"PDRUNEN" wrote in message
...
Hi Group,

Do cable modems modulate the data on a carrier and send down the line or
do
they just use baseband?

If they are carrier control, what frequency do they operate?

Thanks,

De KJ4UO

They do not operate (downstream data) on standard cable TV video channels
used by the local cable provider, but usually for upstream select from
available bandwidth below 42 MHz) The service is bi-directional (which
requires upgrades to some older CATV equipment to support this
functionality)

Cable Modems since about 1996 operate under the DOCSIS standard (there is
also a Euro-DOCSIS)
http://www.cablemodem.com/

The three successive versions of the DOCSIS cable modem specifications:
DOCSIS® 1.0, DOCSIS® 1.1, and DOCSIS® 2.0, provide increasing levels of
capabilities and functionality, while maintaining multi-vendor
interoperability and full backward and forward compatibility of DOCSIS.
a.. DOCSIS 1.0 specifications include technology that was available in the
1995-1996 timeframe, and have become very widely deployed around the world.


b.. DOCSIS 1.1 specifications provide improved operational flexibility,
security, and Quality-of-Service (QoS) features that enable real-time
services such as voice-over-IP telephony (VoIP), interactive gaming, and
tier-based services.


c.. DOCSIS 2.0 specifications provide dramatically increased upstream
throughput for symmetric services. DOCSIS 2.0 provides 30 Mbps of capability
Digital data signals are transmitted over radio frequency (RF) carrier
signals on a cable system. For two-way communication, there is one carrier
signal that carries data in the "downstream" direction (from the cable
network to the customer), and another that carries data in the "upstream"
direction (from the customer to the cable network). Cable modems are devices
at the subscriber premises that convert digital information into a modulated
RF signal in the upstream direction, and convert the RF signals to digital
information in the downstream direction. Another piece of equipment, called
a Cable Modem Termination System (CMTS), performs the converse operation for
multiple subscribers at the cable operator's headend.

Typically, a few hundred users may share a 6-MHz downstream channel and one
or more upstream channels. The downstream channel occupies the space of a
single television transmission channel in the cable operator's channel
lineup. It is compatible with digital set-top MPEG transport stream
modulation (64 or 256 QAM), and can provide up to 40 Mbps. In DOCSIS 1.0 and
1.1, the upstream channels can be up to 3.2-MHz wide, and can deliver up to
10 Mbps-per-channel. In DOCSIS 2.0, upstream channels can deliver up to 30
Mbps over channels as wide as 6.4 MHz. A media access control (MAC) layer
coordinates shared access to the upstream bandwidth.

Google on DOCSIS for thousands of references and technical documents.

gb