Cutting through the maze

Choosing a local loop network topology can mean life or death to a local exchange carrier in today's competitive telecommunications world. With pressure to deliver more services over the same plant, system planners have been winding their way through the maze of choices available to integrate video and telephony capabilities into one network.

Hybrid fiber/coax (HFC) technology is turning out to be a leading choice in this area. Already central to a cable television operator's ability to provide data and telephony services, HFC is emerging as a key competitive threat to the LECs' local loop business.

At the same time, telcos are using HFC to make inroads into cable TV. Factors such as service mix, growth needs, infrastructure and business methods will determine the right solution, whether it is HFC-delivered or an alternative such as wireless video distribution or switched high-speed digital access.

Service and Growth Needs Table 1 shows the respective positions of CATV companies and telcos as they start to consider HFC telephony integration. By definition, a cable operator uses a coaxial cable system to deliver video services. Telcos, on the other hand, deploy twisted pair to distribute signals to customers. CATV operators have a relatively high video penetration and expect low telephony service penetration. But the situation is exactly opposite for local telcos. For a telco, the decision to construct an HFC network depends on the attractiveness and feasibility of alternatives such as multichannel multipoint distribution service (MMDS).

From an architectural perspective, telco networks are configured in a star topology using relatively low-bandwidth twisted pair, while CATV networks are set up in a high-bandwidth bus configuration. Consequently, telco technicians are trained on twisted pair, while CATV technicians are familiar with coaxial cable plants.

Cable TV and telephony service growth patterns share similar, but not identical, characteristics. Cable operators need to profitably supply service in an entire franchise at low penetration rates. Additionally, they might look for opportunities where they can cherry-pick high-volume customers such as apartment complexes or small businesses. Similarly, telcos must provide incremental service-caused by new developments and growth in second lines-anywhere in their exchanges.

HFC History Before HFC, CATV distribution networks used a tree and branch topology consisting of coaxial cable and numerous amplifiers in series. As a result, the typical pre-HFC CATV networks had poor reliability and a limited capability for effectively provisioning interactive services.

In the late 1980s, CATV operators began replacing long trunk lines with point-to-point fiber distribution systems. Since then, enough fiber-an estimated 10%-has been deployed in the distribution plant to support a limited offering of compressed digital video, data and telephony services. HFC has transformed the CATV network's topology from a tree-and-branch architecture to more of a star-bus network. A view of a typical HFC network is shown in Figure 1.

The development of highly linear lasers is the key to the success of HFC. These lasers, indicated in Figure 1 as a transmitter assembly, allow direct conversion of radio frequency signals to the optical domain. The RF spectrum, typically 50 to 750 MHz downstream, modulates the laser either directly or indirectly, depending on laser type.

The output of the laser transmitter may be split to serve several optical nodes or configured for point-to-point service to one optical node. An optical receiver, located within an optical node, converts the optical format signal to the original RF spectrum.

The state-of-the-art optical node serving area is dictated by system economics and typically encompasses 500 homes. Many operators are deploying dark fiber-fiber not currently used-deeper into the network and plan on decreasing service area as required by market demands. The advantages of a smaller serving area are that it: • uses existing coaxial cable plant, including amplifiers and system passives. • enables narrowcasting-the targeting of services to a specific neighborhood or node. • makes the return path transmission in the 5 to 42 MHz band practical.

The two fundamental methods of providing telephony services with an HFC network can be termed "HFC telephony-over-coax" and "HFC telephony overlay."

Telephony-Over-Coax Most efforts to integrate services have used two-way HFC plant to transport a combination of telephony and video-modulated RF carriers from a headend or central office to subscribers. This approach, which allows providers to make incremental additions to their existing coaxial cable architecture, has been especially attractive to CATV operators and some Independent telcos.

Integration of telephony over an HFC network uses many of the components required for a stand-alone HFC network (Figure 2). A few elements, however, must be added to enable telephony transmission.

Most system engineers recommend that telephony services be transmitted by a laser different from that which transmits the CATV programming services. A return optics path is required from the optical node to the headend. System engineers also recommend that the node have redundant transmitters and receivers to provide adequate reliability for telephony services.

The implication is that a minimum of four to six fibers are required between the headend and the optical node. Amplifiers must also be upgraded for two-way operation.

Power supplies need to be replaced or modified to provide eight hours of standby power because they provide backup power to the optical node, the amplifiers and occasionally the telephony modems. Assuming the telephony modems are powered by a central power supply, the multitaps must then be replaced with multitaps that pass power from the feeder cable to the drop cable.

At the home, a modem that uses 3 to 8 W per subscriber provides conversion between the telephony interface and the RF coaxial drop. Some versions of this modem are intended for multiple dwelling units and offer multiple telephony lines. At the headend, an RF host digital terminal (HDT) is required to interface between the switch and the HFC system.

An Overlay System Because the telco and the CATV operator have different network needs, they require different methods for integrating telephony and video services via HFC. One approach that is gaining popularity is to integrate CATV and telephony services using an overlay method that combines HFC and fiber-based local loop technologies.

With this technique, telephony services are transmitted over fiber and existing copper pairs. Programming services are transmitted over AM fiber and coaxial cable plant. The outside plant infrastructure remains common for all applications, while separate electronics provide lifeline telephony and entertainment services. Shown in Figure 3, this type of network is very similar to HFC telephony-over-coax.

