A glossary of common terms used within the telecommunications industry.

Click a term for further information

Asymmetric Digital Subscriber Line

An Asymmetric Digital Subscriber Line (ADSL) is a form of digital subscriber line (DSL) in which the passage of information is weighted heavily in a single direction, typically from the service provider to the customer. This means that the bandwidth in the customer's direction (known as the "downstream" signal) is far greater than the bandwidth in the opposite direction (the "upstream" signal). This also means that the downstream bandwidth tends to be far greater with an ASDL service than with an equivalent Symmetric Digital Subscriber Line (SDSL) service, in which the bandwidth is roughly equal in both directions.

ADSL can work in more than one way, but usually signals are divided into multiple, independent channels of information within a line, known as "bins". Throughout its operation, the system will try to determine, at any given moment, through which combination of bins the best signal to noise ratio can be achieved, and will adjust its usage of the bins accordingly. In this way, distortion of signals can be mostly avoided, and quality within the line can remain consistently high.

Mainly due to their impressive downstream speed, ADSL services are used far more widely than their Symmetric Digital Subscriber Line (SDSL) equivalents. As well as this, because of ASDL's comparative low cost and ease of installation, even newer technologies like optical fibre lines, despite superior speed and efficiency, have failed to make a serious dent in the ADSL market.

Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a standard method of sending digital signals (DSL services, such as internet, video, VOIP) over digital lines, and was originally developed for use over copper based (POTS) lines. More recently, with the advent of optical fibre lines, ATM has also become a popular method of data transfer within optical networks.

This transfer is done in Asynchronous Transfer Mode (ATM) by using a form of Time-Division Multiplexing known as asynchronous Time-Division Multiplexing (or statistical-Time-Division Multiplexing), in combination with a cell based system (see: Asymmetric Digital Subscriber Line [ADSL]). This system breaks both simple and complex signals alike into much smaller, simpler packets of data. Each of these packets of data carries a small amount of information (in what is known as its header) about the channel and path of the data it contains, and this means that they can be reordered if necessary, without being lost or misread, making ATM services very flexible.

The mode is referred to as "Asynchronous" because although Time-Division Multiplexing is used, the management of timing and data is not directly synchronised between the two ends of the line. Instead, at the receiving end, the information contained in the each packet's header is used to work out what the data in that packet is for, and to redirect it accordingly.

This means that different signals can be managed effectively, that data can be sent in a wide variety of forms, and that bandwidth in the line can be managed fairly flexibly, but because an Asynchronous Transfer Mode (ATM) system carries so much additional information along with the original signal (which, naturally, takes up bandwidth), it also means, sometimes, that the system can be significantly slower than an equivalent Synchronous Transfer Mode system would.

Nonetheless, because of its flexibility in dealing with a wide variety of data types and combinations, as well as its ability to adapt to circumstances, Asynchronous Transfer Mode still has a number of advantages over other modes of data transfer, and is very popular for a range of applications; Digital Subscriber Line (DSL) services, in particular.

Competitive Local Exchange Carrier

A Competitive Local Exchange Carrier (CLEC) is a telecommunications company that is either too new or too small to be considered an Incumbent Local Exchange Carrier (ILEC), and so is not subject to the same level of financial regulation. Whereas an ILEC is seen as a threat to a competitive telecommunications market, and is highly regulated, a CLEC is seen as a key competitor, a company actively fighting for the best possible position in the market.

Many economic theories suggest that greater competition between companies tends to result in a greater level of investment, meaning that the emergence of new and novel technology becomes much more likely. In this case, the hope is that by limiting the ability of ILECs to stifle competition, CLECs will be more inclined to invest in new technology and ideas, ultimately providing the country with faster services and a more efficient communications network.

There is some evidence, too (albeit contested), that recent trends in the telecoms industry (Local Loop Unbundling [LLU], for instance) may be directly linked to these regulations, and may indeed be helping to encourage investment in technology on a national scale.

Central Office

A telephone exchange is often known as a Central Office. Traditionally, these were centres responsible for connecting customer telephone lines to each other, allowing users to make calls to one another, and they are still used in this way(see: POTS).

However, with the advent of digital communication technology (see: DSL services), the role of the Central Office has become more complex, and nowadays a CO will provide internet, television and a range of other services alongside the more conventional POTS (voice) services. These can be run by the telephone company (usually an ILEC), or by any number of other (CLEC) companies (see: Local Loop Unbundling [LLU]).

