Abstract:
A method of signal communication includes receiving a selection signal, and selecting a selected frequency range used for a multi-carrier signal communication from a set of predetermined frequency ranges for signal communication depending on the selection signal. The set of predetermined frequency ranges includes a first frequency range and a second frequency range including the first frequency range.

Description:
BACKGROUND 
     Digital subscriber line (DSL) communication is used in a variety of situations at a variety of frequency ranges or bandwidths. 
     For example, DSL communication between a central office and an end user typically takes place over copper lines. These copper lines can be used simultaneously for analog telephony and DSL communication. One such analog telephony is plain old telephone system (POTS). 
     Many modern systems now use integrated services digital network (ISDN) communication. ISDN uses a broader frequency range than POTS. When DSL is used together with ISDN, DSL typically uses a frequency range above 138 kHz for communication. In contrast, when DSL is used together with POTS, DSL uses a frequency range of above about 25 kHz. 
     In another situation, DSL is used without any further services. In this situation, DSL may typically use the whole available bandwidth, as in this situation, voice communication and data communication are typically handled via the same service. In all of these situations, the upper boundary of the DSL frequency range is dependent on the DSL standard used. 
     For these and other reasons there is a need for the present invention. 
     SUMMARY 
     One embodiment provides a method of signal communication. The method includes receiving a selection signal. The method includes selecting a selected frequency range used for a multi-carrier signal communication from a set of predetermined frequency ranges for signal communication depending on the selection signal. The set of predetermined frequency ranges includes a first frequency range and a second frequency range including the first frequency range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
         FIG. 1  is a diagram illustrating three frequency ranges used for various types of communication. 
         FIG. 2  is a diagram illustrating the use of a DSL linecard according to an embodiment in an environment with a splitter. 
         FIG. 3  is a diagram illustrating the use of a DSL linecard according to an embodiment in an environment without a splitter. 
         FIG. 4  is a block diagram illustrating a linecard according to an embodiment. 
         FIG. 5  is a circuit diagram illustrating circuitry according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     Embodiments relate to communication circuitry, corresponding communication devices and methods for controlling such communication components or devices. 
     In the following, various embodiments are discussed with reference to the drawings. In particular, embodiments are capable of performing communication selectively in a first frequency range or a second frequency range are discussed. These embodiments comprise communication circuitry like integrated circuits or groups of integrated circuits, communication devices like linecards or central office equipment using such communication devices and may, for example, be employed for digital subscriber line (DSL) communication like asymmetric digital subscriber line (ADSL) or very high bit digital subscriber line (VDSL). However, embodiments are not limited to these applications. 
     To explain the application of embodiments in the context of DSL communication further,  FIG. 1  illustrates the frequency range or bandwidth used for DSL communication in three different situations. 
     In many cases DSL communication between a central office and an end user (e.g., private homes or a company) takes place over copper lines which have traditionally been used for analog telephony, also referred to as plain old telephone system (POTS). These copper lines in a first scenario may be used simultaneously for POTS and for DSL communication. In this case, for POTS a frequency range up to about 4 kHz is used, and DSL communication takes place in a frequency range above about 25 kHz, the upper boundary of the DSL frequency range being dependent on the DSL standard used. 
     While POTS is still in use for voice communication, in modern systems it has been very often replaced by integrated services digital network (ISDN) communication. ISDN occupies a broader frequency range than POTS up to about 138 kHz. In a second scenario, when DSL is used together with ISDN, DSL correspondingly uses a frequency range above 138 kHz for communication. In this case, voice communication like telephony is handled via ISDN, while data communication, for example for connection with the internet, is handled via DSL. 
     A third scenario also illustrated in  FIG. 1  is the use of DSL without any further services, designated DSL (alone) in  FIG. 1 . In particular, in this case the DSL service may basically use the whole available bandwidth, and no separate voice service is available. This scenario is becoming more popular with the increased availability and quality of voice over IP, where voice communication is not handled via the traditional services like POTS or ISDN, but via a data communication network like the internet by sending voice data in the form of data packets. In other words, voice communication and data communication is handled via the same service. 
     In the former two cases (DSL with POTS or DSL with ISDN), splitters are used to separate the DSL signals from the POTS/ISDN signals, whereas for the latter case no splitter is necessary. 
     Embodiments described in the following provides a DSL linecard for a central office equipment which can be used in both cases. These two cases of the use of this embodiment will be explained further with reference to  FIGS. 2 and 3 . 
