Tone frequency channel. The main types of subscriber telephone lines and services What is the standard frequency band in traditional telephony

Bandwidth (transparency) - the frequency range, within which the amplitude-frequency characteristic (AFC) of an acoustic, radio-technical, optical or mechanical device is sufficiently uniform in order to ensure the transmission of a signal without significant distortion of its shape. Sometimes the term effectively transmitted bandwidth (EDP) is used instead of the term "bandwidth". The main signal energy is concentrated in the ESPC (at least 90%). This frequency range is set for each signal experimentally in accordance with quality requirements.

Basic bandwidth parameters

The main parameters that characterize the frequency bandwidth are the bandwidth and the frequency response unevenness within the bandwidth.

Bandwidth

The bandwidth is the frequency band within which the unevenness of the frequency response does not exceed the specified one.

The bandwidth is usually defined as the difference between the upper and lower cutoff frequencies of the frequency response section f 2 - f 1 (\\ displaystyle f_ (2) -f_ (1))where the amplitude of the oscillations is 1 2 (\\ displaystyle (\\ frac (1) (\\ sqrt (2)))) (or equivalently 1 2 (\\ displaystyle (\\ frac (1) (2))) for power) from the maximum. This level is approximately −3 dB.

The bandwidth is expressed in units of frequency (for example, hertz).

In radio communication and information transmission devices, the expansion of the bandwidth allows more information to be transmitted.

Uneven frequency response

Unevenness of the frequency response characterizes the degree of its deviation from a straight line parallel to the frequency axis.

The attenuation of the frequency response unevenness in the band improves the reproduction of the transmitted signal shape.

Distinguish:

• Absolute bandwidth: 2Δω \u003d Sa
• Relative bandwidth: 2Δω / ωo \u003d So

Specific examples

In antenna theory, bandwidth is the range of frequencies at which an antenna operates efficiently, usually around the center (resonant) frequency. Depends on the type of antenna, its geometry. In practice, bandwidth is usually determined by a given VSWR (Standing Wave Ratio) level such as 2.

From the definition of the bandwidth it can be seen that dispersion imposes a limitation on the transmission range and on the upper frequency of the transmitted signals.

Bandwidth requirements various devices are determined by their purpose. For example, for telephone communication, a bandwidth of about 3 kHz (300-3400 Hz) is sufficient, for high-quality reproduction musical works - not less than 30-16000 Hz, and for television broadcasting - up to 8 MHz wide)

Term frequency band regarding signal associated with concepts of effective signal bandwidth, in which 90% of the signal energy is concentrated (by agreement), as well as the lower and upper boundaries of the signal frequency band. These essential characteristics of a signal source are directly related to physics. this source signal. For example, for an inductive vibration sensor, the frequency band of the output signal is actually limited from above by units of kilohertz due to the inertia of the mass of the magnetized metal core inside the inductance coil of the sensor, and from below by the value associated with the inductance of the coil. The upper limit of the signal bandwidth is generally associated with the physical slew rate limitations of the signal, and the lower limit of the bandwidth is associated with the presence of the low frequency component of the signal, including the DC component.

Term frequency band transmissionused in relation to converters and paths (interfaces) for signal transmission. We are talking about amplitude-frequency characteristic (AFC) these devices and the characteristics of the bandwidth of this frequency response, which are traditionally measured at the level of -3 dB, as shown in the figure above. Zero decibel is the maximum (or average, by convention) value of the signal amplitude in the passband. In the figure, the frequencies F 1 and F 2 are the lower and upper frequencies of the passband, respectively. The lower limit F 1 \u003d 0, if the given converter or path passes the DC component of the signal. The more width frequency bands transmission ∆F \u003d F 2 - F 1 of the converter or data transmission path, the higher resolution (detail) of the signal in time , the higher the data transfer rate in the corresponding interface, But at the same time the more interference and noisefalls into the passband.