The emergence of new technologies, then, as well as economic innovations like LLU, have complicated the role of the Central Office to the extent that calling it simply a "telephone exchange" no longer captures the full extent of its function, and so the term 'Central Office' (often expressed simply as "CO") has become increasingly popular.

Double Ended Line Test

Double Ended Line Testing (also known as Dual Ended Line Testing) is a standard set of methods for testing the effectiveness of a copper line. These tests, as with SELT and MELT, are conducted by the DSLAM, but DELT requires (along with a fully functioning DSLAM) the co-operation of a compatible modem at the customer end of the line, a limitation that SELT and MELT do not have.

DELT, however, of the three, is able to give the closest approximation of a working line. In fact, this is effectively what DELT tests are designed to do; they simulate the conditions of a working line under a range of circumstances and, by comparing the sent signals at one end of the line with the received signals at the other, measure the influence of the physical line on these signals.

Ultimately, this technique does not provide the same level of detail that SELT and MELT tests can provide, but it has advantages that they do not. Because it is testing in conditions that are similar to the working conditions of a line, DELT makes (and requires) few extrapolations, and is as such far less vulnerable to errors in this area. DELT is also unaffected by the length of the line, and is, unlike SELT, able to function over whatever distance the original service is operating.

It is because each is limited in its own way that DELT, SELT and MELT are often offered in combination. The idea is that the strengths of each method will cancel out the weaknesses of the others. We explain here, though, why in reality this solution is not as effective, or indeed as inexpensive, as it may first appear, and why a well designed test head is likely to be a better alternative.

Digital Subscriber Line

A Digital Subscriber Line (DSL) is digital data transfer service between a Central Office (CO) and a customer, which operates over standard POTS lines. The term can be used generally to refer to any service operating in this way, and is sometimes also expressed as "xDSL".

All DSL services, regardless of their form, work by operating at a higher frequency than traditional POTS services. They also are entirely digital. This means that time is not wasted converting analogue signals to digital ones, so bandwidth can be greater and the service can be much faster. In all DSL services, a POTS Splitter (or equivalent) separates out the POTS and DSL signals, allowing the two to operate on the same line without any notable interference.

For the sake of economy, the signal for a Digital Subscriber Line (DSL) tends to be sent initially over a single line (typically an Asynchronous Transfer Mode [ATM] line) from the service provider. In copper based systems this line is usually a bundle of copper wires, packed tightly together to form a single, large cable (though of course each insulated to avoid interference). The individual wires comprising this line are then split off to the relevant customer where required.

Though fibre based services are becoming increasingly popular, this is still the standard setup for POTS services, and is the setup into which DSL services are most often incorporated. Significant forms of DSL include the Asymmetric Digital Subscriber Line (ADSL) and Very-High-Bitrate Digital Subscriber Line (VDSL) services, which take different approaches to the management of digital data, and thus offer differing advantages and disadvantages.

Digital Subscriber Line Access Multiplexer

A Digital Subscriber Line Access Multiplexer is a device found in a telephone company's Central Office (CO), and is a chief component of DSL (Digital Subscriber Line) services.

In DSL, digital signals are sent along copper lines to customers. From the Central Office (CO), the DSLAM communicates with multiple customer modems through these copper lines, making sure that each modem is sent the correct set of data, and that each set of return data is expected from the correct modem. By communicating with other Central Offices (COs), the DSLAM can connect a customer modem to the wider network, through which the customer can access internet and other digital services.

With the help of a POTS Splitter, the high frequency digital signal that the DSLAM sends to a customer can be combined with the appropriate low frequency voice (POTS) signal for that customer. The switches in the Central Office then connect this signal to the relevant customer line, ensuring that the customer is always sent the right voice (POTS) and digital (DSL) signals.

Then, at the customer's house, these signals are split apart again (by another POTS Splitter). The telephone is sent only the low frequency (analogue) signals, while the modem receives the high frequency (digital) signals. In this way a full POTS and digital service can be provided to a customer without any interference between the two, all over a standard POTS line.

