Abstract:
A telephone unit device connectable in series between a telephone set and a wire pair coupled to a telephone service and carrying DC power and an AC telephone signal. The device includes: a first connector connectable to the wire pair; a second connector connectable to the telephone set; means connected between the first and second connectors for passing substantially without change the AC telephone signal between the wire pair and the telephone set; and means connected between the first and second connectors for receiving DC power only from the wire pair and converting the DC power to a level suitable for proper telephone operation of the telephone set connected thereto.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a division of copending application Ser. No. 10/403,526, filed on Apr. 1, 2003, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the transmission of electrical power, and, more particularly to the provision of local Direct-Current (DC) power utilizing active telephone wiring. 
     BACKGROUND OF THE INVENTION 
     Conventional analog telephony (also known as “Plain Old Telephone Service”, or “POTS”) is based on a central office (CO) or private branch exchange (PBX) connected to telephone sets using a wire pair known as a ‘subscriber loop’. In most cases, the telephone set side of the wiring in a building is terminated using telephone outlets, into which the telephone sets are connected. The PBX/CO provides a low-voltage (usually around 48VDC) low-power direct current in order to detect a telephone off-hook/on-hook condition of a telephone connected to that line. In a conventional analog telephone, lifting the handset off the cradle (off-hook condition), closes a switch that places a resistive load across the line, causing a direct current to flow in the subscriber loop. This current is detected at the PBX/CO to signal the telephone off-hook condition, and also for legacy ‘Pulse Dialing’ switching systems. Although originally intended only for detecting the telephone off-hook condition and Pulse Dialing, this DC power has also been widely used to provide low-voltage electrical power for other purposes, such as electronic circuitry used in telephonic devices and related apparatus associated with use of the telephone lines for communication and data transmission. 
     For purposes of this invention, any facility providing a local telephone line to one or more telephones is considered to be functionally equivalent to a Private Branch Exchange (PBX), a Central Office (CO), or similar system, and is herein denoted by the term “PBX/CO”. 
       FIG. 1  is a schematic diagram illustrating a basic in-house PBX/CO-to-telephone connection arrangement  10 . A PBX/CO  11  has a 2-wire telephone line local or subscriber loop constituted by wiring  14  to which telephone sets  13   a  and  13   b  are connected via respective 2-wire connecting lines. A “telephone set” includes, but is not limited to, telephones, fax machines, dial up modems, and any other telephonic devices. In most cases, the telephone sets connect to the telephone wiring by means of telephone outlets  12   a  and  12   b , respectively. 
     While wiring  14  in a premises is normally based on a serial or “daisy-chained” topology, wherein the wiring is connected from one outlet to the next in a linear manner; other topologies such as star, tree, or any arbitrary topology may also be used. Regardless of the topology, however, the telephone wiring system within a residence always uses wired media: two or four copper wires terminating in one or more outlets that provide direct access to these wires for connecting to telephone sets. 
     The term “telephone outlet” herein denotes an electro-mechanical device that facilitates easy, rapid connection and disconnection of external devices to and from telephone wiring installed within a building. A telephone outlet commonly has a fixed connection to the wiring, and permits the easy connection of external devices as desired, commonly by means of an integrated connector in a faceplate. The outlet is normally mechanically attached to, or mounted in, a wall. A “telephone outlet”, as used herein, can also be a device composed of a part that has a fixed connection to the wiring and is mechanically attached to, or mounted in, a wall, and a part that is removably mechanically attached and electrically connected to the first-mentioned part, i.e. a device in which the first part is which is a jack or connector used for both electrical connection and mechanical attachment. The term “wall” herein denotes any interior or exterior surface of a building, including, but not limited to, ceilings and floors, in addition to vertical walls. 
     It would be desirable to have the possibility of carrying power over active telephone lines in addition to the telephone signals. This would, among other things, obviate the need to install additional cabling in installations wherein telephone wiring already exists. For example, power carried over a telephone line may be used to power repeaters, as well as any other mediation devices throughout the telephone wiring, multi-features telephone sets, and other telephony related and non-telephony devices. The powering is usually required when the connected telephones are off-hook and on-hook. 
