WO 2004/066518 PCT/GB2004/000162 POWER LINE COMMUNICATION SYSTEM AND METHOD FIELD OF INVENTION The present invention is related to broadband access network. In particular, 5 the present invention is related to deployment methodology, electrical circuitry and devices used for power line communication in a high population density environment and method of doing the same. BACKGROUND OF INVENTION 10 Broadband provides a fast access to digital data for the consumer. The problem in the market has been the relative high cost of providing the service to where the customers live at a price that is acceptable. The signal is typically transmitted via a high-speed broadband communication system such as optical fiber to a local area. Thereafter, various means are available in the "last mile" for the signal to be 15 transmitted to the individual home or entity. These include cables, wireless communication, regular or leased telephone lines and, most recently, electric power lines. Electric power lines are readily available in the household and would be a very useful means for digital signal transmission. For example a communication network 20 may have a signal transceiver device coupled to the electric power line that extends from the electric meter of each household. This signal transceiver device generates a modulated high frequency signal over the normal 50 or 60 Hz electricity power lines. This high frequency signal is distributed by a signal distribution box (SDB) to a plurality of low voltage communication cables. A signal coupler (SC) then couples the 25 high frequency signal to the electric power line via a direct wire connection. Through the electric power line, the power signal is then conveniently available at each socket of each household. This allows users to be easily connected to the communication - 1 - WO 2004/066518 PCT/GB2004/000162 network simply by plugging into an electrical socket like any other electrical appliances. As a part of an international program to develop data-networking protocols and standards, the Open System Interconnection (OSI model) is produced by International 5 Standards Organization (ISO) for overall structure of the complete communication subsystem. The aim of the ISO-OSI reference model is to provide a framework for the coordination of standards development and to allow existing and evolving standards activities to be set within a common framework. The OSI model is comprised of 7 layers. The first layer of the model is the Physical Layer. The second layer of the 10 model is the Link Layer. The third layer is the Network Layer. The fourth layer is the Transport Layer. The fifth layer is the Session Layer. The sixth layer is the Presentation Layer. Finally, the seventh layer is the Application Layer. In such a layered model, the complete communication subsystem is broken down into a number of layers each of which performs a well defined function. 15 Despite the attractiveness of using power line as the means for signal transmission, the high signal attenuation inherent in wire transmission over distances greater than a few kilometers has limited the use of the common power line as the mode of digital signal transmission. In the case of a densely populated area with high rise buildings having multiple units or flats on each floor, the problem is exacerbated 20 due to the complexity of the power grid. Furthermore, in high rise buildings in Hong Kong, each floor typically has many apartment units (e.g. 4-10) and contains its own meter room that serves all the units on that floor (for example, a 40 storey building may have 40 meter rooms spread throughout the respective floors). This creates a problem in that the distance of the cabling between the signal transceiver, the SDB and 25 the SC may be much longer than what is required for prior art systems that have a single meter room for the entire complex. In addition, power line coverage is further reduced by the signal loss at electrical branches on the electrical risers as well as those electrical branches between the Moulded Case Circuit Breakers (MCCB) in the meter room and the Miniature Circuit Breakers (MCB) inside apartment units. To -2- WO 2004/066518 PCT/GB2004/000162 solve this problem, regularly spaced power line communication devices throughout the building complex must be installed to restore and/or retransmit the signal from the main interconnections such as the first signal transceiver device. The need for multiple devices obviously results in high costs, rendering the technology difficult to 5 obtain consumer acceptance. In addition, installation on the power grid requires that electricity be switched off for extensive periods of time, which causes substantial inconvenience to customers, further reducing the popularity of the method. It is therefore an object of the present invention to provide an alternative or improved power line signaling network. -3- WO 2004/066518 PCT/GB2004/000162 SUMMARY OF INVENTION In accordance with the object of the present invention, there is provided a method for transmitting communication signal on a power line network using a high frequency signal transducing device adapted to transmit a high frequency signal to the 5 power line in an inductive manner without direct wire contact therewith (for example by electromagnetic