Incumbent Local Exchange Carrier

An Incumbent Local Exchange Carrier (ILEC) is a telecommunications company considered too large to be a Competitive Local Exchange Carrier (CLEC), because its power and size allows it (in theory) to stifle competition before it emerges. It is therefore subject to strict regulation, which aims to allow smaller, competitive companies (CLECs) to better compete in the market.

A common argument for this approach is that more competition will encourage more money to be invested in infrastructure and technology, causing more of the newer, faster technologies to become financially viable and attractive.

Whether or not the regulations have achieved their stated aims, or whether they could have achieved them more effectively in another way, is a matter of some debate, but it is clear from the emerging popularity of LLU, and systems like VDSL and VOIP, that changes have been and still are occurring in the telecommunications industry, and that this is, at least in part, because of the distinction (and regulatory differences) between CLECs and ILECs.

Local Loop Unbundling

Local Loop Unbundling is an economic and structural model for providing POTS (voice) and DSL (digital, e.g. internet) services to customers. Effectively, an Incumbent Local Exchange Carrier (ILEC) allows a Competitive Local Exchange Carrier (CLEC) to rent its facilities for its own use.

Local Loop Unbundling can be either "partial" or "full". Partial unbundling is where the Competitive Local Exchange Carrier (CLEC) leaves the operation of POTS services to the ILEC, simply incorporating its own digital (i.e. DSL) service into the setup. The CLEC, for example, will be provided with a room or building at or near the ILEC's Central Office (CO), from which it will run a system of DSLAMs, plugged (through the ILEC's telephone switch system) into the ILEC's copper lines. In this way, a CLEC can provide DSL (Digital Subscriber Line) services without the prohibitive cost and delay of laying their own lines, and can do so for any customer in the area with a phone line, but does not need to take any responsibility for the operation of the customer's telephone (POTS) service. This is left to the ILEC.

Full unbundling, on the other hand, is where the CLEC actually takes over the line, still renting from the ILEC (paying for both the line and the facilities) but controlling the POTS, too. In this way, a CLEC can provide a full range of telephone, internet and other services, as well as a range of deals for, and combinations of, these services.

There are two main alternatives to LLU: "Bitstream access" (BA), which allows the CLEC to use the ILEC's existing equipment to provide their service, and "resale access" (RA), which allow a CLEC to brand an ILEC's service as their own, but in either case the CLEC is limited to services that the ILEC already provides, and so has few means with which to differentiate and make a name for itself.

For this reason, since changes to regulations have made it a more attractive and financially viable choice for CLECs, LLU has been an increasingly popular means of access to copper lines, and has had a significant impact on the way POTS and DSL services are operated and sold.

For more information of Local Loop Unbundling, you may wish to visit our dedicated Local Loop Unbundling website, here.

Metallic Line Test

Metallic Line Testing (MELT) is a standard set of methods for testing the quality of a copper line.

As with SELT and DELT tests, MELT tests are conducted by the DSLAM, but MELT tests, unlike SELT and DELT, make no attempt to simulate elements of the line's operation, trying instead to determine the quality of the copper itself.

In order to do this, MELT tests take a range of measurements about the copper line's physical properties, such as its capacitance, conductivity, inductance, etc., and these measurements are used to determine the likelihood of various faults being present in the line.

As with SELT tests, MELT can be conducted entirely by the DSLAM, requiring no modem to be connected at the other end of the line. Unlike SELT or DELT, however, MELT tests cannot run through the DSLAM's standard physical setup, because in order to operate they must bypass the transformer that connects the DSLAM to the line. This both leaves the DSLAM vulnerable to damage (for instance, from voltage spikes), and incurs setup costs (because either a DSLAM must be modified to make this possible, or a MELT-ready DSLAM must be purchased).

Unlike SELT and DELT,  MELT tests do not rely on the DSLAM to send test signals, and as such are not hampered by faults in the DSLAM's function, but because MELT tests do not simulate the operating conditions of a line, they are often only of practical use in conjunction with other tests. It is for this reason that combined SELT, MELT and DELT capability is regularly offered as part of a DSLAM's function.

The problem with this is that it can often be less effective, and more expensive, than simply using an effective test head, and we explain why this is, here.

Multimode Fibre

Multimode Fibre, in contrast to Single Mode Fibre, is a form of optical fibre through which light is sent in a number of different "modes". In simplest terms, we can think of modes as paths that the light can take through the core of the fibre. In other words, with Multimode Fibre the light takes multiple paths, rather than only one, as is the case for Single Mode Fibre.