     U.S. Pat. No. 6,216,160 to Dichter and US Patent Application Publication 2002/0003873 to Rabenko et al. disclose carrying AC power over active telephone wiring, using frequency domain multiplexing (FDM) in order to avoid interference with the telephony signals, as well as other signals carried over the telephone wiring. This approach to supplying power has drawbacks due to the radiation limitation imposed on non-shielded telephone wiring, for example by the FCC. Furthermore, such implementation requires very complex and expensive filtering circuits. 
     U.S. Pat. No. 6,157,716 to Ortel discloses a technique for carrying DC power over active telephone lines. Based on a diode and on the impedances exhibited in the various on- and off-hook states, DC current can be imposed and extracted using the telephone lines. Using DC powering reduces the radiation and filtering problems associated with the AC powering. However, the technique disclosed by Ortel allows only for a very limited amount of power to be carried over the telephone wiring. 
     A general prior art system  20  is shown in  FIG. 2  and differs from system  10  by including a Power Supply Coupler (PSC)  21 , which is supplied with power from power supply  22  and couples that power onto active telephone wiring  14 . Power supply  22  is powered from the utility AC mains via a standard plug  25 . A load Coupler  23  extracts the power from the telephone line and feeds a load  24 . In such a system, power supply  22  feeds load  24  using the active telephone wiring  14 , with the goal of minimum interference to the telephone signals carried simultaneously over the wiring  14 . 
     It would be highly advantageous to have a system for providing increased amounts of DC power to power remote devices via active telephone lines, such as those served by a PBX/CO in a building or within an office. This goal is met by the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a system and a method for achieving the goal of providing adequate amounts of electrical power over active telephone lines to power loads such as extensive data processing and handling circuitry. 
     The invention is based on separating signals supplied to a subscriber loop into an AC signal and a DC signal. The AC signal, or component, is the telephone signal routed over the system from the PBX/CO to the telephone set(s). The DC signal, or component, is blocked and replaced with a separately generated DC signal, carried over the wiring along with the AC signal. A load can be connected to the wiring via a coupler which passes the DC signal, without affecting the AC signal. This allows loads to be powered by a DC signal in a manner independent of the PBX/CO DC characteristics. A telephone set can be connected to the wiring through a conventional telephone coupler that passes the AC signal and blocks or processes the DC signal to adapt to the telephone DC characteristics. The present invention thus allows a DC signal having any desired voltage level and power content to be imposed on a subscriber loop without interfering with the telephone signals. 
     To maintain the ability of the PBX/CO to detect a telephone off-hook condition in a telephone set connected to a line that has, for purposes of the present invention, been separated from the PBX/CO&#39;s DC current loop, the present invention further provides for non-DC dedicated telephone off-hook signaling carried over the wiring from the telephone set to the PBX/CO. For this purpose, there is provided an off-hook detection device connected to the telephone terminals for generation of a off-hook tone over the wiring when an off-hook condition exists. This tone is detected by a device connected to the wiring, and allows generation and transmission of a DC off-hook state signal to the PBX/CO, thus providing full off-hook and on-hook functionalities. 
     A system according to the invention can be partially or fully integrated into a telephone set, a PBX/CO, distinct stand-alone devices, or telephone outlets. 
     A system according to the present invention is suitable for providing local power via telephone lines within a building or office, or in any other environment whereby telephone service is provided by a Central Office, a Private Branch Exchange, or similar facility. Similarly, the system can be used to carry telephony service over DC-carrying wire pair. The descriptions of the present invention are therefore exemplary and do not limit the application of the invention to telephone lines connected specifically to private branch exchanges, central offices, or any other particular facility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a prior-art PBX/CO-to-telephone connection common in houses and offices. 
         FIG. 2  is a schematic diagram illustrating a prior-art scheme for providing electrical power over local telephone lines. 
         FIG. 3  is a schematic diagram of a telephone installation according to the present invention, showing how the DC power originally supplied by the PBX/CO to the telephone is separated and replaced by power from a dedicated local power supply. 
         FIG. 4  is a functional block diagram illustrating the handling of a telephone off-hook condition according to the present invention. 
         FIG. 5  is a functional block diagram illustrating the grouping of the functions according to the present invention into a Power Unit (PU) and a Telephone Unit (TU). 
         FIG. 6  is a functional block diagram illustrating a sample embodiment according to the present invention. 
         FIG. 7  is a functional block diagram illustrating a sample embodiment according to the present invention wherein the power is also used to power a telephone set. 
