If you want to get into the circuit-switched telephony business, you will need a complete room for the switch, another for the battery system, and possibly yet another room or rooms for transport termination equipment and network management, although this can be colocated with the switch.

HSA Corp.’s J.R. Anderson suggests a minimum starting point for headend telephony space requirements of 1,000 to 1,500 square feet, but 3,000 to 5,000 square feet is more common. He adds that many operators underestimate building and real estate needs when considering telephony.

Plan for future growth

Talk to the service provider and equipment manufacturers to get an estimate of how much equipment will be installed, and the amount of space required for all of it, as well as special access requirements such as oversized doorways. Keep future growth in mind as you go through this exercise. As you review equipment requirements, space is only the first thing to consider.

What about electrical requirements? Is your existing electrical service up to the task of operating the additional equipment, or will it have to be upgraded? Don’t forget about the quality and reliability of your headend’s electrical service. Digital transmission equipment often requires the use of power conditioning hardware to clean up "dirty" power. A UPS—possibly in conjunction with a backup generator—will help with reliability. Improved grounding and lightning protection bear consideration as well.

Next on the list is the heating, ventilation and air conditioning (HVAC) system. Its capacity almost certainly will have to be upgraded, especially the air conditioning equipment. If possible, design the HVAC with some measure of redundancy, so that equipment failure won’t put the entire headend air conditioning system out of service. An upgraded humidity control likely will be necessary because too-dry air can lead to damaging static electricity. At the other extreme, excessive humidity also can be harmful.

A problem area that is easily overlooked is site security. When you begin providing digital services, especially something as critical as telephony, physical site security will have to be a major part of your deployment plans. Be sure to include computer and software security. Fences, video cameras and alarm systems may deter headend break-ins, vandalism and equipment theft, but appropriate firewalls will be necessary to keep hackers out of your system.

Finally, you’ll need to carefully evaluate staffing needs. Most conventional headends essentially are stand-alone facilities, requiring only periodic maintenance visits. Depending on the nature of the digital service, you may find it necessary to have the headend permanently manned. Furthermore, as you move into the digital world, training will become an important part of your daily operation (more on this later).

Is your network in good shape?

Although some types of digital services will work fine on just about any network architecture, the system itself must be in good condition. In general, if a system is properly constructed, well-maintained, and provides reliable and high-quality analog service, then digital deployment will be a lot easier. One benchmark is compliance with performance specifications such as the Federal Communications Commission cable TV technical rules. (See, "Minimum Guidelines for Analog Operation.")

Network architecture and bandwidth are especially important for interactive digital technologies such as cable modems and telephony. Compressed digital video and digital audio services will work on traditional tree-and-branch coax architectures as long as the network and drops are in good condition. But interactive digital services benefit from small, segmented service areas provided by hybrid fiber/coax (HFC) architectures.

Smaller service areas, or at least the ability to easily migrate to smaller service areas in the future, are closely related to efficient reverse path frequency re-use and traffic management. They also allow the system to provide targeted services based on demographics and geography.

If you are contemplating telephony, the architecture will have to accommodate powering network interface units (NIUs) on the customer premises. It will require you to analyze the features of centralized vs. distributed powering, the use of power-passing taps, and the economics of siamesed drop cable vs. powering NIUs directly through the drop cable vs. powering the units from inside the home.

Beef up your maintenance plans

As you evaluate the network’s condition, look closely at maintenance practices. If you have an effective preventive maintenance program, then the plant should be in very good condition. Such a program will have addressed broadband sweeping, active device operating levels, signal leakage and other performance criteria.

If you haven’t had particularly good maintenance, you will need to improve the network’s performance before launching a one-way digital service. Interactive digital services that use the cable network for bidirectional operation require that the system’s overall quality and performance be even better, especially in the reverse path.

Even though digitally modulated signals technically are analog signals, they can be more susceptible than analog TV channels to network and drop problems such as ingress, loose connections, frequency response problems and improper equipment alignment. The industry discovered long ago that digital signals, even relatively simple addressable converter control carriers, force a system operator to do a better job.

Speaking of maintenance, once you begin to provide digital services, you’ll quickly discover that you must add new maintenance practices to your daily operations. You still need to continue existing practices, but digital services require additional maintenance efforts.

One of the biggest problem sources when deploying digital services is the subscriber drop. Substandard installation practices and low-quality drop materials will cause performance problems with digital signals. For more information on this subject, check out the article "Bulletproof Your Drops," which appeared in the April 1999 issue of Communications Technology.

General Instrument’s Joe Waltrich has outlined additional system considerations and precautions for deployment of digital services. They include the following:

For the most part, I agree with these precautions. For setting digital carrier levels, I highly recommend test equipment that has digital carrier power measurement capability built in. This type of equipment is readily available from several test equipment manufacturers and will provide more consistency and accuracy while avoiding potential errors that can take place when performing manual measurements using instrument correction factors.

My second comment on these precautions has to do with broadband sweep. I am aware of at least two manufacturers that have low-level sweep equipment claimed to be capable of sweeping through digitally modulated carriers with little or no interference.

My personal preference is to sweep the entire spectrum so that overall frequency response can be monitored for problems. Still, it would be a good idea to check first with the manufacturers of your digital transmission equipment for recommended maintenance practices.

