WDM provides targeted services in CATV networks

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Community antenna television (CATV or cable TV) networks were developed several decades ago to deliver a few channels of analog television to a fraction of the homes in a small area. In the intervening years, these networks have evolved into true communications networks supporting digital video and the delivery of a variety of targeted services to homes and business in a much larger area. This evolution has required, and will continue to require, the application of several technologies to new network architectures.

WDM technology has become an important part of many CATV networks, allowing network operators to provide targeted services over existing fiber. Because of the nature of CATV networks, however, the requirements of the WDM equipment are unique. CATV networks currently transmit information in multiple formats, including analog video, analog audio, encoded digital via quadrature amplitude modulation/quadrature phase-shift keying/frequency-shift keying (QAM/QPSK/FSK)-modulated digital, and baseband digital.

Analog information includes National Television Standards Committee (NTSC), phase alteration line (PAL), or suppressed carrier amplitude modulation (SCAM) modulated analog television signals for video and audio as well as FM radio. Information encoded in QAM/QPSK/FSK carriers would include digital television signals, data-over-cable-service interface specifications (DOCSIS) cable-modem data, and circuit-switched telephony. As QPSK and FSK are basically derivatives of QAM, we will use QAM to refer to all three modulation schemes. (To double bandwidth, QAM combines into one channel two amplitude-modulated signals with the same frequency but phases that differ by 90°.) Baseband digital transmission provides data links for cable modems, telephony, and QAM-modulated carriers on the return path. WDM technology has applications in all three.

Note that QAM carriers transmit digital information. Therefore, this is frequently referred to as "digital." However, QAM modulation carries the digital information on analog-modulated carriers. Therefore, the requirements for QAM carrier transmission are similar to that of analog transmission.

The use of baseband digital transmission for the return path is a highly unique application of digital technology. The return path of a CATV network currently carries information in a limited frequency band at the low end of the frequency spectrum—5 to 42 MHz in North America and several other countries, and 5 to 65 MHz in most other places. The low cost of high-speed digital optoelectronics makes digitizing and baseband digital transmission in the return band economical in many applications. It is relatively easy to digitize two 5- to 42-MHz bands and time-division multiplex (TDM) the digital information onto a single 2.5-Gbit/s optical link.

Typical CATV networks originate at a regional headend serving millions of homes. This location contains the source for the broadcast TV signals—satellite receivers and antennas that receive local broadcast channels. These channels are modulated onto a broadcast transmitter and sent to hub locations.

Hub locations typically serve a few tens of thousands of homes. Fiber from the hubs brings the signals to optical nodes. Field-mounted nodes are the interface between the optical and coaxial (copper) plants. In such a hybrid fiber/cable (HFC) network, a radio frequency (RF) signal is transmitted along the coaxial cable and periodic taps split off signals for individual homes. If the levels drop too low, RF amplifiers boost the signals. The street layout dictates how the coaxial plant branches out (see Fig. 1).

The tree and branch structure of the coaxial plant is difficult to segment. The optical portion is either a ring or a star that allows for easy segmentation. Consequently, the number of homes served by each node is important in defining the maximum bandwidth available per home. CATV operators frequently segment nodes by inserting additional optics so that the node functions as two or more logical nodes. In this manner the available bandwidth per home is increased without changes to the coaxial tree-and-branch portion of the network.

Services that are transmitted to all the homes in a network are called broadcast services, which are generally both analog and digital video. Services that require dedicated, bidirectional bandwidth to send a specific piece of information to a single home are called narrowcast services. Typical narrowcast services would include cable-modem data transmission, telephony, and content on demand. Traditionally, CATV networks only supported broadcast transmission. However, narrowcast services offer a major opportunity for increased revenue. The ability to support narrowcast services gives CATV operators a significant advantage over direct-to-home satellite service providers.

All narrowcast services require some type of server as well as bidirectional communications to the user. Some operators locate the servers to support narrowcast services in the hubs rather than in the headend. This results in physically large hubs, but does limit the transmission requirements between the headend and hubs. Because the link from the hub to each node is direct, WDM technology is not generally needed in these networks.

Placing the servers that support narrowcast services in a central headend requires much greater information transmission between the headend and the hubs. Operators who use this architecture generally use DWDM technology to minimize the fiber needed between the headend and hubs (see Fig. 2).

There are several unique aspects of the CATV architecture that uses DWDM. First, the forward narrowcast information is in the form of QAM carriers, which carry digital information over the coaxial portion of the plant with conversion. Also, both the broadcast and narrowcast information are transmitted from the hub to the node on a single fiber, allowing a single receiver to receive both. The levels and modulation indices of these two wavelengths must be carefully balanced so that the RF levels of all the carriers have the proper levels in the coaxial plant. This also requires defining the performance needed for each service.

The return path in a DWDM CATV network is either a digitized analog or an analog WDM transmitter in the node. An analog transmitter is generally more efficient if a single return is needed from the node. If two returns are needed from the node, the return from the node is segmented, so a digitized analog return is more efficient because two channels are transmitted on a single WDM wavelength.

The performance of an analog transmission link is highly dependent on the optical power on the receiver. Return analog links typically require at least -8 dBm on the receiver to achieve the performance required. The performance of a baseband digital link is independent of received optical power, provided the detector is operating within its specified range. Because analog links must receive high optical powers, the links must use an erbium-doped fiber amplifier (EDFA) for any reasonable distance of fiber. As digital links require much lower powers, the EDFA is generally not required.

Most CATV operators now provide some form of video-on-demand (VOD) services or plan to in the near future. A VOD network requires a low-cost, high-speed, highly asymmetric and bidirectional data transport network. Each stream transmission requires a digital video channel of 2 to 5 Mbit/s, while the upstream path is transmitting commands (pause, next channel, and so on) from the subscriber.

Three possible network architectures can support VOD—centralized, partially centralized, and distributed (see Fig. 3). In the centralized architecture, the servers and the QAM modulation are located in the headend. This requires QAM transmission of the data from the headend to the hubs. The partially centralized architecture has the servers in the headend and the QAM modulation in the hub locations. This requires baseband (digital) transmission of the data from the headend to the hubs. The distributed architecture has both the servers and the QAM modulation in the hubs, eliminating the need for data transmission between these locations.

While the distributed architecture minimizes much of the need for data transmission, storing all the required data in multiple locations is inefficient. This inefficiency generally overwhelms the savings from data transmission, making a distributed architecture expensive to implement. In contrast, a fully centralized architecture is effective, but places great demands on the analog transmission network.

The most cost-effective option is therefore the partially centralized architecture, which places the QAM modulation in the hub and uses lower-cost digital transport between the headend and the hub. The most cost-effective method for transport of this data is Gigabit Ethernet interfaces. Furthermore, DWDM Gigabit Ethernet allows highly efficient use of available fiber. A partially centralized architecture using DWDM technology makes the installation of a VOD network economically feasible.

Eric Schweitzer is senior director of product marketing at Harmonic, 549 Baltic Way, Sunnyvale, CA 94089. He can be reached at eric.schweitzer@harmonicinc.com.

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