Digital telephone link margin verification

Details Digital telephone link margin verification Digital telephone link margin verification

1999-04-16

2001-11-06

Kuntz, Curtis (Department: 2643)

Telephonic communications

Diagnostic testing, malfunction indication, or electrical...

Of trunk or long line

C379S001010, C379S009000, C379S010010, C379S015010, C379S022000, C379S027010

Reexamination Certificate

active

06314162

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to digital telephone systems and, more particularly, to the measurement of link margin between a digital PBX line circuit and a telephone set.
BACKGROUND OF THE INVENTION
The transmission medium extending between a line circuit in a switching facility such as a PBX, key service unit or central office and a telephone set must provide a communications link that provides a minimum level of attenuation and impairment in order to assure reliable two-way communications. Among other things, the transmission equipment and protocol determine the minimum signal level threshold required to assure reliable communication over links of varying length and impairments.
Typically, when a digital telephone set is installed on a line which connects it to switching equipment (central office, PBX, key system, etc.) the installer is not provided with a means to determine how well the communications path between switch and telephone set are functioning. The telephone set either works flawlessly or doesn't work at all. Only rarely will the telephone set exhibit marginal performance. These characteristics are due to the nature of digital communications and are well understood in the industry. As long as the data are received above some minimum signal to noise threshold the receiving device works flawlessly. In linear systems with Gaussian noise, the difference in signal to noise ratio between nonoperation and flawless operation of a digital receiving device may be as small as ½ dB. This invention provides a means for an installer (as well as a maintenance center or even the user) to measure and determine a figure of merit regarding the performance of the link between telephone set and switching equipment at installation time, or any time thereafter. If the figure of merit is low, the installer can then investigate the reason for that low figure of merit or turn the matter over to another organization that may have greater expertise and/or more extensive test and diagnostic equipment to correct the situation.
SUMMARY OF THE INVENTION
In accordance with the principles of our invention, in one illustrative embodiment thereof, the transmit path of a PBX line circuit serving a digital telephone set is equipped with a series of switchable attenuators that can selectively be inserted at the output of the digital transmitter under the control of a processor to temporarily reduce the transmitted signal to determine if that reduced transmit level remains adequate to ensure reliable communications with the telephone set. The processor sends a test message over the line to the telephone set. If the test signal is received correctly, the telephone set returns an acknowledgment message with normal (maximum) signal level, within a predetermined interval. The PBX line circuit processor responds to the telephone set's answering message by switching in an attenuator to lower the transmit level of the next test message to be transmitted over the line from the PBX to the telephone set. If the telephone set detects the test message it again responds with an acknowledgment message. The PBX line circuit processor responds to this second acknowledgment message by inserting additional attenuation in the transmit path that will further lower the level of the next test message to be transmitted over the link to the telephone set. This process is repeated until the test message signal level is lowered so much that it is not correctly decoded by the telephone set. When the telephone set fails to return an acknowledgment signal within the required response time, the PBX line circuit processor removes all the inserted attenuation, thus returning the system to normal operation. The maximum amount of attenuation that could be inserted at the PBX line circuit's output and still have the telephone set respond to test messages is the figure of merit for the link between the PBX and the telephone set. In this manner the figure of merit can be determine and reported to the installer. If it is too low, the installer can then take corrective action. The same process can be used to determine the figure of merit for the telephone set transmitting to the PBX.


REFERENCES:
patent: 4625077 (1986-11-01), Biffle et al.
patent: 5063563 (1991-11-01), Ikeda et al.
patent: 5473666 (1995-12-01), Szczebak, Jr. et al.
patent: 5768684 (1998-06-01), Grubb et al.
patent: 6061427 (2000-05-01), Ryoo
patent: 6226356 (2001-05-01), Brown

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