For a typical twisted pair phone line this maximum is around 30Kbps. Early attempts to reach this theoretical maximum made use of an odd fact - after the local loop the phone system is digital. This is faster than the theoretical maximum allowed for a purely analogue modem and it does seem to represent the fastest that can be achieved without installing new hardware at the exchange.
The key to getting the speed even higher on the local loop does depend on changing the hardware at the exchange. The limitation on using modems on standard phone lines is mostly due to the way that the local loop is used. As a voice call only uses up to 4KHz the local loop is limited to delivering just this narrow range and 4KHz represents the highest frequency used by any stages further up the line. The wires that go back to the exchange are capable of being used at a much higher frequency than is normally utilised in a standard phone call.
All we have to do is to change the equipment at the exchange and possibly remove any loading coils on the line which where used to improve the voice quality and the maximum frequency you can transmit rises to 1MHz or more. The quality of the line goes down with the distance to the exchange and many other factors but, if the wire is less than 5. However the maximum frequency that a line can work at depends on its length.
The closer you are to the exchange the higher the data rate you can expect. An earlier system, called Carrierless Amplitude Phase CAP , was able to use all the bandwidth above 4KHz as a single transmission channel, had the advantage of being closely related to the Quadrature Amplitude Modulation QAM technique used by high-speed modems at rates above 9.
However, DMT is a more reliable, sophisticated and flexible technology, and has succeeded in becoming the universally adopted standard. When the service first became commercially available, the only additional equipment subscribers needed to use ADSL was a suitable modem. While these devices were still being developed, prototypes had three connectors on the back of the unit: one going to the wall jack and then out to the phone company; one was a standard RJ11 phone jack for analogue phone service; and one was an Ethernet twisted-pair connector that connected the ADSL modem to a PC.
At the user end, the ADSL modem collects high-frequency digital data and assembles it for transmission to the PC or network. The phone signals are sent to the switched telephone network and the digital data routed data to the Internet via a high-speed backbone implemented as T1, fibre, ATM, or DSL. Your DSL modem is the middle man between your Internet service provider and your computer, acting as a data translator that connects your device to the Internet.
Unlike dial-up Internet, which also uses a phone line, DSL allows you to use your phone while connected. Was this Helpful? Yes No By continuing to use our website, you acknowledge the use of cookies. Out of these cookies, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website.
These cookies will be stored in your browser only with your consent. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3, Hertz cycles per second -- see How Telephones Work for a great demonstration of this.
This range of frequencies is tiny. For example, compare this to the range of most stereo speakers , which cover from roughly 20 Hertz to 20, Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity.
DSL does just that. A DSL internet connection is one of many effective communication tools for keeping employees in touch with the office. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit most of the time.
ADSL is a distance-sensitive technology : As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18, feet 5, meters , though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service.
At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream Internet to customer speeds of up to 8 megabits per second Mbps at a distance of about 6, feet 1, meters , and upstream speeds of up to kilobits per second Kbps. In practice, the best speeds widely offered today are 1. You might wonder -- if distance is a limitation for DSL, why is it not also a limitation for voice telephone calls?
The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:.
There are two competing and incompatible standards for ADSL. CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the 0 to 4 KHz kilohertz band, as they are in all POTS circuits. The upstream channel from the user back to the server is carried in a band between 25 and KHz. The downstream channel from the server to the user begins at KHz and goes up to a point that varies depending on a number of conditions line length, line noise, number of users in a particular telephone company switch but has a maximum of about 1.
This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.
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