FILTER AND METHOD THEREFOR

Abstract: A system and mechanism substantially to slow a transient generated during a loop-disconnect activity of a telephone set in a telephone system. The telephone system comprising a plurality of telephone sets (110, 126) coupled in parallel to a subscriber loop (100) and a data modem (138) coupled to the subscriber loop (100) providing data facilities at a frequency above that employed for a voice facility. The filter circuit (116) comprises an inductance (402) and an impedance (404) connected in series with a line, and a shunt element (409) connected in parallel with the line. The filter circuit (116) enables a Calling Line Identification (CLI) facility to be utilised in the telephone system of a subscriber.

Description: FILTER AND METHOD THEREFOR Field of the Invention This present invention relates, in general, to a filter for an apparatus coupled to a communications networks, for example a filter circuit for coupling between telephone handsets and a subscriber loop. Background to the Invention Fixed networks installed by telecommunications operators were originally constructed entirely from metallic transmission media (predominantly copper) carrying voice band signals in the region 300Hz to 3.4kHz, a network structure commonly known as Plain Old Telephone Service (POTS). While the backbone of the transmission network that interconnects switching centres is now mainly based on optical technology, the access portion of the network that connects switches to premises of customers is still dominated by twisted copper pairs of wire. Many telecommunications operators wish to use the existing POTS copper access portion of the network to deliver broadband services such as data delivery (e.g. Internet) and video on demand. In order to satisfy this objective, a number of alternative technologies have been developed to allow high bit rate services to be delivered via the copper access portion of the network to the premises of customers. One example of the alternative technologies is Asymmetric Digital Subscriber Line (ADSL). POTS traffic is carried over a conventional 0 to 4 kHz frequency band and ADSL traffic occupies a frequency band above the conventional frequency band in the region 24 kHz to 1.1 MHz. Both POTS traffic and ADSL traffic can be simultaneously carried over the same wire pair. Full rate ADSL traffic can be deployed by a customer installing a filter in front of each telephone set in the premises of the customer. An overview of this technology is described in an article "Broadband Multimedia Delivery over Copper" by G Young, K T Foster and J W Cook in BT Technology Journal, volume 13, number 4, October 1995. As well as ADSL, higher capacity systems have been proposed such as very high bit rate digital subscriber line (VDSL/VADSL). The generic term for these technologies is xDSL. In order to deliver high bit rate services to customers (hereinafter referred to as "subscribers"), high speed modems are coupled to devices receiving and/or generating ADSL traffic. In order to generate ADSL traffic, the high speed modems use frequencies above the normal voice or POTS frequencies so that normal telephony can co-exist on the subscriber loop with the data service. A problem resulting from the deployment of high bit rate services with normal voice telephony is that the high bit rate services can interfere with the voice telephony service, and vice versa. Consequently, High Pass Filters (HPFs) can be installed within the premises of customers between the modems and the subscriber loop to mitigate the interference by removing or blocking unwanted interfering signals between the modems and the telephone sets and Low Pass Filters (LPFs) can be installed between each telephone set in the premises of the customer and the subscriber loop. Another class of technology developed to allow high bit rate services to be delivered over the copper access portion of the network is known as G.Lite. G.Lite is a modified version of ADSL and designed to mitigate the need to have a splitter located at a central incoming telephone point of the premises of customers. G.Lite technology can utilise existing wiring to the premises of customers and requires a single LPF to be installed by the customer in front of each telephone set in the premises of the customer. Thus, each LPF is positioned in parallel with, and causes interference with, the other LPFs coupled in front of telephone sets at the premises of customers. Additionally, full rate ADSL can be deployed using the existing wiring using LPFs in front of each telephone set in the premises of the customer (such an architecture is known as a "distributed splitter" architecture, and the LPFs are known as "micro-filters"). However, in order to prevent the mutual interference described above in relation to ADSL and G.Lite between the LPFs installed in front of each telephone set, the LPFs are required to have the following properties. At frequencies used by xDSL technologies, the input impedance of the telephone set is undefined when the telephone set is either in an on-hook or off- hook condition. If the telephone set coupled to the subscriber loop is off-hook, the impedance presented to the xDSL modem is low, resulting in an incoming xDSL signal being attenuated. When a condition of the telephone set changes (for example, the telephone set is in the process of going into an off-hook condition) then the impedance of the telephone set changes. Consequently, the hybrid balance of the xDSL modem is altered. This causes a self near-end cross-talk (NEXT) response. Each LPF must therefore present a high input impedance to the subscriber loop at frequencies used by