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10/5/2009
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| Information on these pages copyright of Ian Dufour | If you use this Encyclopedia regularly please make a donation to Tearfund |
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P
There are two main forms of packet-switching - connectionless (e.g. Internet Protocol [IP]) and connection-oriented (e.g. frame relay [FR], asynchronous transfer mode [ATM] and multi-protocol label switching [MPLS]).
The connectionless datagram form of packet switching is the one most commonly associated with packet switching and is similar to message switching using very short messages. Any message exceeding a network-defined maximum length is broken up into shorter units, known as packets, for transmission. The packets for a common destination, each with an associated header, are then transmitted individually through the network, possibly taking different routings, and delivered in an order which is independent of the order of entry. At the destination the packets have to be re-ordered to reconstruct the original message. As there is a variable transit delay from packet to packet it is less suitable for delay sensitive services. See also An Introduction to IP networks, BTTJ, Vol 18, No 3.
A message sent to someone or something at an unknown location. Whilst at one level it can be a broadcast message over a public address system it is usually associated with a personal wireless device called a pager with a tone or vibrator that alerts the owner to an incoming message. Sometimes the message is just that - a tone or vibrator which is known to mean 'call the office' but variants include numeric and alpha-numeric messages displayed on a screen. The pager coverage could be through a wide-area public radio system or a smaller area induction system (e.g. in a hospital). Public wireless paging systems have been largely superseded by cellular radio systems due to the low cost and small size of devices with much greater functionality but their place in the history of encouraging a mobility culture should not be under-rated. Paging as a term lives on in cellular radio systems as the location of a mobile device is first established by the system paging the mobile device long before (in milliseconds) it establishes a call or sends data.
A pair of wires is still one of the basic building blocks of telecommunications even after more than one hundred years. Whilst they have long since been replaced in long distance transmission they are still ubiquitous in the access network, in the home and even in Ethernet distribution up to at least 100 Mbit/s. The two wires are usually twisted together and combined with others in a cable, giving rise to the term twisted pair but the pairs can be spaced apart and parallel for some distance in overhead construction.
A cable containing one or more twisted pairs of wires usually made of copper and insulated with a plastics material (but other insulants can be used notably paper). They can be used internally in the home or in commercial buildings or externally. The number of pairs and the construction of the cable will depend on the purpose. A cable for internal home use or for Ethernet distribution might typically have four pairs in a light plastics sheath. An external cable would have a much harsher environment to deal with and would include a thicker plastics sheath, probably with a thin metallic moisture barrier underneath and then multiple twisted pairs. A distribution cable to the house may include just two pairs but nearer to the telephone exchange building these will have been aggregated into a cable containing thousands of pairs. See also quad cable.
A term used where electronic equipment is used in conjunction with copper pair cables to provide more channels than the underlying pairs provide at baseband. This can include access systems from simple 1+1 carrier systems that provide 1 extra channel on top of the baseband audio channel to digital loop carrier systems providing 120 channels over 8 pairs. For inter-exchange circuits PCM systems provide pair gain with up to 30 circuits over 2 pairs of wires. The term is generally used for speech circuits and is not usually applied to ISDN and for Broadband systems.
A pair or quad cable in which the copper conductors are insulated with dry paper.
A telecommunications API (application programming interface) allowing third-party application providers the opportunity to connect to existing network technologies. See The Parlay API, BTTJ, Vol 21, No 3 and Parlay X Web services, BTTJ, Vol 22, No 1
Passive Optical Networks (PONs)
A means of providing access network capabilities to smaller business customers and even residences using optical fibres. The essence lies in sharing one single-mode feeder fibre from the exchange over several customer's premises using a number of passive (i.e do not require power) optical splitters in the street network. Time Division Multiplex (TDMA) is used on one wavelength to support telephony and other services whilst the other wavelengths are used later to support future service upgrades.
The basic form is known as TPON (telephony over a PON) with a range of other varieties, including ATM over PON (APON), broadband over PON (BPON) and Ethernet over PON (EPON).
PONs were first described by BT in 1987 (Reference: Stern J R, Ballance J W, Faulkkner D W, Hornung S, Payne D B, Oakley K A; Passive Optical Local networks for telephony applications and beyond. IEE Electronic Letters, 19/11/1987, Vol 23, No 24).
