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The Case for Coaxial in Broadband Transmission.

The arrival of deregulation in the telecommunications marketplace has caused a drastic alteration of traditional pricing structures. As operating companies scramble to replace revenue subsidies formerly provided by AT&T Long Lines, corporate data processing and telecommunication managers expect that telcos' local-line charges will soon escalate. Already faced with high costs and long lead times for installation of local T1 voice/data transmission circuits, many large users are taking a hard lood at their options.

Among the alternatives being seriously considered by many large users is private ownership of local-transmission facilities. Organizations that have achieved important cost savings by owning and managing their own telecommunications switching systems are comparing line charges, leasing fees and capital costs for various kinds of local-bypass transmission technologies. Those comparisons are producing results that point to a new kind of future for corporate telecommunications.

In addition to copper-span lines, as traditionally provided be telcos, there are three choices for local broadband voice/data transmission at the T1 rate: fiber-optic, microwave and coaxial-cable systems. Although some possess clear advantages over others in situations involving rights-of-way and line-of-sight limitations, the four can be compared fairly in terms of fundamental implementation costs for single-circuit, multi-circuit and distributed multi-circuit applications, exclusive of continuing service charges.

When evaluating the alternatives for a private transmission system, consider these variables: distance, number of circuits, number of connections (locations), physical operating constraints (such as line of sight, water crossing and rights-of-way), labor costs, transmission types and their capabilities, and test equipment costs.

Beyond basic cable and/or microwave tower requirements, the major cost tradeoffs in single-circuit applications include the number and types of repeaters required for each technology over a given distance and the need for T2-type terminal equipment for transmission over both fiber-optic and most microwave facilities. Multi-Circuit Applications

However, since many large organizations cannot meet their telecommunications requirements with just one local T1 circuit, a more-realistic cost comparison begins to emerge when the we consider multi-circuit applications between two points.

For a large network utilizing copper, microwave or fiber, the addition of T1 circuits requires an increase in and eventually a duplication of plant facilities. With copper T1 span line, utilizing 56-pair copper cable (four pairs per T1), incremental costs for increasing circuits between locations A and B is minimal up to 14 T1s. Once capacity is reached, however, the entire plant must be duplicated.

Microwave is similar to copper, in that duplication of plant facilities becomes necessary when capacity is reached, at four T1 circuits.

As capacity is outstripped, fiber offers two options. Additional fibers at lower speeds can be utilized, or additional T1s can be multiplexed on the existing fiber.

Coaxial cable requires only the addition of a pair of modems per additional T1, which communicates over a newly designed frequency slot. The incremental costs are linked only to additional modems needed. Distance remains the primary variable affecting implementation costs for these latter three, with relationships similar to those described for single-circuit applications.

It's in a third type of application--multiple circuits to distributed sites--where significant costs differences begin to occur between broadband coaxial cable and the three alternatives.

Assume the addition of a second circuit somewhere adjacent to the path of an original T1 link between two office sites. Again, copper-cable--based systems will require duplication of plant facilities to provide the necessary additional bandwidth if the third site is "removed in distance" from the original T1 span.

In this case, the same is true for both fiber-optic and microwave facilities, because each suffers the fundamental disadvantage of being a time-divided transmission medium. Obviously, microwave systems cannot accommodate "taps" into their communications pipelines along an existing link.

As for fiber optics, additional plants must be constructed to connect the third site with one of the original two, with communications among the three sites controlled from a central point. Thus fiber becomes a cost-prohibitive medium for this application.

Coaxial cable's cost advantage for local-area transmission at the T1 rate now becomes important. Rather than adding new plant facilities, the third site is connected by means of additional paired modems transmitting via a new frequency slot. In terms of basic capital requirements, no other alternative can compete with broadband cable in this configuration.

If implementation costs were the only critical factor in choosing a broadband transmission technology, the selection process would be fairly simple. But there are at least two additional and equally important questions that require positive answers: Can this technology handle future change and growth cost-effectively? Will it deliver reliable performance? On both counts, coaxial cable networks score an impressive yes.