       FIG. 2  illustrates the use of a DSL linecard  17  according to one embodiment in a central office equipment  10  in case both DSL and a voice service like POTS or ISDN are present on a subscriber line. Subscriber line, in this case designates a line leading to customers premises, for example to communication equipment in a private home or a company. 
     In the situation illustrated in  FIG. 2 , the subscriber line, which for example is formed by a pair of copper wires, is connected to a splitter  11  serving to separate the DSL signals from the POTS or ISDN signals. To achieve this, a low-pass filter  15  and a high-pass filter  16  are provided in splitter  11 . High-pass filter  16  according to an embodiment may be a first order high-pass filter, whereas low-pass filter  15  may be a low-pass filter of higher order, for example a sixth order filter. However, in other embodiments, high-pass and low-pass filters may have other orders. 
     Low-pass filter  15  has a corner frequency to let the POTS or IDSN signal pass and to block the DSL signal, whereas high-pass filter  16  correspondingly has a corner frequency to let the DSL signal pass and to block the POTS or ISDN signal. The separation of the frequency band for POTS or ISDN and the frequency band for DSL performed in splitter  11  in some splitters is only a comparatively rough separation, in particular for the DSL signal if only a first order high-pass filter is used. In these cases, further filters may be employed in the elements connected to the splitter to further separate the signal and in particular to filter out parts of the respective undesired frequency range in case the filtering performed within splitter  11  needs further filtering. 
     In the structure illustrated in  FIG. 2 , a voice switch  12  is connected which comprises a plurality of POTS linecards  18  or ISDN linecards  18  (depending on the system used for voice communication) or combinations of POTS and ISDN linecards  18 . One of these POTS/ISDN linecards  18  is connected with low-pass filter  15  of splitter  11 . Further splitters connected with further subscriber lines may be connected to other POTS/ISDN linecards within voice switch  12  or to other connectors of POTS/ISDN linecard  18 . In other words, one linecard may have the corresponding components to handle one or more voice connections, and a plurality of linecards may be present in voice switch  12 . Voice switch  12  distributes voice signals received from a backbone network of the telecommunications provider (not illustrated) to the appropriate subscriber line and coalesces incoming signals from the subscriber line. 
     On the other hand, high-pass filter  16  of splitter  13  is connected with a digital subscriber line access multiplexer (DSLAM)  13  and in particular to DSL linecard  17  installed therein. Similar to voice switch  12 , linecard  17  may have more connectors to be connected (via splitters or not) with a plurality of subscriber lines, and DSLAM  13  may comprise a plurality of linecards. According to an embodiment, DSLAM  13  may comprise one or more conventional linecards in addition to one or more linecards  17 . Like voice switch  12  for voice signals, DSLAM  13  serves to coalesce DSL data signals received from subscriber line  14  and to forward the data to a backbone network and distribute data received from the backbone network to the appropriate subscriber line  14 . Furthermore, while  FIG. 2  illustrates high-pass filter  16  external to DSL linecard  17 , high-pass filter  16  may according to an embodiment may be integrated on POTS/ISDN linecard  18 . 
     The second scenario of operation of the DSL linecard  17  according to one embodiment is illustrated schematically in  FIG. 3 . Here, no POTS or ISDN service is present, but subscriber line  14  is only used for DSL communication. Consequently, subscriber line  14  is directly connected with DSL linecard  17  without a splitter inbetween. Also, in the embodiment of  FIG. 3 , DSL linecard  17  may be located in a DSLAM which is not illustrated in  FIG. 3  for simplification. 
     DSL linecard  17  of this embodiment may be used in both operation modes by switching the usage frequency range on the linecard (i.e., by changing the frequency range used for DSL communication depending on the environment (with or without splitter) of the linecard). An examplary implementation of DSL linecard  17  according to an embodiment is illustrated in  FIG. 4 . 
       FIG. 4 , in block diagram form, illustrates one embodiment of linecard  17  for the connection to one subscriber line  14 . In the embodiment illustrated in  FIG. 4 , subscriber line  14  is connected to a 2 wire/4 wire conversion unit  25  which serves for 2 wire/4 wire conversion and vice versa, (i.e., which “splits up” the subscriber line  14  which serves both for sending and receiving data into separate lines for sending and receiving data) which, in  FIG. 4 , are represented as arrows on the left side of 2/4 wire conversion unit  25 . 2/4 wire conversion unit  25  is coupled with an analog unit  24  followed by a digital unit  23 . For data signals received from subscriber line  14 , analog unit  24  performs an analog/digital conversion, possibly together with analog filtering, and forwards the signals to digital unit  23 . In digital unit  23 , further signal processing like digital filtering may be performed. The signals are then forwarded to a backbone network as indicated by arrow  26 . Conversely, when data is received from backbone network, digital unit  23  performs digital filtering if necessary, analog unit  24  converts the digital data to analog data and forwards the analog signal to 2/4 wire conversion unit  25  to be sent over subscriber line  14 . In the embodiment illustrated, 2/4 wire conversion unit  25  comprises a switchable high-pass filter, the function of which is described below. 