Video signals are transmitted as a broadband RF spectrum, which modulates a laser transmitter. Telephony signals are carried on a separate fiber. The HDT provides the interface between the switch and the fiber cable and is either collocated with the laser used for the video signals or remotely located. This allows video and telephony services to be co-deployed or introduced on a staggered basis. For a telephony provider, video services can be added on a location-by-location basis as is economically feasible.

The optical network unit (ONU) contains receivers for both video and telephony. In this case, the optical nodes yield anywhere from four to 192 telephony lines. Only the telephony portion of the ONU is provided with stand-by power.

The fiber component of the telephony system is based on a counter-rotating ring, so additional ONUs may be added anywhere along the fiber as system growth requires. Alternatively, a point-to-point or passive optical, fiber-based network can be deployed. The counter-rotating ring, however, provides for the thinnest fiber route. Because the system uses a ring architecture, only two fibers are used to provide full transport redundancy for telephony signals.

Video transport is enabled through an additional fiber within the same cable as the telephony fibers. Optical couplers are collocated with ONUs as needed, allowing a single fiber and transmitter to be shared among multiple video receivers. Video receivers may be placed at locations whenever economically feasible. To implement a two-way system, another fiber-configured point-to-point from the ONU to the headend-would be added to the fiber cable. The broadband output of the video receiver is distributed via a typical coaxial network. This coaxial network does not require stand-by power or two-way capability. Standard multitaps are also employed.

Comparing Coax and Overlay Table 2 compares telephony-over-coax vs. a telephony overlay. The number of active serial elements indicates the number of power-consuming devices between the switch and the last subscriber. As Figure 4 demonstrates, the overlay approach has only two active elements between the switch and any subscriber.

Slightly more bandwidth is available for video services with the overlay approach because telephony services are carried on another fiber. The power difference between the two systems is due to the need for a modem at every subscriber in the coax approach. Additionally, the telephony-over-coax approach requires stand-by power for all amplifiers, which adds approximately 1 W per passing (assuming an average of 30 passings per amplifier and 30 W per amplifier).

Although an upstream return path is possible with an overlay method, it is not required. The overlay approach also offers the possibility of returning service alarms for "free" from the video optical receivers via the telephone system. An HFC telephony overlay is deployable today because all HFC components required for this implementation are standard, off-the-shelf products.

For local telcos, the overlay method of HFC implementation is best for delivering telephony and video services. It uses their existing infrastructure, which is familiar to telco technicians.

Growth in new services is accommodated by adding a fiber infrastructure where needed. Telcos can selectively offer video services to subscribers on demand. The cost of the entire fiber infrastructure is justified by telephony services, which pay for the addition of video services.

The HFC telephony-over-coax method works best for incumbent CATV providers. Again, this approach uses an infrastructure familiar to CATV technicians. At the expected low-penetration telephony rates, the use of a modem allows costs to be apportioned on a per-subscriber basis to some extent.

However, the costs associated with upgrading the HFC system to make it telephony-ready are significant. In fact, the true costs of deploying telephony-over-coax may not yet be known. To meet the reliability requirements of telephony, one industry expert has suggested that a separate coaxial return network between the subscriber and the optical node may be necessary.

Alternatives to HFC Wireless transmission provides one alternative to HFC for either telephony or video service delivery. In general, wireless transmission complements existing terrestrial networks.

For example, cellular and personal communication services networks are ways an incumbent CATV operator can provide a level of telephony service. These networks can share some portions of the CATV operator's infrastructure but have historically been independent of the existing CATV network.

From a telephony operator's point of view, direct broadcast satellite, MMDS and local multichannel distribution service offer methods of providing broadcast video transmission without affecting their installed outside plant network. Here are some advantages to wireless broadcast systems:

• They require no outside plant coax or copper for signal distribution. • Entry into the video business is relatively easy for a telco. • They offer lower operating costs than wired solutions. • They require no local community franchise. • They offer global coverage from the very beginning.

These wireless methods have been embraced by many carriers. Direct broadcast satellite is currently being marketed by approximately 140 Independent telcos in partnership with the National Rural Telecommunications Cooperative. The MMDS approach is validated by Nynex, Bell Atlantic and Pacific Telesis with their recent investments in existing MMDS systems. In all of these cases, the wireless systems complement the telcos' current local loop investment.

In the future, broadcast wireless networks such as these may be supplemented with fiber optic-fed local loops to provide interactive services such as high-speed Internet access and video-on-demand over existing twisted pairs. Figure 4 shows a generic view of a local loop upgraded for high-speed interactive services.

This scenario assumes that a fiber-based counter-rotating ring provides basic telephony service. Additional bandwidth is available for higher speed services. To maximize network efficiency, switching is distributed in the local loop and is added to the ONUs as necessary. Access to higher-speed services is supported by existing twisted pairs and achieved through complex modulation and encoding techniques.

Implementation of an upgrade will fundamentally be driven by future demand for high-speed interactive services and the cost of upgrading the local loop. With such uncertainty, it is important that network upgrades are cost-effective for today's telephony services, while providing the necessary expansion means for tomorrow's offerings.

The needs of incumbent telco providers and CATV operators are clearly different because of their embedded infrastructures and contrasting methods of doing business. The incumbent CATV provider will probably benefit most with the telephony-over-coax solution. For telcos that choose an HFC approach for telephony and video services, an overlay approach will most likely offer the best answer.

For some telcos, however, the best method of delivering telephony and video services may not reside in HFC but in another solution such as wireless or switched high-speed digital access. Before this renaissance in the local network can begin, however, telco planners must decide whether or not HFC is the answer.

Ken Pyle is Manager of Product Planning at E/O Networks, Hayward, Calif.