         FIG. 8  is a pictorial view of an outlet according to the present invention. 
         FIG. 9  is a schematic diagram illustrating a prior-art scheme for providing data communication over telephone lines. 
         FIG. 10  is a functional block diagram illustrating a sample embodiment according to the present invention wherein the power is also used to power data communication circuitry. 
         FIG. 11  is another pictorial view of an outlet according to the present invention. 
         FIG. 12  is a diagram showing a specific exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles and operation of systems for providing local power over telephone lines according to the present invention may be understood with reference to the drawings and the accompanying description. The drawings and description are directed to principles of the invention. In actual practice, a single component can implement one or more functions; alternatively, each function can be implemented by a plurality of components and circuits. In the drawings and description, identical reference numerals indicate those components that are common to different embodiments or configurations. 
     An analog telephony signal may comprise many components, including voice (audio signal), ringing, DTMF signals, and DC (commonly −48VDC) used for on-hook and off-hook indicators, pulse dialing and powering the telephone set. The invention calls for separating the DC signal (hereinafter referred to ‘DC telephony signal’ or ‘DC signal’) from all other signals (non-DC signals, hereinafter collectively referred to ‘AC telephony signal’ or ‘AC signal’), and while the AC signal is being transparently carried over the telephone wiring, distinctly processing the DC signal, as will be explained in detail below. 
     AC/DC Separation 
     The invention will be now explained in greater detail with reference to system  30  shown in  FIG. 3 . In this system, an AC Pass/DC Stop device  34  is inserted in series between the PBX/CO  11  and telephone wiring  38 , which corresponds to wiring  14  of  FIGS. 1 and 2 . Device  34  is fully AC transparent to allow the AC signal to follow through to wiring  38 , while stopping the DC component of the telephone signal. Similarly, a DC Pass/AC Stop device  35  is also connected to PBX/CO unit  11 . This device is fully DC transparent to allow the DC signal to follow through to a connection  37 , while stopping the AC component of the telephone signal coupled to PBX/CO  11 . AC Pass/DC Stop device  34  may include, but is not limited to, components such as capacitors and transformers. Similarly, DC Pass/AC Stop device  35  may include, but is not limited to, components such as inductors and chokes. A DC power supply  22  receives power from the utility AC mains via a plug  25  and is coupled to wiring  38  via a power supply coupler  33  whose main function is to impose DC power on telephone wiring  38  without interfering with, or being interfered with by, the AC signal carried over wiring  38 . A load  32  is connected to wiring  38  via load a coupler  31 , which extracts the power from wiring  38  without interfering with, or being interfered with by, the AC signal carried over these lines. Telephone set  13   a  is optionally connected to telephone wiring  38  via a telephone coupler  36 , operative to present a conventional telephone interface to telephone set  13   a , without interfering with the DC power carried over wiring  38 . For example, the coupler  36  may comprise a DC/DC converter allowing for providing DC power suitable to the telephone operation. Other functions of coupler  36  will be described below. 
     AC Pass/DC Stop device  34  and telephone coupler  36  are transparent to the AC signal, and DC Pass/AC Stop device  35 , power supply coupler  33  and load coupler  31  do not affect that signal. Thus, the AC signal properly connects the PBX/CO  11  to telephone set  13   a . The DC telephony signal from PBX/CO  11  is decoupled from the DC power signal produced by the power supply  22 . The DC telephony signal is not routed over telephone wiring  38 , but rather routed to connection  37  via DC Pass/AC Stop device  35 . As a substitute, the DC power signal generated by power supply  32  and consumed by load  32  is carried over wiring  38 . It should be noted that in such configuration there is no telephony related limitation to the power that can be carried by the telephone line  38 , thus allowing for high DC voltage and power levels to be carried. 
     Telephone Off-Hook Condition Handling 
     As noted previously, a telephone off-hook state and also pulse dialing signals are detected by the PBX/CO  11  on the basis of a current flow in the subscriber loop. However, since system  30  shown in  FIG. 3  does not allow direct DC coupling between the PBX/CO  11  and telephone set  13   a , an off-hook condition cannot be detected by the PBX/CO. Thus, system  30  may be used in telephone systems not using off-hook signaling, such as intercom or continuous connection. 
