Video over fiber plays center stage at cable-TV show
Video over fiber plays center stage at cable-TV show
n GEORGE LAWTON
Sponsored by the Society of Cable Telecommunications Engineers (Scte), the Cable-Tec Expo `96 Conference, held in Nashville, TN, reinforced fiber`s key role in enabling cable-TV operators to increase network reliability, add more video channels and deliver telephony services. More than 450 vendors showcased their latest products in cable-TV technology to a crowd of approximately 8000 cable-TV engineers.
Several companies provided products that pushed the limits of commercial video-transmission technology. For example, ADC Video Systems in Meriden, CT, has developed a dense wavelength-division multiplexing (WDM) system for its DV6000 video transport system that can carry 20 Gbits/sec (see figure). A commercial version that handles 10 Gbits/sec --much faster than the 2.4-Gbit/sec systems now in use for video transport--is expected to ship this year.
In this commercial system, dense WDM technology combines four 2.4-Gbit/sec signals in the 1550-nm region onto a single fiber. Because each signal carries 16 channels, the fiber can transport 64 channels. A total of 80 channels can be handled if an additional signal is added at 1310 nm.
John Holobinko, vice president of strategic planning at ADC Video, claimed the DV6000 and its dense WDM capability set a new industry standard for digital-video transport. In keeping with the company`s preference for modular architecture, this technology is also compatible with existing DV6000 products.
Time Warner pact
Also at the show, Lucent Technologies in Murray Hill, NJ, and Antec, based in Rolling Meadows, IL, signed a contract with Time Warner for 50 miles of high-fiber-count AccuRibbon cable (see Lightwave, August 1996, page 1). This cable combines 24 ribbon sections of 12 fibers each for a total of 288 fibers. The cable design represents the highest-fiber-count ribbon cable available, according to Lucent.
A key problem cable operators face as they deploy fiber and digital technologies in their networks is how to fill the additional channel capacity. Help to solve this problem comes from TCI in Denver, with a product called digital Headend in the Sky (Hits). Charles Kennamer, senior director of technical operations at TCI Hits, plans to begin service for the company`s first commercial customers next month.
Initially, Hits is expected to carry five channels of digital video within the 6 MHz of bandwidth that is normally used to carry analog video and will provide only about 40 channels of compressed video; but channel capacity should expand as more programming is added. Kennamer said the company is currently exploring technologies such as statistical compression, which could put 20 compressed channels in each 6 MHz of bandwidth.
A major problem of cable-TV networks is the industry`s poor reputation for reliability. However, network reliability has been improving as the industry has developed practices to deal with cable cuts, which cause more than 50% of all major outages, according to Bell Communications Research and Federal Communications Commission statistics. John Chamberlain, vice president of Norscan in Conover, NC, gave a presentation on optimizing fiber-optic availability. He noted that there are ways to increase network reliability using proactive management techniques.
Chambers pointed out that most cable-TV systems that have network management software are only able to find catastrophic failures. However, it is possible to create a monitoring system that looks for failure before it happens and alerts technicians to perform preventive maintenance.
For example, Norscan has developed a system for monitoring the integrity of a fiber-optic cable. It puts a 48V current onto the fiber sheathing. If integrity of the cable jacket is disrupted (for example, by a rodent), an alarm activates and indicates the approximate location of the problem before the fiber is damaged.
As cable-TV operators prepare for new services, they need to create a return-path channel for carrying data to the headend. This channel is required for two-way services such as telephony and Internet access; but today, only 10% of all cable systems provide interactivity because of the high cost of setting up the return path.
One way of reducing costs is by using cheaper lasers. Larry Stark, vice president of broadband market development at Ortel Corp., discussed benefits of using distributed-feedback (DFB) lasers, as opposed to Fabry-Perot lasers for return-path communications. He noted that the difference between the two lasers at a performance level is caused by mode-distortion noise. The Fabry-Perot laser uses a grating to concentrate most of its energy into several distinct wavelengths. The power in each wavelength is always changing due to noise, but, when measured, the power adds up to a constant value because the noise effects cancel out. Dispersion caused by the fiber acts on each wavelength differently, so after the wavelengths have traveled down a length of fiber, the noise effects do not cancel out, resulting in signal distortion.
DFB lasers do not suffer from this problem nearly as much because their energy is more focused. However, DFB lasers are more sensitive to reflections. To address the need for such a laser, Ortel showed its model 1651/B laser line. These lasers come in a cylindrical package with a monitor photodiode and an internal optical isolator to prevent reflections. More importantly, they do not have to be cooled, which enabled Ortel to shrink their size and, hence, their cost.
Because most hybrid fiber/coaxial-cable networks use a single optical node to feed multiple coaxial-cable legs in the distribution segment of the network, another approach to reducing the cost of return-path systems is to place multiple return-path signals onto a single fiber. Most cable systems in the United States reserve 5 to 50 MHz for return-path communications on the coaxial-cable plant side; however, a single fiber is capable of carrying more bandwidth.
For example, Phasecom Inc. in Cupertino, CA, has developed the Spectrum Organizer, which puts the return-path signals from four coaxial-cable legs onto a single fiber. At the Scte show, the company demonstrated a new version for the U.S. and European markets that carries three sets of return-path signals at a lower cost.
As many different companies have created equipment for offering different digital services, cable-TV operators are faced with the task of integrating these into a single system. Albert Young, engineering manager at Cox Communications in Atlanta, noted, "When you allocate upstream bandwidth for each service, you may find you don`t have enough spectrum to allocate. On the other hand, many of these digital applications are bursty, and by allocating a single data stream that is shared, you can get around that problem. Furthermore, all digital systems currently for sale were designed against analog video but not against each other. There may be some interactions we have not predicted that may make them difficult to control."
Single digital architecture
To address this issue, Cox is working with Northern Telecom in Atlanta to develop a single digital architecture to handle all their services. Jack Terry, associate vice president of access architectures at Northern Telecom, gave an overview. The basic concept is that a network interface unit (NIU) on the side of the home would manage all digital communication between the home and the headend. Digital signals would come down the network as 256- or 64-quadrature amplitude modulation (QAM) signals, which carry a lot of data but are expensive and sensitive to noise.
The NIU converts all signals into binary phase-shift keying (Bpsk) signals that enter the home. These signals are cheaper and more robust than QAM. Although Bpsk is not as efficient in its use of spectrum, that is not a problem because it can deliver 54 Mbits/sec over the coaxial cable inside the home. Return-path signals are sent from a telephone or set-top to the NIU, where they are converted into quadrature phase-shift keying signals and sent up to the headend.
Also at the Cable-Tec Expo, Continental Cablevision of Boston, the nation`s third-largest cable company, and LANcity Corp. of North Andover, MA, jointly agreed on a two-year contract for 50,000 LANcity personal cable modems for installation throughout Continental`s systems in New England, Florida and Illinois. (See related news article on page 7.)
According to David Fellows, senior vice president of engineering and technology for Continental, "Over the past five years, our company and LANcity have worked together successfully, deploying cable-modem-based data services at multiple pilot sites and in the large-scale Boston College `Project Agora` site in Massachusetts." q
George Lawton writes from Brisbane, CA.