Data over Cable Service Interface Specification (DOCSIS) is a family of specifications that permits CATV systems to have two-way capability: voice, data, and broadcast-type television downstream and data and voice upstream.

Table 17.2 DOCSIS Physical Layer, Upstream and Downstream


Upstream, Toward CMTS

Downstream, Toward CM


5-42 MHz USA

42-850 MHz USA

5-65 MHz Europe

65-850 MHz Europe


200 kHz to 3.2 MHz


8 MHz Europe


QPSK or 16-QAM

64-QAM with 6 bits/symbol (normal)

or 256-QAM with 8 bits/symbol

Data rate

0.32 to 10.24 Mbps

27-56 Mbps (4-7 Mbytes/sec)

Transmission mode

Transmits bursts in time slots (TDM) with

Continuous stream of data received

reserved and contention time slots

by all modems

Table 17.2 relates DOCSIS 2.0 to the seven-layer OSI reference model. The physical plant itself is often called "layer 0." Original CATV networks were entirely based on 75-Q coaxial cable signal delivery systems. Once fiber-optic cable technology matured, one excellent application was to use fiber-optic cable to bring the CATV signal spectrum from the headend to the first node by fiber optics. A number of advantages derived from this conversion. Among these are:

• Improved signal quality at the end-user because there are less coaxial cable amplifiers in tandem

• Improved reliability (availability)

• Extended coverage area

The CATV industry uses the abbreviation "CMTS" for cable modem termination system, which is housed in the CATV headend and CM (cable modem) for the interface device that resides in the customer end of the system.

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