     A system  40  providing off-hook detection is shown in  FIG. 4 . In order to allow detection of the off-hook state, an off hook detector  41  is added between telephone coupler  36  and telephone set  13   a . Detector  41  functions to detect an off-hook condition of telephone set  13   a . Such detection may be effected by measuring the current flowing through the telephone set  13   a  (similar to the method used by the PBX or CO) or measuring the voltage drop across the telephone set  13   a  connections. While off-hook detector  41  in  FIG. 4  is connected in series to with telephone set  13   a , a parallel connection may also be appropriate, according to the detection method used. 
     Upon sensing an off-hook state, detector  41  notifies off-hook transmitter  42  of such state. Transmitter  42  is operative to transmit a signal over telephone wiring  38  indicating the off-hook detection. This signal is picked up by an off-hook receiver  44 , which is connected across telephone wiring  38 . Off hook receiver  44  then triggers an off-hook simulator  43  that produces a signal simulating the off-hook state and transmits that signal to the PBX/CO connected via DC Pass/AC Stop device  35 . In a simple implementation, off-hook simulator  43  induces a current flow in connection  37  that is similar to the current flow that would have occurred if telephone set  13   a  were to be directly DC connected to PBX/CO  11 . In this way, an off-hook condition of telephone set  13   a  is reliably sensed by PBX/CO  11 . Since on-hook state exists at all times when off-hook state is not sensed, the full telephony service operation is a fully functional equivalent to the performance of the system shown in  FIG. 1 . 
     The off-hook indication signal flowing from transmitter  42  to receiver  44  does not make use of any DC signals as used in the prior-art, but rather uses AC type signaling. Non-limiting examples of such transmission method may include:
         a. A tone sent over wiring  38  (similar to DTMF signaling).   b. In the case wherein a data network signal is also carried over the telephone wiring, the off-hook condition may be encoded as a message carried over the data network.   c. Transmitter  42  can exhibit an impedance change with respect to a specific frequency, sensed by the receiver as drawing current (or current change) at this frequency.       

     In all of the above cases, the off-hook signaling may be continuous, wherein the off-hook indication is being transmitted as long as the off-hook condition exists, or may be used as a status change indication, wherein shifting from on-hook to off-hook and vice versa is indicated. 
     While the invention has been described with respect to a single telephone set  13   a , it will be appreciated that the invention equally applies to any number of such telephones, wherein each telephone is coupled to a coupler  36 , detector  41  and transmitter  42 . Alternatively, multiple telephone sets can be connected to a single set of the above units. Similarly, while the invention has been described with respect to a single load  32 , it will be appreciated that the invention equally applies to any number of such loads, wherein each such load is coupled by a coupler  31 . 
     The Off-Hook signaling mechanism described above in system  40  is composed of Off-Hook detector  41  and Off-Hook transmitter  42  on the telephone side, together with Off-Hook receiver  44 , Off-Hook simulator  43  and DC Pass/AC Stop device  35  on the PBX/CO side. While this Off-Hook signaling mechanism is shown to work as an adjunct to power delivery system  30  as described in  FIG. 3 , it should be obvious that such a mechanism can equally be used in any embodiment wherein DC powering and loading is not used for Off-Hook state detection. For example, in some environments a galvanic isolation is required, obviating the need for carrying DC power over the wiring. In such a case the Off-Hook signaling mechanism described can equally apply. 
     While the invention has been described with respect to an exchange (PBX) or a PSTN (Public Switched Telephone Network) Central Office (CO), it will be appreciated that the invention equally applies to any telephone line source into which a telephone set can be connected. Both circuit switching and packet switching can be used to originate the telephone signal. For example, a VoIP MTA (Multimedia Terminal Adaptor), either as standalone or integrated within a cable-modem or set top box can be used, wherein the telephony service is originated as part of the CATV network. Similarly, VoDSL (Voice over DSL) based telephony can also be used as a telephone line source. 
     While the invention has been described with respect to DC power supply  22  being fed from the AC utility mains via plug  25 , it will be appreciated that the invention equally applies to any AC power supply than converts AC to DC. Furthermore, direct DC feeding may also be used, either fed directly to coupler  33  and obviating the need for power supply  22  or wherein power supply  22  perform DC/DC conversion. In both cases, the power may be originated in the AC utility mains, a battery or externally fed from any network (e.g. HFC network). For example, a UPS (Uninterruptible Power Supply) system can be used, ensuring the telephony service operation even in the case of mains power outage. Also, the input to power supply  2  can be hard-wired to the power system rather than being connected by a plug. 
