Cable Modem Technology

Current Internet access via a 33.6 or 56 Kbps modem is
referred to as “voice-band” modem technology. Like voice-band modems,
cable modems modulate and demodulate data signals. However, cable modems
incorporate more functions suitable for today’s high-speed Internet services.

a cable network, data from the network to the user is referred to as downstream,
whereas data from the user to the network is referred to as upstream. From a
user perspective, a cable modem is a QAM receiver capable of delivering up to 30
to 40 Mbps of data in one 6-MHz cable channel. This is approximately 500 times
faster than a 56 Kbps modem. Data from a user to the network is sent in a
flexible and programmable system under control of the head-end. The data is
modulated with data rates from 320 Kbps up to 10 Mbps. The upstream and
downstream data rates may be flexibly configured using cable modems to match
subscriber needs. For instance, a business service can be programmed to receive
as well as transmit higher bandwidth. A residential user, however, may be
configured to receive higher bandwidth access to the Internet while being
limited to a low-bandwidth transmission to the network.

A subscriber can continue to receive cable television service
while simultaneously receiving data on cable modems to be delivered to a PC with
the help of a simple one-to-two splitter (Figure 1). The data service offered by
a cable modem may be shared by up to sixteen users in a LAN.

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.
In this mode, a satellite or wireless cable television network can also function
as a data network.

At the cable head-end, data from individual users is filtered
by upstream demodulators (or telephone-return systems, as appropriate) for
further processing by a cable modem termination system (CMTS). A CMTS is a data
switching system specifically designed to route data from many cable modem users
over a multiplexed network interface. Likewise, a CMTS receives data from the
Internet and provides data switching necessary to route data to the 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 Star News, CNN or MTV for
broadcast to all users

cable head-end combines the downstream data channels with the video,
pay-per-view, audio and local advertiser programs that are received by
television subscribers. The combined signal is then transmitted throughout the
cable distribution network. At the user location, the television signal is
received by a set-top box, while user data is separately received by a cable
modem box and sent to a PC.

A CMTS is an important new element for support of data
services that integrates upstream and downstream communication over a cable data
network. The number of upstream and downstream channels in a given CMTS can be
engineered based on serving area, number of users, data rates offered to each
user and available spectrum (Figure 3).

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. The operations tasks 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 the geographic region.

Cable data system features

Beyond modulation and demodulation, a cable modem
incorporates many features necessary to extend broadband communications to wide
area networks (WANs). The network layer is chosen as Internet protocol (IP) to
support the Internet and World Wide Web services. The data link layer is
comprised of three sublayers: logical link control sublayer, link security
sublayer conforming to the security requirements and media access control (MAC)
sublayer suitable for cable system operations. Current cable modem systems use
Ethernet frame format for data transmission over upstream and downstream data
channels. Each of the downstream data channels and the associated upstream data
channels on a cable network form an extended Ethernet WAN. As the number of
subscribers increases, a cable operator can add more upstream and downstream
data channels to support demand for additional bandwidth in the cable data
network. From this perspective, growth of new cable data networks can be managed
in much the same fashion as the growth of Ethernet LANs within a corporate

link security sublayer requirements are further defined in three 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 a cable modem identity to paying subscribers and thereby
authorize subscribers access to data network services. Thus, the privacy and
security requirements protect user data as well as prevent theft of cable data

Early discussions in the IEEE 802.14 Committee referred to
the use of the asynchronous transfer mode (ATM) over cable data networks to
facilitate multiple services including telephone, data and video, all of which
are supported over cable modems. Although current cable modem standards
incorporate Ethernet over cable modem, extensions are provided in the standards
for future support of ATM or other protocol data units. IP—telephony support
over cable data networks is expected to be a new value-added service in the near

Cable data network architecture

data network architecture is similar to that of an office LAN. A CMTS provides
an extended Ethernet network over a WAN with a geographic reach up to 100 miles.
The cable data network may be fully managed by the local cable operations unit.
Alternatively, all operations may be aggregated at a regional data center to
realize economies of scale. A given geographic or metropolitan region may have a
few cable television head-end locations that are connected together by fiber
links. The day-to-day operations and management of a cable data network may be
consolidated at a single location, such as a super hub, while other head-end
locations may be economically managed as basic hubs.

basic distribution hub (Figure 4) is a minimal data network configuration that
exists within a cable television head-end. A typical head-end is equipped with
satellite receivers, fiber connections to other regional head-end locations and
upstream RF receivers for pay-per-view and data services. The minimal data
network configuration includes a CMTS system capable of upstream and downstream
data transport and an IP router to connect to the super hub location.

A super hub (Figure 5) is a cable head-end location with
additional temperature-controlled facilities to house a variety of computer
servers, which are necessary to run cable data networks. The servers include
file transfer, user authorization and accounting, log control (syslog), IP
address assignment and administration (DHCP servers), DNS servers and data over
cable service interface specifications (DOCSIS) control servers. In addition, a
super hub may deploy operations support and network management systems necessary
for the television as well as data network operations.

