DSL Phantom Mode. Bell Labs, the research facility of Alcatel-Lucent has developed a technique by which they reached speeds of 300 megabits per second over traditional DSL copper lines, a goal that was considered possible only with the fibrt optic transmission technology.
DSL Phantom Mode - Overview
Fiber optics is undoubtedly the most advantageous choice in the broadband transmission filed, being able to support multimedia services requiring high data traffic such as IPTV (Internet Protocol Television) and Voice Over IP (VoIP). The question that is legitimate to ask is this: to what extent is it possible to "exploit" the normal copper connections that are available in our homes? The Bell Labs division of Alcatel-Lucent seems to have an answer to this question: it is possible to push connections on copper lines up to reach downstream speed equal to 300 Mbps. This technique, called DSL Phantom Mode, has the advantage to result in significantly lower installation costs compared to the installation of new optical fibers where they are requested. Although this speed is quite impressive, there are certain issues that must be taken into account.
A first consideration is related to the distance, a factor that we must take into account when dealing with DSL (Digital Subscriber Line) transmissions. The distance that the transmitted data must travel over copper links (or metals that may be) produces infact a signal degradation, and that's why DSL technologies have been developed always combining them to signal regeneration stations located along the routes traveled by the data. Not only that, the signal degradation is directly related to the amount of data transmitted at any time.
In other words, the higher the bit rate, the faster the signal degradation that ensues. The DSL Phantom Mode technique can achieve a top speed of 300 Mbps but only over a distance of approximately 400 meters. Since only a limited number of dwellings or commercial structures are located within a radius of 400 meters with respect to a multiplexing station, Alcatel-Lucent has also thought of developing a more "soft" DSL Phantom Mode version with which it is possible to achieve a speed of 100 Mbps over a distance up to 1000 meters, a distance sufficient to ensure a significant coverage of users. A second aspect to consider is that in order to apply this technology it takes two DSL lines, that is two pairs of wires; that, however, is not a very big limitation for many types of applications.
Currently, DSL connections can achieve a data rate up to 50 Mb/s using the VDSL2 technology. This technology is characterized by a symmetry between upload and download speeds, which may come to assume almost the same values, allowing applications where it is essential to have a high upload speed (such as VoIP, video and file sharing, etc.). In this sense, VDSL2 can be clearly distinguished from ADSL (Asynchronous DSL), characterized by its high speed in downstream, opposed to an uninspiring upstream speed.
DSL Phantom Mode - How it works
The Phantom Mode is actually based on an old ploy, more than 100 years old, used to increase the transmission rate on normal telephone lines. The technology combines together three existing techniques: bonding, vectoring, and phantom mode. The first two components of the new technology, vectoring and bonding, are two standard ways to increase the speed of DSL broadband connections: vectoring is used to eliminate the noise (cross-talk) from the DSL line, while bonding allows the treatment of multiple lines (each consisting of two wires) as if they were a single line, which allows to increase the available bandwidth by a factor almost equal to the number of used connections (it's actually a little lower, so that from two pairs of 100 Mbps wires you get a connection at about 180 Mbps).
Bonding is currently used in various areas of Europe and Asia. The third component, the Phantom Mode, is based on a "trick" in the networking field discovered in 1886 by the engineer (and pioneer of telephony) John J. Carty, who later became vice president of AT&T.
The "trick" is the following: a digital signal is typically transmitted on two wires twisted together, one positive and one negative. Carty discovered that it is possible to transmit a third signal on four wires, separated in two pairs of two wires each. The negative half of this "phantom" link is transmitted on a pair of wires (in addition to the signal that it is already carrying), and the positive half is transmitted on the other pair. On the receiving end, appropriate analog circuits must separate all the three signals, two real and a "phantom" one, from the two pairs of wires.
Phantom Mode technique is important because it can satisfy the ever increasing demand for high bandwidth applications and services, such as the video-on-demand, HDTV, cloud computing, etc., all applications that require data to be transmitted over the network in a short time. The final solution could be represented by fiber optics, but that has some considerable costs. While waiting for the fiber optics extends coverage, a solution such as DSL Phantom Mode can act as a bridge and manage very well a migration phase.
Alcatel-Lucent's DSL Phantom Mode technology was awarded in 2010 as the "Broadband Innovation of the Year."