If the signal bandwidth partially or completely does not fall within the bandwidth of the converter or path, then this leads to distortion or complete suppression of the signal in the path.

On the other hand, if the effective bandwidth of the signal is many times narrower than the bandwidth of the converter or channel, then this case cannot be considered optimal, since in this physically implemented system there are always noise and interference of various nature, which in the general case are dispersed over the entire bandwidth ... Frequency regions that have no useful signal components will add noise, degrading the signal-to-noise ratio in a given signal conversion or transmission channel. Based on these premises, we came close to term: optimal signal bandwidth Is the bandwidth, the boundaries of which are consistent with effective signal bandwidth.

In the case of an ADC, the upper end of the bandwidth can be provided by an anti-aliasing filter and the lower end can be provided by a high pass filter.

As you can see, the general term frequency bandused in any context is strongly related to the issue of equipment selection according to its frequency characteristics, and is also related to the issue of optimal matching of converters and transmission paths with signal sources.

With the term frequency bandthe following articles are linked:

Basic bandwidth parameters

The main parameters that characterize the frequency bandwidth are the bandwidth and the frequency response unevenness within the bandwidth.

The width of the line

The bandwidth is usually defined as the difference between the upper and lower cutoff frequencies of the frequency response section at which the oscillation amplitude (or for power) is from the maximum. This level is approximately -3 dB.

The bandwidth is expressed in units of frequency (for example, Hz).

Expanding the bandwidth allows more information to be transmitted.

Uneven frequency response

The unevenness of the frequency response characterizes the degree of deviation from a straight line parallel to the frequency axis.

Frequency response is expressed in decibels.

The attenuation of the frequency response unevenness in the band improves the reproduction of the transmitted signal shape.

Specific examples

In antenna theory, bandwidth is the range of frequencies at which an antenna operates efficiently, usually around the center (resonant) frequency. Depends on the type of antenna, its geometry. In practice, bandwidth is usually determined by the SWR (Standing Wave Ratio) level. SWR METER

In optics, the passband is the reciprocal of the broadening of a pulse as it travels a distance of 1 km through an optical fiber.

Since even the best monochromatic laser still emits a certain spectrum of wavelengths, dispersion leads to broadening of pulses as they propagate along the fiber and thus generates signal distortions. When evaluating this, the term bandwidth is used. The bandwidth is measured (in this case) in MHz / km.

From the definition of the bandwidth it can be seen that dispersion imposes a limitation on the transmission range and on the upper frequency of the transmitted signals.

see also

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See what "Frequency band" is in other dictionaries:

frequency band - Frequency range, limited by the lower and upper limits [GOST 24375 80] frequency band The set of frequencies within the considered limits [Non-destructive testing system. Types (methods) and technology of non-destructive testing. Terms and Definitions… …

frequency band - 06.01.16 frequency band: A continuous set of frequencies, limited by upper and lower limits. Note 1 A frequency band is characterized by two values \u200b\u200bthat define its position on the frequency axis, for example, its lower and ... ... Dictionary-reference book of terms of normative and technical documentation

Frequency band - 1. Frequency range, limited by the lower and upper limits Used in the document: GOST 24375 80 2. Continuous frequency band, enclosed between the two limits Used in the document: GOST R 51317.4.3 99 Immunity to radio frequency ... ... Telecommunication Dictionary

frequency band - dažnių juosta statusas T sritis fizika atitikmenys: angl. frequency band vok. Frequenzband, n rus. frequency band, f; frequency band, f pranc. bande de fréquences, f ... Fizikos terminų žodynas

frequency band - dažnių juosta statusas T sritis automatika atitikmenys: angl. frequency band vok. Frequenzband, n rus. frequency band, f pranc. bande de fréquences, f… Automatikos terminų žodynas