the xDSL technologies irrespective of the input impedance of the telephone set. The LPF must prevent xDSL signals from entering the telephone set. When the telephone set coupled to the subscriber loop is in an off-hook condition, xDSL signals can be received by the telephone set and generate audible noise in an earpiece of the telephone set. This is a result of intermodulation products due to non-linearity of the telephone set. The telephone set coupled to the subscriber loop has two modes of operation: a signalling mode and a talking mode. Both modes of operation are realised by modulating the impedance of the telephone set as "observed" by the subscriber loop. By modulating the impedance of the telephone set, a line voltage is consequently modulated. Examples of the signalling mode are: when the telephone set is put in the off-hook condition, or when loop disconnect dialling is carried out using the telephone set. In both of the above examples, the impedance of the telephone set changes abruptly from high impedance to low impedance. The abrupt impedance change generates a voltage step comprising frequency transients in the xDSL frequency band that can cause errors in data transmission or reception of xDSL modems. Consequently, the LPF must reduce the slew rate of the voltage step and hence attenuate frequency transients in the xDSL frequency band. To ensure maximum power is received by the LPFs it is necessary that the impedance of the filter is exactly matched to the impedance of both the subscriber loop and also the impedance of telephone set. If correct impedance matching is achieved then there is no reflection loss due to mismatch. Such a power loss due to reflection is also known as return loss. In practice, the LPF must have minimal return loss at both an input of the LPF and an output of the LPF, otherwise there may be audible echoes on the subscriber loop or a change in a level of feedback to an earpiece (known as "side tone"). Many subscribers have multiple telephone sets coupled to a single subscriber loop in parallel so that telephone calls can be made and received at several locations within the premises of the subscribers. It is necessary to couple LPFs, as described above, between each telephone set and the subscriber loop; the LPFs are consequently arranged in parallel. In a case where one of the telephone sets coupled to the single subscriber loop is in the off-hook condition, the LPF coupled to the one of the telephone sets in the off-hook condition is correctly terminated. However, remaining on-hook telephone sets will have unterminated LPFs. The unterminated LPFs will present a very low impedance at pass-band frequencies of the LPF corresponding to the telephone set in the off-hook condition resulting in poor voice signal quality. "POTS filters for ADSL - A Tutorial and Case Study" (Bob Beeman, Universal ADSL Working Group (UAWG), TG-98-062) discloses a technique that attempts to prevent the effects of unterminated filters. The technique involves shifting the above described low impedance at the pass-band frequencies out of the pass- band of the LPFs by altering a ratio of filter component values of the LPFs . However, the shifting of the low impedance results in an unwanted impedance mismatch between the LPF and the subscriber loop. The unwanted mismatch introduced alters return loss of the telephone set resulting in increased side tone. Summary of the Invention According to a first aspect of the present invention, there is provided a filter for a device coupled to a communications network, the filter comprising at least one filtering component and a switching element coupled to a selectable filter component, the switching element being arranged to couple the selectable filtering component to at least one of the at least one filtering component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. Preferably, the filtering components are arranged to reject selected input frequencies. Preferably, the loop disconnect activity corresponds to the communications device being in one of an on-hook condition and an off-hook condition. Preferably, the loop disconnect activity corresponds to a transition of the communications device from an on-hook condition to an off-hook condition. Preferably, the filtering components comprise inductance and impedance elements connected in series with a subscriber loop. Preferably, the switching element is coupled in parallel with the subscriber loop. Preferably, the switching element is a voltage-dependent switching device. Preferably, the selectable filtering component is a capacitor. According to a second aspect of the present invention, there is provided a filter for a device coupled to a communications network, the filter comprising: a first filter terminal, a second filter terminal, a third filter terminal, and a fourth filter terminal; the first filter terminal is coupled to an impedance, the first impedance being coupled to a inductance, and the inductance being coupled to the third filter terminal; the impedance and the inductance are coupled to the second filter terminal and the fourth filter terminal via a switching element coupled to a shunt impedance, wherein the switching element is arranged to couple the shunt impedance to the impedance and the inductance in response to a loop- disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. According to a fourth aspect of the present invention, there is provided a network access port apparatus for coupling a device to a communications network, the network access port apparatus comprising a filter, the filter comprising at least one filtering component and a switching element coupled to a selectable filter