7.4 Cable Modem
Historically, CATV was a unidirectional medium designed to carry broadcast analog video channels to the maximum number of
customers at the lowest possible cost. The DOCSIS interface specification basically enables the deployment of data-over-cable
systems on a non-proprietary, multi-vendor, interoperable basis for transparent bidirectional transfer of Internet Protocol (IP) traffic
between the cable system head end and customer locations over an all-coaxial or Hybrid-Fiber/Coax (HFC) cable network.
In its simplest form, the system consists of a Cable Modem Termination System (CMTS) located at the headend, a coaxial or HFC
medium, and a Cable Modem (CM) located at the customer premises. In addition, DOCSIS defines physical, signalling and protocol
layers to support interoperability and evolutionary feature capabilities to permit future value-added services.
The DOCSIS layers are briefly outlined as follows:
IP network layer
Data link layer comprising:
Logical Link Control (LLC) sublayer conforming to Ethernet standards
Link security sublayer for basic privacy, authorization, and authentication
Media Access Control (MAC) sublayer for operation supporting variable-length Protocol Data Units (PDU)
Physical (PHY) layer comprising:
Downstream convergence layer conforming to MPEG-2 (Recommendation H.222.0)
Physical Media Dependent (PMD) sublayer for:
Downstream based on ITU-T Rec J.83 Annex B with either 64 or 256 Quadrature Amplitude Modulation (QAM),
concatenation of Reed-Solomon and Trellis Forward Error Correction (FEC), in addition to variable-depth
interleaving
Upstream employing Quadrature Phase Shift Keying (QPSK), 16, 64 or 128QAM, support for multiple symbol
rates, cable modem controlled and programmable from the CMTS, frequency agility, fixed-frame and variable-
length PDU formats, Time-Division Multiple Access (TDMA), programmable Reed-Solomon FEC and
preambles, and capability to support future physical layer technologies.
CX380C User Manual RevA00
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