frequency band - dažnių juosta statusas T sritis Standartizacija ir metrologija apibrėžtis Signalų generatoriaus dažnių diapazono dalis, kurioje dažnį galima keisti tolydžiai arba pakopomis. atitikmenys: angl. frequency band vok. Frequenzbereich, n rus. band… … Penkiakalbis aiškinamasis metrologijos terminų žodynas

frequency band - rus frequency band (w), frequency range (m) eng frequency band fra bande (f) de fréquence deu Frequenzband (n) spa rango (m) de frecuencias, banda (f) de frecuencias ... Occupational safety and health. Translation into English, French, German, Spanish

frequency band (in telecommunications) - frequency band frequency range The area of \u200b\u200bthe signal frequency variation, limited by the lower and upper limits. In practice, the definition of the upper limit by the formula flower (n) \u003d 3 · 10n 1 Hz is widely used, while the lower limit is equal to the upper ... ... Technical translator's guide

frequency band (in vibration) - frequency band A set of frequencies within the considered limits [GOST 24346 80] Topics vibration EN frequency band DE frequenzband FR bande de frequence ... Technical translator's guide

microwave diode frequency band - Δf / f Δf / f Frequency interval in which the microwave diode tuned to a given frequency provides the specified parameters and characteristics in a constant operating mode. [GOST 25529 82] Topics semiconductor devices Generalizing terms ... ... Technical translator's guide

Usually we do not care about how the telephone line works (but not when we have to shout with all our might into the telephone receiver: "Please repeat, you can not hear anything!").

Telephone companies provide a variety of services to customers. It is not so easy to understand the price lists of these services - what is actually offered and how much you should pay for which service. In this article, we will not say a word about prices, but we will try to find out what is the difference between the most frequently offered products and services in the field of telephone communications.

ANALOG LINES, DIGITAL LINES

First, there are analog and digital lines. The analog signal changes in a continuous manner; it always has a certain value, representing, for example, the volume and pitch of the transmitted voice or the color and brightness of a certain area of \u200b\u200bthe image. Digital signals have discrete values \u200b\u200bonly. As a rule, the signal is either on or off, or it is there or it is not. In other words, its value is either 1 or 0.

Analog telephone lines have been used in telephony since time immemorial. Even phones of fifty years ago, most likely, it will be possible to connect to a subscriber loop - the line between the home telephone socket and a central telephone exchange. (The central office is not a gleaming skyscraper in the middle of a city; the average loop length is less than 2.5 miles (four kilometers), so a “central office” is usually located in some nondescript building nearby.)

During a telephone conversation, a microphone built into the handset converts speech into an analog signal transmitted to the central telephone exchange, from where it goes either to another subscriber loop, or to other switching devices, if the called number is outside the coverage of this station. When dialing a number, the telephone set generates signals transmitted over the same main channel (in-band signals) indicating who the call is intended for.

Throughout their existence, telephone companies have accumulated extensive experience in the transmission of speech. It has been found that the frequency range from 300 to 3100 Hz is generally sufficient to accomplish this task. Recall that hi-fi class audio systems are able to reproduce sound without distortion in the frequency range of 20-20000 Hz, which means that the telephone range is usually enough only for the subscriber to recognize the caller by voice (for other applications, this range is likely to be too much narrow - for transferring music, for example, telephone communications is completely unsuitable). Telephone companies provide smooth roll-off at high and low frequencies using a 4000 Hz analog telephone channel.

The central telephone exchange, as a rule, digitizes the signal intended for further transmission over the telephone network. With the exception of Gilbet County, Arkansas and Rat Fork, Wyoming, all American telephone networks carry digital signals between central stations. Although many companies use digital PBX and data communications, and all ISDN facilities are digitally encoded, loopbacks are still the “last resort” of analog communications. This is explained by the fact that most telephones in private homes do not have a means of digitizing the signal and cannot work with lines with a bandwidth of more than 4000 Hz.

WHAT IS 4000 HZ ENOUGH?