There are two main types of Multimode Fibre: step index and graded index. Step index Multimode Fibre relies on the phenomenon of Total Internal Reflection, in which light passing through a material (i.e. the glass in an optical fibre's core) hits the inside edge of that material and is reflected back inside of it, rather than being refracted out. When, as in step index Multimode Fibre, this process repeats, light can continue to be reflected for a very long time, and in this way light (and so signals) can pass along the length of the fibre.

There are, however, complications to this idea, because an effect known as modal dispersion occurs. Modal dispersion means that in any given fibre, because some light is taking a longer path through the fibre than other light, the different signals encoded in that light become increasingly spread out (or dispersed) over time and distance, overlapping one another and becoming ever more difficult to interpret. For this reason, Multimode Fibre in general (but particularly step index Multimode Fibre) tends to provide very poor signals when used over long distances.

Graded index Multimode Fibre is a form of Multimode Fibre designed to suffer less seriously from modal dispersion. In this form, the core of the fibre is graded rather than stepped, meaning that the refractive index of the fibre does not suddenly change as it passes from core to cladding, but smoothly transitions from a high refractive index at the centre of the core to a low refractive index at the edge. This has the effect that the light is made to curve, to the extent that it never actually reaches the edge of the fibre core. Instead, it is bent back in towards the centre, and so to the other side of the fibre core, where the same occurs again.

This has an interesting effect, because light travels more rapidly at the edge, where the refractive index is lower, and so as it moves through the fibre core, passing up and down the refractive index gradient, will always tend towards an average path. This drastically reduces the effects of modal dispersion, meaning that graded index Multimode Fibre can operate over significantly longer distances than its step index equivalent.

But even this trick is not perfect, and there is still a fair amount of attenuation, from modal dispersion and other factors, in graded index Multimode Fibre (at least when compared to Single Mode Fibre). For this reason, UTEL do not use Multimode Fibre, but it should still be noted that graded index Multimode Fibre does offer significant improvement on the distance capabilities of step index Multimode Fibre, which, coupled with the reduced operational cost of the Multimode technique (when compared to the high laser costs of Single Mode Fibre), can make graded index Multimode Fibre a useful tool for some data transfer applications, for instance CCTV (Closed Circuit Television), LAN (Local Area Networks) and University Campus Networks.

Multiplexing

In telecoms (telecommunications), "Multiplexing" refers to the process of combining several different signals into one, unified signal, to be sent along a single line. This is performed by a multiplexer, and at the other end of the line a compatible demultiplexer can be used to unscramble these combined signals, and direct them down the appropriate number of cables in the required order. In a company's Central Office (CO), a DSLAM often performs both of these functions, providing a two way connection between multiple Digital Subscriber Lines (DSLs) and a single line (or a smaller number of lines) from the Central Office (CO) to the wider network.

This multiplexing and demultiplexing process can be performed by a wide range of methods, of which code-division multiplexing, polarisation-division multiplexing, Time-Division Multiplexing and frequency-division multiplexing are just a few examples.

Optical Fibre Faults

No optical fibre is perfect, and faults and losses of various types are fairly common.

Some of these forms of loss are fairly constant, and virtually impossible to avoid. There is, for instance, an effect known as intrinsic absorption, in which the intrinsic molecular properties of the fibre core and cladding cause certain wavelengths of light to be absorbed.

Extrinsic absorption is a similar phenomenon, but is caused by impurities in the core and cladding, rather than the intrinsic molecular properties of the materials.

Both of these effects, as well as Rayleigh scattering (see: OTDR) and others, cause a steady level of loss within a fibre, but there are also more drastic (or at least more sudden) forms of loss that can occur in a fibre.

Macrobends, for example, are a phenomenon in which a fibre is bent too far, and the radius of that bend becomes too narrow for all of the light in the fibre core to be properly reflected (see: Total Internal Reflection). This results in "leakage" of light from the bend.

There are also microbends: tiny (but often extreme) bends or faults in a fibre, caused mainly by improper handling (pinching, squeezing) of fibre. They can take the form of minute bumps and/or dips at the edge of the fibre core, disrupting the path of the light within the core. This causes discrepancies in signals, various forms of dispersion, and ultimately loss of light.

