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10/5/2009
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T
A widely used abbreviation for "Telephone Company". Occasionally also spelt teleco. From the early part of the 20th century national telephone companies were generally government departments with a monopoly on telephony services and were usually associated with the postal authorities. They were often referred to as PTT's, short for Postal, Telegraph and Telephone administrations. In the later part of the 20th century three trends made companies with this origin less important: de-regulation (for the newcomers, manifested as regulation for the former monopoly), competition and privatisation. Deregulation and competition meant that numerous new companies could enter fields previously occupied by the state monopolies. Privatisation meant that ownership of the former state monopolies moved to shareholders via stock market flotations. Whilst the erosion of monopoly through regulation meant that the passage of time should enable newcomers to reach similar size this has not yet happened in fixed-line telephony. In practice the term telco is used more to refer to the former monopolies than to the newcomers.
The ubiquitous presence of wooden poles at the sides of streets and roads is largely taken for granted and they still referred to by their original title reflecting the days before telephony. Nowadays they are also known as telephone poles. They are usually wood, deeply treated with creosote for longevity, although a few (hollow) metal poles (see picture) were used at one stage. In the early days of the 20th Century long-distance transmission systems were carried alongside trunk roads (and railway lines for railway communications) on poles carrying multiple heavy gauge copper wires. These were gradually replaced by underground copper cables, which in turn were replaced by carrier cables, then microwave systems and co-axial cables and finally by optical fibres. In recent times almost all poles have been used for the final distribution to customers, usually used singly, known as a DP or distribution point, fed by an underground cable up to the 'final drop' to the house. The pair of wires going to the house used to be a pair of open wires but for many years it has been usual to use a dropwire comprising a pair of wires with plastic insulation. Occasionally a route of several poles is used (e.g. along a farm lane) with the final one being the DP. There are still some pole routes left in remote parts the UK carrying all or a part of the circuits to a small rural exchange.
Telegraphy is a term that goes back to the origins of telecommunications itself, with morse-code senders and receivers and other similar devices used to transmit information over wires using interrupted DC current. The term was extended to include teleprinters (teletypewriters) and even early facsimile machines. At some stage as bit rates increased, electronic terminals became the norm and transmission systems used modems and then digital transmission, the term became hopelessly old-fashioned and was replaced with the more generic term 'data'. The term was alive and well in the post second World War era but had slipped peacefully away by the early 1960s.
Wireless data transmission from and to vehicles.
A terminal device, including a microphone and earpiece, that provides a human interface for communicating by voice over a distance. Whilst two telephones (and a battery) allow communication over a pair of wires a telephone is usually connected to a private or public telephone network. This allows global communication using the public switched telephone network (PSTN).
A generic title used for many years in the UK and many other countries for the building housing telephone switching equipment. This ranges from the smallest rural local exchange serving just a few customers to buildings in cities housing multiple local units and other major switching centres as well as much other telecommunications equipment. A building housing just transmission equipment but no switching equipment was called a repeater station. With the growth in packet-switching (IP) equipment the boundaries are blurred as the traditional usage of the term related to circuit switching.
The term was also applied more specifically to the local exchange, i.e. the one where the local network copper-pairs terminate. This is also known in the USA as the central office, the end office or the class 5 exchange (as the end office in a five tier switching hierarchy).
The transmission of live messages in the audio frequency range between distant users.
The illusion of presence from a distance achieved by specialised terminal devices linked by telecommunications. Often associated with virtual reality.
An electromechanical device taking a digital input and converting it to a printed output. Early teleprinters produced a a printed strip, later ones provided outputs on roll paper, including pagination. The input was usually a 5 unit Baudot code. The code could be stored by punching holes in paper tape and later read by a punched-tape reader. Teleprinters were used for transmitting the content of telegrams, where the printed tape was stuck on to a telegram form for hand delivery, for all forms of point-to-point text communication, both private and military, and were also the mainstay of the public-switched telex network. Teleprinters were also known as Teletypewriters or Teletype machines, especially in the US, where they were sometimes abbreviated to TTY.
The science of performance engineering in telecommunications networks originating from the work of Erlang.
A specific variety of a teletypewriter made by the Teletype Corporation of America. See teleprinter.
A circuit-switched public data network separate to the public switched telephone network(PSTN). Originally set up for the exchange of traffic at a 50 Baud line-rate between two teleprinters anywhere in the world.
TBA.