Patents - see Intellectual Property
Most commonly associated with a coin operated telephone in the street in a piece of street furniture known in the UK as a telephone kiosk. Also widely available in public places such as stations, hotels as well as shops, cafes and pubs etc. The widespread adoption of mobile phones together with vandalism and theft at unattended sites has turned the economic operation of payphones into a challenge. The initial response included coinless operation involving calling cards but more recently they have evolved by incorporating screens and computer capability to the extent that some only handle Internet communications to the exclusion of speech. Some would regard the Internet Cafe as another extension of the theme.
A network where devices can communicate with each other without requiring a single controlling entity.
Peering is simply defined as an interconnection between two networks that facilitates the interchange of data. Specifically for Internet Service Providers (ISPs) it means that the customers of one ISP can interchange traffic with the customers of another ISP without transiting a third, and usually higher order, ISP. See The art of peering, BTTJ, Vol 18, No 3.
A form of testing aimed at telecommunications or computer networks where an authorised tester will make attempts to find lapses in security in the design and/or implementation of the network with a view to remedying them well before a hostile attack can take place on a live network.
Whenever resources are shared there are performance questions to be answered and a performance engineering job to be done. Typical areas include: the design of a solution to meet specific delay and loss requirements; working out how a solution scales and where the 'break' points and bottlenecks are; how the performance will be measured, monitored and managed; and determining control systems to optimise performance when the system is overloaded.
Performance is about understanding how networks perform by:
- Measuring their performance.
- Modelling their performance.
- Designing their performance.
- Preventing performance collapse under exceptional conditions.
Performance engineering is a key contributor to how the customer perceives the network and how much it costs to build and operate a network. See Performance - a retrospective view, BTTJ, Vol 20, No 3. See also availability, five nines, quality of service.
A very localised local area network (LAN) typically used by an individual to connect various devices over very short range - such as when seated at a desk or in a car. As terminal devices become more distributed with the component parts eventually incorporated into everyday objects the concept of wearable computing will evolve with a PAN used for interconnection.
A broad definition is that when something is modified in its configuration or behaviour by information about the user. It can be grouped into four areas: targeted marketing/advertising; customer relationships; service integration; and knowledge management.
Some definitions of personalisation are very limited and focus on targeting content at users based on their expressed interests or buying patterns. In BT, considerable effort has been applied to exploring the wider uses of personalisation and how services, tasks, devices, interfaces, etc can adapt to information in a user profile. Information in a personal profile need not be static but could change quickly with context. An example of this might be the user's current location. Mobile telephones can be used as an example to look at the success factors and, in particular, the value proposition for the user. The reality is that mobile telephones are very personal devices, hold personal contact information, fit into a person's pocket or handbag, and go everywhere with them. They provide a communications medium for contacting a person rather than a location or an organisation. The following paper provides an overview of the use and potential of personalisation. See Personalisation - an overview of its use and potential, BTTJ, Vol 21, No 1.
Phase in its common English usage meaning "stage of" finds many applications in telecommunications such as the life cycle phases of project management. However, it is a fundamental technical concept related to the sinusoid waveform that finds many applications of which one is phase modulation (PM). PM is one of the three basic analogue modulation techniques - the others being amplitude modulation (AM) and frequency modulation (FM). PM and FM are closely related.
Two sinusoids with the same amplitude and frequency can still vary in time such that one signal lags behind the other. Two such signals are said to be out of phase. If they were identical in time they would be said to be in phase. Taking a full cycle as 360 degrees and if the signals are 180 degrees out of phase they are said to be in anti-phase and the summation of the amplitudes is zero. The phase difference can vary infinitely between 0 and 360 degrees but some conditions such as 90 degrees, 180 degrees and 270 degrees out of phase are easier and cheaper to detect. Changing the phase of a signal is called phase shift. A change in phase that is undesired is called phase distortion. Two signals can be locked in phase provided they are of the same frequency and a commonly found circuit element to do this that is often realised as a semiconductor chipis is a phase locked loop (PLL): this is something of a misnomer as it is usually more concerned with frequency stabilty.