     In general, the functions performed by system  40  can be grouped into two groups: power insertion functions and telephone set functions. The power insertion functions refer to power supply  22 , power supply coupler  33 , AC Pass/DC Stop device  34 , DC Pass/AC Stop device  35 , Off-Hook simulator  43  and Off-Hook receiver  44 . For simple installation and implementation, it can be beneficial to integrate part or all of these functions into a single device, referred to hereinafter as a ‘Power Unit’ (PU)  51  shown as part of system  50  in  FIG. 5 , performing all of the above functions. PU  51  comprises of at least three ports, as follows:
         a. Incoming telephone line interface  52  that allows for the connection of power unit  51  to a PBX/CO  11 .   b. Outgoing telephone line interface  53  for connecting power unit  52  to telephone wiring  38 , into which load  32  and telephone set  13   a  are coupled.   c. Power interface  54  that allows power to be supplied to power unit  51 , e.g., from AC utility mains via plug  25 .       

     As explained above, in the case wherein suitable DC power is directly fed, power unit  22  may be obviated. Similarly, other functions included in power unit  52  may be eliminated or integrated into other devices. 
     While the invention has been described with respect to Power Unit  51  being a standalone and distinct device, it will be appreciated that the invention equally applies to cases where Power Unit  51  is partially or fully integrated into other devices. Specifically, power unit  51  may be integrated into any unit having a telephone line interface. In one embodiment, power unit  51  is integrated into PBX/CO  11  unit. For example, power unit  51  may be integrated into a cable modem or set top box used in the CATV industry and employing VoIP MTA, as well as VoDSL equipment. 
     In another embodiment, power unit  51  is integrated into wiring devices, such as demarcation points, communication closets, outlets and junction boxes. In a preferred embodiment, power unit  51  is integrated into a telephone outlet. In such a system, the 2-wire cable connecting the telephone set to the outlet can carry both the telephony signal and power (as well as other signals, such as data communication signals, if applicable). 
     A system  60  similar to system  50  modified to include such an outlet is shown in  FIG. 6 . System  60  includes a PU  61  that may be identical to power unit  51  integrated within an outlet. Interface  62 , correlated with interface  52  of power unit  51 , connects PU  61  to telephone wiring  14 , preferably an existing in-wall telephone wire pair. PU  61  is AC powered from mains via plug  25 , connected to interface  64 , which correspond to interface  54  of power unit  51 . Telephone set  13   a  and load  32  are connected via a wire pair  65 , connected to interface  63  of PU  61 , corresponding to interface  53  of power unit  51 . In such a configuration, single wire pair  65  is used to carry both telephony and power to telephone set  13   a , simultaneously with power to load  32 . 
     Similarly, the functions associated with the telephone set can be grouped into a standalone, distinct device. Such a unit  55  is shown as part of system  50  in  FIG. 5 . Telephone coupler  36 , Off-Hook detector  41  and Off-Hook transmitter  42  are all integrated into Telephone Unit (TU)  55 . Such a TU  55  comprises two ports:
         a. Telephone wiring interface  57 , connecting TU  55  to PU  51  via telephone wiring  38  and outlet  12   a ; and   b. Telephone set interface  56 , preferably using a standard telephony connector (e.g. RJ-11 in North America), connecting the TU to a telephone set  13   a.          

     According to one embodiment, TU  55  is a standalone, distinct device. However, since an additional device may be neither aesthetic nor easy to handle, other embodiments involve integrating TU  55  into telephone set  13   a , wherein the need for an external module is obviated. However, in both cases of standalone and telephone integrated implementations, there is a risk of directly connecting a telephone set (regular, non modified) directly to the outlet, thus connecting to a power level that may damage the unit. Hence, according to preferred embodiments TU  56  is integrated into an outlet, preferably a telephone outlet. In such a configuration, no external distinct device is required, and the telephone set can be connected to the outlet without any need for special measures. 