User data from basic and super hub locations is received at a
regional data center for further aggregation and distribution throughout the
network (Figure 6). A super hub supports dynamic host configuration protocol (DHCP),
DNS (domain name server) and log control servers necessary for the cable data
network administration. A regional data center provides connectivity to the
Internet and the World Wide Web and contains the server farms necessary to
support Internet services. These servers include e-mail, Web hosting, news,
chat, proxy, caching and streaming media servers.

addition to cable data networks, a regional data center may also support dial-up
modem services (for instance, 56 Kbps service) and business-to-business Internet
services. A network of switching, routers and servers is employed at the
regional data center to aggregate dial-up, high-speed and business Internet

A super hub and a regional data center may be co-located and
managed as a single business entity. A super hub is managed by a cable
television service provider (such as TCI in the US), while the regional data
center is managed as a separate and independent business (such as @Home in the
US). In some regions, an existing ISP may provide regional data center support
for many basic and super hub locations managed by independent cable data network

A regional data center is connected to other regional data
centers by a national backbone network (Figure 7). In addition, each regional
data center is also connected to the Internet and World Wide Web services.
Traffic between the regional networks, the Internet and all other regional
networks is aggregated through the regional data center.

Cable data network standards

A cable data system comprises many different technologies and
standards. To develop a mass market for cable modems, products from different
vendors must be interoperable.

To accomplish the task of interoperable systems, the North
American cable television operators formed a limited partnership, Multimedia
Cable Network System (MCNS) and developed an initial set of cable modem
requirements (DOCSIS). MCNS was initially formed by Comcast, Cox, TCI, Time
Warner, Continental (now MediaOne), Rogers Cable and CableLabs. The DOCSIS
requirements are now managed by CableLabs. Vendor equipment compliance to the
DOCSIS requirements and interoperability tests are administered by a CableLabs
certification program.

Some of the details of cable modem requirements are listed

Physical Layer

DOWNSTREAM DATA CHANNEL: At the cable modem physical layer,
downstream data channel is based on North American digital video specifications
(International Telecommunications Union [ITU]—T Recommendation J.83 Annex B)
and includes the following features:

  • 64 and 256 QAM

  • 6 MHz—occupied spectrum that coexists with other
    signals in cable plant

  • Concatenation of Reed-Solomon block code and Trellis
    code, supports operation in a higher percentage of the North American cable

  • Variable length interleaving supports, both
    latency-sensitive and latency-insensitive data services

  • Contiguous serial bit-stream with no implied framing,
    provides complete physical (PHY) and MAC layer decoupling

  • UPSTREAM DATA CHANNEL: The upstream data channel is a
    shared channel featuring the following:

  • QPSK and 16 QAM formats

  • Multiple symbol rates

  • Data rates from 320 Kbps to 10 Mbps

  • Flexible and programmable cable modem under control of

  • Frequency agility

  • Time-division multiple access

  • Support of both fixed-frame and variable-length protocol
    data units

  • Programmable Reed-Solomon block coding

  • Programmable preambles

MAC Layer

MAC layer provides the general requirements for many cable modem subscribers to
share a single upstream data channel for transmission to the network. These
requirements include collision detection and retransmission.

PRIVACY: Privacy of user data is achieved by encrypting
link-layer data between cable modems and CMTS. Cable modems and CMTS head-end
controller encrypt the payload data of link-layer frames transmitted on the
cable network. A set of security parameters including keying data is assigned to
a cable modem by the Security Association (SA). All of the upstream
transmissions from a cable modem travel across a single upstream data channel
and are received by the CMTS. In the downstream data channel a CMTS must select
appropriate SA based on the destination address of the target cable modem.

Network Layer

Cable data networks use IP for communication from the cable
modem to the network. The Internet Engineering Task Force (IETF) DHCP forms the
basis for all IP address assignment and administration in the cable network. A
network address translation (NAT) system may be used to map multiple computers
that use a single high-speed access via cable modem.

Transport Layer

Cable data networks support both transmission control
protocol (TCP) and user datagram protocol (UDP) at the transport layer.

Application Layer

All of the Internet-related applications are supported here.
These applications include e-mail, ftp, tftp, http, news, chat, and signaling
network management protocol (SNMP). The use of SNMP provides for management of
the CMTS and cable data networks.

Operations System

The operations support system interface (OSSI) requirements
of DOCSIS specify how a cable data network is managed. To date, the requirements
specify an RF MIB. This enables system vendors to develop an EMS to support
spectrum management, subscriber management, billing and other operations.


Cable modem technology offers high-speed access to the
Internet and World Wide Web services. Cable data networks integrate the elements
necessary to advance beyond modem technology and provide such measures as
privacy, security, data networking, Internet access and quality-of-service
features. The end-to-end network architecture enables a user cable modem to
connect to a CMTS which, in turn, connects to a regional data center for access
to Internet services. Thus, through a system of network connections, a cable
data network is capable of connecting users to other users anywhere in the
global network.

Copyright IEC
Reproduced with permission

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