Abstract:
According to a disclosed embodiment, the ideal magnitude and phase of an ideal impedance for interfacing a modem with a telephone line is determined. This ideal impedance can be determined in the presence of an external impedance load and associated circuitry, such as an ADSL modem and accompanying POTS splitter. Thereafter, a model for an interface circuit inside the modem is utilized to arrive at an appropriate impedance for an impedance network inside the interface circuit. As an example, the interface circuit can be a DAA circuit inside a V.90 modem. Then, a relationship between the ideal impedance and an input impedance of the impedance network is established. Thereafter, the impedance network is synthesized to that the relationship between the input impedance of the impedance network and the ideal impedance is in fact satisfied. The impedance network can be, for example, a circuit comprising at least one capacitor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is generally in the field of data communications. More specifically, the present invention is in the field of modem communication over a telephone line. 
   2. Background Art 
   The increased demand and availability of a variety of data communications solutions has resulted in the use of different data communications devices and services, such as V.90 modems and ADSL services, over the same POTS (“plain old telephone service”) telephone line. By way of background, a typical POTS telephone line has an impedance in the range of approximately 600.0 to 900.0 ohms. The input impedance of a data communications device, such as a modem, connected to a POTS telephone line needs to closely match the impedance of the POTS telephone to meet minimum regulatory return loss requirements. 
   When a single data communications device, such as a V.90 modem, is connected to a POTS telephone line, the regulatory requirements can be met by making the input impedance of the V.90 modem approximately 600.0 ohms. However, if an ADSL device, such as an ADSL modem, is also connected, via a POTS splitter, to the same POTS telephone line as the V.90 modem, the resulting input impedance of the coupled data communications devices (i.e. the ADSL modem, V.90 modem, and POTS splitter) no longer matches the POTS telephone line impedance. In fact, the return loss of the above coupled communications devices falls below the regulatory minimum requirements. 
     FIG. 1  shows a block diagram of an exemplary system comprising a V.90 modem, a POTS splitter, and an ADSL load. In system  100  in  FIG. 1 , V.90 modem  102  is connected to POTS splitter  104  via lines  116  and  118 . POTS splitter  104  can be a low pass filter. POTS splitter  104  is connected to TIP terminal  108  and RING terminal  110 , respectively, via lines  112  and  114 . TIP terminal  108  and RING terminal  110 , respectively, are connected to the tip and ring terminals of a POTS telephone line. A first terminal of ADSL load  106  is connected to TIP terminal  108 , and a second terminal of ADSL load  106  is connected to a first terminal of relay  120 . A second terminal of relay  120  is connected to RING terminal  110 . Thus, when relay  120  is closed, ADSL load  106  is connected in parallel with POTS splitter  104  at TIP terminal  108  and RING terminal  110 . As an example, ADSL load  106  can be an ADSL modem. When system  100  is connected to a telephone line at TIP terminal  108  and RING terminal  110 , the input impedance looking into line  112  must closely match the telephone line impedance for the return loss of system  100  to meet regulatory requirements. 
   When V.90 modem  102  is connected directly to a telephone line by itself, i.e. without either POTS splitter  104  or ADSL load  106  connected to the same telephone line, the input impedance of V.90 modem  102  must closely match the telephone line impedance for the return loss of V.90 modem  102  to meet regulatory requirements. This means that, for modems utilizing a Data Access Arrangement (“DAA”) circuit to interface with a telephone line, the input impedance of the DAA circuit must closely match the telephone line impedance. For example, to meet a desired telephone line impedance of 600.0 ohms, the DAA circuit inside V.90 modem  102  needs to have an input impedance that is close enough to 600.0 ohms to meet regulatory return loss requirements. However, in system  100 , with POTS splitter  104 , ADSL load  106 , and V.90 modem  102  connected to the same telephone line, the combined impedance of POTS splitter  104 , ADSL load  106 , and V.90 modem  102  deviate from the telephone line impedance sufficiently such that the return loss of system  100  does not meet regulatory requirements. For example,  FIG. 2  shows a graphical comparison of the simulated return loss of system  100 , represented by waveform  204 , and the minimum required return loss, represented by dashed line  202 , for a 600.0 ohm impedance telephone line. 