     While the invention has been described with respect to a single general load  32 , it will be appreciated that multiple loads may be employed. In some embodiments of the invention, load  32  represents the power required to operate some or all functions of telephone set  13   a . For example, telephone set  13   a  may consists of a fax machine, cordless telephone, answering machine, multi-function telephone, or any other power consuming functionality, wherein the conventional power derived from the telephone line during off-hook condition may not suffice. In the prior-art, such power requirements are supplied by either a battery or via the AC power mains, usually using a small transformer. Powering via the telephone lines according to the invention obviates the need for any additional power supply such as battery or AC power mains connection. 
     In all cases wherein the power carried over the telephone wiring according to the invention is used for powering telephone set functions, load coupler  31  may be implemented in the following forms:
         a. A standalone distinct unit powered by connection to the telephone wiring and feeds the telephone set.   b. Integrated into Telephone Unit (TU)  55  device.   c. Integrated into an outlet.       

     The latter case is represented by system  70  in  FIG. 7 . The telephone sets shown are fax machine  71   a ,  71   b , which commonly require an external power source for their normal operation. In most cases, such power is provided by a transformer (which may be internal to the unit), which is fed from the AC utility mains. Such an arrangement is shown for fax  71   a , powered from the mains by plug  25   a , via transformer  72   a  connected to fax  71   a  via connection  73   a . The telephone connection of fax  71   a  makes use of TU  55   a  as described above with reference to  FIG. 5 . 
     System  70  also has an outlet  77 , integrating a TU  55   b , a load coupler  31  and, optionally, a converter  76 . Outlet  77  is coupled via interface  78  to telephone wiring  38 , carrying both power and telephony signals according to the invention. Within outlet  77 , the telephone wiring is routed to TU  55   b , via its interface  57   b . Port  74  of outlet  77 , preferably using a standard telephone connector (e.g. RJ-11 in North America) connects to output  56   b  of TU  55   b , thus allowing for normal telephone connection according to the invention. Simultaneously, telephone-wiring port  78  is also connected to load coupler  31 . Coupler  31  extracts the power only from the connection, and feeds it via port  75  to power interface  73   b  of fax  71   b . As such, transformer  72  is rendered unnecessary. Furthermore, the connection of fax  71   b  to outlet  77  is simpler, since there is no need for a nearby power outlet, and both cables connected to fax  71   b  are terminated in the same outlet  77 . Since in most cases the voltage level required for the fax is much lower that the voltage level carried over the telephone lines (e.g. 9-12VDC required for the fax operation, while the voltage level over the telephone lines may exceed 40VDC), DC/DC converter  76  may be required in order to adapt the different voltage levels. 
     While outlet  77  has been described in  FIG. 7  with respect to powering telephone equipment, it will be appreciated that power interface  75  can equally feed any general load. Furthermore, load coupler  31  within outlet  77  may be used to power the active circuitry of TU  55   b , if required. 
     A pictorial view of one example of outlet  77  is shown in  FIG. 8 . The outlet is shaped to be usable as a substitute for a regular telephone outlet in North-America, including two screws  81   a  and  81   b  for tightening. Outlet  77  is provided with a connector  74 , which can be of RJ-11 type, and a circular type power connector  75 , similar to common DC jacks. In addition, an indicator  82  is provided to show the presence of power at the outlet. 
     Home Networking Over Telephone Wiring 
     It is often desirable to use existing telephone wiring simultaneously for both telephony and data networking. In this way, establishing a new local area network in a home or other building is simplified, because there is no need to install additional wiring. 
     The concept of frequency domain/division multiplexing (FDM) is well-known in the art, and provides means for splitting the bandwidth carried by a wire into a low-frequency band capable of carrying an analog telephony signal and a high-frequency band capable of carrying data communication or other signals. Also widely used are xDSL systems, primarily Asymmetric Digital Subscriber Loop (ADSL) systems. 
     Examples of relevant prior-art in this field are the technology commonly known as HomePNA (Home Phoneline Networking Alliance), disclosed in international patent document Wo 99/12330 to Foley and in U.S. Pat. No. 5,896,443 to Dichter. Dichter and others suggest a method and apparatus for applying a frequency domain/division multiplexing (FDM) technique for residential telephone wiring, enabling the simultaneous carrying of telephony and data communication signals. The available bandwidth over the wiring is split into a low-frequency band capable of carrying an analog telephony signal, and a high-frequency band capable of carrying data communication signals. In such a mechanism, telephony is not affected, while a data communication capability is provided over existing telephone wiring within a home. 
