Nowadays, high rate digital telecommunication devices over cable, such as for example xDSL, Ethernet technologies etc, are even more diffused. Classically, telecommunication devices that use these technologies are connected to a line for transmitting/receiving data and are supplied through another line connected to the electric mains.
A problem tied to the use of these telecommunication devices consists in that they may be installed only where an electrical power supply line is available.
In order to overcome this limitation, it is possible to connect these telecommunication devices to purposely installed lines adapted to convey data transmitted to or received from the telecommunication system, and to provide the required supply current. Installing these lines is in general expensive and hardly can be done in existing buildings.
Theoretically speaking, it would be possible to use twisted pairs already available in all buildings, or coaxial cables, for conveying data and remote powering the above mentioned high rate digital telecommunication devices. Remote powering telecommunication devices through cable or twisted pair is known. For example, analog telephones, that are even less diffused, were powered and transmitted voice signals through twisted pairs.
To the best knowledge of the applicant, so far it has not been possible to use the same twisted pairs for transmitting wide-band digital signals (for example xDSL) and at the same time for powering telecommunication devices. Attempts for remote powering telecommunication devices using the same twisted pair used for transmitting/receiving digital data, by implementing the same technique used for powering classic analog telephones, have failed, thus showing beyond any doubt that wide band digital telecommunication devices have peculiar characteristics that make them substantially different from classic analog telephones.
More particularly, in classic analog telephones, twisted pairs were used for DC powering and at the same time for transmitting a voice signal, that may be modeled as a low-pass signal concentrated in the frequency range from about 0 up to 4 kHz. Moreover, classic analog telephones needed relatively low supply voltages and currents, thus it was relatively simple to decouple the voice signal from the DC supply and making telephones powered as needed.
Differently, xDSL digital telecommunication devices need higher supply voltages and currents and further the frequency band of conveyed digital signals is in the order of several megahertz, that is of various order of magnitudes greater than the analog voice signal. The fact that the transmitted digital signals have a wide band makes particularly critical remote powering telecommunication devices with relatively high voltage and currents and ensuring a correct decoding of the digital signal and relatively small supply power losses along the line.
The published US patent application No. 2006/238250 provides for dynamic insertion loss control for a 10/100/1000 megahertz Ethernet power on differential cable pairs. A power feed circuit supplies power to a network attached device (PD). An insertion loss control circuit limits power loss in a coupled power feed circuit. The insertion loss control circuit determines an insertion loss limit and senses an average power of the power signals to produce a common mode feedback signal to the power feed circuit.
The published PCT application WO 2007/121148 discloses a network device for supplying power over Ethernet networks through a network device comprising a transformer with a primary winding and a secondary winding. The primary winding is coupled to receive input signals from a network connector and supply data signals to a physical layer (PHY) module. An inductance boost circuit is coupled to the secondary winding and operable to increase the impedance of the primary winding.
The international patent application PCT/IT2011/000241 in the name of the same applicant discloses devices for injecting and drawing a supply current in/from a telephone line or coaxial cable (shown in FIGS. 1a and 1b, respectively) that, according to studies carried out before realizing a tested prototype, would have been capable respectively of:                injecting/drawing a DC supply current in/from a twisted pair for a plurality of telecommunication devices connected thereto together with a stream of xDSL digital data; and        separating the power supply of each telecommunication system from the respective stream of xDSL digital data.        
In an inexplicable manner, the disclosed devices were not capable of remote powering telecommunication devices connected to the twisted pair and at the same time complying with the specifications ruled by international laws for this particular use of twisted pairs, unless the distance between the transmitting device and the receiving device was relatively short. As a matter of fact, the disclosed devices did not comply with the specifications in all case of practical because, for having the desired voltage level in input to the remote powered telecommunication devices, interest it was necessary to raise the supply voltage upstream beyond the maximum limits imposed by specifications, in order to take into account unexpected voltage drops, the origin of which was not understood.