coupling based on Faraday's Law of electromagnetism). The power line communication system according to another aspect of the present invention contains a signal coupling device operating in the physical layer according to the OSI model and containing an input to receive a communication signal 10 from a communication network; and an output to transmit the communications signal to a high frequency signal transducing means. Each of the high frequency signal transducing means is couplable to a power line in an inductive manner that allows the high frequency communication signal (e.g. 1MHz to 30 MHz) to be transmitted (for example, by electromagnetic coupling) without directly connecting to the standard low 15 voltage (e.g. 11 OV or 220V) normal AC electrical power lines. In one implementation, the transducing device is a pair of ferrites, and a high frequency coupler (HFQ) may be used as the signal coupling device to couple signal from the communication cable to the power line. Due to the transducing or inductive method of signal transmission, the HFQ circuitry may be reduced to an input simply 20 connected to an output by a direct wire connection, such as a metallic track facilitating high frequency signal conduction on a printed circuit board. The input may be a RJ45 connector suitable for connecting communication cables such as a Category 5 cable, and the output may include a connector configured to connect to a pair of signaling wires, with each of the signaling wires connected to one of the pair of ferrites. 25 In another aspect of the invention, a distribution means is provided to split the signal generated from the signal transceiver to a plurality of subsignals via output ports. In the preferred embodiment, at least one of the output ports transmits a first -4- WO 2004/066518 PCT/GB2004/000162 signal having a first strength while at least another one of the output ports transmit another output signal having a strength that is higher than the first signal strength. In another aspect of the present invention, a method for transmitting high frequency communication signal onto a power network is provided comprising the 5 step of coupling a communication signal in the physical layer to an electric power line in an inductive manner whereby signal current and voltage are mutually induced therebetween. In this method the coupling step preferably involves receiving a modulated high frequency input signal in the physical layer; and passively distributing the input signal into a plurality of output signals of a plurality of signal strengths. In an 10 alternative embodiment, the input signal is distributed to a plurality of output signals of equal strength. In the preferred embodiment, the method further contains the steps of coupling the output signal to a signal coupling device; and transducing the output signal to a high voltage electric cable. In a further aspect, the present invention provides a method for distributing a 15 communication signal comprising the steps of (a) utilizing a signal coupling device operating in the physical layer for receiving a communication signal from a communication network and transmitting an output thereof; (b) utilizing a high frequency signal transducing device for receiving the communication signal from the signal coupling device, the high frequency signal transducing device operating in the 20 physical layer; and, (c) coupling the high frequency signal transducing device to a power, the high frequency signal transducing device being adapted to transmit a high frequency signal to the power line without direct wire contact therewith. The advantage of the present invention is that the system provided herein can be used to reduce the number of devices needed to provide digital network access to 25 the same number of users within the same physical distance as compared to a pure conductive- installation method over the power grid (for example, direct wire connection between the transceiver device and the bus bar tap point along the electrical riser segment). As mentioned above and further described later, other -5- WO 2004/066518 PCT/GB2004/000162 associated equipment, such as the signal distribution means and the signal coupling means can also be modified to a simpler and cheaper form. Such modifications not only reduce cost, but also improves performance by minimizing signal attenuation and signal interference between signal transceiver devices when a multiple of them are 5 installed in the same building over a common power grid. Such advantages cannot be achieved by the prior art device and systems. On the other hand, the cost of installing the present devices is much lower than using coaxial cables required for broadband access using conventional methods. The hassle to customer is also minimized because all the installation procedures may be performed without the need to switch off the 10 power supply. Thus, the method and system of the instant invention is able to benefit from the use of power line communication while solving the problem associated therewith. -6- WO 2004/066518 PCT/GB2004/000162 BRIEF DESCRIPTION OF FIGURES Figure 1A is a schematic drawing of a prior art power line communication system. Figure 1B is a schematic drawing showing further details of the prior art system of Figure 1A. 5 Figure IC shows the circuit diagram of a signal distribution box (SDB) according to the prior art system. Figure ID shows the circuit diagram of a prior art signal coupler. Figure 2A is a diagram of an embodiment of a PLC system according to the present invention. 