Hybrid fiber/coaxial-network technology serves cost-efficiently in the local loop

Aug 1st, 1996

Hybrid fiber/coaxial-network technology serves cost-efficiently in the local loop

Based on a proven and well-understood cable-TV broadcast structure, HFC-network technology transports and distributes highly reliable and economical telecommunication, interactive multimedia and video services to residences and homes via local fiber nodes

carl podlesny

Scientific-atlanta inc.

Hybrid fiber/coaxial-cable (HFC) network technology employs transmission and distribution characteristics that suit the cost-effective delivery of interactive services to residential and small business subscribers. Recent progress in compressed digital video and multimedia technologies, combined with legislative and regulatory actions that have opened the local loop to competition, has compelled cable-TV system operators and local exchange telephone operating companies to reassess the capabilities of their current and planned service distribution networks. Both industries, driven by the economics of competition, are converging toward a single, integrated-services switching and distribution platform. The HFC distribution technology solves this dual-mode distribution requirement and meets the needs of yet-to-be identified services, future capacity and reliability requirements, with minimal impact on the as-deployed distribution plant (see Fig. 1).

Several approaches have been offered by equipment suppliers to achieve a viable fiber-in-the-loop distribution solution. Telephone operating companies have also made compromises to cost-justify fiber-loop network technology. To date, however, a satisfactory, economical solution has not been realized. Commercially available solutions have proven too expensive and incapable of simultaneously distributing broadband analog and digital services over the same distribution plant in a cost-effective manner.

The economics of local service distribution critically depend upon the installed first cost (IFC) and life cycle cost (LCC) of the network. Networks such as HFC, which incur low IFCs and LCCs costs and can also defer customer-dedicated costs, offer additional advantages.

For example, consider the competitive scenario where Network Operator A spends $100 more per home passed on its distribution plant than does Operator B. Assume further that both operators obtain a 15-year loan at 8% per year (the bank assesses both networks with a 15-year service life). Because Operator A paid an extra $100 per home passed, it must continually earn $307.69 per home passed or $1025.63 per subscriber--at a 30% penetration rate--more than Operator B just to break even (see Fig. 2). At a 30% customer capture (equivalent to 50% of typical cable-TV operators` take of 60% of homes passed), Operator A must average more than $9.80 from each subscriber every month for 15 years just to break even with lower-cost Network Operator B.

According to industry cost-analysis studies, for distributing broadcast National Television Standards Committee (Ntsc) and cable-telephone services, the installed first cost of a basic 750-MH¥HFC system with a subsplit reverse path capability is approximately $240 to $260 per home passed, or $400 to $435 per subscriber at a 60% basic cable-TV service take (see Fig. 3). Upgrading this plant to provide high-reliability node power and fiber-feeder redundancy increases the per- subscriber cost to approximately $650. Assuming that cable-telephone service is provided only to subscribers who also subscribe to basic cable-TV service, the total cost to provide basic video (fiber feeder, coaxial-cable distribution and coaxial-cable drop) and cable-telephone service (headend interface unit, high-reliability network power and customer interface unit) is, therefore, almost $920 at a 25% telephone service take rate.

Compatibility with the existing base of approximately 300 million Ntsc television sets in the United States is the key advantage of HFC technology over all baseband digital distribution solutions. This technology can simultaneously accommodate all of the baseband digital signal technologies used by the telephone industry, as well as standard television signals with no adverse effects. Furthermore, other switching, multiplexing and transport technologies, such as Synchronous Optical Network (Sonet), Asynchronous Transfer Mode (ATM), switched multimegabit data service (Smds) and frame relay, are all compatible with HFC technology. Baseband digital distribution technology, however, must always employ a twofold signal conversion (analog/digital) to carry Ntsc video for viewing. This conversion is expensive and is expected to remain a roadblock to the successful deployment of baseband digital technology for perhaps the next decade.

Broadband carrier distribution technology is flexible because it does not have to provide either rate or protocol adaptation for information providers to communicate with information users. The only prerequisites for communications are:

The information provider and the information user must use compatible equipment (that is, bit rate, framing and communications signaling protocol).

The communications equipment used by the information provider and the information user must use a suitable network interface device to connect to the distribution network.

For a basic definition, HFC technology comprises a configuration of fiber-optic and coaxial cables that are used for the distribution of local broadband communications. HFC networks provide high transmission and distribution performance at relatively low cost because they can be designed to closely match the asymmetrical bandwidth needs of most broadband distribution systems.

