Browse Prior Art Database

External Clock Failure Detection in Communication Networks

IP.com Disclosure Number: IPCOM000042118D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Bigo, F: AUTHOR [+3]

Abstract

In present communication networks wherein DTE (data terminal equipment) units are being interconnected through modems the operations have to be perfectly synchronized. For that purpose, the modem is provided with a phase locked oscillator (PLO) belonging to a so-called internal clock. On the other hand each DTE is also provided with a clock (so-called external clock) operating at the corresponding DTE rate. In most systems, one of the external clocks is used to control the internal clock PLO. The remaining DTE clock signals are derived from the PLO-controlled output and provided to the remaining DTEs. In most known systems there is no checking against the effective presence of the external clock signal at the PLO input.

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External Clock Failure Detection in Communication Networks

In present communication networks wherein DTE (data terminal equipment) units are being interconnected through modems the operations have to be perfectly synchronized. For that purpose, the modem is provided with a phase locked oscillator (PLO) belonging to a so-called internal clock. On the other hand each DTE is also provided with a clock (so-called external clock) operating at the corresponding DTE rate. In most systems, one of the external clocks is used to control the internal clock PLO. The remaining DTE clock signals are derived from the PLO-controlled output and provided to the remaining DTEs. In most known systems there is no checking against the effective presence of the external clock signal at the PLO input. Also, the PLO is so designed as to deliver a free-running clock (unlocked PLO) signal any time the external clock signal is absent from the PLO input. The drawbacks of such a system are: no immunity against external clock glitches and lack of information to report this critical failure of the attached DTE. This situation could be avoided by providing the modem with an external clock detecting system. The system should include, first, a latch L which is unconditionally set by an instruction withing the modem operating microcode. Then the modem microcode starts a software timer of one or several transmit baud times. If during one transmit baud time no external clock transition has been detec...