   As seen in  FIG. 2 , waveform  204 , representing the simulated return loss of system  100 , falls below dashed line  202 , representing the required return loss, in the 0.0 to 4.0 kHz frequency range, i.e. the frequency range where the return loss of system  100  is required to be above 20.0 dB. Thus, even when the input impedance of V.90 modem  102  can sufficiently match a telephone line impedance to enable V.90 modem  102  to meet regulatory return loss requirements, the input impedance of V.90 modem  102  combined with additional impedances of POTS splitter  104  and ADSL load  106  result in system  100  not meeting regulatory return loss requirements. 
   Thus, there is need in the art for impedance compensation to interface a modem to a telephone line in the presence of an external impedance load, such as an ADSL load, in order to meet regulatory return loss requirements. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to impedance compensation in interfacing a modem to a telephone line. The present invention overcomes the need in the art for impedance compensation in interfacing a modem to a telephone line in the presence of additional communications devices and associated circuitry, such as an ADSL load and POTS splitter, in order to meet regulatory requirements. 
   According to one embodiment of the present invention, the ideal magnitude and phase of an ideal impedance for interfacing a modem with a telephone line is determined. This ideal impedance can be determined in the presence of additional circuitry, such as an ADSL modem or POTS splitter. Thereafter, a model for an interface circuit inside the modem is utilized to arrive at an appropriate impedance for an impedance network inside the interface circuit. As an example, the interface circuit can be a DAA circuit inside a V.90 modem. 
   Then, a relationship between the ideal impedance and an input impedance of the impedance network is established. Thereafter, the impedance network is synthesized so that the relationship between the input impedance of the impedance network and the ideal impedance is in fact satisfied. The impedance network can be, for example, a circuit comprising at least one capacitor. Utilizing the above procedures, the present invention results in meeting regulatory requirements in interfacing a modem, such as a V.90 modem to, for example, “600.0 ohm” and “complex impedance” telephone lines, while the telephone lines are loaded by an additional communications device, such as a DSL modem. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a block diagram of an exemplary system including a V.90 modem, a POTS splitter, and an ADSL load connected to a telephone line. 
       FIG. 2  illustrates waveforms representing a simulated return loss of the exemplary system in  FIG. 1  without impedance compensation. 
       FIG. 3  illustrates a model for determining an impedance network for a DAA circuit in accordance with one embodiment of the present invention. 
       FIG. 4  illustrates waveforms representing the magnitude of the input impedance of a DAA circuit utilizing an embodiment of the present invention&#39;s impedance network. 
       FIG. 5  illustrates waveforms representing the phase of the input impedance of a DAA circuit utilizing an embodiment of the present invention&#39;s impedance network. 
       FIG. 6A  illustrates a schematic diagram of an impedance network in accordance with one embodiment of the present invention. 
       FIG. 6B  illustrates a schematic diagram of an alternative impedance network in accordance with one embodiment of the present invention. 
       FIG. 7  illustrates waveforms representing a simulated return loss of impedance model  300  in  FIG. 3  utilizing an embodiment of the present invention&#39;s impedance network. 
       FIG. 8  illustrates a flowchart of an exemplary procedure for determining an embodiment of the present invention&#39;s impedance network in a DAA circuit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to impedance compensation in interfacing a modem to a telephone line. The following description contains specific information pertaining to various embodiments and implementations of the invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order not to obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skills in the art. 
   The drawings in the present application and their accompanying detailed description are directed to merely example embodiments of the invention. To maintain brevity, other embodiments of the invention that use the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings. 
     FIG. 3  shows a model for designing an “impedance network” inside a modem, for example a V.90 modem or a V.92 modem, which when combined with a POTS splitter and an ADSL load, presents the desired impedance to a telephone line. For the purpose of a specific example, in the present application it is assumed that the modem, such as the V.90 modem or a V.92 modem, utilizes a DAA circuit to interface with the “outside world.” As such, in the present application, it is assumed that the impedance of the modem&#39;s DAA circuit, in combination with the impedances of the POTS splitter and the ADSL load, must result in a desired POTS interface impedance, i.e. an impedance which will result in meeting the regulatory requirements in interfacing with a given telephone line. The impedance of the DAA circuit is modeled and shown by dashed box  302  in impedance model  300  in FIG.  3 . Dashed box  302  is referred to as “DAA circuit  302 ” in the description below. 
