Preparing the drop for digital services

Sat, 09/30/1995 - 8:00pm
John Grothendick, Product Development Manager, Antec Corp.

Despite new technological advances in all parts of the cable network, the subscriber drop remains the weakest link. The drop, which is the most labor intensive and costly part of the network, is probably the least equipped to deliver interactive digital services. Today, even with more forgiving analog signals, problems with the drop generate seven out of 10 service calls. If today's drop generates so many problems, imagine the failure rate if operators attempt to use today's drop for tomorrow's more demanding digital services. Customers will demand high quality, interactive digital services, or they'll go elsewhere.

What's emerging

Most advanced cable architectures today have driven fiber optics deep into the plant, transitioning networks from a tree-and-branch beginning to star architectures that rely on coaxial cable with no more than two amplifiers feeding each home. Optical node sizes have shrunk from the original 10,000 or 5,000 homes to 2,000, and now, to 500 homes or less. The fewer homes served by a single fiber, the greater the spectrum availability, and the lower the level of maintenance required. Operators positioning their systems in this way are future-proofing their networks for interactive services and two-way digital communications.

Yet, coaxial cable continues to provide the most robust medium to the home in the delivery of voice, video and data services. A good, high quality drop, installed and maintained correctly, could, theoretically, download the entire Library of Congress in just 15 minutes (experts suggest that normal twisted pair transmission would take more than two years).

New modulation techniques are emerging that are moving the industry toward digital transmission. In today's analog world, amplitude modulation (AM) provides the most basic form of electronic signal delivery. New advancements in Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM) and Vestigial Side Band (VSB) are beginning to emerge. Each has been developed by a different manufacturer for the purpose of squeezing more digital information into a single 6-MHz analog channel. This results in brand new considerations for cable operators in establishing a drop system that will remain transparent despite the type of modulation technique used.

In order to provide a reliable, seamless transmission path for video, voice and data services, the drop system must successfully pass a 1 GHz spectrum, digitally compressed signals and two-way interactive services.

Most industry experts agree that a 5 MHz to 1000 MHz spectrum is required to provide all the services subscribers will be looking for over the next 10 to 15 years. Allocation of the 1 GHz platform varies only slightly among the major multiple system operators (MSOs) involved in hybrid fiber/coax network deployment. A typical spectrum allocation may look like this:

  • The traditional sub-low band has been expanded to cover 5 MHz to 40 MHz and will provide the return path for telephony and other related smaller payload deliveries in a QPSK modulation format.
  • The traditional 50 MHz to 550 MHz band will be reserved for forward analog signal transmission to accommodate traditional receiving devices.
  • The spectrum area from 550 MHz to 750 MHz would be used for digitally compressed video, digital music, interactive games, advertising, etc. The modulation scheme for these services would primarily be 64 and 256 QAM and 16 VSB, and will handle higher bit payloads of up to 45 Mbps or better.
  • The upper end of the 1 GHz platform would be reserved for high-speed, two-way digital services and PCN, with various payloads and modulation formats of different "bit to hertz" ratios.

AM analog signals degrade on a graduated scale before a failure threshold is reached. Digitally delivered signals, with their associated modulation formats, will perform at optimal level until their failure threshold is realized. These thresholds become lower and lower as the modulation format becomes more sophisticated.

Once the bit error rate threshold is reached, the signal becomes virtually unrecoverable. Where an analog signal will simply grow snowy or look "ghosty," bit errors will create a tiling or checkerboarding effect or incorporate other artifacts that will virtually ruin a digital picture.

Of the advanced modulation formats, QPSK is the least sophisticated and the most robust. It offers a failure threshold of 3 x 10-6. That means signals can experience no more than three bit errors per 1 million bits (as tested by a bit error rate detector). Thresholds for QAM and 16 VSB modulation are in the 10-7 or 10-8 range (three bit errors per 10 million to 100 million bits).

Antec has tested drop systems using the QPSK digital modulation format at speeds of up to 2.3 megabits per second. Components tested in the drop included cable connectors, splitters, passives, amplifiers and hardware. In a test comparing standardized, high quality drop components against non-standardized, off-the-shelf devices, the low-quality components consistently performed more poorly. The reasons primarily rest in a lack of proper shielding, impedance matching problems, high attenuation, and substandard environmental protection.

Tests of 16 and 256 QAM as well as 16 VSB will be performed within the next four months. These tests will be done with a variety of payloads and in different bands of the spectrum. The performance parameters detailed in the remainder of this article are the results from data collected when testing with the QPSK modulation technique.

The proper selection of drop cable, connectors, passive devices, house amplifiers, and related hardware is critical to the long-term viability of a drop. Proper installation and maintenance of each component can mean the difference between a high quality drop and one that will need to be modified, or at worst, completely reinstalled when digital services become more prevalent.

Recommended performance parameters

Drop cable: In selecting drop cable, operators should use cable no smaller than RG-6 for systems over 550 MHz. For drops spanning over 150 feet, operators should rely on RG-7 or RG-11 coaxial drop cable. Each drop cable should be sweep tested to 1 GHz to ensure that the cable can handle signals from 5 MHz up to 1 GHz.

