Centralized clock synchronization for time division multiplexed traffic transported over Ethernet networks

A method of distributing a master clock source to a plurality of circuit emulation service (CES) modules over an asynchronous Ethernet transport network, said method comprising the steps of:

• receiving clock synchronization packets comprising modified Ethernet packets sent from a clock distribution unit and extracting network timestamps and a clock quality indication therefrom, wherein said network timestamps and clock quality indication are derived from said master clock source and distributed via modified Ethernet packets over said network to said plurality of CES modules;
• generating local timestamps utilizing a reconstructed clock generated by a local clock source;
• coarse filtering differences between said local timestamps and said network timestamps to generate a clock control signal in a digital time locked loop (DTLL);
• fine filtering differences between said local timestamps and said network timestamps to generate a fine tuning clock control signal once said DTLL achieves lock using said coarse filtering;
• generating said reconstructed clock in accordance with said fine tuning clock control signal;
• wherein a DTLL implements a closed loop tracking system whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said DTLL;
• and
• wherein said liner tuning clock control signal is generated based on only the difference between two timestamps comprising said network timestamp sent when said clock synchronization packet is transmitted and said local timestamp generated when said clock synchronization packet arrives at a receiver.

A method of distributing a clock from a clock distribution unit to a plurality of circuit emulation service (CES) modules over an asynchronous Ethernet transport based network, said method comprising the steps of:

• generating first network timestamps from a primary master clock source at said clock distribution unit;
• establishing first connections for carrying clock synchronization traffic from said clock distribution unit to said plurality of CES modules;
• transmitting first clock synchronization packets comprising modified Ethernet packets incorporating said first network timestamp and a first clock quality indication over said first connections to said plurality of CES modules to be synchronized, said clock quality indication adapted to convey a measure of the quality of said primary master clock source;
• generating second network timestamps from a backup master clock source;
• establishing second connections for carrying clock synchronization traffic from said clock distribution unit to said plurality of CES modules;
• transmitting second clock synchronization packets comprising modified Ethernet packets incorporating said second network timestamps and a second clock quality indication over said second connections to said plurality of CES modules to be synchronized, said second clock quality indication adapted to convey a measure of the quality of said backup master clock source;
• wherein said first clock quality indication and said second clock quality indication are used to select between first and second network timestamps at each CES module;
• wherein a closed loop tracking system is implemented at said plurality of CES modules whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said closed loop tracing system;
• and
• wherein a clock control signal is generated at said plurality of CES modules based on only the difference between two timestamps comprising said network timestamps sent when clock synchronization packets are sent and local timestamps generated when said clock synchronization packets arrive at said CES modules.

A system for synchronizing a plurality of circuit emulation service (CES) modules over an asynchronous Ethernet transport network, comprising:

• a primary clock distribution unit comprising:
• first clock distribution transmitter operative to generate first network timestamps derived from a primary master clock source and to insert said first network timestamps and a first clock quality indication into clock synchronization packets comprising first modified Ethernet packets;
• a first distributor operative to transmit first modified Ethernet packets containing said first network timestamp and said first clock quality indication to said plurality of circuit emulation service (CES) modules over said network;
• a backup clock distribution unit comprising:
• second clock distribution transmitter operative to generate second network timestamps derived from a backup master clock source and to insert said network timestamps and a second clock quality indication into clock synchronization packets comprising second modified Ethernet packets;
• a second distributer operative to transmit second modified Ethernet packets containing said second network timestamp and second clock quality indication to said plurality of circuit emulation service (CES) modules over said network;
• each CES module comprising:
• a first receiver operative to receive said first modified Ethernet packets and extract said first network timestamp and said first clock quality indication therefrom;
• a second receiver operative to receive said second modified Ethernet packets and extract said second network timestamp and said second clock quality indication therefrom;
• a clock source selector operative to select either said first network timestamp or said second network timestamp in accordance with said first clock quality indication and said second clock quality indication;
• and
• a digital time delay loop (DTLL) operative to generate a local timestamp utilizing a reconstructed clock generated by a local clock source, to initially apply quick coarse filtering followed by application of slow fine filtering to time differences between said local timestamps and either said received first or second network timestamps to generate a clock control signal thereby, and to correct the frequency of said reconstructed clock in accordance with said clock control signal, said DTLL operative to implement a closed loop tracking system whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said DTLL;
• and
• wherein said clock control signal is generated based on only the difference between two timestamps comprising said network timestamps sent when said clock synchronization packets are transmitted and said local timestamp generated when said clock synchronization packet are received by a CES module.

