Traditional Cable Modem Termination System Architecture

The long-established CMTS was designed as a single unit to accelerate the development of the first data-over-cable network systems. Various technologies—internet protocol (IP) switching and routing, DOCSIS MAC (media access control), downstream quadrature amplitude modulation (QAM) and Radio Frequency (RF) upconversion as well as upstream QPSK/QAM demodulation—are all encased in one box.

As shown in Figure 1 below, every cable modem within a network connects to the CMTS. As a cable modem is turned on, it must first go through a ranging process to synchronize its frequency and timing to the CMTS. This process ensures that all cable modems sharing the coax/hybrid fiber coax (HFC) cable and the CMTS do not interfere with each other.

In asynchronous time division multiple access (ATDMA) mode, each cable modem gets a specific timeslot to transmit, and all timeslots are aligned among hundreds of cable modems. In synchronous code division multiple access (SCDMA) mode, all cable modems within the network are perfectly aligned to transmit at the same time.

Perfect alignment is mandatory to ensure that the CMTS correctly demultiplexes the bursts to determine the data transmitted from the various cable modems. In either mode, if cable modems are not synchronized, transmissions will be completely lost.

FIG. 1 Cable Modem Termination System Overview

Reference: http://www.symmetricom.com