From a data-networking perspective, the cable modem is an IEEE 802.1d bridge
(with some modifications), supporting an 802.3 (10BASE-T, 100BASE-T, or 1000BASE-T)
Ethernet link on the customer side and the DOCSIS RF link on the operator side. The
CMTS can simply be a bridge as well, but it often is implemented as a full IP router.
A bridge is a Layer 2 device that operates on Ethernet frames, making forwarding
decisions based on hardware (MAC) addresses and a bridging table (also known as a
forwarding database), whereas a router is a Layer 3 device that operates on IP packets,
making forwarding decisions based on IP addresses and a routing table.
Communications Protocols
At their core, the DOCSIS specifications define physical layer and Media Access Control
layer protocols to provide Ethernet transport across the HFC plant.
152 Chapter 6
The DOCSIS Physical Layer The downstream physical layer in the DOCSIS specifications
uses the same MPEG-2 transport technology originally developed for carrying
digital video programming. That technology is standardized as ITU-T J.83,and utilizes
64-QAM or 256-QAM as well as a concatenation of Reed-Solomon coding and trelliscoded
modulation (TCM) for forward error correction (FEC).
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