component, the switching element being arranged to couple the selectable filtering component to at least one of the at least one filtering component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. According to a fifth aspect of the present invention, there is provided a method of filtering signals for an apparatus coupled to a communications network, the method comprising the steps of: increasing a rise time of a first signal in a first frequency band in response to the first signal corresponding to a loop- disconnect activity. Preferably, the method further comprises the steps of: attenuating a second signal in a second frequency band above the first frequency band in response to the apparatus being in one of an on-hook condition and an off-hook condition. Preferably, the step of increasing the rise time of the first signal in response to the first signal corresponding to the loop-disconnect activity further comprises the step of: switching a shunt impedance across a pair of terminals providing the first signal. According to a sixth aspect of the present invention, there is provided a method of filtering a signal using a filter comprising at least one filtering component coupled to a switching element, the switching element being coupled to a selectable filter component, the method comprising the step of: actuating the switching element so as to couple the selectable filtering component to at least one of the at least one filtering component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop- disconnect activity. According to a seventh aspect of the present invention, there is provided computer executable software code stored on a computer readable medium, the code being for filtering signals for an apparatus coupled to a communications network, the code comprising: code to increase a rise time of a first signal in a first frequency band in increase to the first signal corresponding to a loop-disconnect activity. Preferably, the computer executable software code further comprises: code to attenuate a second signal in a second frequency band above the first frequency band in response to the apparatus being in one of an on-hook condition and an off-hook condition. Preferably, the code to increase the rise time of the first signal in response to the first signal corresponding to the loop-disconnect activity further comprises: code to switch a shunt impedance across a pair of terminals providing the first signal. According to an eighth aspect of the present invention, there is provided a programmed computer for filtering signals for an apparatus coupled to a communications network, comprising memory having at least one region for storing computer executable program code, and a processor for executing the program code stored in memory, wherein the program code includes: code to increase a rise time of a first signal in a first frequency band in response to the first signal corresponding to a loop-disconnect activity. Preferably, the program code further comprises: code to attenuate a second signal in a second frequency band above the first frequency band in response to the apparatus being in one of an on-hook condition and an off-hook condition. Preferably, the code to increase the rise time of the first signal in response to the first signal corresponding to the loop-disconnect activity further comprising: code to switch a shunt impedance across a pair of terminals providing the first signal. According to a ninth aspect of the present invention, there is provided a computer readable medium having computer executable software code stored thereon, the code being for filtering signals for an apparatus coupled to a communications network and comprising: code to increase a rise time of a first signal in a first frequency band in response to the first signal corresponding to a loop-disconnect activity. Preferably, the computer readable medium further comprising: code to attenuate a second signal in a second frequency band above the first frequency band in response to the apparatus being in one of an on-hook condition and an off-hook condition. Preferably, the computer readable medium further comprises: code to switch a shunt impedance across a pair of terminals providing the first signal. According to a tenth aspect of the present invention, there is provided computer executable software code stored on a computer readable medium, the code being for filtering a signal using a filter comprising at least one filtering component coupled to a switching element, the switching element being coupled to a selectable filter component, the code comprising: code to actuate the switching element so as to couple the selectable filtering component to at least one of the at least one filtering component in response to a loop- disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. According to an eleventh aspect of the present invention, there is provided a programmed computer for filtering a signal using a filter comprising at least one filtering component coupled to a switching element, the switching element being coupled to a selectable filter component, the programmed computer comprising memory having at least one region for storing computer executable program code, and a processor for executing the program code stored in memory, wherein the program code includes: code to actuate the switching element so as to couple the selectable filtering component to at least one of the at least one filter component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. According to a twelfth aspect of the present invention, there is provided a computer readable medium having computer executable software code stored thereon, the code