the principles and operation of a network according to the present invention may be understood with reference to the drawings and the accompanying description . the drawings and descriptions are conceptual only . 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 descriptions , identical reference numerals indicate those components that are common to different embodiments or configurations . fig3 illustrates a signal canceling unit ( scu ) 30 , which includes two external terminal connections , a terminal 34 a ( a ) and a terminal 34 b ( b ). coupled to these terminals is a sensor 31 , which measures the differential voltage ( constituting a “ first signal ”) between terminal 34 a and terminal 34 b . the value measured by sensor 31 is input into a processing unit 33 , which in turn provides input to a differential driver 32 ( constituting a “ first driver ”), whose outputs are coupled to the terminal 34 a and terminal 34 b . driver 32 can sink or source enough current ( constituting a “ second signal ”) to cancel the first signal at the terminals . processing unit 33 along with sensor 31 and driver 32 forms a closed negative feedback loop , which attenuates and cancels any signal sensed over terminal 34 a and terminal 34 b . fig4 illustrates a network 40 , with scu 30 used as a terminator . network 40 is based on network 10 ( fig1 ), but modified to use scu 30 as a terminator in place of terminator 13 b . signals transmitted to transmission line 15 a and 15 b ( by line driver 11 a , for example ) propagate along the transmission line . upon reaching the end of the transmission line , where terminals 51 a and 51 b of scu 30 are connected , scu 30 senses and acts to cancel the signals . as a result , the signal energy is absorbed by scu 30 , and neither reflection nor any other mismatch occurs . hence , scu 30 acts as a termination device . however , since the structure of scu 30 is generic and is not tailored to the specific transmission line ( e . g ., characteristic impedance ), this same scu can be used for many types of transmission line , such as twisted pair wiring , coaxial cables , etc ., obviating the need to match a specific termination to a specific transmission line . this , of course , provides simple installation and easy logistics , due to the employment of common components for various different applications . a further advantage of using an scu as a terminator stems from the fact that the scu performs the termination function even when not connected at the ends of the transmission line , but at any point throughout the transmission line run , as illustrated in fig5 for a network 50 , which is based on transmission line 15 a , 15 b , 15 c , and 15 d . as with network 10 ( fig1 ), terminator 13 is located at one end ( left side of the figure ), and line driver 11 a and line receivers 12 a and 12 b are coupled to the transmission line . data units 16 a , 16 b , and 16 d are coupled to line units 11 a , 12 a , and 12 b , respectively . if scu 30 were not present in network 50 , network 10 of fig1 would be obtained , wherein data unit 16 a can transmit data to the entire transmission line via line driver 11 a . the transmitting signals would then propagate in the transmission line and would be received by data units 16 b and 16 d via line receivers 12 a and 12 b , respectively . in this case , however , where scu 30 is connected to the transmission line at connection points 51 a and 51 b , the network 50 is modified such that signals transmitted to line driver 11 a , are propagated in the transmission line in two directions . part of the signal energy is propagated towards terminator 13 ( towards the left side of the figure ), where they are absorbed . the other part of the signal energy propagates towards points 51 c and 51 d , representing the other end of the wiring . when the signal reaches points 51 a and 51 b ( connected to the terminals of scu 30 ), scu 30 operates to attenuate , cancel , and absorb the signal energy . thus , little or no signal will propagate from the points 51 a and 51 b towards the end points 51 c and 51 d . in such case , while line receiver 12 a will receive the transmitted signals , line receiver 12 b will not sense any such signals , which are attenuated by scu 30 . thus , scu 30 functions as a terminator for the network segment 15 a and 15 b , extending from terminator 13 to points 51 a and 51 b , helping to avoid reflections in this part of the transmission line . as a result , scu 30 modifies the functionality of the continuous transmission line to be virtually separated into two distinct segments , one using the transmission line from terminator 13 to points 51 a and 51 b , while the other uses the transmission line from points 51 a and 51 b to the end - points 51 c and 51 d . the two network segments are isolated in the sense that signals in one segment cannot pass to the other , even though electrical continuity of the transmission line is fully retained . one