10 Figure 2B is a schematic diagram to show the overview of a PLC system according to the present invention. Figure 2C is a diagram of a variation of embodiment of a PLC system according to the present invention. Figure 3 shows the details of a high frequency coupler (HFQ). 15 Figure 4A shows a circuit diagram of a 6-port high frequency processor (HFP). Figure 4B shows a circuit diagram of a 8-port high frequency processor (HFP). Figure 5A is a top plenary view of ferrite 132a as shown in Fig. 2B. Figure 5B is a side view of the same ferrite as Fig. 5A. Figure 5C is the side view of the same ferrite that has the two halves opened to 20 show the inner structures. Figure 6A shows a frequency response curve of one ferrite sample. Figure 6B shows a magnetization chart of various ferrite samples, the names of the samples being labelled in the chart accordingly. Figure 7A shown a schematic representation of the laboratory testing 25 configuration of the ferrite. Figure 7B shown a schematic representation of the site testing equipment and configuration of the ferrite. Like reference symbols in the various drawings indicate like elements. -7- WO 2004/066518 PCT/GB2004/000162 DETAILED DESCRIPTION Figures 1A and 1B show a prior art system 10 for providing broadband signals to a multi-story building 20. In this example, a broadband signal is transmitted to the vicinity of a transformer facility room providing power to the target building 20 via 5 an infrastructure or backbone network 12. A broadband signal from the infrastructure or backbone network is sent to a first signal transceiver via Ethernet cable 16 (e.g., a Category 5 cable) and the broadband signal is further distributed to a second signal transceiver inside the building 20 via the power grid 21. A first signal transceiver 22 converts the ethernet signal to power line signal that is transmitted through a power 10 line to another transceiver 23 in the building 20. Inside the building, a second signal transceiver 23 converts the power line signal of a first signal transceiver device to a PLC signal of a different frequency band. The Signal Distribution Box (SDB) 26 then distributes the incoming signal into multiple signals for connection to a plurality of Signal Couplers (SC) 32a - 32e. The signal coupler is connected by using either a 15 short-circuit proof wire or wire with in-line fuse to the conducting surface (i.e. conductive coupling) of the electric feeders that bring electricity from the electric meter 27 to the apartment unit. The dotted line shown in Figure 1A represents the meter room 29 and in a typical apartment building in Europe, most of the features described above, including the electric meters, are found in a single meter room on the 20 ground floor. From the meter room 29, the electric cable then runs up the high-rise building and to each apartment unit as shown by the arrows. Still referring to Figures 1A and 1B, Cat 5 cable 24 with RJ-45 connectors on both ends transmits communication signals from the signal transceiver device 23 to the signal distribution box (SDB) 26. SDB 26 distributes the incoming signal from 25 cable 24 to a plurality of RJ-45 output ports 34. Each output port 34 may be connected to a Cat 5 cable 30 with RJ-45 connectors on both ends that connect to one of the signal couplers (SC) 32a-32c. Each signal coupler 32a-32c is coupled to a set of incoming power terminals 38a-38c, respectively. Each set of power terminals is connected to the power line network of an apartment (or business unit) which may be -8- WO 2004/066518 PCT/GB2004/000162 connected to a power line modem 33 or a user. In this example of system 10, signal coupler 32a is connected by a pair of wires 39a and 39b to the incoming power terminals 38a. Pair of wires 39a and 39b are typically connected to power terminals 38a by conventional means, for example, by direct wire to terminal contact, a screw to 5 wire contact, etc. Figure 1C shows a circuit diagram of a signal distribution box (SDB) 26 as may be implemented in the prior art system shown in Figures 1A and 1B. Both the physical structure of the 6-port SDB and the conductive connection method of the SC at the electricity feeders allow only 6 apartments to have power line broadband access 10 with one signal transceiver device in the building. As there is usually 8 to 10 apartment units on a floor in a typical building in Hong Kong, the connection method from prior art requires at least one signal transceiver to be installed on every floor. As there is usually 20 to 25 floors in a typical building in Hong Kong, the cost of signal transceiver devices and the necessary interruption of electricity service to individual 15 apartment unit for wiring up SCs to the conducting surface at the MCCB makes the scheme prohibitive in Hong Kong. In addition, SDB 26 includes attenuation components 50 that are used to equalize signals on output ports 52 and 54. In more detail, attenuation components 50 reduce the strength of output signals on 2 output ports 52 such that output signal strength from these ports