     It should be noted that in systems according to the invention, the AC signal carried over the wiring is unaffected by the DC signal, so that phoneline-based data communication, which uses the high frequency spectrum, is not degraded. An example of a prior-art HomePNA system  90  is shown in  FIG. 9 . Low Pass Filters (LPF)  91   a  and  91   b  are connected in series with telephone sets  13   a  and  13   b  respectively, in order to avoid interference and loading in the frequency spectrum used by the data communication signals. Computers  95   c  and  95   d  (or any other data units) can communicate using the HomePNA technology using Phoneline Communication modems (PNC)  93   c  and  93   d , respectively. Modems  93   c  and  93   d  communicate over phoneline wiring  14  via High Pass Filters (HPF)  92   c  and  92   d , respectively, which avoid interference with the telephony signal using the lower spectrum. PNCs  93   c  and  93   d  connect to computers  95   c  and  95   d , respectively, via respective connections  94   c  and  94   d , commonly standard data interface protocols (e.g. USB, Ethernet10/100BaseT). 
     Each PNC  93  comprises active circuits, and as such requires power for its operation. This power may be supplied by a computer  95 , which is usually the case wherein a PNC  93  is integrated into a computer  95 , or supplied via a link  94 , such as in the case of a USB (Universal Serial Bus) connection. However, in many cases a PNC  93  (commonly integrating an HPF  92 ) is a stand-alone unit, being powered from the AC utility mains (not shown in  FIG. 9 ). This involves complex installation requiring many connections to be made. Such complexity can be avoided, according to the present invention, wherein the telephone wiring is used also to carry the power required for the PNC  93  operation. 
     A system  100  wherein a PNC  93  is fed via telephone wire  38  according to the invention is shown in  FIG. 10 . PNC  93   c  is shown to connect to computer  95   c  via link  94   c . PNC  93   c  allows for networking over telephone wiring  38  and is connected thereto via HPF  92   c  and telephone outlet  12   c . A load coupler  31   c  is also connected to outlet  12   c , and its power output is used to power PNC  93   c  via link  101 . In such a configuration, there is no need for any additional power supply or connection to AC utility mains. 
     In order to reduce the complexity of installation and use, and also to save space and wiring, it has been suggested to integrate PNC  93  into a telephone outlet. Such outlet is described in the patent document WO 01/71980 entitled: “Telephone outlet and system for a local area network over telephone lines” in the name of the present Applicant. In such a configuration, it is preferred to also integrate the PNC powering functions into the outlet as well. Such an outlet  102  is shown in  FIG. 10  as part of system  100 . Outlet  102  is based on outlet  77  shown in  FIG. 7 , modified to include the PNC functionality. Telephone support via interface  74  using TU  55   b  is maintained, wherein LPF  104 , which blocks data signals is added in series to TU  55   b  in order to avoid loading and interference with the data networking signals. Powering of external units via interface  75  is also provided. PNC  93   d  (together with HPF  92   d ) is also included in outlet  102 , allowing for data devices such as computer  95   d  to connect thereto via port  103  and link  94   d . Port  103  is preferably a standard data communication interface such as an Ethernet IEEE802.3 10/100BaseT or USB. As shown in  FIG. 10 , PNC  93   d  is powered by load coupler  31 , via connection  105 , thus obviating the need for a dedicated power supply. 
     A pictorial view of outlet  102  is shown in  FIG. 11 . Added to the outlet of  FIG. 8  is interface  103 , shown as an RJ-45 connector, commonly used for IEEE802.3 10/100BaseT connection. 
     While outlet  102  is configured to support three distinct ports: Telephone set interface  74 , power feeding interface  75  and data unit port  103 , it is understood that any subset of one or two ports can also be implemented according to the present invention. 
     In addition to feeding a PNC  93  integrated into the outlet, the power supplied by a load coupler  31  can also be used to power other functions within the outlet, networked to PNC  93   d . For example, patent document WO 01/80543 in the name of the present Applicant discloses a RF transceiver integrated into an outlet and patent document WO 01/80595 also in the name of the present Applicant discloses a telephone switching network using outlets. In all such cases, the circuits integrated into the outlet can be powered as well by load coupler  31 . 