   Impedance model  300  comprises DAA circuit  302 , diode bridge  304 , filter  306 , impedance load  308 , and relay  310 . DAA circuit  302  comprises “impedance network”  312  (also referred to as Zvi 600 ), “impedance network”  314  (also referred to as Zvi complex ), switches  316  and  318 , NPN transistor  320 , op amp (“operational amplifier”)  322 , resistor  324  (also referred to as “Rb”), and resistor  326  (also referred to as “Re”). 
   A first terminal of resistor  324  is connected to the “+” input of op amp  322  at node  328 , and a second terminal of resistor  324  is connected to ground. The “−” input of op amp  322  is connected to a first terminal of resistor  326  at node  330 . A second terminal of resistor  326  is connected to ground. The emitter of NPN transistor  320  is also connected at node  330 . The output of op amp  322  is connected to the base of NPN transistor  320 . The collector of NPN transistor  320  is connected to the DC positive terminal of diode bridge  304  (shown as “+”) at node  332 . The DC negative terminal of diode bridge  304  (shown as “−”) is connected to ground. 
   A first terminal of switch  316  is connected to a first terminal of impedance network  312  at node  334 . A second terminal of switch  316  is connected to a first terminal of switch  318  at node  332 . A second terminal of switch  318  is connected to a first terminal of impedance network  314  at node  336 . A second terminal of impedance network  312  is connected to a second terminal of impedance network  314  at node  328 . 
   Filter  306  comprises inductors  338 ,  340 ,  342 , and  344 , and capacitor  346 . A first terminal of inductor  338  is connected to a first AC signal terminal of diode bridge  304  at node  348 . A second terminal of inductor  338  is connected to a first terminal of capacitor  346  at node  352 . A first terminal of inductor  340  is also connected at node  352 . A second terminal of capacitor  346  to connected to a first terminal of inductor  342  at node  354 . A first terminal of inductor  344  is also connected at node  354 . A second terminal of inductor  342  is connected to a second AC terminal of diode bridge  304  at node  350 . 
   A second terminal of inductor  340  is connected to a first terminal of impedance load  308  at node  356  (also referred to as TIP node  356 ). In the present embodiment, impedance load  308  can represent an ADSL load, such as an ADSL modem. In another embodiment, impedance load  308  can represent a different impedance load. A second terminal of impedance load  308  is connected to a first terminal of relay  310 . A second terminal of relay  310  is connected to a second terminal of inductor  344  at node  360  (also referred to as RING node  360 ). 
   The function and operation of impedance model  300  in  FIG. 3  will now be discussed. In impedance model  300 , DAA circuit  302  is coupled to diode bridge  304  at node  332 . Diode bridge  304  is in turn coupled to filter  306  at nodes  348  and  350 . Filter  306  is further coupled to impedance load  308  at nodes  356  and  360 . As shown in  FIG. 3 , an impedance load, such as an ADSL load, can be connected across TIP node  356  and RING node  360 . DAA circuit  302  can model the DAA circuit of a modem, such as a V.90 or a V.92 modem. In one embodiment, DAA circuit  302  can model the DAA circuit of a SmartDAA™ V.90 modem. Diode bridge  304  can model a diode bridge circuit in a modem, such as a V.90 modem, and filter  306  can model a POTS splitter (i.e. a POTS filter). Impedance load  308  can represent (i.e. model) an ADSL load, such as an ADSL modem. In another embodiment, impedance load  308  can represent a different impedance load. Thus, for example, impedance model  300  can model a V.90 modem coupled to a POTS splitter, which is in turn connected in parallel with an ADSL modem. 
   In  FIG. 3 , Zin represents the input impedance of impedance model  300  between TIP node  356  and RING node  360 . For example, a value of Zin equal to 600.0 ohms provides optimal impedance, i.e. maximum return loss, when connected to a 600.0 ohm impedance telephone line. Relay  310 , when closed, connects impedance load  308 , such as an ADSL load, between nodes  356  and  360 , i.e. in parallel with filter  306 . For example, impedance load  308  can model the input impedance of an ADSL modem. In another embodiment, impedance load  308  can model the input impedance of another communication device. Filter  306  can model a POTS splitter, for example a low pass POTS filter, by attenuating signals with frequencies above approximately 6.0 KHz, and passing signals in a voice band below 4.0 KHz. Since the cutoff frequency of filter  306  can be close to 4.0 KHz, filter  306  can produce a phase shift in the impedance presented by DAA circuit  302  to the POTS interface port (i.e. TIP node  356  to RING node  360 ). The phase shifts produced by filter  306  and impedance load  308  degrade the return loss of impedance model  300 . 
