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.

In

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.

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.

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

A

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

environment.

The

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

services.

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

term.

Cable data network architecture

Cable

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.

A

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.

In

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

services.

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

providers.

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

below.

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 —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
  
  plants

• 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
  
  CMTS

• 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

The

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.

Conclusion

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 www.iec.org



Reproduced with permission