Phase difference can therefore be seen as being between two reference points on two sinusoidal signals and the concept of phase of a single signal is thus hard to comprehend unless it is related to the rate of change of that signal. This is what happens in phase modulation where it is very similar to frequency modulation. The modulating signal changes the phase in relation to the phase of the unmodulated signal. In a digital context the two stages of a binary signal can be used to modulate the phase (phase shift keying, fsk) as a two phase form. However, as with the other forms of modulation, more complex forms of phase shifting can be used to increase the bit rate compared to the symbol (or Baud) rate.
PHP (a recursive acronym standing for PHP: hypertext preprocessor) is an HTML-embedded scripting language. Much of its syntax is borrowed from C, Java and Perl with a couple of unique PHP-specific features added. The goal of the language is to allow web developers to write dynamically generated pages quickly.
A very small version of a microcell in a cellular network, serving a very limited area.
A flexibility point in the local access network used in the UK in the 1950's and 1960's that became a familiar piece of street furniture. They were called secondary connection points (SCP) as a second flexibility stage after the cabinet (primary connection point - PCP). As the system size grew they added complexity and were replaced with larger cabinets. The cabinet remains the standard PCP flexibility point to this day.
A personal identification number, usually used as a security feature in accessing computer systems. One example of the second of the security trio of "something you have, something you know and something you are". Sometimes used on its own (e.g. to log in to a computer system) but often used with something you have (e.g. a credit card).
A tool, or utility, that sends a packet to an Internet Protocol address to see if the device at that address is on-line. The response reveals the time delay of the packet in reaching the other device. The term is thought to originate from the sound made by a sonar receiver: the technique of sending a signal and receiving a reflected response was thought analogous to sonar and radar. See also traceroute.
The basic unit of colour in a computer image or display. A colour pixel really comprises three dots (red, green and blue) and the variations in them define the colour of the displayed image. The size of a pixel depends on the resolution of the display medium. The name derives from the term picture element, dating back to the time when black and white photographs were printed using dots, notably in newspapers.
Plastic insulated cables
Cables in which the conductors are insulated with plastics - see also paper core cables.
Two or more signals are said to be plesiochronous if their corresponding significant instants occur at nominally the same rate but not in the same phase. Typically the signals would have nominal synchronisation but over time there could be variations due to the two signals not being derived from a common clock. More strictly they are one form of isochronous, asynchronous digital signals. See also mesochronous and heterochronous.
The Plesiochronous Digital Hierarchy (PDH) is an ITU-T standard allowing digital multiplexing - see Units and Symbols . See also Synchronous Digital Hierarchy (SDH).
The unique address, or 14 bit code, for a node in the network used by the Signalling System No 7 in routing calls. The originating and destination point codes are called OPC and DPC respectively.
Radio waves leave an antenna and travel through space in straight lines with the velocity of light. The waves consist of interdependent electric and magnetic fields which act in directions mutually at right angles to each other and at right angles to the direction of propagation of the wave. If the electric field acts in a vertical direction the field is said to have vertical polarisation and such waves are best launched and received by a vertical antenna. If the electrical field is horizontal the waves are said to have horizontal polarisation and horizontal antennas are best. When an advancing wavefront encounters the surface of the earth or becomes deflected by one of the layers of the atmosphere a degree of cross-polarisation can be introduced which results in signals with both vertical and horizontal components present. When these components are equal the signal is said to have circular polarisation.
Poles - see Telegraph Poles
In networking, a policy is a set of statements that define how the network's resources are to be allocated among its clients. Clients can be other networks, enterprises, individual users, departments, host computers, or applications. Resources can be allocated based on time of day, client authorization priorities, availability of resources and other factors. Allocation can be static or dynamic.
All networks have policy, even if it does not claim to be a policy based network. For example the following policies are common in IP networks:
- Routing policy. Defines how the routing policy picks the best routes.
- Security policy. Defines who is allowed to use which resources.
- Performance policy. Defines the ratio of network resources to customers under normal operations and under failure.
Polyethylene/Polyethylene sheath
The usual material for providing the outer sheath of underground cables. Because of molecular migration over time they are not impervious to water so a metallic moisture barrier is usually bonded to the inside of the sheath.