     The embodiment illustrated in  FIG. 4  further comprises a control unit  20  for controlling the switching between operation with a splitter as illustrated in  FIG. 2  and operation without a splitter as illustrated in  FIG. 3 . The necessary instructions for performing these tasks may be stored in a firmware memory associated with control unit  20 . Control unit  20  is coupled with a Z measurement unit  22  for measuring an impedance Z of subscriber line  14 . Furthermore, the embodiment illustrated comprises a central processing unit (CPU)  21  which controls the DSL communication via subscriber line  14  and in particular is responsible for establishing and terminating DSL connections via subscriber line  14 . 
     In the following, an exemplary operation of the embodiment of  FIG. 4  for switching between the situations of  FIGS. 2 and 3  is explained. 
     When CPU  21  starts to establish a DSL connection via subscriber line  14  with a subscriber (which may be initiated by a request sent from the subscriber via subscriber line  14 ) it informs control unit  20 . Control unit  20  then controls Z measurement unit  22  to perform impedance measurements on subscriber line  14  for determining whether a splitter is present, for example whether splitter  11  illustrated in  FIG. 2  is presented. Possible measurement sequences for achieving this is explained in greater detail below. According to other embodiments, automatic detection whether a POTS/ISDN service is implemented may be achieved by testing for equipment other than splitters at the central office side or subscriber side or for configurations of equipment at the central office or subscriber side. For example, according to one embodiment, the presence of a POTS/ISDN linecard may be directly tested and reported to control unit  20 . 
     When it is detected that a POTS or ISDN service is used, for example if the impedance measurements performed by Z measurement unit  24  yield the result that a splitter is present, control unit  20  sets 2/4 conversion unit  25  and digital unit  23  in a “combined mode” (i.e., a mode of operation for the parallel use of POTS or ISDN over subscriber line  14 ). In particular, an analog high-pass filter in 2/4 wire conversion unit  25  and/or digital high-pass filters in digital unit  23  are set to appropriate corner frequencies as illustrated in  FIG. 1 , for example a corner frequency of 138 kHz for use together with ISDN or a corner frequency of 25 KHz for use together with POTS. Furthermore, digital unit  23  is controlled to perform de-modulation of data only using carriers in the corresponding frequency range used for DSL, for example by using discrete multitone modulation as a modulation technique for DSL connections. 
     On the other hand, when it is detected that no POTS or ISDN service is implemented, for example if the impedance measurements yield the result that no splitter is present, control unit  20  controls 2/4 wire conversion  25  and digital unit  23  to operate in “single mode” (i.e., a mode where subscriber line  14  is used only for DSL). Then, high-pass filters, as mentioned above, are switched to a lower corner frequency (e.g., 25 kHz or 15 kHz) or even switched off completely in order to use the full available bandwidth for DSL transmission. The corner frequency used in single mode is, in embodiments, a predetermined constant. Correspondingly, digital unit  23  is controlled to use a correspondingly extended frequency range for de-modulation of data. 
     Alternatively or additionally to the switching between single mode and combined mode based on the measurements made by Z measurement unit  22 , the switching can also be effected by sending a corresponding control signal c to control unit  20 , for example for performing the mode switching based on an user input either remotely via data communication or directly at the central office side. In other words, embodiments may employ an automatic switching, for example using an impedance measurement, a manual switching, for example by control signal c, or both possibilities. 
     Furthermore, the embodiment illustrated comprises a wetting current unit coupled with subscriber line  14  via a DC measurement unit  28 . Wetting current unit  27  is activated by control unit  20  in single mode. When activated, wetting current unit  27  applies a predetermined fixed voltage to subscriber line  14 . Applying such a fixed voltage causes a “wetting current” to flow via subscriber line  14  which prevents or slows the corrosion of contacts in circuitry connected to subscriber line  14 . In combined mode, wetting current unit  27  in the embodiment illustrated may be disabled as POTS or ISDN standards dictate applying a voltage to the subscriber line. In this case, in the combined mode when a splitter and corresponding POTS/ISDN equipment (e.g., voice switch  12  of  FIG. 2 ) is present a constant DC voltage is supplied to subscriber line  14  by this unit. The wetting current thus generated in an embodiment is between 0.2 mA and 20 mA, while other embodiments may use other welting currents. 