are approximately the same 20 as output ports 54. Therefore, SDB 26 must be located approximately equi-distant from the power line distribution point of each apartment unit within the building. In reality, apartment units are usually distributed evenly on every floor in a building, the farther away the apartment unit from its electric meter and electrical connections, the farther it is located from the SDB and SC and as a result a significant drop in signal 25 strength may be experienced due to attenuation in the power cables and electrical junction points such as the miniature circuit breaker and socket points inside the relevant apartment. Figure 1D shows a circuit diagram of a signal coupler 32a as may be implemented in the prior art system shown in Figures IA and 1B. Signal coupler 32a -9- WO 2004/066518 PCT/GB2004/000162 includes several component sub-systems 60-63 that are used to compensate for alternating current (AC) components of the incoming signal derived from the connection to a power line network. For example, sub-systems 60-63 are used to block the 50 or 60Hz low voltage AC component, couple the power line signal 5 between the RJ45 interface and the connector, provide a signal reference and provide over-voltage protection, respectively. Figure 2A shows an embodiment of the overall power line installation architecture inside a typical high rise multi-tenant building with a plurality of electrical risers according to the present invention. In this example, the power meters 10 135 for each apartment are located on the corresponding floors, and the main power line brings electric power to each meter (not shown). The horizontal dotted lines show the separation of each floors. However, for the ease of illustration, only a few horizontal dotted lines are shown (between floors 31 and 32; 24 and 25; 16 and 17; and 8 and 9). It is understood that the HFP and electric meters shown along the same 15 row are on the same floor. Furthermore, the identification of the lower floors are not shown in order not to obscure the details of the present invention. The installation of this system does not require modification of the existing power line network. In this embodiment, communication signals are switched to a signal transceiver 106 and high frequency processor (HFP) 110 at the Ethernet Switch 102 that interconnects several 20 signal transceiver devices with Ethernet cables. HFP 110 distributes the modulated carrier wave (such as Ethernet) signal received on a first communication cable 112 to a series of high frequency couplers (HFQ), such as HFQ 120 via a series of output cables 112b. Each HFQ 120 (i.e. signal coupling device) is connected to a respective second communications cable 130. Communications cable 130 are looped around the 25 ferrite pairs 132a, 132b and 132c (only shown in Figure 2B) at an entry point of the power network into the respective apartment or building unit. Fastening the ferrites to the power lines with signal wires provide an electromagnetic coupling of communications signals in and out of power lines. - 10 - WO 2004/066518 PCT/GB2004/000162 Figure 2B shows in greater details an embodiment of a power line communications (PLC) system that may be used to transmit communication signals over an existing power line distribution network to a home or building such as the one shown in Figure 2A. The installation of system 100 does not require modification of 5 the existing power line network. In this embodiment of PLC system, a single signal coupler 120 (i.e. HFQ) is connected to a first pair of double insulated wires 130a and 130b that is looped through three pairs of ferrites 132a to 132c. The double insulated wires 130a and 130b between the 3 pairs of ferrites 132a to 132c are twisted doublets and are shown in a single line for ease of illustration. In operation, ferrites 132a to 10 132c are fastened to power line wires 134a to 134c respectively that feeds electricity into an apartment. Fastening the power lines and double insulated cables carrying the power line signal with ferrites provide an electromagnetic coupling or induction of communications signals in and out of the power lines, and therefore in and out of the power network. In this diagram, one output signal from HFQ 120 is transferred 15 through a pair of double insulated wire 130 and the ferrite pairs according to the direction of the arrows to allow multiple distribution of communication of signals to multiple apartment units of a power network. In an embodiment, a capacitor 136 is connected between each ferrite pair to reduce possible interference of signals, for example capacitor 136 is connected to the cables 134a connecting between ferrite pair 20 132a. As further shown in the example in power line 134a, communication signals are transmitted over the power line into a household with a power line modem 140 provided to demodulate the power line signal and convert to packetized TCP/IP data to send to a computer 142 or the power line modem 140 modulates the packetized TCP/IP data from the computer to power line signal through the standard network 25 interface like IEEE 802.3 LAN or USB interface on the computer and power line modem. Internet access and data transmission can be made by simply connecting the computer to the power line modem and plugging the