   The telephone line voltage polarity at TIP node  356  and RING node  360  that is also provided to nodes  348  and  350  via filter  306  is arbitrary. However, diode bridge  304  is added to ensure that a positive voltage is always applied to DAA circuit  302  at node  332 , regardless of the line voltage polarity present at TIP node  356  and RING node  360 . 
   The function and operation of DAA circuit  302  is now discussed in relation to impedance model  300 . Zin* represents the input impedance of DAA circuit  302  at node  332 . The value of Zin* can be calculated by applying a test voltage “VT” to node  332  and determining a test current “IT” that will be drawn by node  332 . When TIP node  356  and RING node  360  are connected to a telephone line impedance in a range of approximately 600.0 to 900.0 ohms (defined as the “600.0 ohm mode” in the present application), switch  316  is shorted to connect impedance network  312  (i.e. Zvi 600 ) between nodes  328  and  332 . Similarly, when TIP node  356  and RING node  360  are connected to a “complex” telephone line impedance (defined as the “complex mode” in the present application), switch  318  is shorted to connect impedance network  314  (i.e. Zvi complex ) between nodes  328  and  332 . 
   In the 600.0 ohm mode, the magnitude of the voltage at the “+” input of op amp  322  is determined by the equation:
 
 V (+)= Rb/ ( Rb+Zvi   600 )* VT   equation (1)
 
where “V(+)” is the voltage at node  328 , and “VT” is the test voltage applied at node  332 . The voltage (“Ve”) at the emitter of NPN transistor  320  (i.e. node  330 ) is also equal to the right side of equation (1). “Ve” (i.e. the emitter voltage) induces a current (“IT”) through “Re” (i.e. the emitter resister of NPN transistor  320 ). “IT” (i.e. the induced current through “Re”) also flows through NPN transistor  320 .
 
   The impedance of Zvi 600  can be made very large such that very little current flows through Zvi 600  (i.e. the feedback path from node  332  to node  328 ). Since very little current flows through Zvi 600  (i.e. almost all of the current induced at node  332  flows through the collector of transistor  320 ), the collector current (“Ic”) can be made equal to “IT,” the current flowing though “Re.” Thus, “Ic” is equal to “Ve/Re,” which is determined by the following equation:
 
 Ve/Re=Rb/ ( Rb+Zvi   600 )*( VT/Re )  equation (2)
 
   Since “IT” is equal to “Ic,” “IT” is also equal to the right side of equation (2). Thus, since Zin* (i.e. the input impedance of DAA circuit  302  at node  332 ) is equal to “VT/IT,” Zin* can be determined by the following equation:
 
 Zin*=Re (1 +Zvi   600   /Rb )  equation (3)
 
Simplifying equation (3) results in the following equation:
 
 Zin*=α+β*Zvi   600   equation (4)
 
where α is a scaling factor equal to “Re,” and β is a scaling factor equal to “Re/Rb.” As such, equation (4) is a relationship between the input impedance of an impedance network, for example, Zvi 600 , and the “ideal” or “desired” impedance, i.e. Zin*. Thus, Zin* is directly proportional to Zvi 600 , the impedance of the feedback path from node  332  to node  328  in the 600.0 ohm mode. Zvi complex  can be substituted for Zvi 600  in equation (4) to determine Zin* in the complex mode (i.e. with a complex telephone line impedance at TIP node  356  and RING node  360 ). The ideal magnitude and phase of Zin*, i.e. the desired input impedance of DAA circuit  302  at node  332 , can be determined by simulation using impedance model  300  and a mathematical program, such as Mathcad®, by MathSoft Engineering &amp; Education, Inc. for either the 600.0 ohm mode or the complex mode. It is noted that Zin* is also referred to as the “ideal impedance” in the present application, and DAA circuit  302  is an exemplary “interface circuit” in the present application.
