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Adding Digital Television Services to Cable Systems
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From:
OpenCable Architecture
Author: Michael Adams
Publisher: Cisco Press (53)
More Information
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Cable operators are adding many types of digital services to their systems,
including digital television, cable modem services, and telephony. This chapter
focuses on digital services delivered through the television receiver. Such
services might include broadcast, interactive, and on-demand services, but
they have some important characteristics in common:
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The service fits into existing television viewing habits. This is sometimes
called the lean-back
model to contrast it with the lean-forward model of personal computer
use. For example, the audience expects to be entertained, educated, and
informed—viewing often coexists with other social activities, such
as conversation, drinking, and eating.
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Video and audio quality are important to the service, to captivate the
viewer despite other demands on his or her attention. (In the entertainment
world, these are called production
values and actually extend far beyond technical measures of quality,
such as signal-to-noise ratio and video bandwidth.)
For convenience, I will refer to all these services generically as digital
television to suggest a much broader category than digital picture and
sound.
The first part of this chapter discusses the technical and market drivers
for digital television. The second part of the chapter concentrates on the
necessary enhancements to the cable network to support digital television
services, which can be summarized as
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Hybrid fiber coaxial (HFC) upgrade (see Chapter 2,
“Analog Cable Technologies”)—Strictly speaking, it
is not necessary to upgrade the cable system to carry digital television
services. However, the additional spectrum required to carry both analog
and digital television is a welcome side effect of an HFC upgrade.
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Broadband transmission (see Chapter 4, “Digital
Technologies”)—All cable systems are broadband
analog networks, which means that
any digital signals must be modulated onto a radio-frequency (RF) carrier
before they can be carried by the cable system.
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Out-of-band communications—Cable systems were originally conceived
as one-way broadcast networks. Adding a forward out-of-band channel, which
is routed (to the distribution hub or fiber node), provides significant
operational advantages. Adding a reverse out-of-band channel completes
the two-way signaling path, effectively placing each digital set-top on
a local area network (LAN).
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Data communications overlay—Interconnecting all the distribution
hub LANs to the headend equipment with a data communications overlay network
completes the infrastructure required for digital television and also
enables interactive and on-demand services (see Part
II, “Interactive and On-Demand Services”).
Digital channels
have some major technical advantages over analog channels:
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Each additional digital channel
can carry from 10 to 14 services in the same bandwidth that is occupied
by a single analog channel (see the section Video Compression in Chapter
4).
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Digital channels are less sensitive to noise and distortion compared
to analog channels, allowing the cable operator to run the digital channels
at lower power levels than the analog channels to reduce laser loading
and overall distortion (see the section Broadband Transmission in Chapter
4).
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Digital channels can carry data as easily as they carry video and audio.
This means that new services can be added relatively easily to the payload
(see Chapter 8, “Interactive Services”).
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Premium digital channels are
secured using digital cryptographic techniques that are much harder
to break than analog scrambling (see Chapter 6,
“The Digital Set-Top Converter”).
However, the customer requires a relatively expensive digital set-top to
receive digital channels (a capital outlay by the cable operator of about
two to three times the cost of an analog set-top converter). Moreover, the
customer cannot receive digital channels with an analog cable ready receiver.
(See Chapter 18, “OCI-C2: the Security Interface,”
for a discussion of cable ready digital television.)
This section addresses the drivers for digital television by addressing
the following:
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Channel expansion—Digital
television squeezes many more channels into a given amount of cable
spectrum than analog.
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DBS competition—Matching DBS channel line-ups and providing the
all-important “digital picture and sound” sticker.
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High
definition television (HDTV)—Compressed
digital formats that provide the only way to send HDTV in North America.
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Consolidation with other digital services—Sharing the network
infrastructure necessary for digital television with cable modem and
telephony services.
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RF return traffic—The aggregation of reverse traffic.
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Business communications—Opportunities to reduce existing data
communications costs by using a shared exterprise network.
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Network management—The increased need for network management to
provide a highly reliable cable system.
It is a fundamental
truth that no cable operator can ever have too much bandwidth or too many
channels. New channels appear each year (for example, the Radio City Music
Hall channel, BBC America, Food Network, Lifetime, OVATION, International
Channel, Nick-At-Nite TV Land, Outdoor Channel, Discovery Kids, and Game
Show Network, to name just a few), and this trend shows no sign of slowing
down. Because each additional analog channel requires an additional 6
MHz of valuable spectrum, cable operators spend significant time and energy
considering which channels to carry. (If you visit the Web site of a new
channel, you will probably be asked to help lobby for carriage on your
local cable system.) In New York, Manhattan Cable already carries more
than 100 analog channels.
By using digital channels to carry new services, the pressure on cable
system spectrum is reduced. At some future time, it will be possible to
collapse existing analog channels into digital channels.
