1. Field of the Invention
The present invention generally relates to telecommunications circuitry, and more particularly to a circuit that controls the application of power to a line driver circuit.
2. Discussion of the Related Art
In recent years telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications includes not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately DC to approximately 3.4 kilohertz.
New, high-speed data communications provided over digital subscriber lines, such as Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), etc. (more broadly denoted as XDSL) provide for high speed data transmissions, as is commonly used in communicating over the Internet. As is known, the bandwidth for XDSL transmissions is generally defined by a lower cutoff frequency of approximately 30 kilohertz, and a higher cutoff frequency which varies depending upon the particular technology. Since the POTS and XDSL signals are defined by isolated frequency bands, both signals may be transmitted over the same two-wire loop.
A "line-card," containing line interface circuitry, is provided at the central office. The line interface circuitry provides the interconnections among xDSL circuitry, POTS or PSTN voice circuitry, off-hook (or tip/ring) detection circuitry, ring generator circuitry, and the local loop. The line interface circuitry also includes appropriate electronic filtering circuitry that operates to minimize any noise transmission across the local loop. As is known, the line interface circuitry includes a POTS filter that is interposed between the various POTS circuits and the xDSL circuit. This filter protects the POTS circuitry from the high frequency signals of the XDSL transmission circuitry. It also serves to minimize noise transmissions across the local loop from the central office to the customer premises.
As is known, a central office typically includes numerous line cards to service a number of local loops. Each line card includes a line driver circuit that drives signals across the local loop. It should be appreciated that, for any given local loop, the line driver spends a significant percentage of time in standby operation (ie., not transmitting). During this "standby" time, however, the line driver circuit continues to consume power. Although the power consumed by a single line driver circuit is relatively small (often on the order of approximately 30 mAmps), this power consumption, multiplied by the large number of line drivers in a central office location, realizes a relatively large overall power consumption.
Accordingly, there is a desire to provide line driver circuitry that dissipates less power than line driver circuitry known in the prior art.