Also of note are reflection events, and these are quite common in the joins between fibres. This is usually because the tips of the two fibres are not properly mated, and a tiny air gap has been left between the two. The change in refractive index between the fibre core and the air causes partial internal reflection (the name for the phenomenon in which some light is reflected back inside of a material, while some passes through into the second medium [compare: Total Internal Reflection]). Specks of dirt can often be the cause of this effect, getting caught between fibre tips, parting them and allowing space for air.

A device such an OTDR (see UTEL's Fastlight OTDR, here) can detect these events, many of them can be limited, and some can be almost entirely removed, but there is no such thing as a perfect optical fibre. There is always loss, and always the potential for faults, and for this reason, careful testing is often crucial to the success of optical fibre communications.

Optical Time Domain Reflectometer

An Optical Time Domain Reflectometer is a device that can be used for finding faults in optical fibre, which works by sending a signal (a pulse of light) out through the fibre, and waiting for this signal to be reflected back to it.

An Optical Time Domain Reflectometer is able to measure time very accurately, meaning that it is able to time the light's journey through the fibre, and can make very detailed measurements of the returning signal's strength at specific points in time. Light returning from different points along the fibre will arrive at different times, so these measurements can be used to determine how far along a fibre a reflection has occurred.

Because of an effect known as Rayleigh Scattering, which occurs throughout the glass, there is always some light returning from every point along the fibre. This means that an OTDR can provide a detailed graph of how the light from the signal is behaving at every point along the fibre.

The reason for Rayleigh Scattering is that molecular structures themselves can contain imperfections, and these imperfections cause tiny variations in the refractive index of the substance (in this case, the core of the fibre). These tiny variations disrupt the path of light within the core of a fibre, causing a small amount of light to be scattered outwards in all directions (including back along the fibre, hence the OTDR can detect it).

This effect shows up as a downward sloping line on the OTDR's graph, which the effects of any reflection or loss within the fibre (see: Optical Fibre Faults) can be compared to. A skilled operator can interpret these results very effectively, identifying not only a great deal of detail about the nature of faults within the fibre, but their exact locations too.

For all of this capability, and a range of powerful additional features, see UTEL's Fastlight OTDR, here.

Plain Old Telephone Services

The term "Plain Old Telephone Services" ("POTS") refers to copper-and-voice-based analogue communication services that predate modern digital communication technology (e.g. optical fibre, mobile phones), and for the vast network of these that still exists today in the form of landline telephone services.

Many modern technologies (UTEL's DSL optimised Test Heads, for example, and indeed DSL itself) are compatible with Plain Old Telephone Services (POTS), and POTS still offers certain advantages over many other telecommunication services. They to be more reliable, for instance, particularly in adverse environmental conditions (e.g. extreme weather) and because the systems are generally already in place, are typically considered far cheaper to run (although the increasing popularity of fibre based hardware, along with the rise of services such as Voice Over IP (VOIP), may over time be rendering such claims steadily less accurate). For these reasons and others, various services that can be considered Plain Old Telephone Services (POTS) are still in widespread use today.

Find out more about POTS, here.

POTS Splitter

A POTS Splitter is a device designed to split POTS (voice) signals, which are low frequency and analogue, from high frequency digital (DSL) signals.

It does this by the use of frequency filters. A low pass filter allows only the lower frequencies (containing the analogue voice data) through to the telephone, while the high frequency signals are directed to the DSL modem and contain the digital data. To avoid interference between these two signals, an unused (and therefore safe) range of frequencies is left between the two.

A POTS Splitter can also play a crucial role in combining POTS and DSL signals, ensuring that any imperfections in the higher frequencies of a POTS signal are stripped out, and do not interfere with the DSL signal.

Today, many digital services are run on POTS lines, alongside analogue voice services, and this means that a POTS Splitter is a very useful device, allowing these services to run simultaneously over the same line without any significant interference.

Single Ended Line Test

Single Ended Line Testing (SELT) is a standard set of methods for testing a copper line.

These tests, as with MELT and DELT tests, are conducted by the DSLAM. Unlike DELT tests, however, they do not require a modem to be connected at the other end of the line, and, unlike MELT, they require no physical modification to the DSLAM itself.