power line modem into a power socket 144 in the apartment. - 11 - WO 2004/066518 PCT/GB2004/000162 Figure 2C is an equivalent embodiment of the present system but using the 6 port HFP device 110a. If all else being equal in the building, more bandwidth for broadband access will be available for each apartment unit if the 6-port HFP is used instead of the 8-port HFP as the 6-port connects to a smaller number of floors. As 5 compared with prior art 6-port SDB, the 6-port HFP provides two output ports with higher strength for signal coverage to remote floors. Figure 3 shows a circuit diagram of high frequency coupler (HFQ) 120 that may be used in an embodiment of the PLC system shown in Figure 2B. In an embodiment, HFQ 120 includes circuit lines 240a and 240b that are implemented as 10 metallic tracks on a printed circuit board with the RJ45 jack input connector 242 at one end and the output connector 244 on the other end. HFQ 120 is a relatively simple implementation compared to prior art signal coupler 32a (see Figure ID). It does not require the relatively complicated and expensive sub-systems 61-63 as described in Figure ID that are included in signal coupler 32a since it is not directly 15 connected to an alternating current (AC) line. Instead HFQ 120 is electromagnetically coupled to a power network using ferrites as shown in Figure 2B. The HFQ according to the present invention with reduced number of components and substantial cost savings exhibits, depending on the frequency, signal loss comparable to or slightly better than a prior art device (signal coupler) as shown 20 in the data in Table 1. The measurement presented in the table below was obtained by using at the transmitting end: a Rohde & Schwarz SML 01 Signal Generator with a 500hm matched connector transmitting a sinusoidal signal of OdBm from 1 to 30Mhz with a step size of 1MI-Iz, at the receiving end: a Rohde & Schwarz FSP-B3 Spectrum Analyzer with a 50Ohm matched connector running from 1 to 30Mhz with 25 a Frequency Span of 1MHz, Center Frequency of 1MHz, VBW & RBW of 30KHz for the reception of the sinusoidal signal coming through the HFQ and SC . - 12 - WO 2004/066518 PCT/GB2004/000162 Table 1 : Comparison of HFQ of the present invention and prior art Signal Coupler (SC) Frequency HFQ Loss Signal Coupler Improvement (MHz) (db) Loss (db) (db) 1 1.055 2.695 1.64 2 0.405 1.12 0.715 3 0.25 0.59 0.34 4 0.21 0.38 0.17 5 0.175 0.33 0.155 6 0.175 0.325 0.15 7 0.175 0.375 0.2 8 0.2 0.4 0.2 9 0.225 0.415 0.19 10 0.255 0.455 0.2 11 0.245 0.52 0.275 12 0.26 0.535 0.275 13 0.185 0.525 0.34 14 0.235 0.645 0.41 15 0.205 0.66 0.455 16 0.2 0.705 0.505 17 0.235 0.705 0.47 18 0.42 0.585 0.165 19 0.815 0.625 -0.19 20 0.705 0.77 0.065 21 0.56 0.8 0.24 22 0.55 0.76 0.21 23 0.615 0.805 0.19 24 0.545 0.95 0.405 25 0.465 1.065 0.6 26 0.405 1.1 0.695 27 0.35 0.77 0.42 28 0.33 1.15 0.82 29 0.315 1.17 0.855 30 0.28 1.175 0.895 Figure 4A shows the circuit diagram of a 6-port high frequency processor (HFP) 110 signal distributor device that may be used to distribute an incoming signal to a 5 plurality of output ports. It contains a first stage signal splitting transformer 171a coupled to the input port 148 for splitting the signal received by the input port 148 to two first- stage subsignals 173a. Two second-stage signal splitting transformers 171b - 13 - WO 2004/066518 PCT/GB2004/000162 are coupled to the first-stage signal splitting transformers for splitting each first stage subsignals into two second-stage subsignals via a total of four second-stage subsignal tracks 173b. For ease of description, two of the four second-stage subsignals tracks are identified as 173b1 and the remaining two of the second-stage subsignal tracks are 5 identified as 173b2. Two third-stage signal splitting transformers 171c are further coupled to two of the second-stage subsignal tracks 173b1 for splitting two of the second stage subsignals into four third-stage low strength subsignals via four third stage low strength subsignal tracks 173c. The four third-stage low strength subsignal tracks are further connected to four of the low strength output ports via four coupling 10 transformer each having a 1:1 transformation ratio. Finally, the remaining two second stage subsignal tracks 173b2 are coupled to two high strength output port via two coupling transformers each having a 1:1 transformation ratio. The 14.7Ohm resistor is a thick film chip resistor with a tolerance of 1% and size of 0603 where rated power is 1/16Watts. 15 This embodiment of HFP is different than the prior art device SDB 26 (see Figure IC) since it does not have components for equalizing the output signal such as the attenuation component 50 as shown in Figure 1C. Such additional components cause undesirable attenuation of the input port signals transmitted from signal transceiver 23. Instead, signals received through input port 148 and transmitted from 20 output ports 150 (the 9 dB loss ports) may be of a relatively higher strength than the output signals from ports 160. This allows the HFP to be placed in the middle floor within a group of floors that are being served by the same HFP. Signal transmission to apartments on floors farthest away from the HFP may use the two higher strength output ports 150 (the 9 dB loss ports) to obtain the same signal as the closer floors. 