A modem is a device that converts digital signals from a computer into analog signals with frequencies within the bandwidth of a telephone line. The maximum channel bandwidth is directly related to the bandwidth. More precisely, the amount of throughput (in bits / sec) is determined by the bandwidth and the signal-to-noise ratio tolerance. Currently, the maximum throughput of modems - 33.6 Kbps - is already close to this limit. Users of 28.8 Kbps modems are well aware that noisy analog lines rarely provide full throughput, which is often much lower. Compression, caching and other evasions help to rectify the situation somewhat, and nevertheless, we would rather live to see the invention of a perpetual motion machine than to the appearance of modems with a bandwidth of 50 or at least 40 Kbps on ordinary analog lines.

Telephone companies solve the opposite problem - they digitize the analog signal. To transmit the resulting digital signal, channels with a bandwidth of 64 Kbps are used (this is a world standard). Such a channel, called DS0 (digital signal, level zero), is the basic building block from which all other telephone lines are built. For example, you can combine (the correct term is to multiplex) 24 DS0 channels into a DS1 channel. By leasing a T-1 line, the user actually gets a DS1 circuit. When calculating the total throughput of DS1, you must remember that after every 192 information bits (that is, 8000 times per second), one synchronization bit is transmitted: a total of 1.544 Mbps is obtained (64000 times 24 plus 8000).

DEDICATED LINES SWITCHED LINES

In addition to the T-1 line, the client can lease leased lines or use ordinary switching lines. By leasing a T-1 channel or a low-speed data line from a telephone company, for example, a dataphone digital service (DDS), a subscriber actually leases a direct connection and as a result becomes the only user of a channel with a bandwidth of 1.544 Mbps (T-1 ) or 56 Kbps (low speed line).

Although frame relay technology involves switching individual frames, the corresponding services are offered to the user in the form of virtual communication channels between fixed endpoints. From a network architecture perspective, frame relay should be viewed as a dedicated line rather than a switched line; important is the fact that the price of such a service with the same bandwidth is significantly lower.

Switching services (an example of which is the service of a regular apartment telephone) are services purchased from the telephone company. On demand, the subscriber is provided with a connection via a network of public switches to any node of the telephone network. In contrast to the situation with leased lines, the fee in this case is charged for the connection time or the actual volume of traffic and depends mostly on the frequency and volume of network use. Digital switching services can be provided based on X.25, Switched 56, ISDN Basic Rate Interface (BRI), ISDN Primary Rate Interface (PRI), Switched Multimegabit Data Service (SMDS) and ATM protocols. Some organizations, such as universities, railways or municipalities, create private networks using their own switches and leased or sometimes even their own lines.

If the line received from the telephone company is digital, then digital-to-analog conversion is not required to exchange data between the telephone network and the terminal equipment (which is the term telephone companies refer to equipment such as computers, fax machines, video phones, and digital telephones). therefore, there is no need for a modem. Nevertheless, in this case, the use of the telephone network imposes certain requirements on the subscriber. In particular, you should ensure that the loop is terminated correctly, that traffic is routed correctly, and that telephone company diagnostics are supported.

An ISDN BRI line must be connected to a device named NT1 (network termination 1). In addition to terminating the line and supporting diagnostic procedures, the NT1 device negotiates a two-wire subscriber loop with a four-wire digital terminal system. When using leased digital lines T-1 or DDS, and digital services, use a channel service unit (CSU) as the line load. The CSU acts as a terminator, ensures the correct line load and processes diagnostic commands. The customer's terminal equipment interacts with a data service unit (DSU), which converts digital signals to standard form and transfers them to the CSU. Structurally, CSU and DSU are often combined into one unit called CSU / DSU. The DSU can be built into a router or multiplexer. Thus, in this case, too (although modems are not needed here), certain interface devices will need to be installed.

