System for synchronizing circuitry in an access network

• US 7,394,830 B2
• Filed: 09/11/2003
• Issued: 07/01/2008
• Est. Priority Date: 09/11/2003
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Chapman, John T.
• Primary Examiner(s): Orgad; Edan
• Assistant Examiner(s): HALIYUR, VENKATESH N
• Application Number:
  US10/662,025
• Publication Number:
  US 20050058159A1
• Time in Patent Office: 1,755 Days
• Field of Search: 370/324, 370/503, 375/354, 713/400, 713/503, 713/600
• US Class Current: 370/503
• CPC Class Codes: H04J 3/0655 using timestamps
• CPC Class Codes: H04J 3/0682 by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
• CPC Class Codes: H04J 3/0694 Synchronisation in a TDMA node, e.g. TTP

Abstract

A master Timestamp Synchronization Circuit (TSC) in a Cable Modem Termination System (CMTS) estimates a master timestamp value for an upcoming time reference. The master TSC sends the master timestamp value asynchronously over an Internet Protocol (IP) network to slave TSCs in other CMTSs. The slave TSC compares a local timestamp value with the master timestamp value when the upcoming time reference occurs. If the local timestamp value does not match the master timestamp value, the slave TSC is resynchronized using the master timestamp value.

Claims

1. A synchronization circuit, comprising:

a local timestamp counter configured to generate a local timestamp value;

a processing circuit to receive externally generated synchronization pulses and to receive a predicted master time stamp value associated with a future one of the externally generated synchronization pulses, wherein the processing circuit receives the predicted master timestamp value asynchronously in Internet Protocol (IP) packets received over an IP connection,

the processing circuit to identify the local timestamp value and synchronize the local timestamp value upon receipt of the future one of the externally generated synchronization pulses; and

one or more line cards in a same Cable Modem Termination System (CMTS) chassis that each have local timestamp counters that is adjusted according to the received predicted master timestamp value and local timestamp value at the future received synchronization pulse and wherein the processing circuit identifies an error condition according to a number of times the local timestamp counter is synchronized with the received predicted master timestamp value.

2. The synchronization circuit according to claim 1 wherein the processing circuit is located in a Cable Modem Termination System (CMTS) and receives the predicted master timestamp value from another CMTS.

3. The synchronization circuit according to claim 1 wherein the Internet Protocol (IP) packets containing the master timestamp value use a multicast address.

4. The synchronization circuit according to claim 1 including a holding register configured to store the received master timestamp value.

5. The synchronization circuit according to claim 2 further comprising the processing circuitry to forward the predicted master timestamp value to the another CMTS.

6. The synchronization circuit according to claim 1 wherein the synchronization pulse cycle at a rate of somewhere between 8 Kilo Hertz and 1 Hertz.

7. The synchronization circuit according to claim 1 including a first CMTS including one or more line cards that are used for downstream channels and a second CMTS including one or more line cards that are used for upstream channels, cable modems receiving data on the downstream channels of the first CMTS and sending data on the upstream channels of the second CMTS.

15. The synchronization circuit of claim 1 wherein the predicted master timestamp value is equal to a sum of an actual master timestamp value for a previous synchronization pulse and a predetermined amount multiplied by a difference between two previous actual master timestamp values.

16. The synchronization circuit of claim 15 wherein the predetermined amount is equal to a quotient of a difference in the time between the previous synchronization pulse and a next synchronization pulse divided by a period between synchronization pulses that corresponds to the two previous actual master timestamp values.

8. A synchronization system, comprising:

a first Cable Modem Termination Systems (CMTS) having a first chassis containing a master synchronization circuit;

a second CMTS having a second separate chassis containing a slave synchronization circuit;

a master synchronization circuit including;

a master counter to generate respective master timestamp values varying with cycling of a clocking signal;

a processing circuit to determine, for a given count of consecutive ones of synchronization pulses cycling less often than the clocking signal, the corresponding difference between the master timestamp values and to

calculate a future master timestamp value by adding the corresponding difference to an initial one of the master timestamp values corresponding to an initial one of the synchronizing pulses and the master synchronization circuit being configured to

forward the calculated future master timestamp value to the slave synchronization circuit over a wide area network for synchronizing the value of a slave counter in the slave synchronization circuit with the future master timestamp value at a future synchronization pulse generated independently of operations by the master counter and slave counter and corresponding to a given count of consecutive ones of synchronization pulses following the initial one of the synchronization pulses; and

one or more line cards in at least one of the first and second CMTS that include one or more slave circuits each synchronized with the future master timestamp value at the future synchronization pulse when the difference between an actual master timestamp value and the future master timestamp value is within the predetermined range.

9. The system according to claim 8 wherein the master synchronization circuit is further configured to:

identify the actual master timestamp value corresponding to the future synchronization pulse when the future synchronization pulse occurs;

determine whether a difference between the actual master timestamp value and the future master timestamp value is within a predetermined range; and

send an error message to one of the slave synchronization circuit when the difference between the actual master timestamp value and the future master timestamp value is not within a predetermined range that causes the slave synchronization circuit to take over operations as the master synchronization circuit.

10. The system according to claim 9 wherein the slave synchronization circuit is configured to calculate and forward new future master timestamp value in response to receiving the error message.

11. The system according to claim 8 wherein the slave synchronization circuits adjust a received future master timestamp value according to an amount of delay associated with receiving the synchronization pulses.

12. A method for synchronizing circuitry, comprising:

identifying a period between synchronization pulses;

extrapolating a time for a future synchronization pulse by adding one of the synchronization pulses to the period multiplied by a predetermined amount,

extrapolating a master timestamp value by adding a master timestamp value for the one of the synchronization pulses and the predetermined amount multiplied by a difference between two previous master timestamp values,

receiving the extrapolated master timestamp value for an upcoming time reference in an Internet Protocol (IP) packet over an asynchronous Internet connection;

generating a local timestamp value;

comparing the local timestamp value at the upcoming time reference with the extrapolated master timestamp value;

synchronizing the local timestamp value with the extrapolated master timestamp value according to the comparison; and

receiving the extrapolated master timestamp value from a first cable modem termination system (CMTS) and using the extrapolated master timestamp value to synchronize a timing circuit in a second CMTS.

13. A method according to claim 12 including:

synchronizing the timing circuitry in the first Cable Modem Termination System (CMTS) with the timing circuitry in the second CMTS;

using the first CMTS to send data to cable modems; and

using the second CMTS to receive data from the same cable modems.

14. A method according to claim 12 further including:

receiving an error message indicating that the extrapolated master timestamp value is not equal to an actual master timestamp value for a next synchronization pulse;

predicting a new master timestamp value in response to the error message; and

sending the predicted new master timestamp value to a generation source of a message including the received extrapolated master timestamp value.