Architecturally, HFC networks employ a star/star-bus-type architecture. Transmission signals are distributed from the headend or local distribution hub in a star-like fashion, using fiber-optic feeders to provide economical reach to the serving area node. The fiber node, in turn, cost-effectively distributes these signals throughout the last mile or two of the network with multiple coaxial-cable buses (star-bus). The coaxial bus distribution plant is cost-effective because each section of coaxial cable not only brings signals to the subscribers, but also carries the signals for the subscribers served by all the sections that are located further down the bus. Therefore, a given section of coaxial-cable bus serves not only an initial four or eight subscribers, but also another possible 40 or 50 subscribers down the star-bus.

HFC distribution networks rely upon radio frequency (RF) modulation technology to put information onto a carrier that connects the information supplier to the information user. Media adapters process the various digital format signals for compatibility with the HFC network. These adapters are primarily high-speed digital modems that operate similarly to computer modems that send data over analog telephone lines. The typical modulation schemes employed by these modems are 16- and 64-level quadrature amplitude modulation (QAM). Complex high-order modulators provide a degree of RF signal compression, thereby increasing the bit-carrying capacity of a given carrier. The North American standard RF channel consists of a 6-MH¥bandwidth; in other countries, the bandwidth is 8 MHz. A typical 750-MH¥North American television transport system delivers about 116 downstream channels.

HFC broadband carrier distribution systems can interface seamlessly with standard telecommunications switching and distribution network facilities. For example, an HFC network can be deployed to interface with high-speed switching equipment such as ATM, frame relay and packet switches, as well as with narrowband Integrated Services Digital Network and voice-switching equipment (see Fig. 4). It can also interface directly with 155-Mbit/sec Sonet OC-3 facilities and TR-8 local digital switches. The basic HFC network easily forms part of an infrastructure that can manage more than 35 Gbits/sec of digital-video information services in real time.

Passive coaxial-cable networks

The ultimate HFC architecture is called a fiber-to-the-last-device, or a passive cable network (see Fig. 5). Today, these networks prove economical only in service areas that can generate the high revenues required for profitable operation. They provide the greatest possible per-subscriber bandwidth of any HFC design.

Using techniques developed for providing interactive switched-digital services, these networks deliver 2-Gbit/sec signals to a fiber node using as few as 70 6-MH¥video channels. Also, this information does not have to be switched at the fiber node, which converts the signals from an optical to an electrical format suitable for driving the subscriber`s distribution coaxial-cable platform. The subscriber`s electronics equipment is instructed by the network traffic controller to tune to a specified 6-MH¥channel and look for its data in a 28-Mbit/sec data stream. Both functions can be implemented reliably and cost-effectively.

Although the rulings of the Federal Communications Commission limit the return-path bandwidth from approximately 5 to 40 MH¥because Channel 2 must be carried untranslated in the downstream path, passive cable networks can circumvent this constraint by expanding the signal spectrum to 1 GH¥for carrying return-path information.

Adaptation challenges

HFC technology originally evolved to meet the narrowcasting needs of cable-TV distribution service operators. However, this technology had to be modified to meet the new requirements of fully interactive broadband distribution networks. Some problems that had to be solved included service availability and interdiction, network powering, subscriber privacy, network management, power passing taps, noise-ingress control, capacity expansion and minimizing reverse-path signal differences.

Consider how the distribution capacity of HFC networks can be increased to accommodate either unforeseen new subscriber services or increased service demands. Downstream capacity can be extended by increasing the forward-channel bandwidth, employing narrowcasting distribution techniques, using more efficient modulation techniques and combining these basic techniques.

Most broadband carrier distribution technology being deployed is capable of being expanded from a 750-MH¥to a 1-GH¥bandwidth in the forward direction, with no change in amplifier spacing. In addition, some product lines can subdivide the original servicing area size several times without causing fundamental changes in the original distribution plant to increase the per-home-passed forward and reverse bandwidth.

The traffic-carrying requirements of the distribution plant`s reverse path must also be carefully engineered. The standard way to increase the channel carrying capacity (forward or reverse) is to split the serving area. This split reduces the number of subscribers served by each fiber node. Therefore, the information bandwidth available to each subscriber can be increased 2 to 16 times using this technique. The available bandwidth increases linearly with the reduction in the number of subscribers served by the fiber node.

Smaller service areas also enable the use of modulators with higher bit-rate efficiencies, such as 64-level QAM, which further improve the information-carrying capacity of the available bandwidth. For example, a fifteenfold increase in per-subscriber bandwidth can be achieved in both the downstream and upstream directions if a 275-home service area using quadrature pulse shift keying modulation (base case) is upgraded to 75-home nodes using 64-QAM modulation.

When there is a large amount of contention on the reverse-path channels to transmit large amounts of bursty high-speed data, a media access protocol similar to those used by local area networks can be adopted . u

Carl Podlesny is strategic marketing director, telecommunications systems, at Scientific-Atlanta Inc. in Atlanta.

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