TELEPHONE CARRIERS

Most analog subscriber loops can only provide 33.6 Kbps throughput under very favorable conditions. On the other hand, the same twisted pair that connects the office to the central office may well be used for ISDN BRI, giving 128 Kbps data throughput and another 16 Kbps for management and configuration. What's the matter here? The signal transmitted over analogue telephone lines is filtered to suppress all frequencies above 4 kHz. When using digital lines, such filtering is not required, so the bandwidth of the twisted pair turns out to be significantly wider, and, consequently, the bandwidth also increases.

Leased lines with a bandwidth of 56 and 64 kbps are two-wire or four-wire digital lines (in the latter case, one pair is used for transmitting and the other for receiving). These lines are also suitable as carriers for the provision of digital communications services such as frame relay or Switched 56. T-1s, as well as ISDN PRI and frame relay, often carry four-wire lines or even optical cables. T-3 lines are sometimes coaxial, but more often they are still optical.

Although ISDN continues to attract the widest attention as a means of high-speed signal transmission over long distances, recently there have been newer means of communication for the "last mile" (ie loopback). PairGain and AT&T Paradyne offer products based on Bellcore's high bit-rate digital subscriber loop (HDSL) technology. These products allow you to equalize the capabilities of all available subscriber loops; By installing HDSL devices at both ends of the line, you can get DS1 throughput (1.544 Mbps) on virtually all existing subscriber loops. (HDSL with a length of up to 3.7 km can be used on subscriber loops without repeaters in the case of standard 24 gauge wires. For normal T-1 lines to work, repeaters must be installed every kilometer and a half). An alternative to HDSL in achieving DS1 throughput on the last mile is to either use fiber optic cable (which is quite expensive) or install multiple repeaters on each line (not as expensive as fiber optic technology, but still not cheap). In addition, in this case, the costs of the telephone company, and therefore the client, for maintaining the line in working order increase significantly.

But even HDSL is not the latest in last-mile capacity enhancement. The successor to HDSL, asymmetrical digital subscriber line (ASDL) technology, is expected to be able to provide 6 Mbps unidirectional bandwidth; the bandwidth of the other is significantly lower - something around 64 Kbps. Ideally, or at least in the absence of anyone's monopoly - assuming that the cost of the service to the customer is roughly the same as the cost to the telephone company - a large proportion of customers could use ISDN PRI (or other T-1 based services) at a cost comparable to the current price of ISDN BRI.

However, ISDN advocates today probably have nothing to worry about; in most cases, telephone companies will choose to increase the capacity of their lines and put all the profits in their pockets without compromising the cost of services to the customer. It is not at all obvious that service tariffs should be based on common sense.

Table 1.Types telephone services

Usually we do not care about how the telephone line works (but not when we have to shout with all our might into the telephone receiver: "Please repeat, you can not hear anything!").

Telephone companies provide a variety of services to customers. It is not so easy to understand the price lists of these services - what is actually offered and how much you should pay for which service. In this article, we will not say a word about prices, but we will try to find out what is the difference between the most frequently offered products and services in the field of telephone communications.

ANALOG LINES, DIGITAL LINES

First, there are analog and digital lines. The analog signal changes in a continuous manner; it always has a certain value, representing, for example, the volume and pitch of the transmitted voice or the color and brightness of a certain area of \u200b\u200bthe image. Digital signals have discrete values \u200b\u200bonly. As a rule, the signal is either on or off, or it is there or it is not. In other words, its value is either 1 or 0.

Analog telephone lines have been used in telephony since time immemorial. Even phones that are fifty years old can most likely be connected to a local loop - the line between a home telephone socket and a central telephone exchange. (A central telephone exchange is not a gleaming skyscraper in a downtown area; local loop lengths average less than 2.5 miles (four kilometers), so a "central office" is usually located in some nondescript building nearby.)

During a telephone conversation, a microphone built into the handset converts speech into an analog signal transmitted to the central telephone exchange, from where it goes either to another subscriber loop, or to other switching devices, if the called number is outside the coverage of this station. When dialing a number, the telephone set generates signals transmitted over the same main channel (in-band signals) indicating who the call is intended for.