SELT gathers information through a range of different testing methods, for example Quiet Line Noise tests and TDR tests. A SELT Quiet Line Noise test attempts to determine the level of electrical noise that is present in a line when no signal is sent through it. This can be useful in determining a line's quality, but because SELT takes readings from only a single end of the line, can often give an incomplete picture of the noise level within it.

TDR (Time Domain Reflectometry [see: OTDR, for its optical equivalent]), is a form of testing in which a signal pulse is sent out along the line, and the nature of any returning (reflected) signal is analysed in relation to the time it has taken to return. This is possible to do because any sudden change in a line's impedance (meaning the extent to which it restricts the flow of current within it) along its length will cause part of the signal to be reflected back along the line, at the border of that change.

Because a change in impendence causes a reflection, it also causes a loss of signal within a line when it is operating (for the simple reason that the reflected part of a signal will not reach the other end of the line). This means that, by detecting and locating these changes, a TDR test is able to detect and locate losses within a line.

This, in conjunction with other test methods, can produce detailed results, but its capability is ultimately limited by distance in a way that DELT and MELT are not, and SELT, like DELT, is still reliant on the DSLAM's ability to send test signals. In other words, if the DSLAM is not sending signals properly, SELT is only of very limited use.

It is because of these limitations that SELT is often offered as part of a combined SELT, MELT and DELT setup, but even this can have significant limitations, as we explain here.

Single Mode Fibre

Single Mode Fibre is a form of optical fibre through which light is passed only by a single, narrow path (or "propagation mode"). This is managed by the use of precise, pure lasers, and by the fibre's very narrow core.

In contrast, with Multimode Fibre the core is fairly wide (at least, compared to Single Mode Fibre), and this means that the equipment used to send light into the fibre core can be far less precise. Light entering the fibre at a range of different angles is able to pass through by an equivalent range of different paths.

This is not the case with Single Mode Fibre. The propagation of light is restricted, allowing light to pass only (or almost only) through the very centre of the fibre.

The key advantage of this is that loss of signal is far less of a problem with Single Mode Fibre. Because of the light's narrow path there is very little change for it to escape from the fibre (see: Optical Fibre Faults) and because it takes only a single path through the fibre, rather than multiple paths, modal dispersion (see: Multimode Fibre) is pretty much eliminated.

The downside to this is cost. Sending signals through Single Mode Fibre requires a narrow, precise beam of light, and this means expensive lasers. Such costs are often harder to justify over short distances, where the disadvantages of Multimode Fibre would be far less apparent, and for this reason Single Mode Fibre tends to be used for long distance communication, where the cost of such lasers is more easily justifiable, and Multimode Fibre largely useless.

Because of its high quality and reliability, all of UTEL's Fastlight solutions are based around Single Mode Fibre.

Synchronous Transfer Mode

As with Asynchronous Transfer Mode (ATM), Synchronous Transfer Mode (STM) relies on Time-Division Multiplexing, but in STM this process is synchronous, meaning that both ends have agreed upon a set of time slots (or an algorithm for determining a set of time slots) for the data cells from each channel to fall within.

This technique means that only a small amount of additional information is required in order to read the signal (see: ATM), and is often advantageous to STM, ensuring that significant bandwidth is not wasted on carrying these additional signals, but at times it can also serve as a disadvantage. When different signals in the line are not taking up the same amount of bandwidth, some bandwidth is wasted maintaining time slots that are not in use.

However, a variant of Synchronous Transfer Mode (STM) known as Dynamic Synchronous Transfer Mode (DTM) has been developed as a solution to this problem, so called because it dynamically reallocates bandwidth as it operates. This ensures that users or activities needing only a small amount of bandwidth at a particular time are given an equivalently smaller portion of the bandwidth than users or activities requiring a large amount. For this reason, where there are significant losses within a line, Dynamic Synchronous Transfer Mode (DTM) can deal very effectively with this, simply reallocating bandwidth to optimize performance in the new context. In many circumstances, then, modern forms of Synchronous Transfer Mode (STM) can be notably faster than Asynchronous Transfer Mode (ATM).

Asynchronous Transfer Mode (ATM) offers the significant advantage of flexibility, however, of being faster under normal circumstances than standard STM, and of being far cheaper and easier to implement than DTM. For this reason, ATM is often seen as a more favourable choice than STM and its variants.