A key stage in the delivery of any project whether it is for a new electronic component or a circuit board or a piece of equipment or a fully interconnected telecommunications system or a new service. It therefore covers a huge range of activities but usually is to confirm that the design meets its intended purpose. This can include verification that the design of a component meets its specified tolerances and can be manufactured. It can also cover confirmation that each component or part that is produced meets the specification - by the use of automatic test equipment on all items, or by sampling. In telecommunication networks a key stage in the delivery of new functionality and new services is integration testing which proves that a number of parts can be integrated into a larger system and work under at the performance levels needed for live operation. Another variant of testing is penetration testing where an authorised tester will look at a network for lapses in security with a view to remedying them well before a hostile attack.
The paper Automated testing as an aid to systems integration (BTTJ - ref) gives an insight into the main elements required for the successful automation of testing large complex software systems.
Tetherless, in the context of communications, is generally taken to mean that the user has total freedom of location and total freedom from wires. All digital communications devices need to take in bits and electrons (and many need to emit bits as well) and frequently the bits and/or electrons travel along wires to the device. So in practice tetherless means locally-powered wireless terminal devices. From a systems point-of-view this implies inclusion of their base station which will usually not be battery-powered and will have wired connections. This definition of tetherless covers the ubiquitous mobile phone, the pager, infra-red remote controllers, PDAs (personal digital assistants) and many other devices.
This stands for terrestrial trunked radio, which is a set of standards developed by ETSI (European Telecommunications Standardisation Institute) as a common mobile radio infrastructure throughout Europe and principally aimed at public safety services such as the police, fire and ambulance services. It draws on experience from private mobile radio (PMR) despatch systems and also cellular wireless systems.
See Professional mobile radio - the BT Airwave public safety service and the path for technology and service evolution, BTTJ, Vol 19, No 1.
Probably the ultimate form of an International standard. It forms a basis for telecommunications and without it digital interconnection would be impossible. The earliest forms of mankind recognised days, the movement of shadows and the phases of the moon. This led to increasingly accurate forms of measurement and the development of clocks from the likes of Stonehenge, through sundials and many other unique and innovative contraptions to the timepieces we know today. One trend has been that the time interval that matters has reduced: thousands of years ago accuracy to a day would be good; a thousand years ago an hour; 500 years ago a minute; a hundred years ago a second. There is no reason to assume we have arrived. The second was only redefined in terms of a Ceasium atom's resonant frequency in 1967 and the current time standards used within the Global Positioning Satellite GPS) system are accurate to within 100 billionth's of a second. As electronics get ever faster (for example clock speeds in computers and multi-Gigabit/s transmission systems) the time intervals used get ever shorter and the need for synchronisation gets more challenging.
A specific and identifiable time interval in a digital transmission transmission system. Most commonly used in the context of frames in pulse code modulation (PCM) multiplexing, where for instance, timeslot 16 in a 32 timeslot frame will carry signalling data.
This stands for Telecommunications and Internet Protocol Harmonisation Over Networks. It was a project of the European Telecommunications Standards Institute and is a registered trade mark of theirs. It investigated the issues of service level interworking between circuit-switched and packet-switched networks and how inter-networking takes place. It also developed the means that generic services could be defined independently of the underlying network technology through a common architecture, appropriate protocols, QoS recommendations and conformance testing standards. Its work was absorbed into the ITU-T in 2003 due to its International, rather than just European, importance.
A token is a combination of bits used on a particular type of local area network (LAN) to control the transmit privileges of a computer on the network. The token is passed from computer to computer so that they can take turns in transmitting. Two forms of networking were defined: token bus (IEEE 802.4) and token ring (IEEE 802.5). In practice these form of LAN largely represented proprietary interests and Ethernet (IEEE 802.3) became the standard widely adopted around the world.
A diagnostic tool, or utility, that is used to trace the route of a packet between one computer and another over an Internet Protocol network. It sends a sequence of packets with characteristics that induce each router in the path to identify itself and thereby builds up a picture of the number of hops and time delays. Most operating systems have a traceroute utility built-in but others are available and web-based versions are available as well. See also ping.
Generically this refers to meaningful information or communications being carried by a transmission channel or switch. It has some specific meanings in the context of teletraffic and IP networks. See also Erlang.
See article on MPLS: Traffic engineering, BTTJ, Vol 18, No 3.
A combined transmitter and receiver, often in the context of a radio frequency device in a single enclosure but equally applicable to functions on a silicon chip.
