Cable Modem Technology Overview

by Leon Tufallo.
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Data Over Cable Service Interface Specifications (DOCSIS) is a project that was developed to provide a set of necessary communications and operations support interface specifications through which cable companies can achieve cross-platform functionality. In essence, DOCSIS can guarantee interoperability by establishing standards for carrying IP packets over an HFC cable TV network.

The Hybrid Fiber Coax (HFC) portion refers to any configuration of fiber optic and coaxial cable that is used to distribute broadband communications such as voice, video, and data. The HFC network connects subscribers to the cable headend and video flows as analog radio frequency or optical signals. Optical fiber brings the signal from the headend to fiber nodes that serve 500 to 2000 homes. Fiber optic is used because it has lower signal power loss and is less susceptible to noise compared to coaxial cable for long distances. Fiber node converts optical signals from fiber to electrical signals on 75-ohm coaxial cable. Coaxial cable has higher signal power loss than fiber, but it is a more cost-effective way to reach subscribers.

The regional headend and local headends are connected via the high-speed fiber network. The video signal flows in analog or digital formats over the fiber network. Usually the regional headend receives national channels from satellites and transmits them to various local headends. Local headends may receive local channels as well as national channels from the regional headend. They selectively process and transmit them to subscribers based on individual requests or demographic group needs. All the video channels are modulated and sent to the combiner at the local headend. The downstream port of the CMTS is connected to the up-converter, and the output signal goes into the combiner. In essence, the output signal (6 MHz wide) becomes one of the video channels that is sent downstream for data communication. Please note that to achieve two-way data network, bidirectional amplifiers are required.

Downstream and Upstream

Downstream is the term used for the signal received by the cable modem. In other words, the signal flows from the headend toward the subscribers. It is also called forward path. Upstream is the term used for the signal transmitted by the cable modem. The signal flows from the subscribers to the headend. It is also called the return path or reverse path.

Modulation Modes

Digital modulation is the physical layer of the DOCSIS protocol stack. The different types of modulation modes are as follows:

  • Quadrature Phase Shift Keying (QPSK) A digital modulation method in which 2 data bits are represented with each baud symbol. QPSK is used for upstream transmission.

  • Quadrature Amplitude Modulation (QAM) A digital modulation method in which the value of a symbol consisting of multiple bits is represented by a carrier's amplitude and phase states. Typical QAM types are

    - 16-QAM (4 bits per symbol) Used for upstream transmission.

    - 64-QAM (6 bits per symbol) Used for downstream transmission.

    - 256-QAM (8 bits per symbol) Used for downstream transmission.

Spectrum Sharing

Time-division multiplexing (TDM) permits timeslots within a channel to be shared by multiple subscribers. TDM is used for downstream transmission, in which only one transmitter is involved. Time-division multiple access (TDMA) allows multiple subscribers to transmit sequentially to a common receiver. It is used for upstream or return transmission in which a number of transmitters need to communicate with the headend.

DOCSIS Hardware Specifications

The tables below show the DOCSIS hardware specification. The DOCSIS hardware must meet or exceed the published specifications for the cable access solution to work properly.

Table 1 summarizes key parameters of the upstream signal.

Table 1 - Upstream Characteristics
Frequency Range 5 to 42 MHz
Bandwidth 200, 400, 800, 1600, 3200 KHz
Modulation Mode QPSK or 16-QAM
Symbol Rates 160, 320, 640, 1280, 256 Ksym/sec

Table 2 summarizes key parameters of the downstream signal.

Table 2 - Downstream Characteristics
Frequency Range 88 to 860 MHz
Bandwidth 6 MHz
Modulation Mode 64-QAM or 256-QAM
Symbol Rates 5.056941 or 5.360537 Msym/sec


Table 3 summarizes the incoming upstream signals that need to be supported by the CMTS receiver. Downstream RF output is also specified in this table

Table 3 -CMTS Power Level Range
Upstream Power Level Range
200 KHz 16 to +14 dBmV
400 KHz 13 to +17 dBmV
800 KHz 10 to +20 dBmV
1600 KHz 7 to +23 dBmV
3200 KHz 4 to +26 dBmV
Downstream Power Level Range
6 MHz +50 to +61 dBmV


Table 4 summarizes input and output signal levels for the cable modem

Table 4 - Cable Modem Power Level Range
Output Power Level Range
QPSK +8 to +58 dBmV
16-QAM +8 to +55 dBmV
Input Power Level Range
64-QAM/256-QAM 15 to +15 dBmV

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