This invention relates to a signal splitter device for separating telephony traffic from further traffic occupying a higher frequency band, such as digital subscriber line traffic. It also relates to a filter for use in filtering telephony traffic.
Telecommunications operators"" fixed networks were originally constructed entirely from metallic transmission media (predominantly copper) carrying voice band signals in the region 300 Hz to 3.4 kHz, a service 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 customers is still dominated by twisted copper pairs. Many telecommunications operators wish to use their existing POTS copper access network to deliver broadband services such as data delivery (e.g. Internet), teleworking and video on demand. The current technologies for delivering data over the access network are (a) voice band modems which code data into signals lying within the voice band, supporting data rates of up to 50 kb/s, and (b) ISDN.
A number of alternate technologies have been developed to allow high bit rate services to be delivered over the copper access network. One such technology is asymmetric digital subscriber loop (ADSL). POTS traffic is carried over the conventional 0 to 4 kHz frequency band and the ADSL traffic occupies the frequency band above POTS in the region 24 kHz to 1.1 MHz. Both POTS traffic and ADSL traffic can be simultaneously carried over the same wire pair. An overview of this technology is described in an article xe2x80x9cBroadband Multimedia Delivery over Copperxe2x80x9d by G Young, K T Foster and J W Cook in BT Technology Journal, volume 13, number 4, October 1995. A number of different digital modulation techniques have been proposed for carrying the digital data, such as discrete multitone modulation (DMT) where data is carried by a large number of frequency carriers, Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude/Phase Modulation (CAP). 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.
At the telecommunications operator""s exchange, and at the subscriber end, the POTS traffic and xDSL traffic needs to be separated. This is achieved by a splitter device comprising a low-pass filter which delivers POTS signals to a telephone or POTS line card, and a high pass filter which delivers xDSL signals to an xDSL modem or line card.
An article xe2x80x9cADSL and VADSL Splitter Design and Telephony Performancexe2x80x9d by John Cook and Phil Sheppard, in IEEE Journal on Selected Areas in Communications, volume 13, number 9, December 1995, describes two approaches to designing splitters. The article describes the use of a passive low-pass filter and concludes that this can have an adverse affect on telephony transmission performance. The article also proposes an active, powered, filter which uses generalised immittance converters (GIC) at both the input and output of the filter. This, however, requires powering which must be obtained separately and can lead to a filter which is costly and bulky.
U.S. Pat. No. 5,627,501 xe2x80x9cSignal Coupler with Automatic Common Line Attenuation Compensationxe2x80x9d describes a signal coupler for use with a line carrying POTS and ADSL traffic, where a variable impedance circuit is placed in series with a low-pass filter and the POTS terminal, the impedance circuit being switched on or off to minimise the detrimental effect on filter performance which strong signals can have.
There is a need to provide adequate low-pass filtering of the POTS traffic to prevent corruption of xDSL traffic operating in the frequency bands above the voice frequencies.
The present invention seeks to provide an improved splitter device.
According to a first aspect of the present invention, there is provided a splitter for separating telephony traffic from further traffic occupying a higher frequency band, the telephony traffic comprising speech and signalling traffic, the splitter device comprising:
an input for transmitting and receiving the telephony and further traffic;
a high-pass filter coupled to the input for passing the further traffic; and,
a low-pass filter coupled to the input for passing the telephony traffic, the low-pass filter being operable to vary its filtering response between:
a first low-pass response for use during telephony speech traffic; and,
a second, more restrictive, low-pass response for use during at least part of the time that telephony signalling traffic is present.
The second, more restrictive low-pass response can be used for the whole of the time occupied by the telephony signalling traffic, at the beginning and end of signalling traffic periods, or for some other duration.
Brief Description of the Drawings
For a better understanding of the invention, and to show by way of example how it may be carried into effect, embodiments will now be described with reference to the accompanying drawings, in which:
FIG. 1 shows a network for delivering POTS and broadband services to a subscriber;
FIG. 2 shows how POTS and ADSL traffic is carried over the line in the network of FIG. 1;
FIG. 3 shows a splitter for use in the network of FIG. 1;
FIGS. 4A, 4B and 4C show types of POTS signalling where transients occur;
FIG. 5 shows filter responses for a filter in accordance with an embodiment of the invention;
FIG. 6 shows one way of implementing first and second filter responses;
FIGS. 7A and 7B show two further ways of implementing different filter responses;
FIG. 8 shows an example of a filter using a non-linear element;
FIGS. 9A and 9B show equivalent circuits for the filter of FIG. 8 at different signal amplitudes;
FIG. 10 shows a response of the non-linear element;
FIG. 11 shows an example of a higher-order filter;
FIG. 12 shows a first filter using a current-sensing element;
FIG. 13 shows another filter using a current-sensing element.