Distributed CMTS Clock Synchronization via Timestamp Smoothing
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Solution Overview
Problem
Conventional cable modem communication systems face challenges in synchronizing distributed Cable Modem Termination System (CMTS) components due to the introduction of significant jitter in packet data networks, making it impractical to meet the required accuracy without adding excessive latency.
Innovation Solution
A method is introduced where the head end transmits data packets with measured delay Quality of Service (QOS) markings and time stamps to the downstream transmitter hub, which smoothes the time stamps based on delay tags to synchronize its clock with the head end clock, and subsequently synchronizes the upstream receiver hub clock, using ranging operations supported by cable modems to maintain synchronization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the packet data network is used to synchronize distributed CMTS components, then the system can achieve distributed operation, but the packet data network introduces significant jitter that makes it impractical to meet synchronization accuracy requirements without adding excessive latency
Solution Approach 1:
The patent introduces an intermediary synchronization mechanism that operates independently of the packet data network's jitter-prone path. By using a dedicated synchronization channel and intermediary timing messages, the system achieves accurate clock synchronization between distributed CMTS components without being affected by the packet data network's variable latency and jitter.
Solution Approach 2:
The patent replaces the mechanical approach of relying on the packet data network's inherent timing with a more precise electronic synchronization mechanism. By substituting the network-based timing with a dedicated synchronization protocol that uses timestamped messages and delay compensation, the system achieves superior timing accuracy while maintaining distributed operation.
2Measurement precision
If delay compensation is applied to synchronize clocks, then timing accuracy is improved, but the complexity of the synchronization protocol increases
Solution Approach 1:
The patent applies preliminary action by having the packet data network elements pre-measure and pre-report their delays to the synchronization server before the actual synchronization process. This advance delay information is stored and used during synchronization, eliminating the need for complex real-time delay calculation and reducing the overall protocol complexity while maintaining high timing accuracy.
Data Source
AI summary
A distributed CMTS includes a head end, a downstream transmitter hub, and an upstream receiver hub. The head end transmits data packets to the downstream transmitter via a coupling packet data network. Each data packet is marked with a “measured delay” Quality of Service (QOS) and includes a time stamp that is based upon a clock of the head end and a delay tag. The downstream transmitter hub receives the plurality of data, adjusts the time stamps based upon the delay tags, smoothes the time stamps of the plurality of data packets and, based upon the smoothed time stamps, synchronizes its clock with the clock of the head end. The downstream transmitter hub then synchronizes a clock of the upstream receiver hub with the clock of the downstream transmitter hub. The upstream receiver hub may synchronize its clock to the clock of the head end using this technique.