Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1997-09-25
2001-05-22
Luther, William (Department: 2664)
Pulse or digital communications
Transceivers
Modems
C375S356000
Reexamination Certificate
active
06236675
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data communications, and more particularly, to a system and method for maintaining synchronization between a central office modem and a plurality of remote, or user, modems in a multipoint network topology.
BACKGROUND OF THE INVENTION
The field of data communications typically uses a device, such as a modem, to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables devices to communicate rapidly large amounts of data. Modems communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation and differentially encoding the information to reduce errors and improve throughput. The constellation can include both analog and digital information or often merely digital information.
In the above mentioned communications system, typically both digital data and an analog signal are transmitted. The data signal to be transmitted is represented by a sequence of data symbols, where each data symbol is associated with a particular N-dimensional signal point value taken from a signal space. Similarly, the analog signal, which is represented by a voice signal, is processed so that it is mapped into the N-dimensional signal space to provide a voice signal point. This voice signal point defines the magnitude and angle of a voice signal vector about the origin of the signal space. The data symbol and the voice vector are then added together to select a resultant N-dimensional signal point. These N-dimensional signal points are grouped into signal space constellations and then transmitted to a far-end modem.
Upon reception of the transmitted N-dimensional signal point, the receiver of the far-end modem detects the embedded data symbol and subtracts the data symbol from the received N-dimensional signal point to yield the voice signal vector. This voice signal vector is then used to recreate the voice signal.
In the above mentioned communications environment, a control modem is located at a telephone company central office location. Connected to the control modem via a conventional copper wire pair are a plurality of remote modems. The remote modems reside at a common location, such as a residence or a business. The communication technique between the control modem and the remote modems is generally half duplex. In order to establish a communications connection between the central office modem and any of the remote modems, a lengthy preamble, which allows the modems to synchronize, at the start of each message is required. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two modems at a time. In a multipoint environment, greater circuit efficiency is possible because of the ability to connect multiple modems to the same communication channel.
In a 4 wire communications environment, the control modem can transmit continuously so that outbound preambles (from central office to remote location) are unnecessary, thus assuring remote modem synchronization. In a 2 wire multipoint environment, as contemplated by the present invention, when a remote modem transmits, it disrupts the outbound control signal from the control modem making it impossible for all remote modems connected to the communication line to maintain synchronization with the control modem. It is desirable to allow all remote modems to continuously maintain synchronization with the control modem in order to significantly reduce the training preamble necessary to establish a communication path between a remote modem and the control modem.
SUMMARY OF THE INVENTION
The present invention provides an improvement to a half duplex multipoint communication environment by enabling a control modem to continuously transmit a pilot tone, which can be simultaneously received by all remote modems on the communication line. By employing the present invention, all remote modems will maintain a continuous lock on the pilot tone from the control modem, thereby maintaining timing synchronization. By maintaining timing synchronization, special start up preamble signals are unnecessary during each poll sequence, thus significantly reducing the time required for communicating data.
The present invention provides a system for providing a pilot tone in a multipoint communication environment in a modem. The system includes a first modem, or control modem, having a pilot tone generator. The first modem also contains a transmitter for transmitting the pilot tone along a communications channel. The system further includes an additional modem, or remote modem, having a receiver for receiving the pilot tone and a decoder for decoding the pilot tone. The pilot tone remains active, and is received by the additional modem during a period when the additional modem is transmitting to the first modem. A plurality of additional modems can be connected to the communications line, all remaining in synchronization with the first modem.
The pilot tone is generated at a frequency that is outside of the normal communication bandwidth of the first modem and any additional modem, thereby allowing the additional modem to continuously receive the pilot tone, even while transmitting to the first modem.
The remote modems also include a voltage controlled crystal oscillator (VCXO) which allows the remote modems to transmit to the control modem using a time base that is locked to the frequency of the pilot tone, thereby eliminating the requirement that the control modem synchronize to each additional modem for each communication session. In this way, all remote modems can communicate to the control modem using the pilot tone time base, thereby eliminating the need for the control modem to lock to each remote modem. A pilot tone frequency that is exactly ½ the sampling rate of the modem is useful because it is outside the data bandwidth and allows all modems to easily reproduce exactly the same sample rate as the control modem.
The present invention also provides a method for communication in a modem, including the step of generating a pilot tone in the control modem. The control modem then transmits the pilot tone to all remote modems on the communications line. Each remote modem also includes a receiver for receiving the pilot tone and a decoder for decoding the pilot tone. In order to allow the control modem receiver to remain in synchronization with all remote modems, each remote modem contains logic designed to operate on the pilot tone in order to generate a low jitter clock signal, which is used to derive a sample rate signal used by the remote modem to transmit signals to the control modem. These transmit signals are thereby already synchronized to the control modem by the transmitted signals' phase alignment with the pilot tone, thus allowing the control modem receiver to maintain synchronization with the remote modem.
The invention has numerous advantages, a few of which are delineated hereafter, as merely examples.
An advantage of the present invention is that it minimizes connection time between a control modem and a remote modem by eliminating the need for a lengthy start up preamble.
Another advantage of the present invention is that it allows a control modem to continuously maintain synchronization with a plurality of remote modems.
Another advantage of the invention is that by employing a voltage controlled crystal oscillator (VCXO) in order to develop a low jitter clock signal in a receiver, it allows the control modem to maintain constant synchronization with a plurality of remote modems.
Another advantage of the present invention is that it is simple in design, reliable in operation, and its design lends itself to economical mass production in modems
Bedingfield John
Betts William L.
Bingel Thomas J.
Chapman Joseph Q.
Coston Charles G.
Luther William
Paradyne Corporation
Thomas Kayden Horstemeyer & Risley
LandOfFree
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