ANSI SCTE 137-1:2017 pdf free download – Modular Headend Architecture Part 1: DOCSIS Timing Interface
Figure 1–1 shows frequency and timing distribution examples for both headend and Hub. The DTI Server establishes the reference for the timing distribution network and synchronizes all connected DTI Clients via point- to-point connections between the server and each client. A single protocol initiated by the DTI server permits the client to perform frequency and time synchronization. As shown, upstream receive, Edge QAMs, and the M-CMTS CORE may have different uses for the synchronized frequency and time, but utilize a common client function. The DTI protocol and server-client interactions are described in detail in Sections 6 and 0. The essential characteristics are:
• The DTI server initiates the protocol, which the DTI client uses to establish its time and frequency synchronization.
• Using a ping-pong scheme, the client always immediately replies to the DTI server when it receives a transmission from the DTI server. The server uses this response to auto-compensate any delays with the effect that the client becomes precisely synchronized to the server.
• The server-to-client-to-server handshake continually repeats, assuring that a tight synchronization can be maintained.
The DTI protocol and components support accurate and robust transport of the server 10.24 MHz master clock and 32-bit DOCSIS timestamp to the client within a node or building. The protocol is structured to minimize the complexity and cost of the client clocks and the per port cost of the shared server function while supporting all the DOCSIS S-CDMA, TDMA, and future T-Services timing requirements in a modular system. The high accuracy (<5 ns) and high stability (<1 ns timing jitter budget) is achieved by using a simple ping-pong layer 2 timing protocol over a single twisted pair connection using common passive PHY components in both directions. This structure provides delay reciprocity so that all cable delay processing can be performed in the server. The client’s role in delay correction is to provide a fixed delay response to the server frame and to use the cable advance supplied by the server to advance the local 10 kHz DTI frame clock to correct for cable delay. To ensure reliable transport and client clock operation, the client clock is required to report the current phase error of its local clock (frame clock) with respect to the delay-corrected server frame clock. This measurement is reported to the server at the 10 kHz frame rate. The server’s role is to process this measurement data and verify the client’s timing operation. This protocol supports real-time detection and mitigation of client clock faults. The DTI client can be realized with a single digital component, a simple PHY and a low cost local oscillator, as holdover and filtering are supported in the shared server. A common definition of the DTI high-speed clock is necessary to ensure compatibility between all DOCSIS DTI client components.