DirecTV, EchoStar, and others use “digital picture and sound
quality” to market their services. In fact, poorly compressed digital
video can be markedly inferior to analog (see the section Video Compression
in Chapter 4). Digital video compression
allows the operator to precisely control the amount of quality granted
to each video channel without regard to the intervening transmission.
This gives a digital operator the flexibility to offer some channels at
higher quality than analog and some at lower quality—a flexibility
that is quite valuable in terms of maximizing revenue.
Because marketing is often a game
of “spec”-manship, it is important for cable operators to
match the specifications claimed by digital broadcast from satellite (DBS)
operators.
High definition
television (HDTV) offers a resolution of up to 1,920 horizontal
pixels by 1,080 vertical lines. Multiplying these numbers gives a pixel
count of 2,073,600 pixels per frame. This is exactly six times as many
pixels as a standard-definition (or full-resolution) picture with 720
horizontal pixels and 480 vertical lines.
HDTV offers a stunning increase in picture quality, and at normal viewing
distances is almost as good as the human eye; for this reason, an HDTV
picture has been described as “like looking through a window.”
Because of its high bandwidth requirements, digital compression provides
the only practical way to deliver HDTV.
HDTV Bandwidth Requirements
Without compression, an
HDTV picture would require a bit rate of about 746 Mbps (assuming
8-bit, 4:2:0 sampling—see the section Picture Resolution in Chapter
4). If this signal was analog-modulated with the same efficiency
as NTSC, it would require six times the 4.2 MHz video bandwidth of
NTSC, or 25.2 MHz. In practice, the European HDMAC-60 system requires
24 MHz and the Japanese Multiple
Sub-Nyquist Sampling Encoding (MUSE) system requires 9 or 12 MHz.
Each of these systems is designed to use the entire bandwidth of a
satellite transponder.
There is insufficient radio frequency spectrum to carry many analog
HDTV channels, even on upgraded cable systems. In fact, HDTV is so
bandwidth-hungry that no North American standard exists for analog
HDTV transmission, and digital television provides the only form of
HDTV carriage.
Using MPEG-2 main
profile at high level (MP@HL) compression, the bit rate of an HDTV
signal can be reduced to 18.8 Mbps, an MPEG compression ratio of nearly
40:1. (This is similar to a typical 31:1 compression ratio for SDTV—see
the section Video Compression in Chapter 4.)
The North American standard (ITU J.83 Annex B) for QAM-256
modulation is designed with HDTV
in mind. The QAM-256 transport payload is 38.48 Mbps, enough to carry
two HDTV channels. Similarly, the VSB-8 (which is the North American
terrestrial broadcast modulation standard defined by ATSC A/53) has
a transport payl oad
of 19.24 Mbps, enough to carry a single HDTV channel. (See the
section Broadband Transmission in Chapter 4.)
Cable operators
have offered many new service offerings
based on digital technology—for example, local-area network (LAN)
interconnection, telephony, cable modem services, meter reading, and power
management. Some of these services cannot justify their own network infrastructure
but generate significant revenue if they ride on a network infrastructure
deployed for digital television.
Many cable operators
have deployed advanced analog converters and are already struggling with
an efficient way to aggregate radio-frequency (RF) return traffic and
transport it back to the headend controller. Terminating the RF return
at the hub and using a digital network to transport the return traffic
to the headend is an attractive solution.
Nearly all
cable operators have some existing leased or dial-up lines that are used
to connect to billing providers and other services. The connection charges
on these lines are an ongoing expense, and the ability to run legacy
data communications, such as IBM's Systems Network Architecture (SNA),
over a shared network infrastructure is an important consideration. Like
any other business, cable operators are adopting enterprise network approaches
to consolidate their internal communications needs to interconnect computers
systems, LANs, and PBXs. Some of the larger cable systems span multiple
area codes, and operators can save long-distance charges by using their
own network infrastructure to carry voice calls.
As digital services
develop, cable operators have to address a new area: network management.
New services being offered require higher availability (for example, telephony),
and it is no longer sufficient to react to outages by truck rolls and
to maintain headend equipment by responding to customer phone calls. A
more proactive approach to network management often requires real-time
monitoring of hub equipment, power supplies, and, in some cases, amplifiers.
The network management traffic is digital in nature and often uses the
Simple Network Management Protocol
(SNMP) and requires a digital network overlay on top of the existing analog
network.
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Breaking News
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One of the primary architects of OpenCable, Michael
Adams, explains the key concepts of this initiative in his book
OpenCable Architecture.
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Just Published
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 Residential
Broadband, Second Edition
by George Abe
Introduces the topics surrounding high-speed networks
to the home. It is written for anyone seeking a broad-based familiarity
with the issues of residential broadband (RBB) including product
developers, engineers, network designers, business people, professionals
in legal and regulatory positions, and industry analysts.
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