Today, applications are increasingly making more use of the
upstream transmission path, for example, peer-to-peer sharing of multimedia content
Hybrid Fiber-Coax 151
and transmission of digital images. In addition, business customers may require a symmetric
connection to the Internet in order to host a website or for virtual private network
connections to remotely located employees. Furthermore, there are applications on the
horizon that will absolutely require symmetric bandwidths (e.g., video telephony, video
conferencing, and so on). All of these factors underscore the need for the higher capacity
upstream technology introduced in DOCSIS 2.0 and the channel bonding introduced in
DOCSIS 3.0.
Cable Modems
Digital data is transmitted over RF carrier signals on a cable system. For two-way
communication, one carrier signal carries data in the downstream direction, and another
carries data in the upstream direction. Cable modems (CMs) are devices at the
subscriber premises that convert digital information into a modulated RF signal in the
upstream direction and convert the RF signal to digital information in the downstream
direction.
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