Very high bit-rate Digital Line Subscriber (VDSL) is next generation DSL at super-accelerated rates of 52 Mbps downstream and 12 Mbps upstream. Downstream data rates refer to download speeds, or the speed at which data travels to a computer, while upstream data rates refer to upload speeds, or the speed at which data travels from a computer to the Internet. The architecture is based one of two technologies: QAM (Quadrature amplitude modulation) or DMT (Discrete multitone modulation). These two technologies are not compatible with each other and according to many manufacturers, DMT is more commonly used.
VDSL is so hardy that it is capable of providing services like HDTV and video-on-demand along with Internet access, and it can be bundled with HDTV packages as it establishes a presence in the marketplace. It is the first high-speed technology that can provide an entire home-entertainment package, making it entirely unique. As demand grows, the price of packages will likely fall.
This technology is able to deliver incredible bandwidth over standard telephone lines because voice communications through the telephone require only a fraction of the wire's capability. For a rough analogy, users can consider a multilane freeway where only the slow lane is being used for traffic traveling at very slow speeds. By opening the other lanes to faster hybrid traffic, the entire freeway can be used, or in this case, the entire wire pair. A telephone or fax can also be used simultaneous for Internet access or other VDSL services.
VDSL, based on discrete multi-tone modulation (DMT), creates 247 virtual channels within the available bandwidth. Each channel's integrity is monitored and data is switched to an alternate channel when signals become degraded. In this way, data is constantly shifted to the best route for transmitting or receiving data, making DMT a robust, albeit complex, technology.
As with other broadband technologies, end-user speeds will depend upon the distance of the feed or loop to the local telephone company or telco office. Shorter distances afford faster rates, while longer loops degrade signal and speed. One drawback of this technology is that it requires a very short loop of about 4,000 feet (1,219 meters), or three-quarters of a mile. Another complication can inadvertently create a solution for the distance problem: the complication of fiber optic lines.
Many telcos are installing fiber optic lines in place of copper lines. If a stretch of line between the customer and telco is fitted with fiber optic, VDSL signals get "lost in translation" converting from analog (copper), to fiber optic (digital), and back to analog. A VDSL gateway device installed at the junction box will translate the signals to pulses of light able to traverse the fiber optic cable. Through this process, the distance barrier associated with copper wire and VDSL is "bridged" or bypassed. When the telco receives the light impulses, it sends data back to the junction box gateway, which converts the signal to forward along the copper wires a short distance to the modem. In this scenario, distance is not a limiting factor.
VDSL is available worldwide in specific regions and growing all the time, though it's not easily found in the United States. A second generation version known as VDSL2 boasts speeds up to 100 Mbps.