Telephonic communications – Subscriber line or transmission line interface
Patent
1996-08-22
1998-10-06
Matar, Ahmad F.
Telephonic communications
Subscriber line or transmission line interface
379 2, 379 29, 379 30, 379400, H04M 124, H04M 322
Patent
active
058189262
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a switching device, particularly a maintenance termination unit (MTU) for use in a telephone network.
BACKGROUND OF THE INVENTION
Recent de-regulation of the telecommunications industry has resulted in greater freedom for subscribers to install their own equipment, and as a result telephone companies have insisted that their responsibility for maintenance of the network end at the subscribers' premises. It has therefore become necessary for clear demarcation points to be established and for the telephone companies to be able to determine, preferably remotely, whether any fault lies on the telephone company's or on the subscriber's side of the demarcation point. In order to do this, a so-called maintenance termination unit is installed, generally in or on a subscriber's premises at that point where the telephone cable enters. The intention is that the telephone company can send a special signal along the line to discover not only where any fault lies but also what kind of fault exists, for example whether the fault is an open or short circuit. An MTU is preferably able to disconnect one or both of the lines to the subscriber and also to connect those lines together to allow a loop-back test to be carried out. It is clearly necessary that such disconnection and testing be carried out only when desired and not, for example, in response to a ringing signal or other normal line voltage. Some form of filtering is therefore desirable. Also, it is desirable that the system be fail-safe such that failure of the MTU does not prevent normal operation of the telephone or other subscriber equipment. The present invention therefore provides a particularly advantageous form of filtering, and in a preferred embodiment, employs depletion mode field effect transistors (FETs) as series switches in the lines which will remain conducting in the presence of a fault in their control circuits.
Various designs of MTU have been proposed, of which the following may be mentioned.
GB 2149274 (Teradyne) discloses a pulse-operated remote isolation device comprising normally conducting field effect transistors together with detection circuitry that detects a 130 volt command pulse from the telephone company that activates a control circuit which, by means of a transformer circuit, changes the state of the FETs from a low to a high impedance. In this way, the subscriber's equipment is disconnected. After a fixed period of 16 to 18 seconds, which is determined by a pulse detection timer, the control circuit allows the FETs to return to their conductive state. The circuit required is rather complicated and involves a special counter chip producing an AC output which is fed to a transformer isolating circuit, an output of which is then rectified to provide a control voltage to a chip containing the FETs.
U.S. Pat. No. 4323799 (King) discloses an impulse activated switch having a controlled time-delayed self-restoration from its complimentary state to its initial state. It comprises a photo-diode array connected to gate and source electrodes of a FET to cause the FET to switch. A capacitor that forms part of an RC timing network is also charged by the diode array.
U.S. 4710949 (Ahuja) and GB 2097632 (Phillips) may also be mentioned.
Many of the circuits of the prior art are complicated and require large numbers of electronic components and are consequently likely to be expensive, large and potentially unreliable.
We have now designed an improved MTU in which a timing circuit for delayed reconnection of a subscriber can act as a back-up filter to a primary filter that prevents a normal ringing signal on the line from activating the MTU. Whilst this back-up feature might not be required under perfect conditions, it can allow for greater tolerance in component characteristics, it can allow for initially ideal components to deteriorate with age or to operate outside expected temperatures and it can reject spurious line voltages.
SUMMARY OF THE INVENTION
Thus, the prevent invention provides a
REFERENCES:
patent: 4558182 (1985-12-01), Perry et al.
patent: 4710949 (1987-12-01), Ahuja
patent: 4796290 (1989-01-01), Perry
patent: 5604785 (1997-02-01), Pryor et al.
patent: 5651048 (1997-07-01), Leeuw
patent: 5652575 (1997-07-01), Pryor et al.
Burkard Herbert G.
Matar Ahmad F.
Raychem Corporation
Richardson Timothy H. P.
Saint-Surin Jacquies
LandOfFree
Switching device for telecommunications channel does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Switching device for telecommunications channel, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switching device for telecommunications channel will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-87637