Method of transmitting clock signals, method of synchronizing clock generators or network elements, as well as a network element and a clock generator

A method of synchronizing a network element of a synchronous digital communications network with a transmitted clock signal, said method comprises:

• receiving a message signal at the network element from a neighboring network element,
• deriving a clock signal from the message signal, and analyzing a synchronization status message contained in the received message signal to determine the clock quality of the message signal,
• applying to the derived clock signal a phase modulation characteristic of the clock quality of the message signal, and transferring the phase-modulated clock signal through a clock output to a clock generator,
• receiving at the clock generator a respective phase-modulated clock signal from each of a number of network elements,
• evaluating the phase modulation to determine the respective clock quality therefrom, and selecting one of the received clock signals as a reference clock signal based on the clock quality,
• sending said reference clock signal to the number of network elements or to further network elements for synchronization,
• receiving at the network element said reference clock signal from the clock generator via a clock input of the network element, and
• synchronizing the network element with the reference clock signal.

A network element for a synchronous digital communications network, comprising:

• a clock supply circuit for supplying the network element with an internal clock signal,
• a clock interface for receiving an external clock signal from a clock source, said external clock signal having a phase modulation applied thereto prior to transmission to the network element, said phase modulation characteristic of the quality of the external clock signal, wherein said phase modulation lies within the maximum permissible phase-variation limits for the synchronous digital communications network, and
• synchronizing circuitry synchronizing the clock supply circuit to the received external clock signal, said network element further comprising:
• a phase modulation detector detecting said phase modulation applied to the external clock signal, said phase modulation being indicative of the quality of the external clock signal, wherein the synchronizing circuitry synchronizes the clock supply circuit to the external clock signal based on the quality of the external clock signal.

A network element for a synchronous digital communications network, comprising:

• a clock supply circuit for supplying the network element with an internal clock signal,
• a clock interface for receiving an external clock signal from a clock source, said external clock signal having a phase modulation applied thereto prior to transmission to the network element, said phase modulation characteristic of the quality of the external clock signal, wherein said phase modulation lies within the maximum permissible phase-variation limits for the synchronous digital communications network, and
• means for synchronizing the clock supply circuit to the received external clock signal, said network element further comprising:
• means for detecting said phase modulation applied to the external clock signal, said phase modulation being indicative of the quality of the external clock signal, wherein the means for synchronizing synchronizes the clock supply circuit to the external clock signal based on the quality of the external clock signal.

A network element for a synchronous digital communications network, comprising:

• a clock supply circuit for supplying the network element with an internal clock signal,
• a clock interface for receiving an external clock signal from a clock source, said external clock signal having a phase modulation applied thereto prior to transmission to the network element, said phase modulation characteristic of the quality of the external clock signal, wherein said phase modulation exhibits one of two or more periodic phase modulation patterns depending on the quality of the external clock signal, and
• synchronizing circuitry synchronizing the clock supply circuit to the received external clock signal, said network element further comprising:
• a phase modulation detector detecting said phase modulation applied to the external clock signal, said phase modulation being indicative of the quality of the external clock signal, wherein the synchronizing circuitry synchronizes the clock supply circuit to the external clock signal based on the quality of the external clock signal.

A network element for a synchronous digital communications network, comprising:

• a clock supply circuit for supplying the network element with an internal clock signal,
• a clock interface for receiving an external clock signal from a clock source, said external clock signal having a phase modulation applied thereto prior to transmission to the network element, said phase modulation characteristic of the quality of the external clock signal, wherein said phase modulation lies within the maximum permissible phase-variation limits for the synchronous digital communications network, and
• means for synchronizing the clock supply circuit to the received external clock signal, said network element further comprising:
• means for detecting said phase modulation applied to the external clock signal, said phase modulation being indicative of the quality of the external clock signal, wherein the means for synchronizing synchronizes the clock supply circuit to the external clock signal based on the quality of the external clock signal.