Throughout their existence, telephone companies have accumulated extensive experience in the transmission of speech. It has been found that the frequency range from 300 to 3100 Hz is generally sufficient to accomplish this task. Recall that hi-fi class audio systems are able to reproduce sound without distortion in the frequency range of 20-20000 Hz, which means that the telephone range is usually enough only for the subscriber to recognize the caller by voice (for other applications, this range is likely to be too much narrow - for transferring music, for example, telephone communication is completely unsuitable). Telephone companies provide smooth roll-off at high and low frequencies using a 4000 Hz analog telephone channel.

The central telephone exchange, as a rule, digitizes the signal intended for further transmission over the telephone network. With the exception of Gilbet County, Arkansas and Rat Fork, Wyoming, all American telephone networks carry digital signals between central stations. Although many companies use digital PBX and data communications, and all ISDN facilities are digitally encoded, loopbacks are still the “last resort” of analog communications. This is explained by the fact that most telephones in private homes do not have a means of digitizing the signal and cannot work with lines with a bandwidth of more than 4000 Hz.

WHAT IS 4000 HZ ENOUGH?

A modem is a device that converts digital signals from a computer into analog signals with frequencies within the bandwidth of a telephone line. The maximum channel capacity is directly related to the bandwidth. More precisely, the amount of throughput (in bits / sec) is determined by the bandwidth and the signal-to-noise ratio tolerance. Currently, the maximum throughput of modems - 33.6 Kbps - is already close to this limit. Users of 28.8 Kbps modems are well aware that noisy analog lines rarely provide their full bandwidth, which is often much lower. Compression, caching and other evasions help to rectify the situation somewhat, and nevertheless, we would rather live to see the invention of a perpetual motion machine than to the appearance of modems with a bandwidth of 50 or at least 40 Kbps on ordinary analog lines.

Telephone companies solve the opposite problem - they digitize the analog signal. To transmit the resulting digital signal, channels with a bandwidth of 64 Kbps are used (this is a world standard). Such a channel, called DS0 (digital signal, level zero), is the basic building block from which all other telephone lines are built. For example, you can combine (the correct term is to multiplex) 24 DS0 channels into a DS1 channel. By leasing a T-1 line, the user actually gets a DS1 circuit. When calculating the total throughput of DS1, you must remember that after every 192 information bits (that is, 8000 times per second), one synchronization bit is transmitted: a total of 1.544 Mbps is obtained (64000 times 24 plus 8000).

DEDICATED LINES SWITCHED LINES

In addition to the T-1 line, the client can lease leased lines or use ordinary switching lines. By leasing a T-1 channel or a low-speed data line from a telephone company, for example, a dataphone digital service (DDS), a subscriber actually leases a direct connection and as a result becomes the only user of a channel with a bandwidth of 1.544 Mbps (T-1 ) or 56 Kbps (low speed line).

Although frame relay technology involves switching individual frames, the corresponding services are offered to the user in the form of virtual communication channels between fixed endpoints. From a network architecture perspective, frame relay should be viewed as a dedicated line rather than a switched line; important is the fact that the price of such a service with the same bandwidth is significantly lower.

Switching services (an example of which is the service of a regular apartment telephone) are services purchased from the telephone company. On demand, the subscriber is provided with a connection via a network of public switches to any node of the telephone network. In contrast to the situation with leased lines, the fee in this case is charged for the connection time or the actual volume of traffic and depends mostly on the frequency and volume of network use. Digital switching services can be provided based on X.25, Switched 56, ISDN Basic Rate Interface (BRI), ISDN Primary Rate Interface (PRI), Switched Multimegabit Data Service (SMDS) and ATM protocols. Some organizations, such as universities, railways or municipalities, create private networks using their own switches and leased or sometimes even their own lines.