 
   Using the 600.0 ohm mode by way of an example, graph  400  in  FIG. 4  shows waveform  402 , which represents the ideal magnitude of Zin* plotted against frequency in the 600.0 ohm mode. As discussed above, waveform  402 , i.e. the ideal magnitude of Zin*, can be determined by simulation using impedance model  300  in the 600.0 ohm mode. Similarly, graph  500  in  FIG. 5  shows waveform  502 , which represents the ideal phase of Zin* plotted against frequency in the 600.0 ohm mode. As discussed above, waveform  502 , i.e. the ideal phase of Zin*, can also be determined by simulation using impedance model  300  in the 600.0 ohm mode. 
   As shown above in equation (4) , Zin* is directly proportional to Zvi. Thus, once the ideal magnitude and phase of Zin* have been determined for a desired impedance at TIP node  356  and RING node  360 , the present invention&#39;s impedance network, i.e. Zvi, can be accurately synthesized to match the ideal magnitude and phase of Zin*. For example, Zvi 600 , the present invention&#39;s impedance network in 600.0 ohm mode, can be accurately synthesized to match waveforms  402  and  502 , respectively, the ideal magnitude and phase of Zin* at a 600.0 ohm impedance at TIP node  356  and RING node  360 . Thus, by establishing a relationship between Zin* and Zvi, and simulating the ideal magnitude and phase of Zin* for a desired Zin, the present invention allows Zvi to be accurately synthesized. 
   Referring now to  FIG. 6A , impedance network  600  illustrates an exemplary impedance network of a DAA circuit in accordance with one embodiment of the present invention. Impedance network  600  represents a desired impedance network that can be synthesized by using an ideal magnitude and phase of Zin* in the 600.0 ohm mode, such as waveforms  402  and  502 , in equation (4) discussed above. Thus, impedance network  600  represents one implementation of Zvi 600 , the present invention&#39;s impedance network in the 600.0 ohm mode. However, it is manifest to one skilled in the art that many other implementations of Zvi 600  can be synthesized from the ideal magnitude and phase of Zin* in the 600.0 ohm mode and by utilizing equation (4) . 
   In impedance network  600 , a first terminal of capacitor  602  is connected to a first terminal of resistor  604  at node  610 , also referred to as a first terminal of impedance network  600 . A second terminal of capacitor  602  is connected to a second terminal of resistor  604  at node  612 . For example, the value of capacitor  602  can be approximately 68.0 picofarads (“pF”), and the value of resistor  604  can be approximately 348.0 kilo ohms. 
   A first terminal of capacitor  606  is connected to a first terminal of capacitor  608  at node  612 . A second terminal of capacitor  606  is connected to ground. For example, the value of capacitor  606  can be approximately 6.8 nanofarads (“nF”). A second terminal of capacitor  608  is also referred to as a second terminal of impedance network  600 . For example, the value of capacitor  608  can be approximately 47.0 nF. 
   Referring to  FIG. 6B , impedance network  650  illustrates an exemplary impedance network of a DAA circuit in accordance with one embodiment of the present invention. Impedance network  650  represents a desired impedance network that can be synthesized by using an ideal magnitude and phase of Zin* in the complex mode in equation (4) discussed above. Similar to impedance network  600  discussed above, impedance network  650  represents only one of many possible implementations of Zvi complex , the present invention&#39;s impedance network in the complex mode. 
   In impedance network  650 , a first terminal of capacitor  652  is connected at node  654 , also referred to as a first terminal of impedance network  650 , and a second terminal of capacitor  652  is connected to ground. For example, the value of capacitor  652  can be approximately 150.0 pF. A first terminal of resistor  656  is connected at node  654 , and a second terminal of resistor  656  is connected at node  658 . For example, the value of resistor  656  can be approximately 511.0 kilo ohms. A first terminal of capacitor  660  is connected to a first terminal of capacitor  662  at node  658 . A second terminal of capacitor  660  is connected to ground. For example, the value of capacitor  660  can be approximately 6.8 nF. A second terminal of capacitor  662  is also referred to as a second terminal of impedance network  650 . For example, the value of capacitor  662  can be approximately 47.0 nF. 
