Browse Prior Art Database

Multi-Mode Adaptive Clock Synchronization Based on Jitter Buffer Depths

IP.com Disclosure Number: IPCOM000125326D
Original Publication Date: 2005-Jun-20
Included in the Prior Art Database: 2005-Jun-20
Document File: 1 page(s) / 22K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

This idea allows an integrated access device, to synchronize an on-board 8Khz voltage controlled oscillator (VCXO) with the 8Khz voice clock of the telephone network, where the only interconnection between the two is an asynchronous packetized network carrying digitized voice samples. The integrated access device buffers some number of voice samples received from the network before playing them out a phone part, to avoid underrunning the transmitter due to variation in arrival times of subsequent voice packets received from the network. By tracking the depths of the buffer, known as the jitter buffer, over time, and adjusting the on-board clock (VCXO) accordingly, the clocks may be synchronized very closely, over a wide range network conditions. During subsequent lab tests, it was found that high speed modem operation was very sensitive to clock speed changes. If the clock is adjusted too rapidly or too far over a given period of time, the modem will lose synchronization, and re-train or possibly disconnect. This made modem performance very poor, if usable at all. Likewise, jitter buffer underruns caused re-trains and disconnects, so allowing the VCXO to free-run at a stable value was not desirable. It was also found that sharing the wide area network connection (DSL) bandwidth between voice and "bursty data" applications resulted in significantly more jitter in the jitter buffer. Under these conditions, the jitter buffer depth fails to stabilize at a given value in the "short" term, and the previously described method and parameters results in over-controlling the VCXO. Although the jitter buffer depth is tightly controlled, the rapid clock adjustments result in poor modem operation. To solve the problems described above, the following enhancements were added to the adaptive mode of clock synchronization:

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Multi-Mode Adaptive Clock Synchronization Based on Jitter Buffer Depths

Idea: Mark Gladden, US-Dallas, Texas; Curtis Pastor, US-Dallas, Texas; Guy Dame, US-Dallas,

Texas; Huey Dixon, US-Dallas, Texas; Clyde Musgrave, US-Dallas, Texas

This idea allows an integrated access device, to synchronize an on-board 8Khz voltage controlled oscillator (VCXO) with the 8Khz voice clock of the telephone network, where the only interconnection between the two is an asynchronous packetized network carrying digitized voice samples. The integrated access device buffers some number of voice samples received from the network before playing them out a phone part, to avoid underrunning the transmitter due to variation in arrival times of subsequent voice packets received from the network. By tracking the depths of the buffer, known as the jitter buffer, over time, and adjusting the on-board clock (VCXO) accordingly, the clocks may be synchronized very closely, over a wide range network conditions.

During subsequent lab tests, it was found that high speed modem operation was very sensitive to clock speed changes. If the clock is adjusted too rapidly or too far over a given period of time, the modem will lose synchronization, and re-train or possibly disconnect. This made modem performance very poor, if usable at all. Likewise, jitter buffer underruns caused re-trains and disconnects, so allowing the VCXO to free-run at a stable value was not desirable.

It was also found that sharing the wide area network connection (DSL) bandwidth between voice and "bursty data" applications resulted in significantly more jitter in the jitter buffer. Under these conditions, the jitter buffer depth fails to stabilize at a given value in the "short" term, and the previously described method and parameters results in over-controlling the VCXO. Although the jitter buffer depth is tightly controlled, the rapid clock adjustments result in poor modem operation.

To solve the problems described above, the following enhancements were added to the adaptive mod...