Mike, another article posted in a Lightwaves Magazine in June.
Dense WDM technology comes to cable TV
Cable-television system operators are arming themselves with a new high-bandwidth weapon as they battle with local telephone companies to provide residential customers with high-speed links to the Internet. Like their telco counterparts, the cable guys are turning to dense wavelength-division multiplexing (DWDM) to dramatically increase the information-carrying capacity of their fiber-optic links.
Simply put, DWDM uses the different colors (or wavelengths) of light to send many signals over the same length of optical fiber. Until recently, signal degradation and crosstalk have limited the use of DWDM in cable-television systems.
It appears, however, that the technical obstacles to DWDM have been surmounted. Harmonic Lightwaves Inc. (Sunnyvale, CA) last month introduced a DWDM system designed for hybrid fiber/coax (HFC) cable-television systems. Other cable-TV equipment makers, including Antec and General Instrument, are also expected to roll out DWDM systems in the near future.
In addition to introducing the new DWDM system at the National Cable Television Association's annual conference and trade show in Atlanta, Harmonic Lightwaves announced a major customer for the gear. Tele-Communications Inc. (TCI - Englewood, CO) will deploy Harmonic Lightwaves' METROLink DWDM equipment to dramatically increase the upstream and downstream capacity of its systems in Vancouver, WA, and Dallas, TX. Industry heavyweight TCI has stated its intention to upgrade many of its networks with DWDM systems to offer its other subscribers interactive services as well.
In Vancouver, TCI will use the METROLink system to provide 30,000 homes with video services and high-speed Internet access through the @Home Network. In addition to increasing bandwidth capacity of HFC networks, METROLink enables cable operators to provide narrowcast service directly from the headend, eliminating the need to deploy expensive equipment in hubs. "Advanced DWDM solutions, such as METROLink, will enable us to get the most from our HFC networks and to shrink hubs," says Tony Werner, TCI's executive vice president of engineering and technology operations.
Centralizing optical transmitters and other directed services equipment in the headend, rather than distributing it to hubs, reduces maintenance and real estate costs, according to John Trail, product manger for transmission systems at Harmonic Lightwaves. Rather than equipping hubs with Synchronous Optical Network (Sonet) or Ethernet links, cable operators can use them to house "optical demultiplexers, passive components and a few transmitters for the return path," according to Trail, all of which require only a few cubic feet of space.
The two-way METROLink system is composed of Harmonic Lightwaves's new 1550-nanometer transmitters on the International Telecommunications Union (ITU) grid, gain-flattened optical amplifiers and matching passive components. With METROLink, each narrowcast wavelength carries several channels of digital subscriber service. The multiple narrowcast wavelengths are transported on a single fiber from the headend to the hub, where they are unbundled and optically combined with the broadcast signal. The combination of the two wavelengths of broadcast and narrowcast signals is then sent out to a targeted node or group of nodes. A similar DWDM arrangement is used to bring the return signal back to the headend. The METROLink system provides 72 additional channels, each capable of 27-Mbit/sec two-way transmission, Trail says.
Using high-frequency channels and low signal power are the secrets to overcoming the crosstalk and signal-degradation problems that have slowed the development of DWDM technology for cable-TV systems, according to Trail. Crosstalk is most troublesome, he adds, on "lower-frequency channels and decreases as you get up into higher- frequency channels. It also gets worse as you launch power into the system."
Harmonic Lightwaves resolved these problems by building a low-power system that operates at frequencies above 200 MHz. "We can do this because we use quadrate amplitude-modulated (QAM) channels," states Trail. "The carrier-to-noise ratio only has to be 40 dB for that to work. There is still crosstalk, but the QAM channels are much more tolerant of crosstalk."
The cost of equipping a 20,000-subscriber cable system with METROLink technology will range from about $170,000 to $250,000, according Trail. That outlay promises to give operators a lot of bandwidth for their buck. "We add nine QAM channels per wavelength," states Trail, "and you can put eight wavelengths on a single fiber. That comes out to 72 channels per fiber, with each channel operating at 27 Mbits/sec in each direction."
Tim |