application of such virtual networks separation is for noise isolation , as illustrated in fig6 with a network 60 . network 60 is similar to network 50 ( fig5 ), except that a noise source 61 appears in place of data unit 16 d and line receiver 12 b . the noise generated by noise source 61 propagates ( in the left direction ) towards scu 30 . upon reaching scu terminals 51 a and 51 b , scu 30 operates to attenuate the noise signal , and prevents the noise from reaching transmission line 15 a and 15 b and thereby degrading communication over that network segment . while noise source 61 is described and illustrated as a distinct unit connected at a single point to transmission line 15 c and 15 d , the same noise cancellation function is performed where noise is generated by inductive means from external sources . for example , transmission line 15 c and 15 d may extend over an area near sources of electromagnetic interference . the scu can thus help in isolating the induced noise from a specific portion of the conductive transmission line . bridge - taps are known to cause impedance mismatch and reflections in transmission lines and other wired communication environments . fig7 illustrates a network 70 , which is similar to network 60 ( fig6 ), but with added transmission line 15 e and 15 f , connected to terminals 51 a and 51 b respectively , forming a bridge tap at terminals 51 a and 51 b . without scu 30 , the bridge tap at these points would create an impedance mismatch and cause signal reflections in the communications over transmission line 15 a , 15 b , 15 c , 15 d , 15 e , and 15 f . the presence of scu 30 at the bridge - tap junction , however , cancels and absorbs the signals at terminals 51 a and 51 b , and eliminates such reflections . in doing so , three isolated communication segments are formed , one segment consisting of transmission line 15 a and 15 b , a second segment consisting of transmission line 15 c and 15 d , and a third segment consisting of transmission line 15 e and 15 f . the capability of an scu to isolate electrically connected transmission line enables the formation of multiple distinct communication networks over continuous electrical conducting transmission line , as shown in fig8 . a network 80 is based on transmission line 15 a , 15 b , 15 c , and 15 d . scu 30 connects to the transmission line at terminals 51 a and 51 b , and isolates the transmission line into two communication segments . one segment is based on transmission line 15 a and 15 b , and extends from terminals 51 a and 51 b towards the left in fig8 . the other segment is based on transmission line 15 c and 15 d , and extends toward the right . data unit 16 a transmits across transmission line 15 a and 15 b via line driver 11 a , and provides the signal received by data unit 16 b via line receiver 12 a . similarly , data unit 16 e transmits across transmission line 15 c and 15 d via line driver 11 b , with the signal received by data unit 16 d via line receiver 12 b . being isolated by scu 30 , both transmissions can occur simultaneously , without interfering with each other . additional line drivers , line receivers and transceivers can be added to each communication segment . similarly , adding additional scu &# 39 ; s can split electrically - connected transmission line into more segments , wherein an isolated segment is formed between adjacent scu pairs , or between the scu and the ends or terminators of transmission lines . the function of the scu has been so far been described only as a terminator , but an scu can also be modified to perform a line receiving function , as shown in fig9 , which illustrates a signal canceling and receiving unit ( scru ) 90 . scru 90 is based on the structure of scu 30 , ( fig3 ), but the processing unit 33 is modified to a processing unit 91 , which provides additional output via a terminal 34 c ( c ). the output on terminal 34 c uses sensing function 31 , and together with part of processing unit 91 serves as a line receiver , similar to line receiver 12 a or 12 b . thus , scru 90 simultaneously performs two functions : signal cancellation as does scu 30 , and line receiver functionality , as do line receivers 12 a and 12 b , thus allowing the sensed signal or any function thereof to be output on the terminal 34 c and placed on the transmission line . an example of an application using scru 90 is shown in fig1 , for a network 100 . network 100 is based on network 80 ( fig8 ), but scu 30 is replaced by scru 90 , whose terminal c is connected to a data unit 16 f via a connection 102 . scru 90 further is connected to transmission line 15 a , 15 b , 15 c , and 15 d at junctions 101 a and 101 b . in a manner similar to that of network 80 ( fig8 ), this configuration allows two isolated communication segments to use the transmission line simultaneously without interfering with each other . one segment transports data over transmission line 15 a and 15 b , while the other segment transports data