadband fixed wireless acess (BFWA), mobile communications (2G and 3G cellular), rural telephony broadcasting, emergency/disaster applications, military communications, etc. The platform, which could be tethered or floating ballons or an aircraft, would typically be at altitudes of 17 - 21 km where wind currents are low and commercial aviation is unaffected. Their coverage area is such that they could replace a large number of base stations and have the advantage over satellite systems of lower path loss and better latency due to the shorter transmission path. At the time of writing there are many systems in development but none yet deployed commercially due to the considerable number of challenges to be overcome.
HiperLAN is a wireless local area network (WLAN) standard for the 5 GHz frequency range evolved in Europe and similar to the IEEE 802.11 standard. The European view had been more forward looking than the IEEE standards evolution with an emphasis on future compatibility with ATM, IP and 3G core networks as well as fixed LANs. However, the 802.11 series has had a marked advantage in time to market for widespread consumer use any advantages of HiperLAN in an evolving wireless-centric core network design is still uncertain.
TBA
See also:
Introducing home area networks, BT Technology Journal, April 2002.
Home Area Network Technologies, BT Technology Journal, April 2002.
A service provided to Internet content providers that allows, for example, the data for a web site to be held (hosted) on a server in a data centre - or 'server farm'. The larger data centres will usually be sited close to a major node in the Internet to allow fast interconnection to a point providing some alternative routing options without dependency on an Internet Service Provider (ISP). The high-speed access data links will often be from more than one communications provider to give further redundancy and improve availability. The data centre will be staffed 24 x 7 to provide rapid repair times in the event of failures and have standby power capabilities.
A small business or a consumer's home web-site may be on a server shared with many others whereas a larger corporate site may be either on a dedicated server or on one owned by the company but sited in the hosting centre for maintenance and access to the standby power and communications facilities (this is called co-located hosting).
The same concept applies to data other than web-sites. For instance it is possible to host applications on the servers for remote use by a company or even for general 'pay by use'. Hosting applications as a service for sale is done by an Applications service Provider (ASP) which may or may not be a business combined with the hosting company.
In practice we can use a 2-wire circuit for transmitting in both directions. We do this by using some techniques to combine the two directions of transmission either in the device, the circuit, or both. The principal device for doing this is a 2-wire to 4-wire converter, often called a hybrid.
An architecture adopted in cable-TV systems using optical fibre distribution in the trunk part from the head-end to a series of smaller co-axial cable systems covering the distribution and subscriber drop. Each of the smaller cable systems is fed with its own fibre back to the head-end.
A link from a page viewed in a browser to another. Used extensively in the on-line version of this encyclopedia.
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