Components of a Cable System
Headend
Antenna Site
CM
Receivers, Modulators,
and Scramblers
Amplifier
Fiber
Node
Fiber
Node
Servers CMTS Downstream
Upstream
Internet
Tap Splitter
In Figure 2-1, video signals are received at the antenna site and sent to the
local headend site. Feeds from other headend sites might be used, too. Each TV
channel has receivers, modulators, and scramblers. The signals are transferred
via RF to a fiber transmitter. Data signals from the Internet or various servers
(such as e-mail or content servers) are modulated by the Cable Modem
Termination System (CMTS) router and sent as RF signals to the fiber transmitter.
They are then translated to optical signals and sent downstream, toward
the end users. A fiber node in the distribution network translates them back
to RF signals and sends them over the coaxial cable. Signals are boosted at
intervals by an amplifier. A tap divides the signal for sending to a particular
subscriber�fs residence. At the residence, a splitter divides the signals into data
and video. Video is sent to the TV or set-top box, and data is sent to a cable
modem (CM). The cable modem demodulates the signal back to digital.
Cable Standards
Worldwide, three standards control cable TV systems:
�¡ National Television Standards Committee (NTSC).Analog television
standard used in North America. Specifies a 6-MHz channel width.
�¡ Phase-Alternating Line (PAL).Color television standard used in
most of the rest world. Specifies 6-, 7-, or 8-MHz channel widths.
�¡ Systeme Electronic Couleur avec Memoire (SECAM).Standard
used in France and some Eastern European countries. Specifies an 8-MHz
channel.
The standard for sending data over cable systems is Data-Over-Cable
Service Interface Specifications (DOCSIS). This standard, developed by
Cablelabs, defines physical and data link layer requirements for cable
modems. Cablelabs also certifies cable modems and CMTS systems to work
with the standard. At the physical layer, DOCSIS specifies channel widths
and modulation methods. At the data link layer it specifies access methods,
some QoS capabilities, and some security features.
Cable RF waves use different frequencies for upstream and downstream
signaling. Downstream signals are allowed 810 MHz of bandwidth, in the
50- to 860-MHz range. This is then further subdivided into channels of 6-,
7-, or 8-MHz depending on the standard used. Upstream signals have only
37 MHz of bandwidth, in the 5- to 42-MHz range.