Generally taken to mean the medium by which signals are transported from a source to a destination. The medium can be analogue or digital and and carry voice, data or image. In older times telecommunications disciplines fell into either 'transmission' or switching' but the advent of the digital age and packet-switching have made common technologies ubiquitous and made the distinctions less obvious. Nevertheless, an optical-fibre line-system would be clearly recognised by all as a transmission system as would a point-to-point microwave link. Other transmission systems include digital subscriber lines (DSL), cable television and any wireless link including satellites. These transmission systems can interconnect through switching devices and other transmission systems to form an end-to-end transmission between source and destination. There is also an end-to-end dimension to transmission which includes the bandwidth available and various degradations which can affect analogue and digital signals differently. Degradations and impairments can include attenuation, delay, distortion, echo, error, fading, jitter, noise and wander. See also coding and protocols.
The transmission line and the transmission equipment impose various restrictions on the signal to be transmitted. Where these restrictions are significant, such as in digital systems, it is necessary to match the source signal to the transmission capability by encoding the signal. Many of the requirements of the encoding system are common to general coding requirements, such as transparency, unique decodability and efficiency/redundancy. Other issues like error correction, spectrum shaping and timing have a special relevance at the physical layer although often handled at higher layers as well. Many line codes for digital transmission have been evolved. Some of them include 2B1Q (two binary, one quaternary) 4B3T (four binary, three ternary), Alternate Mark Inversion (AMI) and Manchester coding.
A transmission line is the physical medium for transmission. It is usually a pair of wires, a coaxial line, an optical fibre or a waveguide. Line transmission is the theory of the propagation of electric waves along transmission lines. Usually assuming an infinitely long uniform line the theory covers attenuation, velocity of propagation, phase, characteristic impedance, standing waves, skin effect etc.
In its widest sense it is a sender. It is generally used in terms of a device for transmitting signals over radio frequencies. See also transceiver. It also refers in line transmission to the device sending signals down a cable, for instance the output from an amplifier. In the UK it was also used as the term for the microphone, especially of the carbon granule variety, in a telephone handset.
As a general term in telecommunications it is usually taken to mean the transmission network - the means of transporting signals around the network. It does have a more specific meaning at layer 4 of the OSI 7-layer reference model where it relies on the lower 3 layers (layer 1 physical, layer 2 data link and layer 3, network). The OSI transport layer provides data-transport from a source machine to a destination machine independent of the physical network(s) in use. A good example of why engineers and computer scientists don't always understand each other.
A cable containing copper-pairs which also includes a metallic screen across the diameter of the cable to separate the pairs for the two directions of transmission to improve the crosstalk characteristics for digital transmission systems carried on the pairs.
One of the basic forms of networks.
The launch of a new piece of equipment, new software or new service will usually be preceded by trials. These are sometimes phased, in that the first release is subjected to a trial, then modified in the light of experience and trialled again, These phases are often referred to as alpha and beta trials after the first two letters in the Greek alphabet. Some trials lead nowhere and the new item is abandoned but in general the process leads to better quality and a reduction in the cost of failure before a product or service is replicated and sold.
A term usually used by telecommunications operators, but especially cable-TV operators, to describe the ability to deliver a package of cable-TV, telephony and data (Internet) services to customers.
The lower part of the earth's atmosphere in approximately the first 15km adjacent to the earth surface. The thickness depends on season and latitude. Radio signals that travel wholly in the troposphere are called space waves. See also ground waves, ionosphere, propagation and sky waves.
Above 20 - 30 Mhz radio waves arrive through the troposphere, are called space waves and communication is generally 'line of sight'. During World War II radar operation interference was noticed beyond line of sight distances and investigations showed it to be due to inconsistent refractions caused by minor variations in temperature and humidity. The end result was that a tiny proportion of the power leaked 'over the horizon'. This was used in the 1950's onwards, in the days before satellite communications, for multi-channel telephony systems. These were principally military, including between the USA and Europe via relay stations in Canada, Greenland, Iceland and Scotland. Some civilian uses included communications to deep-sea oil rigs. The high signal losses involved necessitated the use of high-power transmitters, large antennas and low-noise receivers, all at the leading edge of technology in their day. For those in the vicinity of the large unused signal it also led to the rather derogatory rhyming epithet of "catastrophic splatter". There are still some ongoing applications in remote areas and for tactical deployments, including military, and where a satellite is either not available at all or not at a competitive cost.
A US term originating from the generic name for a form filled in by an operator when taking a fault report from a customer. It is still used in a wholly automated and electronic environment and concerns the handling of fault reports from a service viewpoint. The fault reports can be raised by customers or from network alarms, and the system supports them being assigned to users for resolution (or to trouble ticketing systems of other organisations), with built-in workflow and escalation.
A cable containing insulated metallic conductors where each pair of wires are twisted together. The cable can contain a single twisted pair but more usually the cable will include more than one. Some external cables used near to telephone exchanges can include thousands of pairs, made up in a number of units before being collectively sheathed in one cable.
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