being for filtering a signal using a filter comprising at least one filtering component coupled to a switching element, the switching element being coupled to a selectable filter component, and comprising: code to actuate the switching element so as to couple the selectable filtering component to at least one of the at least one filtering component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop- disconnect activity. Advantageously, the present invention provides a high performance filter that can be arranged in parallel with similar filters in a voice and data subscriber network, without impeding network performance by interacting with each other. Further advantage is obtained due to the simple design of the filter of the present invention. The circuit is generally constructed entirely from passive components, thereby minimising power consumption. The small size of the few components enables the filter to be of diminutive size and so may be integral with the telephone set rather than being provided as a separate external component. In another embodiment, the filter may be provided within a telephone socket whereby a telephone set is connected to the line. The filter circuit can be cheap to manufacture and may be simply installed by a customer, removing the need for a skilled telecommunications installer. Furthermore, the present invention may be utilised in a telephone system using a Calling Line Identification (CLI) feature. CLI information is transmitted using a V23 Frequency Shift Keying (FSK) modem capable of operating at 1.3/2.1 kHz. The filter of the present invention does not attenuate CLI signals transmitted to a telephone set, because filtering is carried out by the selectable filtering element when the telephone is in the on-hook condition. The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention. Brief Description of the Drawings At least one embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a subscriber's installation; Figure 2 illustrates bandwidth allocation of POTS and xDSL traffic over a line of Figure 1 ; Figures 3A and 3B show types of POTS signalling where transients occur; Figure 4 is a schematic diagram of the filter of Figure 1 constituting an embodiment of the invention; Figure 5 is a schematic diagram of an alternative filter constituting another embodiment of the invention, and Figure 6 shows a modification to the filter of Figure 4 or the alternative filter of Figure 5. Detailed Description of the Preferred Embodiment(s) Throughout the description, identical reference numerals are used to identify like parts. A subscriber telephone installation (Figure 1) comprises a subscriber loop 100 having a first leg 102 and a second leg 104. A first terminal 106 and a second terminal 108 of a first telephone set 110 are coupled to a first filter terminal 112 and a second filter terminal 114 of a first filter 116, respectively. A third filter terminal 118 and a fourth filter terminal 120 of the first filter 116 are coupled to the first leg 102 and the second leg 104 of the subscriber loop 100, respectively. A first terminal 122 and a second terminal 124 of a second telephone set 126 are coupled to a first filter terminal 128 and a second filter terminal 130 of a second filter 132, respectively. A third filter terminal 134 and a fourth filter terminal 136 of the second filter 132 are coupled to the first leg 102 and the second leg 104 of the subscriber loop 100, respectively. Although the first and second filters 116, 132 have been described as being separate from the first and second telephone sets 110, 126 respectively, the first and second filters 116, 132 can be built into the first and second telephone sets 110, 126 respectively. The first telephone set 110 and/or the second telephone set 126 can be in an on-hook condition or an off-hook condition. When in the off-hook condition, the first telephone set 110 and/or the second telephone set 126 signal by either or both of two methods: (i) by rapidly varying the impedance to create a pulse or a transient from a line voltage of the subscriber loop 100, known as loop- disconnect signalling, or (ii) by sending two selected voice band tones simultaneously along the subscriber loop 100, known as dual tone multi- frequency (DTMF) signalling. Loop-disconnect signalling is an example of a loop-disconnect activity. A first terminal 140 of a modem 138 is coupled to the first leg 102 of the subscriber loop 100, and a second terminal 142 of the modem 138 is coupled to the second leg 104 of the subscriber loop 100. The modem 138 enables a subscriber to be provided with a high bit rate facility, for example by using an ADSL technique or a VDSL technique. Referring to Figure 2, the subscriber loop 100 carries voice traffic in a voice frequency band 200, and digital data traffic in an ADSL frequency band 202 above the voice band 200. The ADSL technique supports communication in a forward direction and a reverse direction by allocating specific frequency bands to a forward channel and a reverse channel (not shown) in the ADSL frequency band 202. In the case of loop-disconnect signalling described above, the pulse or the transient that is generated can contain frequencies outside the voice frequency band 200. The frequencies outside the voice traffic frequency band 200 are capable of interfering with the ADSL traffic. In the subscriber loop 100, the ADSL technique represents just one technique that can be used for transmitting the digital data traffic at high bit rates contemporaneously with the voice traffic. Other methods for transmitting the digital data at high bit rates can be employed, for example, VDSL. Consequently, it should be appreciated