DSLAM Adaptive Clock Recovery for VoDSL Packet Synchronization
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Solution Overview
Problem
In VoDSL systems, the lack of a synchronized clock signal for A/D and D/A converters leads to jitter buffers overflowing or becoming empty, causing slips that can disrupt voice communication, especially in packet networks like Ethernet, where delay variations are significant and clock synchronization is absent.
Innovation Solution
A DSLAM generates an 8 kHz reference clock signal using an adaptive clock method based on the arrival times of RTP packets, which is then connected to the NTR unit of the ADSL modem, ensuring synchronization by identifying and selecting the packet stream with the lowest delay variation for synchronization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a free running oscillator is used to provide a reference clock signal, then the system can operate without external synchronization, but the clock accuracy deteriorates to approximately 100 ppm causing frequent slips
Solution Approach 1:
The patent implements feedback by continuously monitoring the arrival times of RTP packets and using this information to adjust the phase and frequency of the local clock signal. The system measures packet arrival deviations and feeds this information back to the clock synchronization mechanism, enabling dynamic correction of clock drift and maintaining accurate synchronization without requiring external clock signals.
2Adaptability or versatility
If packet networks are used for voice transport, then flexibility and cost-effectiveness improve, but delay variations increase causing jitter buffer overflow or underflow
Solution Approach 1:
The patent applies dynamics by making the jitter buffer size and timing parameters adaptive rather than fixed. The system dynamically adjusts buffer thresholds and timing based on observed packet arrival patterns and delay variations. This allows the system to optimize its performance in real-time according to current network conditions, maintaining reliable voice communication despite the variable delays inherent in packet networks.
3Measurement precision
If adaptive clock methods are used over ADSL links, then clock synchronization can be achieved, but delay variations from long data packets reduce synchronization quality
Solution Approach 1:
The patent implements preliminary action by establishing clock synchronization based on voice packets before data packets arrive, and by using voice packet timing characteristics to set up the synchronization state in advance. The system prepares the clock synchronization mechanism using the more predictable timing patterns of voice traffic, creating a stable baseline that is less susceptible to the variable delays introduced by long data packets.
Data Source
AI summary
A packet forwarding unit such as a Digital Subscriber Line Access Multiplexer (DSLAM) receives Ethernet packets carrying real-time information and forwards the information therein on output lines such as Asymmetric Digital Subscriber Lines (ADSLs). The DSLAM includes a clock device having an extraction unit and an adaptive clock. The extraction unit analyzes received packets to select streams of received packets from one destination to one user, and the adaptive clock generates a reference clock signal according to the arrival times of packets in the selected streams. The clock signal is sent to time reference units in DSL modems for the output lines.