Like telephone modems, cable modems modulate
and demodulate data signals. However, cable modems incorporate more
functionality designed for today's high-speed Internet services. In a
cable network, data flowing from the network to the user is referred to
as downstream and data flowing from the user to the network is referred
to as upstream. From a user perspective, a cable modem is a 64/256 QAM radio
frequency (RF) receiver capable of delivering up to 30 to 40 megabits
per second (Mbps) of data in one 6-megahertz (MHz) cable channel. This
is almost 500 times faster than a 56-kilobit-per-second (Kbps) modem.
The headend manages traffic flow from the user to the network.
Headends have facilities to do the following:
- Receive programming (for example, from
NBC, CBS, and cable networks such as MTV and ESPN)
- Convert each channel to the channel
frequency desired; scramble channels as needed (for the premium
channels)
- Combine all the frequencies onto a
single, broadband analog channel (frequency-division multiplexing [FDM])
- Broadcast the combined analog stream
downstream to subscribers
The data is modulated using a QPSK/16 QAM
transmitter with data rates from 320 Kbps up to 10 Mbps. The upstream
and downstream data rates can be configured to meet the needs of the
subscribers. For instance, a business service can be programmed to both
transmit and receive at relatively high rates. A residential user, on
the other hand, can have their service configured to receive higher
bandwidth access to the Internet while limited to low-bandwidth
transmission to the network.
A subscriber can continue to receive cable television
service while simultaneously receiving data on a cable modem to be
delivered to a personal computer with the help of a simple one-to-two
splitter.
The data service offered by a cable modem can be shared by up to 16
users in a local-area network (LAN) configuration.
Because some cable networks are suited
for broadcast television services, cable modems may use either a
standard telephone line or a QPSK/16 QAM modem over a two-way cable
system to transmit data upstream from a user location to the network.
When a telephone line is used in conjunction with a one-way broadcast
network, the cable data system is referred to as a telephony return
interface (TRI) system. Telephone return means that the consumer (or the
subscriber modem) makes a telephone call to a terminal server when the
consumer requires return-path service. At the cable headend, data from
individual users is filtered by telephone-return systems for further
processing by a cable modem terminal server (CMTS). The CMTS
communicates with the cable modem to enforce the Media Access Control
(MAC) protocol and RF control functions, such as frequency hopping and
automatic gain control.
A CMTS provides data switching necessary
to route data between the Internet and cable-modem users. Data from the
network to a user group is sent to a 64/256 QAM modulator. The result is
user data modulated into one 6-MHz channel, which is the spectrum
allocated for a cable television channel such as ABC, NBC, or TBS for
broadcast to all users. 
A cable headend combines the downstream
data channels with the existing video, pay-per-view, audio, and local
advertiser programs that are received by television subscribers. The
combined signal is now ready to be transmitted throughout the cable
distribution network. When the signal arrives at the user's site, the
television signal is received by a converter box generally located on
the top of a television, while user data is separately received by a
cable modem or router and sent to a PC.
The CMTS, an important new element for
support of data services, integrates upstream and downstream
communication over a cable data network. The number of upstream and
downstream channels in any particular CMTS can be designed and adjusted
based on the size of the serving area, number of users, and data rates
offered to each user.
Another important element in the
operations and day-to-day management of a cable data system is an
element management system (EMS). An EMS is an operations system designed
specifically to configure and manage a CMTS and associated cable-modem
subscribers. These operations include provisioning, day-to-day
administration, monitoring, alarms, and testing of various components of
a CMTS. From a central Network Operations Center (NOC), a single EMS can
support many CMTS systems in a particular geographic region. 
Beyond modulation and demodulation, a
cable modem or router incorporates many features necessary to extend
broadband communications to wide-area networks (WANs). The Internet
Protocol (IP) is used at the network layer to support the Internet
services such as e-mail, Hypertext Transfer Protocol (HTTP), and File
Transfer Protocol (FTP). The data link layer comprises three sublayers,
including the Logical Link Control (LLC) sublayer, link security
sublayer conforming to the security requirements, and MAC sublayer
suitable for cable-system operations. Cable systems use the Ethernet
frame format for data-transmission data channels. The downstream data
channels and the associated upstream data channels on a cable network
basically form an Ethernet WAN. As the number of subscribers increases,
the cable operator can add more upstream and downstream data channels to
meet the additional bandwidth requirements.
The link security sublayer is defined in
three (sub)sets of requirements: baseline privacy
interface (BPI), security system interface (SSI), and removable security
module interface (RSMI). BPI provides cable-modem users with data
privacy across the cable network by encrypting data traffic between the
user's cable modem and CMTS. The operational support provided by the EMS
allows a CMTS to map cable-modem identities to paying subscribers and
thereby authorize subscriber access to data network services. These
privacy and security requirements are designed to protect user data as
well as prevent unauthorized use of cable data services.