A circuit emulation services (CES) module, comprising:

• a first receiver operative to receive, over an asynchronous Ethernet network, first clock synchronization packets comprising first modified Ethernet packets from a primary clock distribution unit coupled to a primary master clock source and extract a first network timestamp and first clock quality indication therefrom, said first clock quality indication adapted to convey a measure of the quality of said primary master clock source;
• a second receiver operative to receive, over said Ethernet network, second clock synchronization packets comprising second modified Ethernet packets from a backup clock distribution unit coupled to a backup master clock source and extracting a second network timestamp and second clock quality indication therefrom, said second clock quality indication adapted to convey a measure of the quality of said backup master clock source;
• a clock source selector operative to select a preferred network timestamp based on said first clock quality indication and said second clock quality indication;
• a digital time delay loop (DTLL) operative to generate a local timestamp utilizing a reconstructed clock generated by a local clock source, to initially apply quick coarse filtering followed by application of slow fine filtering to time differences between said local timestamp and either said received first or second network timestamps to generate a clock control signal thereby, and to correct the frequency of said reconstructed clock in accordance with said clock control signal, said DTLL operative to implement a closed loop tracking system whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said DTLL;
• and
• wherein said clock control signal is generated based on only the difference between two timestamps comprising said network timestamp sent when said clock synchronization packets are sent by a clock distribution unit and said local timestamp generated when clock synchronization packets arrive at a receiver.

An edge switch for use in an asynchronous Ethernet transport based network, comprising:

• a plurality of user ports for interfacing said edge switch to one or more users;
• a plurality of network ports for interfacing said edge switch to one or ore communication links;
• a packet processor comprising ingress packet processing means and egress packet processing means;
• a circuit emulation services (CES) module for centralized clock distribution over said network, said CES module comprising:
• a first clock distribution transmitter operative to generate first network timestamps derived from a primary master clock source and to insert said first network timestamps and a first clock quality indication into clock synchronization packets comprising first modified Ethernet packets;
• a first distributor operative to establish first connections to a plurality of CES modules over said network, and to transmit said first modified Ethernet packets over said first connections to said plurality of CES modules;
• a second clock distribution transmitter operative to generate second network timestamps derived from a backup master clock source and to insert said second network timestamps and a second clock quality indication into clock synchronization packets comprising second modified Ethernet packets;
• a second distributor operative to establish second connections to said plurality of CES modules over said network, and to transmit said second modified Ethernet packets over said second connections to said plurality of CES modules;
• wherein said first clock quality indication and said second clock quality indication are used to select between first and second network timestamps;
• a digital time delay loop (DTLL) operative to implement a closed loop tracking system whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said DTLL;
• and
• wherein a clock control signal is generated at said CES modules based on only the difference between two timestamps comprising network timestamps sent when clock synchronization packets are sent by a distributor and local timestamps generated when clock synchronization packets are received by said CES modules.

An edge switch for use in an asynchronous Ethernet transport network, comprising:

• a plurality of user ports for interfacing said edge switch to one or more users;
• a plurality of network ports for interfacing said edge switch to one or more communication links;
• a packet processor comprising ingress packet processing means and egress packet processing means;
• a circuit emulation services (CES) module comprising:
• a first receiver operative to receive, over an asynchronous Ethernet network, first clock synchronization packets comprising first modified Ethernet packets from a primary clock distribution unit coupled to a primary master clock source and to extract a first network timestamp and first clock quality indication therefrom, said first clock quality indication adapted to convey a measure of the quality of said primary master clock source;
• a second receiver operative to receive, over said Ethernet network, second clock synchronization packets comprising second modified Ethernet packets from a backup clock distribution unit coupled to a backup master clock source and to extract a second network timestamp and second clock quality indication therefrom, said second clock quality indication adapted to convey a measure of the quality of said backup master clock source;
• a clock source selector operative to select either said first or second network timestamps based on said first clock quality indication and said second clock quality indication;
• and
• a digital time delay loop (DTLL) operative to generate a local timestamp utilizing a reconstructed clock generated by a local clock source, to initially apply quick coarse filtering following by application of slow fine filtering to time differences between said local timestamps and said selected network timestamps to generate a clock control signal thereby, and to correct the frequency of said reconstructed clock in accordance with said clock control signal, said DTLL operative to implement a closed loop tracking system whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said DTLL;
• and
• wherein said clock control signal is generated based on only the difference between two timestamps comprising network timestamps sent when clock synchronization packets are sent by a clock distribution unit and local timestamps generated when clock synchronization packets arrive at a receiver.

A method of distributing a master clock source to a plurality of circuit emulation service (CES) modules over an asynchronous Ethernet transport network, said method comprising the steps of:

• receiving network timestamps and a clock quality indication encapsulated in modified Ethernet packets sent from a centralized clock distribution entity over said network, said network timestamps and clock quality indication derived from said master clock source;
• generating local timestamps utilizing a reconstructed clock generated by a local clock source, a local timestamp generated for each received network timestamp;
• generating said reconstructed clock based only on the relative time differences between said network timestamps and said local timestamps;
• wherein coarse filtering is applied initially to said relative time differences in order to quickly acquire said reconstructed clock to a predetermined accuracy;
• wherein fine filtering is applied subsequently to said relative time differences in order to achieve slow fine-tuning of said reconstructed clock;
• and
• wherein a closed loop tracking system is implemented whereby an equilibrium point is achieved comprising a constant value of network timestamp minus local timestamp and any deviations in the equilibrium point are corrected by said closed loop tracking system.