An interface through which information passes. Ports can be physical or non-physical. Almost every electronic device - including computers, switches and routers and consumer devices - has inputs and/or outputs and thus has ports.
Well known non-physical ports are those found in the Internet associated with the transmission control protocol (TCP) where probably the best known is port 80 for the hypertext transport protocol (HTTP). There are, however, 65,536 TCP ports in total and the port numbers are divided into three ranges: the well known ports, the registered ports, and the dynamic and/or private ports. The well known ports are those from 0 to 1023, the registered ports are those from 1024 to 49151 and the dynamic and/or private ports are those from 49152 to 65535. A great deal of information about Internet ports can be found using an Internet search engine starting by entering two words: TCP and ports.
Well known physical ports include sound-in and sound-out on a range of devices and for device ports on a computer such as for the display, mouse, USB and FireWire. Network ports on a computer include the modem and the Ethernet (RJ45) connection but can also include non-physical wireless ports - both radio and infra-red. An optical transmission line terminating equipment will have an optical port for the fibre to be connected. Most physical ports are defined by their electrical or optical characteristics and their physical structure. The latter simplifies interconnection by avoiding the possibilities of, for example, a display being plugged into a USB port.
Power for telecommunications systems
Power systems are invariably taken for granted in telecommunications systems but cover a wide range of components and activities. Traditional telephone systems are powered centrally from 50v dc systems although the primary source of energy is from external ac supplies. Nowadays, a lot of the equipment sited at telecommunications facilities is computing equipmenr which has its own demands. Large telecommunications centres can have energy demands measured in hundreds of kilowatts.
Examples of central power systems are batteries, engine alternators, rectifiers, inverters, converters, circuit breakers, fuses, various distribution systems for ac and dc as well as various controllers and monitoring systems. Each country and operator has taken decisions over the years that differentiate particular solutions although many of the component parts are common.
Modern digital, wireless and data systems and remote electronics in the access network all still require power although the requirements are subtly different to traditional telephone systems. Modern electronics use so much power that the heat has to be dissipated by good power and heat management using airflow within racks and frequently requires heat extraction and fan cooling and in extreme circumstances requires refrigeration (air conditioning). Densely packed very high power consumption and heat generating devices such as high-power rf transmitters can even require water cooling.
Particular practices, such as earthing (grounding) and bonding have an impact on systems at all levels as anyone who has hunted down noise induced from earth loops can attest. Ringing sytems are also usually embraced under the power heading. Related industries are often referred to as HVAC - heating, ventilation and air-conditioning.
Powerline
This is a generic term for telecommunications services provided over plant used for the delivery of electric power, both in the wide area network and in the home.
Fairly self-explanatory, in that this covers a variety of ways of making payments for telecommunications services before they are used associated with a restriction on the service once the pre-payment has been used up. Typical examples are pre-pay calling cards for use in public payphones and pre-pay units credited to a user on a mobile network. Pre-paid services usually obviate the need for a contract between the user and the service provider as well as the need to provide a bill.
A means of providing priority access to telecommunication services for authorized users during emergency events. The overall aim is to provide a high probability of call completion during national emergency and disaster situations. This has been a feature of the public switched telecommunications network (PSTN) for decades but its meaning in next generation and mobile networks is still the subject of discussions in many standards bodies including the IETF, ETSI and ITU. The ITU standards use the title International Emergency Preference Scheme (IEPS).
An ISDN connection for a customer at E1 (2.048 Mbit/s) or T1 (1.536 Mbit/s) rate and connecting directly into a digital switching facility providing the ISDN set of services. See also basic rate ISDN
Private Branch Exchange (PBX)
A telephony switching unit used within an enterprise and not available for public switching. Also known as a PABX - private automatic branch exchange. When automatic switching was first introduced the PBX was classified into two types: the operator controlled PMBX (private manual branch exchange) and the automatic PABX. With the long past demise of the PMBX the generic term has returned.
At one stage the PBX was though to be under threat by Centrex - a service provided by telecom operators to meet similar needs - but it did not become a significant threat outside the US. The PBX is again under threat, this time because of the use of voice over IP and related technologies to combine a company's telecommunications requirements with its IT infrastructure by means of a small softswitch as a part of one common network and set of servers.
