Clock synchronization and dynamic jitter management for voice over IP and real-time data

• US 6,259,677 B1
• Filed: 09/30/1998
• Issued: 07/10/2001
• Est. Priority Date: 09/30/1998
• Status: Expired due to Term

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Jain, Jaswant R.
• Primary Examiner(s): NGO, RICKY QUOC
• Application Number:
  US09/164,431
• Time in Patent Office: 1,014 Days
• Field of Search: 370/389, 370/352, 370/356, 370/400, 370/401, 370/503, 370/507, 370/508, 370/509, 370/516, 370/517, 370/518, 370/519, 370/252, 375/356, 375/359, 375/371
• US Class Current: 370/252
• CPC Class Codes: H04J 3/0632 Synchronisation of packets and cells, e.g. transmission of voice via a packet network, circuit emulation service [CES] queuing arrangements in packet switching elements H04L49/90; synchronising systems for the synchronous transmission of a pulse code modulated video signal with one or more other pulse code modulated signals H04N7/56
• CPC Class Codes: H04L 12/6418 Hybrid transport
• CPC Class Codes: H04L 2012/6481 Speech, voice
• CPC Class Codes: H04L 2012/6489 Buffer Management, Threshold setting, Scheduling, Shaping
• CPC Class Codes: H04L 65/1101 Session protocols
• CPC Class Codes: H04L 65/752 adapting media to network capabilities
• CPC Class Codes: H04L 65/764 at the destination reformatting of additional data in video clients H04N21/4355
• CPC Class Codes: H04L 65/80 Responding to QoS
• CPC Class Codes: H04L 9/40 Network security protocols

Abstract

A real-time receiver and method for receiving and playing out real-time packetized data are disclosed. The receiver includes a packet transmission fixed delay estimator and a packet transmission variable delay estimator. The fixed delay estimator determines, using packets received up to the current point in a conference, the non-variable portion of observed delays. This non-variable portion is subtracted from each packet'"'"'s observed delay to obtain a variable delay estimate for that packet.

Since variable delays actually drive the buffering time needed at the receiver to achieve smooth playout, the packet variable delay estimates can be used directly to adjust playout delay. Adaptive playout delay is preferably set aggressively low, based on observed packet variable delay estimates, to reduce data latency. Playout delay can be adjusted rapidly upwards when higher packet delays are observed, allowing rapid adaptation to network statistical variations and reducing the frequency of late packets.

Claims

1. A packet-based real-time data receiver comprising:

a packet transmission fixed delay estimator, said fixed delay estimator keeping a fixed delay estimate over the duration of a conference connection using said receiver, and adjusting said fixed delay estimate downwards during said conference in response to the arrival of a conference packet prior to a minimum arrival time predicted for that packet by said fixed delay estimate; and

a packet transmission variable delay estimator, said variable delay estimator calculating a variable packet delay for each conference packet by subtracting a predicted minimum arrival time for each packet, based on said fixed delay estimate, from the actual arrival time of each packet.

2. The data receiver of claim 1, further comprising an adaptive playout delay estimator that adapts packet playout delay for said receiver using variable packet delays from said packet transmission variable delay estimator.

3. The data receiver of claim 2, wherein said adaptive playout delay estimator comprises a non-linear packet variable delay filter.

4. A packet-based real-time data receiver comprising:

a playout buffer for queuing packets from a received data stream for playout, said packets in said received data stream each containing a packet timestamp generated by a remote system send clock operating at a send clock rate;

a local timestamp generator operating at approximately said send clock rate;

a packet transmission fixed delay estimator, said fixed delay estimator comparing the packet timestamp from each received packet to a receive timestamp taken from said local timestamp generator at approximately the arrival time of said received packet, and adjusting a fixed delay estimate downwards in response to the arrival of packets prior to an arrival time predicted by said fixed delay estimate;

a packet transmission variable delay estimator, said variable delay estimator calculating a variable delay for each packet by subtracting said fixed delay estimate from the difference between the receive timestamp and the packet timestamp; and

a playout delay estimator that non-linearly adapts a playout delay estimate for received packets based on the relative magnitude of the playout delay estimate as compared to a variable delay calculated by said packet transmission variable delay estimator.

5. The data receiver of claim 4, wherein said local timestamp generator is synchronized with said remote system send clock.

6. The data receiver of claim 4, wherein said packet transmission fixed delay estimator, packet transmission variable delay estimator, and playout delay estimator comprise a programmed microprocessor.

7. A method of receiving and playing packetized real-time data, said method comprising the steps of:

establishing a real-time conference over a packet-switched network;

for a first real-time data packet received during said conference, initializing a packet transmission fixed delay estimate and a playout delay estimate for the conference; and

for each additional real-time data packet received during said conference,

calculating a packet delay estimate,

adjusting said fixed delay estimate downwards if said fixed delay estimate is greater than said packet delay estimate, and

subtracting said fixed delay estimate from said packet delay estimate, thereby obtaining a packet variable delay estimate.

8. The method of claim 7, wherein said step of initializing a packet transmission fixed delay estimate consists of setting said fixed delay estimate to equal the clock offset between a local clock reference and a timestamp affixed to said first data packet by its sender.

9. The method of claim 7, wherein said adjusting said fixed delay estimate downwards step comprises, for a data packet triggering such adjustment, resetting said fixed delay estimate to equal the clock offset between a local clock reference and a timestamp affixed to that data packet by its sender.

10. The method of claim 7, further comprising as a part of said adjusting said fixed delay estimate downwards step, adjusting said playout delay estimate upwards by an equivalent amount.

11. The method of claim 7, further comprising the step of introducing an artificial positive rate bias in said fixed delay estimate.

12. The method of claim 7, further comprising the step of adapting said playout delay estimate throughout the duration of said conference to approximately maintain a preset ratio between said playout delay estimate and said packet variable delay estimates calculated for said real-time data packets.

13. The method of claim 12, wherein said adapting said playout delay estimate comprises applying a correction formula to said playout delay estimate for each packet variable delay estimate, with a correction formula gain determined by the ratio of that packet variable delay estimate to the playout delay estimate.

14. The method of claim 13, wherein said gain is highest for packet variable delay estimates greater than the playout delay estimate and less than a preset maximum variable delay.

15. The method of claim 13, comprising applying said correction formula by mapping said ratio into one of a plurality of ratio zones, each of said zones having a zone-specific gain formula.

16. The method of claim 7, further comprising the step of fixing the playout time for data spurts during said conference using said playout delay estimate.

17. The method of claim 7, wherein said step of adjusting said fixed delay estimate downwards comprises resetting said fixed delay estimate to said packet delay estimate.

18. A method of receiving and playing packetized real-time data, said method comprising the steps of:

establishing a real-time conference over a packet-switched network;

for a first real-time data packet received during said conference, initializing a packet transmission fixed delay estimate and a playout delay estimate for the conference;

introducing an artificial positive rate bias in said fixed delay estimate;

for each additional real-time data packet received during said conference,

calculating a packet delay estimate,

adjusting said fixed delay estimate downwards if said fixed delay estimate is greater than said packet delay estimate, and

subtracting said fixed delay estimate from said packet delay estimate, thereby obtaining a packet variable delay estimate;

adapting said playout delay estimate throughout the duration of said conference to approximately maintain a preset ratio between said playout delay estimate and said packet variable delay estimates calculated for said additional data packets; and

periodically adjusting playout time for data packets received during said conference to include a buffer time, measured with reference to the fixed delay estimate, corresponding to said playout delay estimate.