What is VDSL, and how does it work?

The full form of VDSL is the Very High-Speed Digital Subscriber Line. It is an electronic system that uses pairs of twisted copper wires as used in telephones, to transmit text, sound, and video at very high speed over short distances. This allows people in homes and offices to enjoy multi-media presentations in real-time at their desktop computers, without affecting the ability to receive and to make telephone calls.

Telephone wires have a capacity far in excess of that used for telephone conversations. All Digital Subscriber Line technologies use this spare capacity to transmit data other than sound that is in the form of text and video. Sound is transmitted at frequencies of less than 4 thousand cycles per second. Data in digital form uses very much higher frequencies and hence the same pair of copper wires behind each ordinary telephone can carry digital data and voice conversations at the same time.

VDSL can send up to 55 megabits of data per second as against 10 megabits per second with a conventional modem. However transmission quality falls sharply beyond about 4 thousand feet. Therefore VDSL has to interface with optical fiber lines to receive and to send data over long distances. This interface takes the form of a special transceiver in each home or office and a junction gateway that provides a pathway to a central source or server. Such an arrangement not only overcomes VDSL?s inability to travel over long distances but also allows conversion between analog and digital data through light pulses.

The central source or server can route data from the home or office transceiver, through the gateway junction to the appropriate network of final destination. The system also works in reverse that is from an original source to a central location and from such a central location to individual transceivers through the junction gateway. Transactions can take place at electronic speed with no perceptible time lag between sending and receiving signals or commands.

There are 2 kinds of VDSL-one uses existing ADSL (Asymmetric Digital Subscriber Line) technology while the other has the potential to use less power. The first system is called DMT (Digital Multi-Tone). DMT divides the capacity of the copper wires into 247 sections each with a width of about 4 thousand cycles per second. The system monitors each of the 247 channels and sends data in either connection on the best of the available channels.

The other VDSL system is called QAM (Quadrature Amplitude Modulation). QAM changes the shape and angle of waves generated by signals. This exponentially increases the speed and capacity of a line to receive and to send digitized data. CAP (Carrierless Amplitude Phase) is a variation of QAM in which the line is divided in to three distinct frequencies, each separated from the other two by wide bands.

One channel each is for data from an individual computer to the central server and vice-versa. The third channel is for voice signals. The clear separation between the 3 sub-divisions gives CAPS the advantage of transmission without disturbance and interference. Each kind of VDSL is strongly supported by its inventors and industrial propagators.

VDSL will be used on a wider scale once a binding choice is made between the options. VDSL will enable individual homeowners and office users to enjoy information and entertainment services of better quality and at lower total costs as well.

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