Operators should select drop cable offering a 75 2 ohms or better impedance level. Impedance is determined by the distance between the center conductor of the cable and the outer conductor, and is related to structural return loss. This distance must remain consistent throughout the cable, from its installation at the tap through its connection to the home. For this reason, installers shouldn't bend cable more than 10 times its diameter. Crimping the cable, bending it too tightly, even stepping on it can make the cable oval in spots that will impact its impedance and thus, deliver lower levels of performance into the home.

Structural return loss is specified at 23 dB. Flexing of the aerial drop will impact this structural return loss specification, sometimes by as much as 10 to 15 percent. Poor structural return losses are the result of impedance mismatching and can cause microreflections. In turn, this can cause bit errors and damage to digital signal integrity.

For aerial applications, operators should use a messengered cable so the "messenger" provides the support and removes strain from the coaxial cable itself. For underground applications, a flooded, polyethylene jacketed cable is recommended for weather and water protection.

Shielding on drop cable is equally important. In addition to the aluminum tape, drop cable should have a minimum of 60 percent braid coverage to reduce the impact of ingress (signals getting into the system) or egress (signals getting out of the system).

Connectors: Connectors should offer shield effectiveness at 80 dB or better within the 5 MHz to 1 GHz spectrum. This type of shielding will again prevent ingress/egress problems. A circumferential seal at the port/connector interface is necessary in outdoor cable applications, and the author's company recommends using a 360-degree compression connection at both the tap and home connections (a hex fitting may reduce the impedance or drop cable return loss by crimping the cable). For the F-connections, use torque specifications outlined by the manufacturer or tighten the connection with an extra quarter turn to ensure a tight fit. Seal all connections at the connector/cable interface to provide effective weather protection and prevent ingress/egress.

Passives: Splitters and other passive devices should effectively pass 5 MHz to 1 GHz and provide 110 dB EMI shielding. Each passive should include a 1/2 long mechanical F-port to accommodate the deep threading used for many premium connectors. Each passive should also provide a minimum of 15 dB input/output return loss.

Select passives with a rugged housing and backplate (stainless steel or zinc with tongue and groove construction). Many of today's passive devices use printed circuit boards which can radically improve electrical performance, since PCBs standardize the manufacturing process. Check to see that the PCB is properly mounted and grounded on standoffs that are well connected to the housing. Poor grounding, as well as substandard components, will act as a source of increased return loss in a splitting device.

The primary problems during installation rest in installer abuse of passives. In many cases, splitters and the like are tossed into a box to jostle around during transport, damaging the components.

It's important that passive devices be treated with the same care as optical transmitters. While much less expensive, passives perform an important network task and won't perform as specified if they are damaged.

During the installation process, installers should assess which passive should be used for individual subscribers. For instance, one home may have three TVs, two located in roughly the same area; a third may be a significant distance away. In this case, a balanced splitter may impair the level of signals delivered to each TV. While the closest two will have a proper signal level, the third may not. An unbalanced splitter then can compensate for changes in the distance of each TV's location.

House amplifiers: Select a two-way compatible amplifier that can use the 5-40 MHz return path without modification. The amplifiers should be sweep tested to 1 GHz, deliver less than 7 dB of noise and provide a minimum of 13 dB of return loss. Pay attention to CSO figures; some manufacturers of house amplifiers often do not provide this information directly.

The Society of Telecommunication Engineers' Interface Practices Committee is studying CTB and CSO parameters for house amplifiers and will likely have specifications soon.

Hardware: Select hardware that is Underwriters' Laboratories listed and which meets ASTM galvanizing specifications. Do not purchase hardware that contains staples, clips, or anything else that requires use of a hammer. Hammers are the worst enemies of good drop installers; typically a hammer will hit the cable (even inadvertently) and result in the impedance or structural return loss that leads to a less than perfect drop installation.

Surge protection: Protection from surges in the form of lightning or other "rapid rise time transients" remains another critical concern in the drop. As new devices are added to the network-telephony interfaces, broadband modems, etc.-inadequate surge protection may prove to be life-threatening to a subscriber. It may no longer be acceptable to show that the drop was properly bonded to alleviate the system operators from surge damage liability. Not only may a customer be lost to the competition, but costly legal problems may also result.

Bonding the drop still remains the primary surge protection method for the drop system. It's wise to check the National Electrical Code and local ordinances on bonding to ensure that your system is effectively bonded. If possible, attach a short #12 or larger bond wire to the grounding devices installed by the local power utility company. Avoid severe bends in the bonding wire that would present extra impedance to the surge going to ground.

In-home wiring: Operators are extremely concerned that homeowners can cable their own additional outlets. Operators are responsible to the FCC for the disposition of the signal up to the subscriber terminal device. Although totally controlling home wiring will be impossible, there are still some measures that the operator can take to ensure its quality. Public service announcements and bill stuffers can educate subscribers as to the importance of using system recommended equipment and procedures.

Subscriber drops still comprise up to 75 percent of the investment made in the broadband infrastructure. By installing non-compatible, poor performing drop components-or failing to properly train installers on how those components should be installed-the network operator, who is otherwise building a reliable and high-quality system, will suffer from lower network performance at the subscriber's home. Without effectively training personnel on proper installation, operators face increased operational costs for drop servicing, potentially lower revenues from consumers dissatisfied with the quality of services, and added capital costs as new drops must be installed to handle future services.

Drops require quality components and a specialized, well-trained staff that can ensure that each drop is installed correctly and performs effectively-at every single home.



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