     Furthermore, in the embodiment illustrated in  FIG. 4 , a DC measurement unit  28  is provided. With DC measurement unit  28 , measurements which are also referred to as “metallic test access” or “line testing” can be performed, like measurements for determining whether a foreign voltage is connected to subscriber line  14 , or measurements of capacitances coupled with subscriber line  14 . Such measurements, in an arrangement having a splitter are conventionally implemented on the POTS/ISDN linecard  18 . In order to be able to perform these measurements also in a splitterless environment like in  FIG. 3 , DC measurement unit  28  is provided in embodiments, such that the measurements can be performed irrespective of whether a splitter and the corresponding POTS/ISDN linecards are present or not. 
     In the following, a realization of Z measurement unit  22 , 2/4 conversion unit  25 , DC measurement unit  28  and wetting current unit  27  according to an embodiment is discussed with reference to  FIG. 5 . 
     In the embodiment illustrated in  FIG. 5 , Z measurement unit  22  comprises a controllable AC voltage source  38 , a measurement resistor Rm, an analog/digital converter  37  and a signal processing unit  36 . Furthermore, coupling capacitors C 3  and C 4  are provided to couple Z measurement unit  22  to subscriber line  14 . Capacitors C 3  and C 4  are dimensioned such as to allow the AC measurements with the frequencies as discussed below, but to basically block signals having a lower frequency or even DC signals. A current flowing via subscriber line  14  dependent on the voltage applied by AC voltage source  38  is measured by measuring the corresponding voltage drop over the measurement resistor Rm, converting it to a digital value using analog/digital converter  37  and calculating the impedance by dividing the applied voltage by the measured current in signal processing unit  36 . To this end, when control unit  20  of  FIG. 4  controls AC voltage source  38  to supply a certain voltage with a certain frequency, SPU  36  is informed by control unit  20  of these parameters so as to be able to perform the measurement and calculation. 
     An exemplary implementation of a measurement procedure for determining whether a splitter is present is explained in the following. 
     The method which is employed in one embodiment uses information based on the frequency dependence of a termination impedence (i.e., that the high-pass filter and the low-pass filter provided in a splitter having a defined frequency behavior and the POTS/ISDN linecard constitutes a defined termination having a defined impedance). In particular, as already discussed, high-pass filter  16  in a conventional realization of a splitter as illustrated in  FIG. 2  may be a first order high-pass filter (e.g., formed by a capacitance) which therefore has a weak dependency of its attenuation on frequency. On the other hand, low-pass filter  15  conventionally may be a higher order filter (e.g., a sixth order filter) which therefore has a strong dependency of its attenuation on frequency and in particular has an almost abrupt change from passband to cut-off region. Therefore, for example in the case of POTS combined with DSL, when the corner frequency (which in the following is designated f 1 ) of low-pass filter  15  and high-pass filter  16  is 25 kHz, below f 1  DSL linecard  17  “sees” the impedance of POTS/ISDN linecard  18 , whereas for frequencies higher than f 1  low-pass filter  15  is in the cut-off region and therefore DSL linecard  17  sees an open connection (i.e., a termination with a very high impedance). Therefore, the following measurement procedure for detecting the presence of a splitter may be employed: 
     1. Measure impedance at a frequency below f 1  (e.g., f 1 −df), wherein for the case of f 1 =25 kHz df may be 18 kHz and therefore the measurement frequency may be 7 kHz. This impedance will be designated Zlow. 
     2. Measure an impedance Zhigh at a frequency above f 1  (e.g., f 1 +df) (in the numerical example given above 43 kHz). 
     3. If |Zhigh−Zlow|&gt;dZ, wherein dZ is a given constant (e.g., 200Ω), then decide that a splitter is present. 
     4. Else decide that a splitter is not present. 
     Therefore, with only two easily implemented measurements the presence of a splitter can be detected. 
