Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – With blocking of normal usage
Patent
1988-02-01
1989-02-21
Schreyer, Stafford D.
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
With blocking of normal usage
379 97, 379 22, 371 22, H04M 308, H04M 324
Patent
active
048072745
ABSTRACT:
A telephone line quality testing system detects line echoes in the telephone connection between two subsystems in a message transmission system. The test for line echoes is performed while the two subsystems exchange identification data by transmitting DTMF signals in a predefined sequence. Each subsystem transmits a marker at the beginning of each DTMF transmission. This marker is constructed so that it can be distinguished, with absolute certainty, from any possible echo of the previous DTMF transmission by the other subsystem. In general, the marker includes at least one DTMF signal selected from the set of DTMF signals not included in the last X DTMF signals sent by the other subsystem. Each subsystem also generates an "Expected marker" that should precede the next DTMF transmission by the other subsystem. When receiving a DTMF transmission, each subsystem compares the initial DTMF signals received with the expected marker. If a match is found, the first DTMF signal which matches the expected marker is denoted as the beginning of the received packet and the DTMF signals preceding that signal, if any, are discarded as line echoes. If none of the initial (e.g., fourteen) signals received match the expected marker the call is terminated because a line quality problem has been detected.
REFERENCES:
patent: 3508144 (1970-04-01), Mayer
Octel Communications Corporation
Schreyer Stafford D.
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