A circuit designed for the exclusive use of an individual or organisation and not connected to the public network (PSTN). Sometimes called a private line or a leased line.
Mobile radios used by businesses and organisations for their internal operations and not forming a part of the public network. Examples of users range from taxi companies, through utilities and delivery companies for work force mangement, to the emergency services. The widespread availability of cellular systems has seen it decline for commercial use but there has been an increased interest in it for emergency use, notably for the TETRA system. See Professional mobile radio - the BT Airwave public safety service and the path for technology and service evolution, BTTJ, Vol 19, No 1.
A network designed for the exclusive use of an organisation, invariably connected to the public network (PSTN) at various points. Calls/connections made wholly within the private network are referred to as on-net and calls/connections made from within the private network to other networks, public or otherwise, are called off-net. See also virtual private network.
Processing can be used in a context such as digital signal processing (where it is implied that something is done by a processor to the signal to change it), or as in word processing (where a word processor is a computer programme which provides advanced features to supplement basic numeric and text typing). However, the usual use of the word processor is in the context of a computer where it is the main element in processing data. Originally it was a short-form for the central processing unit (CPU) in large computers which had multiple functions, for instance arithmetic logic, control of other devices and storage. The advent of microprocessors and other devices has somewhat blurred this relationship - for instance storage is now rarely associated with the processor, even when it is provided on a common chip.
Procurement (equipment purchasing)
See Purchasing
The scale of many activities associated with telecommunications networks can be immense and project management skills are needed by many people working in the business.
A project is a sequence of related tasks or activities undertaken for the purpose of supplying a requirement to a specified time, cost and quality.
Project management delivers the project requirement. It is an essential discipline to successfully meet the requirements for new products and services, to build and enhance capabilities to meet customer needs and to help manage change. It is about planning, directing and controlling tasks or activities and resources - often cutting across functional and organisational boundaries. It calls on a number of specific and disciplined approaches.
The time element of a project is important because it indicates a finite duration for the completion of the project unlike a process, which is on-going. A group of projects, which together achieve a common purpose in support of the strategic aims of a business are often called a programme. A project manager is the person responsible for the delivery of the project to the client.
The overall mechanism by which radio waves travel from a transmitting antenna to a receiving antenna. The more important constituents are ground waves, sky waves and space (or tropospheric/line of sight) waves.
Protocols are a set of agreed formats and procedures that govern the transfer of information between devices to ensure that the received data is identical to the sent data. Most protocols operate at or between one or more layers of the ISO reference model.
A process where a prototype is produced for testing and evaluation and subsequent modification before continuing with either a further stage or the roll-out of the main product or service. the term can apply to hardware, software, processes etc. It is usually a stage in a formal project management process. Sometimes called 'alpha' and 'beta' trials or, for the implementation of a large telecommunications system, a 'first office application'.
Provisioning systems embrace the operational support systems interacting with Network Elements for the provision, modification, cessation and test of a service. The benefits of such provisioning systems are that they:
Public key infrastructure (PKI)
A PKI is an integration of hardware, software and cryptographic components combined with policies and procedures to enable business applications to operate in a secure environment. The policies and procedures provide management with enforceable mechanisms to safely operate the PKI in such a way as to assure due diligence and prudent business practices. In other words, a PKI is a tool to enable you to meet your security policy.
A process where an analogue signal is sampled at a certain rate, then quantised into a certain number of levels which are then encoded.
The design and provision of networks ultimately involves the network provider in contracts with equipment suppliers (also known as vendors). The largest network operators will spend several billions of Euros each year. The handling of the purchasing and commercial relationships needs a professional approach and is usually put in the hands of a specialist department. There are many aspects to be covered. These include:
- Procurement policies, including legal obligations and policies specific to the company.
- Legal compliance, notably local or regional legislation on competitive tendering.
- Ethical considerations.
- Signing powers for appropriate levels of expenditure.
- The training and competence of people with buying authority.
- Negotiation with potential suppliers.
- Terms and conditions of contracts.
- Logistics arrangements for the provision of goods and services.
- Resolution of changes to contracts and disputes.
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