that throughout this description, reference to the ADSL traffic or G.Lite traffic can equally be substituted by reference to any other type of DSL traffic (known by the generic term, xDSL). For the purposes of simplicity and clarity the following description will refer to the first filter 116 and the first telephone set 110, but the skilled person should appreciate that the description can equally apply to the second filter 132 and the second telephone set 126. Referring to Figure 4, a circuit of the first filter 116 comprises an inductor 402 having a first terminal 410 coupled to the third filter terminal 118 and a second terminal 412 coupled in series to a first terminal 414 of an impedance 404. A second terminal 416 of the impedance 404 is coupled to the first filter terminal 112. A shunt element 409, comprising a switching element, for example, a switch 406 and a capacitor 408, is coupled between the inductor 402 and the impedance 404 in parallel with the first telephone set 110. A first terminal 418 of the switch 406 is coupled to the second terminal 412 of the inductor 402 and the first terminal 414 of the impedance 404. A second terminal 420 of the switch 406 is coupled to a first terminal 422 of the capacitor 408. A second terminal 424 of the capacitor 408 is coupled to the second filter terminal 114 and the fourth filter terminal 120. In the present example, an inductance of the inductor 402 is 5mH, an inductance of the impedance 404 is 5mH and a capacitance of the capacitor 408 is 100nF. However, it should be understood that the above described component values are for exemplary purposes only and any suitable values can be used. The switch 406 can be any non-linear element (NLE) with suitable transfer characteristics. The switch 406 can be any suitable voltage dependent device such as a zener switch comprising a plurality of back-to-back zener diodes (of, for example, a value around 5V) or a semiconductor switch, such as a bi- directional silicon switch. Alternatively, the switch 406 can be a low voltage transient suppressor, a bridge rectifier or a single zener diode. The switch 406 enables the first filter 116 to switch between a first state for providing a first filtering action to prevent xDSL signals interfering with voice traffic, and a second state for providing a second filtering action to prevent loop disconnect activities from interfering with the xDSL signals. The performance characteristic of the switch 406 of the first filter 116, is that above a predetermined threshold voltage a current increases sharply. The skilled person will be aware that a typical speech signal has a peak-to-peak amplitude of about 3V, and a typical xDSL signal has a peak-to-peak amplitude of about 30V. However, the xDSL signal is attenuated by the inductor 402 and hence the peak-to-peak amplitude across the shunt element 409 is less than 30V. The skilled person will appreciate that the peak-to-peak amplitude of the voice traffic is present across the shunt element 409. Clearly, the switch 406 must operate with a threshold voltage greater than 3V to avoid activation of the switch 406 by voice traffic signals. Loop-disconnect signalling varies between about 10V and 50V. Hence, a suitable value for the predetermined threshold voltage for the switch 406 is around 3.5V. For low amplitude signals such as a speech signal, the switch 406 has a high resistance and therefore the shunt element 409 is inactive. For high amplitude signals, such as the transients described above, the switch 406 exhibits low resistance and thus the capacitor 408 is active and forms part of the first filter 116. However, the shunt element 409 has no effect without the impedance 404 to work against. Thus in the apparatus of Figure 1 , the first filter 116 and the second filter 132 ensure that the voice traffic and the ADSL traffic do not interfere with each other. In operation, the first filter 116 is capable of switching between a first state and a second state. The first state is required when the first telephone set 110 is in either the on-hook condition or the off-hook condition. When the first telephone set 110 is in the off-hook condition, the first filter 116 operates predominantly as a first order low-pass filter. The order of the first filter 116 is determined by the number of reactive components in the first filter 116. The inductor 402 is the only reactive component in the first filter 116, the first filter 116 having a high input impedance at high frequencies, thus introducing a high impedance to the xDSL signals traversing the subscriber loop 100. Hence, the xDSL signal on the subscriber loop 100 is not loaded by the input impedance of the first filter 116. Furthermore, the first filter 116 gives sufficient attenuation in a stop-band to supply a required rejection of the xDSL signals into the first telephone set 110 and prevent the xDSL signals generating noise in the first telephone set 110. When the first telephone set 110 is in the on-hook condition, there is little or no capacitance on a telephone side of the inductor 402, and hence no resonance between the impedance 408 and the inductor 402. Consequently, the input impedance does not impair performance of the first filter 116 when in use. The second state is required in either of two circumstances. A first circumstance is during a transition (Figure 3A) from the on-hook condition to the off-hook condition, when a line voltage of the subscriber loop 100 rapidly drops from, for example, around 50 Volts to around 10 Volts. The transition from the on-hook condition to the off-hook condition is an example of a loop-disconnect activity. A second circumstance is during loop-disconnect