R
Radio
The wireless transmission of information using radio waves. In telecommunications radio is most widely associated with cellular radio systems, terrestrial microwave, satellites, wireless local area networks (WLANs) and wireless local loop (WLL).
Radio paging - see paging
Electrical energy that has escaped into free space exists in the form of electromagnetic waves. These waves, which are commonly called radio waves, travel with the velocity of light and consist of magnetic and electric fields that are at right angles to each other and also at right angles to the direction of travel. The principal properties of a radio wave are the frequency, direction of travel, intensity and plane of polarisation. Radio waves are classified as follows: Very Low Frequency (VLF) from 10kHz to 30 kHz; Low Frequency (LF) from 30kHz to 300 kHz; Medium Frequency (MF) from 300 kHz to 3 MHz; High Frequency from 3MHz to 30 MHz; Very High Frequency (VHF) from 30 MHz to 300 MHz; Ultra High Frequency from 300 MHz to 3 GHz and Super High Frequency (SHF) from 3GHz to 30 GHz. See also propagation and frequency spectrum.Railway telecommunications
Rapid Applications Development
Rapid application development (RAD) aims to deliver systems in a shorter time than traditional development methods, whilst at the same time producing an deliverable of high quality and at a lower cost. Using RAD can shorten a development lifecycle by gaining intensive user involvement throughout the process and using CASE tools to improve development productivity. As such the approach is well suited to certain telecommunications and ICT projects.
The key elements of the RAD approach are:
- Workshops. All major decisions on the system requirements and design are captured at a series of intensive workshops. To make these workshops effective, it is important that all users who need to have an input are represented by people with the authority to make such decisions
- CASE and productivity tools. The design decisions are input onto a CASE tool during the workshop sessions. The development team use this information to generate and present prototypes during the workshops, and subsequently use the tool to generate the application code during the construction phase of the project; and
- Timeboxing. The system is implemented as a series of releases which are delivered within timeboxes of pre-determined duration. The users agree on which functionality should be delivered within each timebox. If progress is slower than planned, the timebox end-date remains the same, but some of the lower priority functionality is left out of the release, to be re-addressed in a subsequent timebox.
A term used by telecommunications and computing people in sometimes complex ways and not always meaning the same thing. One (telecommunications) definition of it is "when milliseconds matter". It means no perceived delay or latency when something is happening.
The transmission of speech (mostly two way) and video (mostly one way usually with associated speech) provide onerous demands on telecommunications systems. A conversation between two people happens in real-time. When a telecommunications system is interposed between them there should be no perceived delays. If there are delays it becomes disruptive - a geo-stationary orbit satellite channel has delays that are not acceptable to some people. Any longer delays mean avoiding simultaneous two-way transmission and resorting to half-duplex/simplex, i.e one person speaking at a time with verbal handover to the other - or to messaging which implies storage and later processing.
Signalling is another extreme example of the need for millisecond response times and provides a source for the one of the most onerous challenges in real-time communications. Signalling messages sent, received, acknowledged and acted upon (sometimes multiple times) in SS7 must be completed so quickly as to offer no perception of delay to call set-up time - including for complex intelligent network calls that often include messages between multiple distantly located network elements.
The real-time performance of systems is impacted by the number of transactions being completed at the same time.
In the computing world real-time tends to mean that data is processed when it arrives rather than being stored for later (which could be as long as for overnight batch processing). Originally the delays and latency acceptable for general computing were of a different scale to those acceptable in telecommunications (hence the development of special processors and languages for telephone exchange control). With the advent of the Internet and the user delays, manifested as latency, becoming more visible to users the two worlds are converging in their views and their performance capabilities.
There are various meanings of receiver. It can mean: a radio or television receiver (when associated with a transmitter in the same case/structure referred to as a transceiver); the receiving end of a circuit in 4-wire transmission (where send and receive is relative to that point in the circuit); the earpiece that converts electrical signals into sound in a telephone handset.
Reflection is the reversal of direction of an electromagnetic ray or wave or a particle on encountering a boundary. It is particularly relevant to telecommunications in the context of light in optical fibres and with radio waves (see sky waves) and is important in all forms of line transmission where an impedance mismatch causes part of the signal to be reflected (see standing wave). In light and radio communication the following diagram shows the incoming path of the ray (or wave or particle) called the incident path with the outgoing path called the reflected path.
Assuming a plane suface (i.e. perfectly flat), the angle of incidence equals the angle of reflection measured about the normal to the plane (i.e. an imaginary line at a right angle to the plane). The boundary may not be perfectly reflecting, may not be perfectly flat, or may be deliberately shaped - as in a microwave dish - to focus the incident path. A surface that is not flat can cause scattering. An example of how a shaped parabolic reflector is used to focus an incoming signal from a satellite is shown in the following diagram.
Refraction is the change of direction of an electromagnetic ray or wave travelling in one medium when it encounters another medium and the two media support different velocities of propagation. An example is light passing from air into water where, say, a coin at the bottom of a bowl of water looks as though it is displaced from its real position. Refraction is particularly relevant to telecommunications in the context of light in optical fibres and with radio waves. The diagram shows how a ray in one medium is refracted as it enters a different medium.
The amount of refraction depends on the velocities of propagation of the respective media that are expressed as the refractive index (n) of the medium. The angles of incidence and refraction are governed by a simple relationship known as Snell's law. In some circumstances (when n1/n2 > 1) the ray will be reflected rather than refracted and the angle at which this happens is called the critical angle. This is significant in step-index optical fibres. In some cases the medium is not homogenous and will have a varying refractive index that results in curving the ray. This can apply to radio waves in the troposphere (see sky waves) where the refraction varies at different heights and according to atmospheric conditions and in graded-index optical fibres where the varying refractive index is part of the design.