     In case of an ISDN system, f 1  would be set to 138 kHz (see  FIG. 1 ). Moreover, according to another embodiment it is also possible to perform three impedance measurements, namely a measurement of Zlow below a first frequency f 1  (e.g., 25 kHz), of an impedance Zmid between said first frequency and a second frequency f 2  (e.g., f 2 =138 kHz), and of Zhigh above the second frequency. In this case, if Zlow differs from Zmid by more than dZ, a POTS splitter and corresponding POTS linecard is present, if Zmid differs from Zhigh by more than dZ, a ISDN splitter and corresponding ISDN linecard is present, and if none of the two cases applies, no splitter is present. 
     In still further embodiments, a plurality of measurements are performed over a given frequency range, for example from 4 kHz to 200 kHz, to detect the presence of a splitter and its splitting frequency (i.e., the corner frequencies of its filter(s)). In embodiments, the splitting frequency thus determined is used for determining the frequency range to be used for data transmission (e.g., DSL communication). 
     In embodiments, the above-described measurements are performed by using sine signals at the frequencies to be measured. In other embodiments, multitone signals over a wider frequency range are used for obtaining a plurality of measurement values at different frequencies simultaneously. 
     As indicated in  FIG. 5 , Z measurement unit  22  is coupled with control unit  20  of  FIG. 4  which, as already described, evaluates the results and performs the corresponding actions. 
     Next, the realization of 2/4 wire conversion unit  25  according to the embodiment illustrated in  FIG. 5  is discussed. In the embodiment illustrated in  FIG. 5 , 2/4 wire conversion unit  25  comprises a transformer  32  for signal coupling. The 2/4 wire conversion itself takes place in the circuit part illustrated on the left side of 2/4 wire conversion unit  25  in  FIG. 5  and is, in the embodiment illustrated, realized as a bridge circuit comprising resistors R 1 -R 4  and the corresponding inductivities of transformer  32 . An amplifier  30  amplifies the signals received from analog unit  24  of  FIG. 4  which are then coupled into subscriber line  14  via transformer  32 , and an amplifier  31  amplifies the signals received from subscriber line  14  which are to be forwarded to analog unit  24 . In the embodiment illustrated, the resistances R 1 -R 4  are provided to minimize reflecting of the sending signal (i.e., the signal received from analog unit  24 ) back to analog unit  24  via amplifier  31 . To this end, one or more of resistors R 1 -R 4  may be made adjustable. Additional means for echo cancellation (e.g., a corresponding filter) may also be provided. 
     On the side of transformer  32  to which subscriber line  14  is coupled, capacitors C 1  and C 2  are provided which selectively can be coupled between inductors  39 ,  40  of transformer  32  via switch S 1 . Switch S 1 , as indicated by an arrow in  FIG. 5 , is controlled by control unit  20 . Capacitors C 1  and C 2 , respectively, together with inductors  39  and  40  form a high-pass filter for signals received from and sent to subscriber line  14 . Capacitors C 1  and C 2  in the embodiment illustrated have different values, such that by switching between C 1  and C 2  the corner frequency of this high-pass filter can be varied. In the embodiment illustrated, the corner frequency with capacitor C 1  coupled between inductors  39  and  40  may, for example be 15 kHz while with C 2  it may be 138 kHz. In this case, if a splitter with an ISDN linecard is detected or present, capacitor C 2  is coupled between inductors  39  and  40  to provide DSL together with ISDN service, whereas in the absence of ISDN capacitor C 1  is coupled between inductors  39  and  40  to be able to use the full bandwidth for DSL. 
     In a different embodiment, an additional capacitor is provided so as to be able to switch between three possible corner frequencies, one for DSL alone (e.g., 15 kHz), one for DSL together with POTS (e.g., 25 kHz) and one for DSL together with ISDN (e.g., 138 kHz). In a further embodiment, the selectable frequencies may be 25 kHz and 138 kHz, the former being used for DSL together with POTS or DSL alone and the latter being used for DSL together with ISDN. 
     Wetting current unit  27  in the embodiment of  FIG. 5  is realized by a current source  35  coupled to subscriber line  14  via a switch S 2  as needed. Therefore, the wetting current does not necessarily flow the whole time, but may be fed to subscriber line  14  in intervals or only in single mode. 
     DC measurement unit  28  in the embodiment of  FIG. 5  comprises an analog/digital converter  33  coupled between the two wires of subscriber line  14  for measuring a voltage applied on subscriber line  14  and a measurement resistor Rw to provide the voltage drop which is measured by analog/digital converter  34 . Since the voltage drop over measurement resistor Rw corresponds to the current flowing multiplied with the resistance value of resistor Rw, analog/digital converter  34  measures the current flowing on subscriber line  14 . The outputs of analog/digital converter  33  and  34  are coupled with central processing unit  21  of  FIG. 4  for evaluation of the results. 