signalling (Figure 3B), when a succession of sharp-edged pulses or transients 300 are generated. During loop disconnect activities, the transients constitute a high frequency signal component. The transients can interfere with the xDSL traffic. It should be noted that during the two loop-disconnect activities described above, the voice circuit is not established and so a Quality of Service (QoS) of a voice service is not of importance at that time. When the loop-disconnect activities occur and the transients are generated, switch 406 switches the capacitor 408 into the circuit of the first filter 116 in response to at least one sudden voltage step. The first filter 116 is then in the second state and operates as a second order filter (the capacitor 408 being a second reactive component). In the second state, the line of the subscriber loop 100 is effectively shorted out. The capacitor 408 substantially reduces a rise time of the at least one voltage step produced by the loop-disconnect activities. The loop-disconnect activities mentioned above, lasts for a relatively brief period compared to a duration of a telephone call. After the relatively brief period, the first filter 116 returns to the first state and operates as the first order filter again. In the absence of a further high frequency signal, the switch 406 maintains a high resistance and thus the capacitor 408 remains inactive and does not form part of the first filter 116. In a first alternative embodiment, the first filter 116 is modified by including a first additional capacitor 502 (Figure 5). The first additional capacitor 502 has a first terminal 504 coupled between the first terminal 418 of the switch 406 and the first terminal 414 of the impedance 404. The first additional capacitor 502 has a second terminal 506 coupled between the second filter terminal 114 and the second terminal 424 of the capacitor 408. In a second alternative embodiment (Figure 6), a second additional capacitor 602 replaces the first additional capacitor 502 of the first alternative embodiment described above. A first terminal 604 of the second additional capacitor 602 is coupled to the first terminal 418 of the switch 406 and a second terminal 606 of the second additional capacitor 602 is coupled to the second terminal 420 of the switch 406, i.e. the second additional capacitor 602 is coupled in parallel with the switch 406. In operation, the first and second alternative embodiments operate in a similar manner to that previously described above in relation to the first embodiment (of Figure 4). However, the inclusion of the first or second additional capacitor 502, 602 improves filtering of the voice frequency signals when the first telephone set 110 is in the off-hook condition. The above described embodiments are examples of single-ended filter circuits. It should be appreciated that the above described filters can be balanced to reduce conversion of differential mode signals to common mode signals. As an example, the first filter 116 of Figure 4 comprises the inductor 402 and the impedance 404, constituting a first and second series impedance. The series impedances can be split so that the values of the first and second series impedances are halved, and a first additional impedance (not shown) is coupled between the second terminal 424 of the capacitor 408 and the second filter terminal 114 and a second additional impedance (not shown) is coupled between the second terminal 424 of the capacitor 408 and the fourth filter terminal 120. The first and second additional impedances are equal to the halved first and second series impedances, respectively. The above examples of the first filter 116 comprise the impedance component 404. It should be appreciated that the impedance component 404 can be a resistance, an inductance, a capacitance or a combination of two or more of these. Also, the first filter 116 may be located within the first telephone set 110 or alternatively at a socket connecting the subscriber loop 100 to the first telephone set 110. The above examples have been described in relation to a telephone system within the premises of a single subscriber where there is a need to reduce negative effects of transients. However, modifications to the above examples can be utilised in any situation where suppression of a transient is required. For example, the suppression of transients by a mains filter for a Power Line Transmission (PLT) system. In the PLT system, high frequency signals are transmitted on a same wire as a mains electricity supply. Although a simple static filter can be used to eliminate unwanted high frequency signals being presented to, for example, appliances using the mains electricity supply. However, the static filter cannot block a transient voltage on the mains electricity supply and so suppression of the transient voltage is required in order to protect a PLT compatible modem. The above requirements are satisfied by the modifications to at least one of the above-described examples. Alternative embodiments of the invention can be implemented as a computer program product for use with a computer system, the computer program product being, for example, a series of computer instructions stored on a tangible data recording medium, such as a diskette, CD-ROM, ROM, or fixed disk, or embodied in a computer data signal, the signal being transmitted over a tangible medium or a wireless medium, for example microwave or infrared. The series of computer instructions can constitute all or part of the functionality described above, and can also be stored in any memory device, volatile or nonvolatile, such as semiconductor, magnetic, optical or other memory device.