   Referring again to  FIG. 4 , waveform  404  represents the actual simulated magnitude of Zin* using impedance network  600  in impedance model  300  in the 600.0 ohm mode. Thus, as shown in graph  400 , the actual magnitude of Zin* closely matches the ideal magnitude of Zin* in the frequency range of 0.0 to approximately 4.0 KHz when using synthesized impedance network  600  in impedance model  300 . Referring again to  FIG. 5 , waveform  504  represents the actual simulated phase of Zin* using impedance network  600  in impedance model  300  in the 600.0 ohm mode. Thus, as shown in graph  400 , the actual phase of Zin* also closely matches the ideal phase of Zin* in the frequency range of 0.0 to approximately 4.0 KHz when using synthesized impedance network  600  in impedance model  300 . 
     FIG. 7  shows a graphical comparison of waveforms representing the minimum required return loss and the simulated return loss of impedance model  300  in  FIG. 3  at an approximate 600.0 ohm telephone line impedance utilizing an embodiment of the present invention&#39;s synthesized impedance network. As shown in graph  700  in  FIG. 7 , waveform  702  represents the minimum required return loss of a device connected to tip and ring terminals of a telephone line at an impedance of approximately 600.0 ohms, plotted against frequency. Waveform  704  represents the simulated return loss of impedance model  300  in the 600.0 ohm mode using the invention&#39;s synthesized impedance network in DAA circuit  302  with an ADSL load connected between nodes  356  and  360 . Thus, as shown in graph  700  in  FIG. 7 , the present invention&#39;s synthesized impedance network, for example, impedance network  600  in  FIG. 6 , produces a simulated return loss that exceeds the minimum required return loss of 20.0 dB in the frequency range of 0.0 to approximately 4.0 KHz. 
   Referring to  FIG. 8 , the exemplary procedure discussed above for synthesizing the present invention&#39;s impedance network is now summarized. The procedure begins at step  802 . At step  804 , the ideal magnitude and phase of Zin*, i.e. the input impedance of DAA circuit  302 , is determined to meet, for example, a desired 600.0 ohm telephone line impedance by simulation using impedance model  300  in FIG.  3 . For example, the ideal magnitude of Zin* in the 600.0 ohm mode can be represented by waveform  402  in FIG.  4 . By way of another example, the ideal phase of Zin* in the 600.0 ohm mode can be represented by waveform  502  in FIG.  5 . 
   At step  806 , an impedance network is synthesized to match the ideal magnitude and phase of Zin* determined at step  804 . For example, impedance network  600  in  FIG. 6A  can be synthesized to match the ideal magnitude and phase of Zin* at a desired telephone line impedance of 600.0 ohms at TIP node  356  and RING node  360 . As shown in equation (4) above, Zvi 600 , i.e. impedance network  312  in DAA circuit  302 , is proportional to Zin*. Thus, the present invention determines Zin* by simulation using impedance model  300  to meet a desired telephone line impedance, such as a 600.0 ohm telephone line impedance at TIP node  356  and RING node  360 . The proportional relationship between Zvi and Zin* in equation (4) above enables Zvi to be synthesized to match the ideal magnitude and phase of Zin*. 
   At step  808 , the synthesized impedance network, such as impedance network  600  in  FIG. 6A , is used in DAA circuit  302  to verify that regulatory return loss requirements are met by simulating the return loss using impedance model  300 . For example, waveform  704  in  FIG. 7  represents the simulated return loss with the invention&#39;s synthesized impedance network in DAA circuit  302  using impedance model  300  at a 600.0 ohm telephone line impedance. As seen in  FIG. 7 , the simulated return loss represented by waveform  704  exceeds the regulatory return loss represented by waveform  702  in a frequency range of 0.0 to approximately 4.0 kHz. At step  810  the procedure ends. 
   It is appreciated by the above detailed description that the invention provides impedance compensation in interfacing a modem to a telephone line in the presence of a POTS filter and an external impedance load, such as an ADSL modem. From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. The described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention. For example, although the invention was described for an ADSL load, the invention also applies to other impedance loads. By way of another example, although the invention was described to meet the 600.0 ohm and complex mode telephone line impedances, an embodiment of the present invention can determine an impedance compensation network for any desired telephone line impedance. 
   Thus, impedance compensation in interfacing a modem to a telephone line has been described.