Modem operating with central office clock

Details Modem operating with central office clock Modem operating with central office clock

1998-11-02

2001-05-22

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transceivers

Modems

C375S220000

Reexamination Certificate

active

06236677

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a modem system performing full duplex PCM-AM transmission and particularly to a system that uses clock feedback from the central station to synchronize data transmission in the sending mode.
INFORMATION DISCLOSURE AND PRIOR ART
The motivation for improved modem technology originates from the requirement for greater and greater speed in the transmission of data and ability to receive data. The present state of the art system operating with modems utilizes quadrature phase modulation at a bit rate of 33.6 kbs.
FIG. 1
is a schematic diagram of a system that is the current standard in the market place for transmission between two hosts wherein each host is a computer. There is shown a first host
10
having a transmitter section
12
and a receiver section
14
communicating with the receiver section
26
and transmitter section
25
of a second host
28
through a first central station
22
A coupled to a second central station
22
B by digital lines (T-
1
)
23
. Digital signals from the user-transmitter
12
are converted to a QAM signal (quadrature amplitude modulation) by D/A
16
A (digital-analog converter) and coupled by hybrid transformer
18
to hybrid transformer
20
A at the first central Office
22
A that is local to host
10
. An A/D converter
24
A converts the analog signal to digital signal and transmits the digital signal to the D/A converter
21
B of the second central office
22
B that is local to host
28
. Transmission of signals from host
28
back to host
10
occurs in a complimentary manner from transmitting section
25
of the second host through D/A converter
16
B back through the central stations
22
A and
22
B to the receiving section
14
of first host
10
.
The rate of data transmission is limited by the bandwidth of the analog PSTN connection (telephone line) between the hosts and the respective central stations and the use of full duplex QAM (quadrature-amplitude-modulation).
In another arrangement of the prior art, the second host is an ISP (internet server provider) and the ISP is connected to its local central station by a digital (T-
1
) line. The first host is a computer connected to its local central station by a PSTN (analog) line. Using full duplex QAM transmission, the data rate is limited by the PSTN line to 33.5 kbs.
In a white paper published over the Internet by the Rockwell Corporation, there was disclosed a system wherein a host computer received data from an ISP (digital) host using PCM-AM transmission and synchronizing the receiver clock with the clock of the central station. . Data was transmitted from the computer back to the ISP at 33.5 kbs using QAM.
SUMMARY
An object of this invention is to provide a system that exchanges digital data with a remote host at an overall rate that is faster than can be achieved with present state of the art systems including the Rockwell arrangement. This object includes the object of transmitting and receiving data using a full duplex mode at a rate of 56 kbs.
The rate of 56 kilobits/sec represents the maximum bit rate that can be transmitted through a PSTN line using PCM-AM formal. The PCM-AM signal is an analog signal having 256 levels wherein each level represents the value of one byte (eight bits).
This invention is directed toward a first host and a second host communicating through a central office. At least one of the hosts is connected to the central station through a PSTN line. Communication takes place at 56,000 kbs utilizing full duplex PCM-AM modulation. Simultaneous transmission and reception takes place at both hosts utilizing the same bandwidth. This is achieved according to the invention by:
using negative feed back at the host receiving section to cancel reflection of the transmitted signal.
synchronizing the transmit and receive clocks to one another of each host and to the transmit and receive clocks of the other host.
When one of the hosts is connected to the station through a T-
1
(digital) connection, then, according to the invention, the central station is provided with a D/A converter and A/D converter, each with a clock for timing sampling and these clocks are synchronized with the clocks of the host connected to the central station through a PSTN line.
The major challenge is for the receiver to determine which received sample value corresponds to the sample time clocked by the central office station. To do this, the host-receiver section is provided with a receiver clock that is synchronized with a station office clock-and a transmitter clock that is slave to the user receive clock. Synchronizing is accomplished in two steps. In the first step the frequency of the receiver clock is adjusted to equal the frequency of the central office clock In the second step, the phase of user receiver clock pulse is adjusted to zero with respect to the clock pulse received from the central station.


REFERENCES:
patent: 5463616 (1995-10-01), Kruse et al.
patent: 5778022 (1998-07-01), Walley
patent: 5802153 (1998-09-01), Sridhar et al.
patent: 5892792 (1999-04-01), Walley
patent: 6094452 (2000-07-01), Gelblum et al.
patent: 6104730 (2000-08-01), Marks

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