over transmission line 15 c and 15 d . in addition , by utilizing the line - receiving functionality of scru 90 , data unit 16 f can receive signals from both networks . in yet another embodiment of the invention , a line - driving capability is also integrated into the scru . fig1 illustrates an scrtu ( signal canceling , receive and transmit unit ) 110 . scrtu 110 includes all components of scru 90 , but also includes a line driver 111 ( constituting a “ second driver ”), which is fed from an additional scrtu terminal 34 d ( d ) and feeds a third signal to the transmission line . scrtu 110 has two states of operation , denoted as “ receive ” and “ transmit ”. in “ receive ” state , the functionality of scru 90 is fully retained , and scrtu 110 performs signal cancellation and reception . in “ transmit ” state , line terminals 34 a ( a ) and 34 b ( b ) are connected to line driver 111 output terminals as shown , so that scrtu 110 can transmit data received at terminal 34 d to terminals 34 a and 34 b . shifting between the states is performed by two spdt ( single pole double throw ) switches 112 and 113 . switches 113 and 112 are connected to terminals 34 a and 34 b , respectively . in the ‘ receive ’ state , both switches 112 and 113 are in state ‘ 1 ’, thus connecting terminal 34 a and terminal 34 b terminals to sensor 31 and driver 32 , and thereby performing the function of scru 90 . in the ‘ transmit ’ state , both switches 112 and 113 are in state ‘ 2 ’, thus connecting terminal 34 a and terminal 34 b to the outputs of line driver 111 , and thereby performing as a line driver . switches 112 and 113 are controlled by a logic unit 114 , which changes switches 113 and 112 as required to select the desired state . fig1 illustrates an alternative implementation of an scrtu 120 . in this alternative configuration , driver 32 is also used as the line driver . an spst switch 121 is used to route the input into driver 32 . in state ‘ 1 ’, driver 32 is connected to the output of processing 91 , and thereby performing the function of scru 90 . in state ‘ 2 ’, driver 32 is coupled to terminal 34 d , and thereby functions as a line driver . a logic block ( not shown in fig1 ) is used to control switch 121 , shifting it from state to state as required . fig1 illustrates a network 130 using such scrtu &# 39 ; s . network 130 uses network transmission line 15 a , 15 b , 15 c , 15 d , 15 e , 15 f , 15 g , and 15 h , and has a bridge - tap at points 51 a and 51 b . data units 16 f , 16 g , 16 h , 16 i , and 16 j are coupled to the transmission line via scrtu &# 39 ; s 110 a , 110 b , 110 c , 110 d , and 110 e , respectively . as explained above , although the wiring is electrically continuous , the communication segments formed are of point - to - point type between any scrtu pair . scrtu 110 a communicates in a point - to - point topology with scrtu 11 b , over transmission line segment 15 a and 15 b . similarly , scrtu &# 39 ; s 110 b and 110 e communicate over transmission line segment 15 e and 15 f , scrtu &# 39 ; s 110 b and 110 c communicate over transmission line segment 15 c and 15 d , and scrtu &# 39 ; s 110 c and 110 d communicate over transmission line segment 15 g and 15 h . in addition to the benefit of point - to - point , the network also allows for multiple independent communication segments to operate independently , as long as there are not any two scrtu &# 39 ; s transmitting to the same segment . for example , scrtu 110 a can transmit to scrtu 110 b over transmission line segment 15 a and 15 b , while scrtu 110 d can simultaneously transmit to scrtu 110 c over transmission line segment 15 g and 15 h . network 130 demonstrates the scrtu based network capability of point - to - point communications and multiple transmissions over continuous wiring . these capabilities can be useful for existing wiring having unknown topology , and having ‘ bus ’ type connection points . for example , in - wall existing telephone wiring , in - wall existing power lines or catv cabling which are not used for their original purpose . continuity is common to all of these types of wiring , where outlets are provided for connecting to the wiring . hence , coupling scrtu &# 39 ; s to each outlet allows for reliable high bandwidth communication between data units connected to the scrtu &# 39 ; s . while the invention has been described with respect to a digital communication application , it will be appreciated that the invention is equally applicable to analog communication as well , such as video , audio or any other type of communication . in such configurations , data units 16 are replaced by suitable analog units , and the scu &# 39 ; s , scru &# 39 ; s , and scrtu &# 39 ; s are modified accordingly to support such communication . while the invention has been described with respect to a limited number of embodiments , it will be appreciated that many variations , modifications and other applications of the invention may be made .