Operations is the hard part

Dealing with technical issues and making the network and drops digital-ready are, believe it or not, the easy part of deploying digital services. In fact, the launching of digital will impact the entire cable TV system operation. At an SCTE Cable-Tec Expo training workshop, Cox Communications’ Richard White summed it up quite clearly in the introduction to his presentation about digital video services:

"All areas of the business are affected by the introduction of digital TV (DTV). Marketing now has more services to sell—a definite plus. The customer has more programming choices, new services and new features—another plus.

"The technicians, engineers and management information system (MIS) operators on the other hand, have more channels and services to monitor and maintain while customer service representatives and installers have new services and technical requirements to learn.

"The increased time, resources and training required to deliver digital video can place a sizeable burden on the day-to-day operations of the system if one is not prepared to handle these demands."

Consider the warehouse, something seemingly unrelated to digital services. Most contemporary digital set-tops are much larger than analog converters, often approaching the size of a CD player or videocassette recorder (VCR). At the very least, White says, the larger set-tops will require more warehouse space.

If "burn-in" testing of new set-tops is performed in the warehouse, additional space may be required. And because

digital set tops are worth several hundred dollars each, security of warehouses and technician and installer vehicles must be tightened.

Don’t forget training

A critical operations issue is training. The installer, for instance, will have to be skilled in much more than just subscriber drop installations. Because of the variety of new digital services that will be available, the installer will have to thoroughly understand each of these services and, at times, be in a sales and marketing mode.

It will be unacceptable to our subscribers if they ask questions about one or more of the new services, only to be told by the installer—or customer service representative (CSR), for that matter—to call someone else for the answer.

Installers and technicians will require very broad skills that go far beyond RF and traditional cable TV technology. Installers of the future also will need to understand Ethernet, data and telephone networks, personal computers (PCs) and software, home security systems, addressable TV sets, videoconferencing systems, telephone network features, and wiring home appliances.

Customer service and support are important parts of digital deployment, especially for services such as cable modem Internet access. This type of service requires knowledgeable support staff and CSRs who can deal with Internet access questions, computer problems and software issues.

Some operators have partnered with local firms that have this expertise and staffing already available. Depending on market conditions and customer expectations, 24/7 call center support may be necessary.

Get cracking

Preparing for deployment of digital services may not be rocket science, but it does require a lot of preparatory and ongoing work in both technical and operations areas. This article has touched on a few of the major considerations, but obviously many other factors must be taken into account.

As we look for alternative revenue sources in today’s competitive environment, emerging digital technologies appear to hold the most promise. We need to make sure our systems and personnel are ready for digital because if we falter here, our credibility will be at stake. No longer are we the only game in town, and consumers will take their business where they feel they are getting the most value for their money.

Minimum Guidelines for Analog Operation

The Federal Communications Commission technical regulations for cable TV systems (FCC Rules, Part 76, Subpart K, Technical Standards) often serve as the basis for defining minimum acceptable system performance criteria for the launch of digital services. Here is a summary of key FCC technical requirements for analog operation.

If your system meets or beats these numbers everywhere and on every channel, you will have fewer problems deploying digital services than systems that don’t meet these specs.

Parameters

Minimum visual carrier amplitude:

Chrominance-to-luminance delay inequality:

Differential gain:

Differential phase:

Signal leakage (less than and including 54 MHz and greater than 216 MHz):

Signal leakage (over 54 MHz up to and including 216 MHz):

Don’t Fall Off the Digital Cliff

Just when you get used to using your signal level meter (SLM) to set visual and aural carrier levels, using a spectrum analyzer for carrier-to-noise (C/N) and distortion measurements, and interpreting your broadband sweep receiver’s display for troubleshooting frequency response problems, along comes digital!

In many cases, short of a full-blown outage from a cut cable or some other catastrophic problem, analog performance degradation is gradual. That is, as C/N gets worse, analog TV picture quality deteriorates. Even as things go from bad to worse, usually some semblance of picture and sound remains until things eventually fade into oblivion. Not so with digital. Things remain pretty much perfect until the so-called crash point, and then everything goes away. The difference between working and not working usually is on the order of 1 dB or less.

In an analog world, you generally can tell if things are getting bad. With digital, there is no visible warning. Unless you can make certain digital-specific measurements, you really have no way of knowing how close to this "cliff" your digital signals are.

What, you might ask, are those digital-specific measurements, and why can’t conventional test equipment be used?

Many of the measurements we perform on analog TV and similar signals don’t apply in the digital world. For digital, you need to consider a whole new suite of measurements, most of which require some specialized test equipment, or at least an upgraded version of some of the test equipment you’re now using for analog measurements.

Fortunately, several test equipment manufacturers have instruments available that can make a host of measurements on digitally modulated carriers. In a few cases, it’s even possible to upgrade existing test equipment firmware to at least allow accurate digitally modulated carrier power measurements.

If I were thinking about buying test equipment to make digital measurements, here are some capabilities I’d look for:

Additional capabilities might include error vector magnitude, estimated noise margin, adjacent channel power, channel power-to-noise and severely errored seconds. When choosing test equipment for digital, make sure it can measure the specific signals carried on your network: 64-QAM (quadrature amplitude modulation), 256-QAM, coded orthogonal frequency division multiplexing (COFDM), quadrature phase shift keying (QPSK) and so on.

Some instruments can make all or most of the measurements I’ve just listed (and sometimes also analog TV channel measurements) in one box, but only for one or two modulation formats.

This article originally appeared in sister publication "International Cable" and has been updated.

Ron Hranac is vice president of RF engineering for High Speed Access Corp. He also is senior technical editor for "Communications Technology." He may be reached via e-mail at .