25 Figure 4B shows the circuit design of an 8-port HFP which has the same output loss as the 6-port HFP design for deployment in high rise multi-tenant building. In this embodiment, there is one input port 170 and eight output signal ports. A first stage signal splitting transformer 181a is coupled to the input port for splitting the signal received by the input port 170 to two first-stage subsignal tracks 183a. Two second - 14 - WO 2004/066518 PCT/GB2004/000162 stage signal splitting transformers 181b are coupled to the two first-stage subsignal tracks 183a for splitting each first-stage subsignals into two second-stage subsignals via a total of four second-stage subsignal tracks 183b. The four third-stage signal splitting transformers 181c are coupled to the four second-stage subsignal tracks 183b 5 for splitting the four second-stage subsignals into eight third-stage subsignals via eight third-stage subsignal tracks 183c. The 8 third-stage subsignal tracks 183c are further connected to eight output ports via eight coupling transformers each having a 1:1 transformation ratio (also referred to as 1:1 coupling transformers) such that two way signal transmission may be effected. 10 In this embodiment, the input signal received from input port 170 is split and transmitted through 8 output ports 172. The 8-port HFP provides power line signal coverage to more floors than the 6-port without introducing more loss, effectively more floors and apartment units will be covered for building with the same number of floors and same number of apartment units per floor as compared with the 6-port. 15 Figure 5A to 5C show how a ferrite may be used as the signal transducing device according to the present invention. In this example, the ferrite includes a commercially available standard ferromagnetic block encased by a cubic or rectangular plastic housing. The ferromagnetic block, which usually consists of an alloy of iron oxide and other element such as manganese, zinc and nickel assumes the 20 shape of a rectangular block with a cylindrical hollow space inside hereinafter referred to as the core. Figure 5A is the top plenary view of the ferrite block with a hollow core the edge 502a of the ferromagnetic block defining the cylindrical core can be seen through the plastic housing 504 but for clarity of explanation, the external edges of the ferromagnetic block are also drawn in Fig.5A with dotted lines. Also for clarity 25 of viewing and explanation, the ferromagnetic block in Figure 5C is shaded with straight line shading at the flat side 502a facing the open while the surface defining the cylindrical core 502b is shaded with curved lines. Referring again to Figure 5A, lugs 510 are provided at the two ends of the casing to prevent the ferromagnetic block from slipping out of the casing after it has been installed. A further plastic lug 512 (see Fig. - 15 - WO 2004/066518 PCT/GB2004/000162 5C) extending from the casing also acts to secure the ferromagnetic block inside the casing. The casing 504 contains two halves attached on one side by a connecting strip 514. As shown in Fig. 5B, the first half 504a is provided with a catch 506 that extends 5 towards the second half 504b when the two halves are connected in the operating position. The second half of the casing 504b is provided with a pair of lugs 508 that is adapted to mate with the opening in catch 506 and locks theretogether. In this illustration, the position of the signaling cable 130 and the electric cable 134a are also shown with dotted lines to illustrate how they are threaded through the 10 hollow core of the ferrite after the two halves are fastened thereover. In the most preferred embodiment, a mylar sheet 516 (shown as the solid black area in Fig.5C) or a sheet of any electric insulative material is provided along the shaded cross sectional faces 502a to create an air space between the halves. The dimension, size and shape of the ferrite may be determined by the user 15 according to the size of the cable used and the characteristic frequency response of the ferrite in the frequency band of the power line signal to be applied. In one embodiment, a ferrite with minimum impedance of 50 ohm at 25MHz and minimum impedance of 180 ohm at 100MFz is used. The mylar film is used to create an air gap of any thickness, for example 0.18 mm to increase the level of saturation current of the 20 ferrite while maintaining an optimal level of electromagnetic coupling of signal between signal cables and electric wires. The saturation current of the ferrite or the maximum rated current to go through the electric wires can be found by using Ampere's law to relate the magnetizing force with the induced flux density in the ferrite core and air gap 25 where N x Isat = H-core x Lengthcore + H-gap x Length gap and B core = Bgap = flux density in Teslas or Weber per square meters at saturation which is derived from the property chart of the ferromagnetic material used -16- WO 2004/066518 PCT/GB2004/000162 And Hgap = B_gap / uo N = number of turns of wire or cable, I-sat = maximum current on the electric wires in Ampere, H_core = magnetizing force of ferrite at saturation flux density in Ampere 5 turns per meter Lengthcore = mean path length of ferrite core in meter H_gap = magnetizing force of air gap