Claims: CLAIMS: 1. A filter for a device coupled to a communications network, the filter comprising at least one filtering component (116) and a switching element (406) coupled to a selectable filter component, the switching element being arranged to couple the selectable filtering component (408) to at least one of the at least one filtering component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. 2. A filter as claimed in Claim 1 , wherein the filtering components are arranged to reject selected input frequencies. 3. A filter as claimed in any one of claims 1-2, wherein the loop disconnect activity corresponds to the communications device being in one of an on-hook condition and an off-hook condition. 4. A filter as claimed in any one of claims 1-2, wherein the loop disconnect activity corresponds to a transition of the communications device from an on- hook condition to an off-hook condition. 5. A filter as claimed in any one of claims 1-4, wherein the filtering components comprise inductance (402) and impedance (404) elements connected in series with a subscriber loop. 6. A filter as claimed in any one of claims 1-5, wherein the switching element is coupled in parallel with the subscriber loop. 7. A filter as claimed in any one of claims 1-6, wherein the switching element is a voltage-dependent switching device. 8. A filter as claimed in any one of claims 1-7, wherein the selectable filtering component is a capacitor. 9. A filter as claimed in claim 1 , comprising: a first filter terminal (112), a second filter terminal (114), a third filter terminal (118), and a fourth filter terminal (120); the first filter terminal is coupled to an impedance (402), the first impedance being coupled to a inductance (404), and the inductance being coupled to the third filter terminal; the impedance and the inductance are coupled to the second filter terminal and the fourth filter terminal via a switching element (406) coupled to a shunt impedance (408), wherein the switching element is arranged to couple the shunt impedance to the impedance and the inductance in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. 10. A communications device comprising a filter according to any one of claims 1 to 9. 11. A network access port apparatus for coupling a device to a communications network, the network access port apparatus comprising a filter according to any one of claims 1 to 9. 12. A method of filtering signals for an apparatus coupled to a communications network, the method comprising the steps of: increasing a rise time of a first signal in a first frequency band in response to the first signal corresponding to a loop-disconnect activity. 13. A method as claimed in Claim 12, further comprising the steps of: attenuating a second signal in a second frequency band above the first frequency band in response to the apparatus being in one of an on-hook condition and an off-hook condition. 14. A method as claimed in any one of claims 12-13, wherein the step of increasing the rise time of the first signal in response to the first signal corresponding to the loop-disconnect activity further comprises the step of: switching a shunt impedance across a pair of terminals providing the first signal. 15. A method of filtering signals according to any one of claims 12-14 using a filter comprising at least one filtering component (116) coupled to a switching element (406), the switching element being coupled to a selectable filter component (408), the method comprising the step of: actuating the switching element so as to couple the selectable filtering component to at least one of the at least one filtering component in response to a loop-disconnect activity, thereby slowing at least one transient generated during the loop-disconnect activity. 16. Computer executable software code stored on a computer readable medium, the code being for filtering signals for an apparatus coupled to a communications network, the code comprising code to perform the method step of any one of claims 12 to 15. 17. A programmed computer for filtering signals for an apparatus coupled to a communications network, comprising memory having at least one region for storing computer executable program code, and a processor for executing the program code stored in memory, wherein the program code includes: code to perform the method step of any one of claims 12 to 15.

Inventors: HUMPHREY LESLIE, WILLIAMSON ROGER J, HUMPHREY, LESLIE, WILLIAMSON, ROGER, J.
Applicants: HUMPHREY LESLIE, NORTEL NETWORKS LTD, WILLIAMSON ROGER J, HUMPHREY, LESLIE, NORTEL NETWORKS LIMITED, WILLIAMSON, ROGER, J.
Application year: 2001
Main classification: H04B3
IPC codes: H04L25, H04M1, H04M11