If the line received from the telephone company is digital, then digital-to-analog conversion is not required to exchange data between the telephone network and the terminal equipment (which is the term telephone companies refer to equipment such as computers, fax machines, video phones, and digital telephones). therefore, there is no need for a modem. Nevertheless, in this case, the use of the telephone network imposes certain requirements on the subscriber. In particular, you should ensure that the loop is terminated correctly, that traffic is routed correctly, and that telephone company diagnostics are supported.

An ISDN BRI line must be connected to a device named NT1 (network termination 1). In addition to terminating the line and supporting diagnostic procedures, the NT1 device negotiates a two-wire subscriber loop with a four-wire digital terminal system. When using leased digital lines T-1 or DDS, and digital services, use a channel service unit (CSU) as the line load. The CSU acts as a terminator, ensures the correct line load and processes diagnostic commands. The customer's terminal equipment interacts with a data service unit (DSU), which converts digital signals to standard form and transfers them to the CSU. Structurally, CSU and DSU are often combined into one unit called CSU / DSU. The DSU can be built into a router or multiplexer. Thus, in this case, too (although modems are not needed here), certain interface devices will need to be installed.

TELEPHONE CARRIERS

Most analog subscriber loops can only provide 33.6 Kbps throughput under very favorable conditions. On the other hand, the same twisted pair that connects the office to the central office may well be used for ISDN BRI, giving 128 Kbps data throughput and another 16 Kbps for management and configuration. What's the matter here? The signal transmitted over analogue telephone lines is filtered to suppress all frequencies above 4 kHz. When using digital lines, such filtering is not required, so the bandwidth of the twisted pair turns out to be significantly wider, and, consequently, the bandwidth also increases.

Leased lines with a bandwidth of 56 and 64 kbps are two-wire or four-wire digital lines (in the latter case, one pair is used for transmitting and the other for receiving). These lines are also suitable as carriers for the provision of digital communications services such as frame relay or Switched 56. T-1s, as well as ISDN PRI and frame relay, often carry four-wire lines or even optical cables. T-3 lines are sometimes coaxial, but more often they are still optical.

Although ISDN continues to attract the widest attention as a means of high-speed signal transmission over long distances, recently there have been newer means of communication for the "last mile" (ie loopback). PairGain and AT&T Paradyne offer products based on Bellcore's high bit-rate digital subscriber loop (HDSL) technology. These products allow you to equalize the capabilities of all available subscriber loops; By installing HDSL devices at both ends of the line, you can get DS1 throughput (1.544 Mbps) on virtually all existing subscriber loops. (HDSL with a length of up to 3.7 km can be used on subscriber loops without repeaters in the case of standard 24 gauge wires. For normal T-1 lines to work, repeaters must be installed every kilometer and a half). An alternative to HDSL in achieving DS1 throughput on the last mile is to either use fiber optic cable (which is quite expensive) or install multiple repeaters on each line (not as expensive as fiber optic technology, but still not cheap). In addition, in this case, the costs of the telephone company, and therefore the client, for maintaining the line in working order increase significantly.

But even HDSL is not the latest in last-mile capacity enhancement. The successor to HDSL, asymmetrical digital subscriber line (ASDL) technology, is expected to be able to provide 6 Mbps unidirectional bandwidth; the bandwidth of the other is significantly lower - something around 64 Kbps. Ideally, or at least in the absence of anyone's monopoly - assuming that the cost of the service to the customer is roughly the same as the cost to the telephone company - a large proportion of customers could use ISDN PRI (or other T-1 based services) at a cost comparable to the current price of ISDN BRI.

However, ISDN advocates today probably have nothing to worry about; in most cases, telephone companies will choose to increase the capacity of their lines and put all the profits in their pockets without compromising the cost of services to the customer. It is not at all obvious that service tariffs should be based on common sense.

Table 1.Types of telephone services