at saturation flux density in Ampere turns per meter uo = permeability of air 10 Simply as one illustration, the dimensions of the ferrite may be: A defines the width of the ferrite including the outer plastic casing and the catch 506 and may be 31.5mm +/- 2.0mm B defines the breadth of the ferrite including the outer plastic casing and may be 30.5mm+/- 1.0mm 15 C defines the inner diameter of the hollow ferrite core and may be 13.Omm+/- 1.5mm D defines the height of the ferrite including the outer plastic casing 32.5mm+/- 1.0mm The material of the ferrite may be selected from commercially available ferromagnetic material by measuring the frequency response at the operating 20 frequency range of power line transceiver as well as the inductive coupling effect thereof at the operating current of low voltage domestic electric cables. The impedance characteristics of the ferrite should allow optimal coupling of the power line signal to and from the electrical line (e.g. 50Hz or 60Hz for normal household) without saturating the ferrite material. As a non-limiting example, this can be 25 determined by testing to see if a ferrite has a frequency response similar to Fig 6A with increasing reactance in between 1MHz to 30MHz with relative permeability of ferrite not smaller than 400Wb/Am (Webers per Ampere-meter) The ferrites can be, for example, a ring or cube with a core hole about 13mm in diameter for accommodating electric cable size between 16mm-square to 35mm-square. For 30 electric cable size larger than 35mm-square, ferrites that exhibit similar material characteristics with slightly bigger core will be used (a just fit instead of tight or loose fit). The saturation current of the ferrite will be derived from Fig 6B. The -17- WO 2004/066518 PCT/GB2004/000162 Magnetization chart in Fig. 6B is based on the formulae shown in the previous paragraph for (a) no air gap and (b) optimization of the air gap (for example, by using a mylar film with the minimum thickness including the glue thickness] to ensure the ferrite will not saturate by the loading current that runs in electric cables 5 Each ferrite sample 701 is screened by lab measurement using an R&S SMLO1 signal generator 702 to generate a 1-40MHz signal, and an R&S general purpose Spectrum Analyzer 703 to record the response as shown in Fig.7A. Those that show a frequency response with the highest dBm level are selected for further testing in the actual site (such as a household in its natural environment). The performance of the 10 ferrites are ranked according to their dBm levels according to Fig. 7A and Fig. 7B. In particular to the data shown in Table 2, the ferrites were assessed from frequency of 1MHz to 40Mz. The frequency response for each 10MHz segments is compiled as an average and ranked accordingly. Using this method, ferrites that respond to various frequency ranges may be readily selected. 15 Once these ferrites have been selected according to the previously described test, the ferrites are further verified in a household environment according to Fig. 7B by connecting the R&S Signal Generator 801 directly to the ferrites 802 that are connected to the housing hold power supply 803. Once the ferrites 802 are connected to the household electrical cable, their frequency response are further measured using 20 the same Spectrum Analyzer 703 with a high pass filter connected between the electrical output and the input of the Spectrum Analyzer 703. The frequency response of the ferrites as measured therefrom is once again assessed according to the frequency of 1-40MHz. Again, the average was taken for each 1OMFIz segments and the ferrites are ranked accordingly. The ferrites that have the best response (i.e. ranking of 1) are 25 the ones with the highest dBM level. These ferrites are then selected for use in the household as the signal coupler. Examples of site test results of some ferrite samples are shown in Table 3. - 18 - WO 2004/066518 PCT/GB2004/000162 TABLE 2 Lab Test Results of Ferrite Samples Pre-selected Samples Measured Direct Cube - (1) Cube - (2) Ring - (5) Ring - (2) Ring - (3) Frequency connect (dBm) (dBm) (dBm) (dBm) (dBm) (dBm) (MHz) (dBm) I 1 MHz -0.26 -17.68 -15.71 -19.74 -21.29 -24.54 -20.27 2 MHz -0.17 -11.90 -10.09 -13.98 -14.76 -19.96 -14.34 3 MHz -0.23 -8.83 -7.33 -11.09 -11.93 -16.02 -11.42 4 MHz -0.23 -6.96 -5.58 -8.67 -9.47 -13.41 -9.49 5 MHz -0.24 -5.54 -4.45 -7.14 -8.06 -12.05 -8.25 6 MHz -0.28 -4.63 -3.67 -6.15 -7.22 -10.54 -7.3 7 MHz -0.32 -3.98 -3.11 -5.30 -6.45 -9.42 -6.56 8 MHz -0.33 -3.55 -2.69 -4.88 -5.73 -8.58 -5.98 9 MHz -0.23 -3.15 -2.38 -4.47 -5.22 -7.97 -5.51 10 MHz -0.26 -2.88 -2.17 -4.21 -4.82 -7.41 -5.11 Average -0.26 -6.91 -5.72 -8.56 -9.50 -12.99 -9.42 Ranking 2 1 3 5 7 11 MHz -0.23 -2.59 -2.02 -3.89 -4.47 -6.82 -4.82 12 MHz -0.29 -2.43 -1.9 -3.69 -4.31 -6.59 -4.55 13 MHz -0.26 -2.34 -1.83 -3.48 -4.10 -6.24 -4.37 14 MHz -0.23 -2.27 -1.77 -3.45 -3.85 -6.01 -4.19 15 MHz -0.25 -2.13 -1.72 -3.39 -3.76 -5.85 -4.06 16 MHz -0.33 -2.14 -1.68 -3.27 -3.62 -5.49 -3.91 17 MHz -0.32 -2.11 -1.69 -3.24 -3.55 -5.30 -3.84 18 MHz -0.32 -2.00 -1.66 -3.16 -3.37 -5.30 -3.75 19 MHz -0.31 -1.93 -1.64 -3.05 -3.27 -5.09 -3.64 20 MHz -0.26 -1.96 -1.63 -3.03 -3.21 -5.05 -3.58 Average -0.28 -2.19 -1.75 -3.37 -3.75 -5.77 -4.07 Ranking 2 1 3 4 7 5 21 MHz -0.30 -1.84 -1.6 -2.92 -3.22 -4.99 -3.51 22 MHz -0.27 -1.95 -1.59 -2.79 -3.08 -4.82 -3.44 23 MHz -0.31 -1.83 -1.6 -3.01 -3.05 -4.80 -3.4 24 MHz -0.33 -1.91 -1.6 -2.98 -3.11 -4.70 -3.37 25 MHz -0.34 -1.81 -1.6 -2.97 -3.12 -4.61 -3.34 26 MHz -0.34 -1.93 -1.61 -2.84 -3.03 -4.61 -3.3 27 MHz -0.36 -1.89 -1.65 -2.90 -3.09 -4.53 -3.31 28 MHz -0.36 -1.94 -1.66 -2.91 -3.01 -4.52 -3.29 29 MHz -0.39 -1.92 -1.68 -2.83 -3.03 -4.59 -3.28 30 MHz -0.41 -1.97 -1.69 -2.89 -3.14 -4.48 -3.27 Average -0.34 -1.90 -1.63 -2.90 -3.09 4.57 -3.35 Ranking 2 1 3 4 7 5 31 MHz -0.51 -2.01 -1.75 -3.03 -3.18 -4.73 -3.29 32 MHz -0.46 -2.04 -1.71 -2.88 -3.18 -4.48 -3.26 33 MHz -0.38 -2.01 -1.7 -2.87 -3.04 -4.55 -3.2 34 MHz -0.48 -1.92 -1.75 -3.00 -3.06 -4.47 -3.24 35 MHz -0.49 -1.92 -1.78 -2.93 -3.18 -1.53 -3.23 36 MHz -0.54 -1.90 -1.81 -3.03 -3.19 -4.46 -3.24 37 MHz -0.46 -2.09 -1.87 -3.02 -3.30 -4.48 -3.27 38 MHz -0.64 -2.25 -2.1 -3.22 -3.50 -4.71 -3.49 39 MHz -0.50 -3.08 -3.33 -4.56 -4.53 -5. -4.6 40 MHz -0.51 -2.08 -2.03 -3.05 -3.40 -4.54 -3.37 Average -0.50 -2.13 -1.98 -3.16 -3.36 -4,60 -3.42 Ranking 2 1 3 4 7 5 -- 194- WO 2004/066518 PCT/GB2004/000162 TABLE 3 Site Test Results of Ferrite Samples Preferred Samples Non-preferred Samples Frequency Cube - (1) Ring - (2) Ring - (5) Cube - (2) Ring -(1) Ring - (3) Ring - (4) (MHz) (dBm) (dBm) (dBm) (dBm) (dBm) (dBm) (dBm) 1MHz -44.2 -46.67 -36.3 -50.91 -47.47 -46.69 -52.9 2MHz -30.78 -32.46 -23.98 -37.84 -33.86 -33.04 -36.93 3MHz -38.02 -39.85 -31.25 -45.16 -40.82 -40.91 -43.51 4MHz -34.93 -36.66 -29.71 -37.65 -37.83 -33.13 -36.26 5MHz -32.62 -34.17 -27.98 -38.49 -35.85 -34.58 -37.37 6MHz -33.48 -34.87 -28.39 -39.75 -36.76 -36.36 -39.61 7MHz -20.8 -22.9 -18.01 -30.58 -26.95 -25.31 -30.01 8MHz -27.25 -28.8 -23.66 -39.76 -36.44 -30.67 -39.26 9MHz -31.27 -32.8 -28.59 -40.55 -36.31 -35.46 -39.79 Avar ge -3 4 -4 -8' .0 - -35 :. 40" Raningq 2 '~ 6' 10MHz -38.56 -39.84 -36 -45.9 -42.3 -41.84 -44.87 11MHz -34.27 -35.72 -31.81 -39.48 -36.35 -37.19 -39.01 12MHz -32.48 -33.9 -30.04 -37.81 -35.41 -36.11 -37.15 13MHz -37.57 -39.06 -34.98 -42.45 -38.46 -40.91 -41.55 14MHz -35.92 -37.03 -33.29 -37.26 -34.4 -39.31 -36.27 15MHz -33.51 -35.21 -31.07 -38.39 -35.41 -36.59 -37.39 16MHz -30.75 -32.19 -28.64 -35.05 -31.81 -34.07 -34.02 17MHz -28.94 -30.38 -26.83 -32.77 -29.86 -32.06 -32.3 18MHz -32.04 -33.59 -29.84 -36.42 -33.9 -35.29 -35.7 19MHz -37 -38.41 -35.1 -41.88 -38.93 -40.27 -41.26 20MHz -40.21 -40.76 -38.52 -44.82 -41.81 -43.42 -43.88 Average -35 -36 -32 -39 -36 -38 -38 Ranking 2 3 1 5 3 4 4 21MHz -39.02 -40.66 -38 -44.96 -42.09 -42.11 -44.28 22MHz -33.05 -34.52 -32.12 -39.33 -36.58 -35.91 -38.65 23MHz -35.71 -36.62 -34.71 -40.88 -38.09 -38.45 -40.36 24MHz -39.15 -40.12 -37.91 -43.65 -40.85 -42.18 -42.87 25MHz -38.13 -39.01 -36.92 -42.23 -39.74 -40.71 -41.9 26MHz -38.7 -39.98 -36.98 -43.65 -40.56 -41.36 -42.41 27MHz -42.56 -44.22 -41.7 -47.71 -46.07 -45.67 -47.64 28MHz -45.07 -45.62 -42.85 -49.3 -47.25 -47.2 -48.87 29MHz -41.55 -42.66 -39.8 -46.24 -43.89 -44.26 -45.42 30MHz -41.85 -43.01 -40.11 -46 -43.63 -44.57 -45.35 Pnnlin, 62 0 8 _____ £ 31MHz -39.73 -40.84 -38.11 -42.97 -40.64 -42.61 -42.37 32MHz -36.97 -38.03 -35.34 -40.06 -37.72 -39.73 -39.44 33MHz -35.66 -36.73 -34.36 -38 -35.53 -38.39 -37.31 34MHz -33.65 -34.51 -32.36 -36.14 -33.76 -36.2 -35.55 35MHz -32.91 -33.8 -31.77 -36.45 -34.36 -35.1 -35.86 36MHz -35.55 -36.7 -34.71 -39.25 -36.96 -37.89 -38.63 37MHz -39.22 -39.91 -38.39 -42.39 -40.46 -41.44 -41.86 38MHz -39.52 -40.43 -38.82 -42.53 -40.47 -41.96 -41.96 39MHz -40.15 -40.78 -39.13 -43.98 -41.62 -42.58 -43.43 40Mhz -40.9 -41.7 -39.37 -45.59 -43.77 -43.2 -45.41 Average -37 -38 -36 -41 -39 40 Ranking 2 3 -1 - -- > Z 5 5 -20- WO 2004/066518 PCT/GB2004/000162 In some of the embodiments described, the ferrites may be installed while a power line is energized, and may be installed on a wide variety of power line 5 configurations. Furthermore, the transmission of signals have been described in one direction from the backbone network to the household power line merely for ease of description. It is clear that for any communication, bi-directional traffic is possible, and is intended to be available in the aforementioned systems. A number of implementations have been described. Nevertheless, it will be 10 understood that various modifications may be made without departing from the spirit and scope of the claims. For example, a variety of PLC configurations may be used. Although electromagnetic coupling is used as the specific example, other non conducting method may also be used. Further, the structure of the ferrite connectors, communications cables and other components may be changed. The signal distribution 15 device describes an input port and a plurality of output ports for ease of description. It is clear that in a bi-direction signal coupling system, the aforementioned input and output ports also act as output and input ports respectively, when a household user send the communication signals back to server. Furthermore, the signal distribution device is described as having tracks for ease of description. It is clear that the wiring 20 may be in the form of metal tracings or tracks on a PCB board, or any other connection that allows signal coupling such as wiring. Accordingly, other implementations are within the scope of the following claims. -21-