Time Division Multiplexing

Time Division Multiplexing (TDM) is a form of Multiplexing in which data from different signals can share a single line by virtue of being temporally staggered (that is, divided up through time, in such a way as to not overlap). Typically (at least in non-statistical Time Division Multiplexing) the signals that are sharing the line are split up into small packets of information, and these packets are assigned time slots within the overall signal.

At the receiving end, the demultiplexer is able to piece together the original signals because it knows which time slot each packet of the signal will have been sent to by the multiplexer. In this way, large amounts of information can be sent down a single cable without any notable interference between the signals.

Standard Time Division Multiplexing is particularly useful for Synchronous Transfer Mode (STM), while a variant of it, statistical Time Division Multiplexing, is used for Asynchronous Transfer Mode (ATM).

Total Internal Reflection

Total Internal Reflection is a common optical phenomenon. When light, travelling within a particular range of angles, reaches the boundary between the material it is travelling in and a material of a lower refractive index, it will be Totally Internally Reflected. This means that instead of being simply refracted (redirected) as it passes into the second material, all of the light will be reflected back inside of the initial substance.

Optical fibre can encourage Total Internal Reflection by having a core that is of a higher refractive index than the cladding. In this way, light (at an appropriate angle of incidence) can be made to reflect along the length of the fibre core, and in theory, were the fibre infinite, the light would continue to do this forever. In practice, though, imperfections in the fibre make this impossible, and there is always some loss (see: Optical Fibre Faults).

For more information on how light can be passed through a fibre, see: Single Mode Fibre and Multimode Fibre.

Very-High-Bitrate Digital Subscriber Line

VDSL is a digital service (see: DSL) designed to run at very fast speeds through copper based POTS networks. Though the terms VDSL and ADSL (Asymmetric Digital Subscriber Line) are often differentiated, and usually refer to distinct services, VDSL is essentially asymmetric, providing a large amount of bandwidth in the downstream direction, but only a small amount in the upstream direction (for more on this, see: ADSL).

VDSL's speed stems mainly from its level of flexibility. It uses several different frequency bands simultaneously, in both upstream and downstream directions, and adjusts the use of these bands to ensure that the maximum possible bandwidth is always attained. However, while VDSL is indeed fast and flexible, its use of such a wide spectrum of frequencies results in a lot of attenuation within the line, mostly because of the spectral limitations of the copper wires the signals are sent on.

For many purposes, then (e.g. in a conventional DSL setup) VDSL is practically useless, but with the emerging popularity of (optical) fibre based services, VDSL is nonetheless beginning to present a very lucrative possibility for telecommunication companies.

Essentially, the situation is this: a serious problem with fibre based services is that they are far too expensive to take directly to each customer's house, and so are typically taken only from one Central Office (CO) to another (where the cost of deployment can be more easily justified).

Standard DSL services, on the other hand, operating on copper wires (initially deployed with only voice [POTS] services in mind), have much lower bandwidth than fibre services, and so restrict the speed of transfer for these digital signals.

A proposed compromise (and one that is beginning to become a reality) is that optical fibres are sent only as near to the customer as they need to be for VDSL to (effectively) operate. In this way, the loss of speed normally caused by conventional DSL services can be greatly reduced, and quality of service can be significantly improved for a fraction of the cost of laying fibre directly to a customer's house.

With this in mind, then, many companies worldwide are working to move their DSLAMs into cabinets that are within a VDSL operable range of customer's houses, and are using VDSL to provide very fast internet, and other digital services, to customers.

Voice Over IP

Voice Over IP (VOIP) is a system by which voice signals are sent across an internet connection rather than a traditional POTS based system. These services are typically less reliable than POTS, and are dependent on wall power at the receiver end (whereas POTS is often not), but Voice Over IP (VOIP) has the advantage of being very cheap, relying as it does on existing internet structures. Users of VOIP will typically pay only minimal charges on top of their normal internet payments, because they are simply using an existing service (usually ADSL) to carry the signal.

Given that many users of voice services are already connected to the internet, it seems that there are in many cases clear financial reasons for using VOIP in the place of POTS. Current financial models, however, along with customer concerns about reliability and quality, have meant that many homes and businesses are continuing to use POTS in the place of VOIP.