     DC measurement unit  28  as illustrated in  FIG. 5  comprises one measurement resistor Rw located on one side of current source  35 . In a different embodiment, two measurement resistors are provided, one on each side of current source  35 , and each coupled with a corresponding analog/digital converter for measurement. In this way, imbalances in the subscriber line  14 , for example an imbalance to ground, may be detected. 
     As a matter of course, the above-described embodiments are to be taken as examples only and not as limiting the scope of the present invention. Some of the possible modifications to the above-described embodiments are discussed in the following. 
     In the above-mentioned embodiments, a wetting current unit and a DC measurement unit are provided. However, in other embodiments these elements are not present. In such embodiments, the DC measurements for line testing if needed may be for example performed with separate dedicated test equipment which may be coupled to subscriber lines via relays or other means. 
     In  FIG. 4  and to some extent also in  FIG. 5 , the various functions of the embodiment are represented by separate blocks. However, this does not mean that the various units or blocks have to be implemented as separate circuits but various blocks may be integrated together in one circuit or integrated circuit. Moreover, while in the embodiment of  FIG. 4  a central processing unit and a control unit are illustrated as separate entities, the two functions may be combined in a single processing unit. 
     In the embodiments of  FIGS. 4 and 5 , Z measurement unit  22  is coupled to subscriber line  14 . In a different embodiment, the functionality of Z measurement unit  22  is implemented in the part of the circuit responsible for the processing of the DSL signals (i.e., in analog unit  24  and digital unit  23  which, in an embodiment, form a DSL chipset). In this case, when evaluating the impedance measurement to determine whether a splitter is present, the attenuation of the high-pass filter of the 2/4 wire conversion unit which has been described above has to be taken into account. However, since the capacitance and inductance value forming that filter are known, this effect may be compensated by a corresponding evaluation process. 
     In the embodiment illustrated, the presence of a splitter is detected via impedance measurements. In other embodiments, the presence of a splitter may be determined by reflectometry measurements. In this case, a signal is sent via subscriber line  14  and basically the time until the reflected signal arrives back at the circuit is measured to determine a length of the line to a corresponding reflecting element. For a frequency smaller than f 1  (f 1  defined as explained above) the POTS/ISDN linecard connected to a splitter in case of  FIG. 2  would constitute a reflecting element. Therefore, if for these frequencies a length of less than a given length (e.g., 100 m) result, this would be taken as an indication that a splitter is present. In this respect, it should be noted that the drawings are not to scale, and splitter  11  in  FIG. 2  may be located remote from DSLAM  13  and voice switch  12 , and also between voice switch  12  and DSLAM  12  there may be some distance. 
     Furthermore, as already explained, in different embodiments there is no automatic splitter detection, but the information in which mode to operate the system is given by an external control signal, for example dependent on a user input. On the other hand, the method and devices for determining whether a splitter which has been described above may also be employed separately, for example in testing equipment. 
     As has already been mentioned, besides the high-pass filter in the 2/4 wire conversion unit  25  (the corner frequency of which is switched by switch S 1  in the embodiment of  FIG. 5 ), further high-pass filters may be present in analog unit  24  or digital unit  23 . The corner frequency of these additional filters may be switched as well, or the filters may be disabled entirely (i.e., corresponding to a corner frequency of 0) if the whole frequency range is to be used for DSL and/or if no other service is present. 
     In other embodiments, no switching of filters occurs, but the usable frequencies for DSL transmission are still changed from a first frequency range (e.g., above 138 kHz in case of combined DSL and ISDN use) to a second, broader frequency range like the whole frequency range (e.g., for DSL use alone). In such an embodiment, no switchable filters are needed. On the other hand, because of the attenuation of the filters for the corresponding combined mode or mode using the first frequency range, the use of the remaining frequency range is limited, (i.e., less additional data may be transmitted via the additional frequencies). 
     Furthermore, in embodiments a lightning protection for the DSL linecard is provided. The lightning protection in this embodiment is designed to work also in the case of no splitter being present (i.e., in the case of  FIG. 3  where a lightning protection provided by high-pass filter  16  of splitter  11  of  FIG. 2  is not available). 
     While the embodiments of the invention have been discussed using an exemplary DSL linecard, other embodiments may generally be employed in communication systems wherein signal transmission may be performed either over a wider frequency range or a narrower frequency range comprised in the wider frequency range. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments illustrated and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.