     Implementation 
     System  120  in  FIG. 12  includes specific exemplary components for implementing system  40  shown in  FIG. 4 , as well as the other embodiments disclosed herein. DC Pass/AC Stop unit  35 ′ shown is a sample embodiment of equivalent unit  35  in system  40 . As shown, inductors  121  and  122  are used to stop the AC signal, while passing the DC signal, as known in the art. In order to obtain better results, a gyrator circuit (active inductor) may be used. Off-Hook simulator  43 ′ shown is a sample embodiment of equivalent unit  43  in system  40 . Resistor  123  is shown, operative to allow a DC current flow when switch  124  is closed, thus simulating the off-hook DC current to PBX/CO  11 . Most PBX/CO&#39;s  11  require 20 milliamp           res as an Off-Hook indication signal; thus assuming a DC level of 48VDC, resistor  123  should have a resistance of 48/0.020=2400 ohms. Switch  124  is operated by a threshold detector  125 , which measures the voltage across resistor  128 . In some cases an optical coupler is recommended for use as part of switch  124  and threshold detector  125 , allowing for galvanic isolation. Capacitor  127  in series with inductor  126  serves as a Band Pass Filter (BPF) for passing the Off-Hook tone, while blocking and not loading all other signals. A notch filter is also recommended for such BPF. In one preferred embodiment, a tone of 18 KHz is used as the Off-Hook signaling tone, being separated sufficiently from the telephony spectrum of 300-4000 Hertz and also separated sufficiently from ADSL signals starting at a lower frequency limit of 100 KHz. As such, the BPF allows for the tone to pass and generate a current level in resistor  128  that allows for threshold detector  125  to operate switch  124 .
     Similar to AC Stop/DC Pass unit  35 ′, Power Supply coupler  33 ′ and load coupler  31 ′, shown as a sample embodiment of the respectively equivalent units  33  and  31  in system  40 , perform the function of passing the DC signal and stopping the AC signal. Power Supply coupler  33 ′ is composed of inductors  131  and  132 , and load coupler  31 ′ is composed of a similar arrangement of inductors  133  and  134 . Again, gyrator implementation of the inductors is preferable. 
     Telephone coupler  36 ′ functions to pass the AC signal to telephone set  13   a , and also receives DC from unit  22 ′. Telephone coupler  36 ′ is a sample embodiment of coupler  36  in system  40 . Coupler  36 ′ makes use of a split center tap transformer  139 . Capacitors  140  and  141  are connected to the center taps in both the primary and secondary transformer  139  windings, thus allowing the AC signal to pass through from outlet  12   a  to telephone set  13   a . The DC power over the telephone wiring is extracted over capacitor  141 , and injected to DC/DC converter  138 . The DC/DC function to adapt the voltage level to the level required by telephone set  13   a  (typically −48VDC). The output from DC/DC converter  138  is connected across capacitor  140 , in order to combine it with the AC signal. DC/DC converter  138  may include other functions common to telephony such as current limit and output impedance. 
     Off-Hook detector  41 ′, which is an example of detector  41  in system  40 , is based on an under-voltage threshold detector  135 . Upon telephone set  13   a  shifting into the Off-Hook state, the DC voltage over its terminals is reduced to less than 20VDC. This voltage level is detected by under-voltage detector  135 , which in turn closes switch  136  within Off Hook transmitter  42 ′, which is an example of transmitter  42  in system  40 . A tone (sine-wave) generator  136  provides the off-hook tone, which is imposed over the telephone wiring when the contacts of switch  135  close. 
     AC Pass/DC Stop unit  34 ′, which is an example of AC Pass/DC Stop unit  34  in system  40 , uses two capacitors  129  and  130  in order to block the DC signal and pass the AC signal. 
     All of the patent documents cited herein are incorporated herein by reference. 
     While the invention has been described with respect to imposing power over an active telephone line, it will be appreciated that the invention equally applies to any installation wherein a telephony connection is to be imposed over DC carrying wires. 
     While the invention has been described with respect to analog (POTS) telephony, it will be appreciated that the invention equally applies to ISDN (Integrated Services Digital Network) telephony, and to any case wherein limited DC power is used to power remotely wired connected units. 
     While the invention has been described with respect to in-house installations, it will be appreciated that the invention equally applies to any installation wherein active POTS telephone wiring is used, such as residential, offices, factories, enterprises or MDUs (Multi Dwelling Units), and may be either in-house or external to a house, or both. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications, derivatives, combinations and other applications of the invention may be made.