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Adding Digital Television Services to Cable Systems


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Adding Digital Television Services to Cable Systems



Drivers for Digital Television



Transmission of Digital Television



Out-of-Band Data Communications



Out-of-Band Channel Termination



Headend-to-Distribution Hub Interconnection







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OpenCable Architecture

From: OpenCable Architecture
Author: Michael Adams
Publisher: Cisco Press (53)
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5. Adding Digital Television Services to Cable Systems

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:

  • 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.

  • 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

  • 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.

  • 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.

  • 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).

  • 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”).

Drivers for Digital Television

Digital channels have some major technical advantages over analog channels:

  • 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).

  • 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).

  • 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”).

  • 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:

  • Channel expansion—Digital television squeezes many more channels into a given amount of cable spectrum than analog.

  • DBS competition—Matching DBS channel line-ups and providing the all-important “digital picture and sound” sticker.

  • High definition television (HDTV)—Compressed digital formats that provide the only way to send HDTV in North America.

  • Consolidation with other digital services—Sharing the network infrastructure necessary for digital television with cable modem and telephony services.

  • RF return traffic—The aggregation of reverse traffic.

  • Business communications—Opportunities to reduce existing data communications costs by using a shared exterprise network.

  • Network management—The increased need for network management to provide a highly reliable cable system.

Channel Expansion

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.

Direct Broadcast Satellite Competition

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

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 payload of 19.24 Mbps, enough to carry a single HDTV channel. (See the section Broadband Transmission in Chapter 4.)

Consolidation with Other Digital Services

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.

Radio Frequency Return Traffic

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.

Business Communications

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.

Network Management

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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