The use of a prism to produce an optical spectrum occurs because of the different velocities of propagation of the different colours - or wavelengths - and they are refracted by differing amounts. This is similar to group delay in transmission lines.
A device used in digital transmission to reconstitute a distorted signal as it passes along the transmission path. It examines the incoming signal and identifies each element as a 0 or 1 and automatically regenerates appropriate 0 or 1 elements for output to line.
In Strowger days a regenerator was an electro-mechanical device used to regenerate 10 pulse-per-second dial pulses over connections from distant telephone exchanges. It did, however, meet the above definition completely, although fortunately all further realisations have been wholly electronic and are now capable of speeds of tens of Gbit/s for digital transmission systems.
An optical regenerator is generally distinguished from an optical repeater by converting from optical to electrical to perform pulse reshaping and then back to optical - this is more properly called an opto-electrical regenerator. Optical regeneration at the photon level is possible but is still largely at the research stage.
Relay
A basic building block of electro-mechanical telephone exchanges - picture required.
It also means relaying signals - as in store and forward message switching.
The probability that a service, network, system or component will perform within certain limits over time. At once both a simple concept and yet a significant area of study known as reliability engineering, based on statistical techniques. In telecommunications reliability can apply to a network, a node, software systems or modules, cables, equipment and sub-units or circuit boards or of any device or component. In fact anything that goes into providing an end-to-end service. A user requirement may be phrased in terms of availability or other quality of service measure which can then be apportioned across the many constituent parts of the service. See also five nines, performance, quality of service.
An amplifier inserted along the route of a transmission channel to overcome the effects of attenuation of the signal and to counter other effects such as crosstalk and noise. Voice frequency circuits would typically have an amplifier inserted in the route every 60km. High frequency systems, such as co-axial cable systems, would have amplifiers inserted more frequently, say between 10 and 20 km depending on the bandwidth of the system. In the modern digital era the equivalent is a digital regenerator.
A building, or part of a building, housing repeaters. Typically a small part of a telephone exchange floor area would be allocated to repeaters and other transmission equipment and be called a repeater station. In larger buildings such as city telephone exchanges a whole floor might have been allocated to transmission equipment. In some cases a small building might be dedicated exclusively to housing repeater equipment - typically many of them along the route of an inter-city co-axial cable.
The advent of smaller electronics that could be housed in underground repeater cases and of optical fibres that would span many inter-city routes without any amplification being needed largely did away with the need for dedicated repeater stations. Whilst areas of exchanges for transmission equipment still persist the blurring of the traditional boundaries between switching and transmission equipment (e.g. with packet switching and IP) have made the term largely obsolete.
Requirements Management is the activity of dealing with requirements throughout the lifecycle of a project. It is a co-operative and iterative process that controls the relationship between the proposed product or enhancement and the solution to that proposal.
Capturing requirements is not an isolated activity at the beginning of the lifecycle. Requirements are organic, they grow and develop as understanding increases. A solution evolves in response to changing requirements and deepening understanding. Requirements therefore need to be managed throughout the lifetime of a product or system by a team that includes customers, suppliers and analysts. The aim is to achieve in partnership an optimum solution, based on requirements that are driven by a business case and by operational constraints.
If the solution is not based on what is required then there is little chance that what is delivered is what is needed. Many studies have made the observation that amending a requirement before delivering the product is many times cheaper than amending a delivered solution. However, a requirements manager must work closely with the larger project management team.
One of the basic forms of networks.
Risk Management (of a project)
A risk is an uncertainty, which could either provide opportunities to increase forecast benefits or prevent the achievement of a project's aims. Managing risk does not simply mean responding to events when they happen, it means doing as much as possible to anticipate events and to plan and act accordingly.
Managing risk in projects breaks down into three main areas: identifying risks and any obvious responses to those risks; assessing the risks in order to get a better understanding of the chance of them happening, their possible effect and choosing the appropriate responses; and finally making decisions and taking action to cope with the risks.
There are many benefits in managing risk in the wider business arena, for example, to make sure that investment opportunities are prioritised; in not starting 'no-win' projects; in making sure that budgets are realistic; gaining significant benefits during the early phases of a project; setting plans for monitoring and control. Project workshops are ideal for carrying out initial risk assessments. More information can be found under project management.
The process of determining and prescribing the path or method to be used for establishing connections or forwarding messages. Routing is about:
- Establishing the network topology.
- Calculating the best path across the network topology. The best path may be based on the following:
- Shortest path.
- Least loaded path.
- Bandwidth available.
- Fastest path.
- Type of traffic to be routed.
In some network technologies the network topology discovery process is separated from the best path calculation process. In IP networks the routing process is conducted by the routing protocols, an example of which is OSPF.
Routing is often confused with "switching". The basic concepts of routers and routing are given in An Introduction to IP networks, BTTJ, Vol 18, No 3.
A test routine used by teleprinter maintenance engineers because keying R and Y continuously incurred the maximum number of mechanical reversals and tested the machinery before putting it back into service. The same string was used to test the transmission signals, notably the finely tuned relays used in MCVF (multi-channel voice frequency) systems. The origin lies in the 5 unit code where both R and Y have alternate Mark and Space (0s and 1s, hole or no hole, in punched tape).
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