In local-area applications involving multiple circuits and distributed sites, any change or addition of site locations requires new plant construction when using copper-cable, fiber-optic or microwave transmission technologies--with attendant installation charges, capital costs and lead times. With broadband-cable networks, however, adding or changing sites simply involves installing a set of modems at the new location and tapping into the installed cable system. It requires only a few network changes or additions to translate cable's inherent flexibility into significant cost-efficiency advantages.

Coaxial cable also accommodates growth between two points more effectively than other technologies. As mentioned, the addition of even a single T1 circuit along an existing point-to-point link requires additional plant construction with copper cable. And while both microwave and fiber-optic systems can provide numerous T1 circuits over a given point-to-point link, users face the periodic need for higher capacity and more expensive terminal equipment to accommodate new circuits.

By contrast, a single coaxial link configured as a 300-MHz mid-split system equipped with quality broadband modems can accommodate up to 76 T1 circuits--merely by adding modems as requirements grow. When operating flexibility and expansion requirements are fully evaluated, cable offers the most cost-effective choice for users who anticipate significant future change or growth on the local level.

But cost efficiency is ultimately less important than dependable performance; a transmission medium prone to reliability problems is no bargain at any price. Interference Resistance Among the Alternatives

Among the four broadband transmission alternatives being considered, fiber optics has the greatest inherent resistance to externally induced transmission interference. Coaxial networks that ie adjacent to large power-generating stations, for example, would probably require a fiber-optic link to ensure acceptable performance levels. Under normal circumstances, however, properly designed coaxial-cable systems provide transmission reliability on a par with fiber optics and superior to copper-cable or microwave systems.

In considering design, it's important to distinguish between cable TV systems that deliver entertainment services to the home and institutional coaxial data-transmission links. Home-entertainment CATV systems typically transmit signals to thousands of individual points, all multidropped from a single cable span. Each tap into a coaxial-cable span offers the potential for signal interference or loss, and that potential increases significantly when very large numbers of signals are multidropped from the cable loop.

By comparison, even very large, fully loaded institutional coaxial systems contain very few taps. That key difference between the two applications--together with far more stringent shielding and maintenance procedures specified for institutional networks--ensures excellent signal integrity within the cable span.

Another critical factor in coaxial-cable network performance involves modem design. Several broadband-cable modems are available in today's marketplace. The best offer very low bit-error rates for a given data-transmission rate even when performing under difficult system-loading conditions.

In developing our own Model 6402 Broadband Data Modem, Scientific-Atlanta duplicated a city-wide system of maximum length and channel loading as a design test-bed for the product. Operating under those conditions--representative of the largest possible system filled to capacity--the 6402 delivers end-to-end performance at a bit-error rate of 10.sup.-9.

Clearly, broadband coaxial cable is technologically equal or superior to the major alternatives for local data transmission at the T1 rate in many applications.

Two important trends in todayhs braodband-cable marketplace are opening the local telecommunications arena to a host of new players--CATV operators, equipment manufacturers, a variety of interconnect firms, and nonregulated carriers. The first trend is the growing willingness of large users to implement private coaxial networks on the local level or, alternatively, to lease coaxial T1 circuits on shared facilities. The second is the integration of local coaxial broadband capabilities into product and service packages offered by equipment manufacturers and nonregulated carriers. Over the long term, these two market forces should provide increased flexibility and competitive pricing for all major users of broadband telecommunications,

Many users are actively approaching large CATV operators in the hope of leasing T1 circuits on existing institutional coaxial networks. Where those networks are already in place, users can benefit from almost immediate availability of T1 links, extremely competitive installation charges, and relatively low circuit lease costs. The difficulty lies in the fact that most in-place coaxial broadband systems were installed several years ago to satisfy start-up franchise agreements, and few serve the newly developed office and business parks that now surround most metropolitan areas. Most prospective users require turnkey network implementation, and it is this latter situation that has opened a broad range for opportunities for prospective users and suppliers alike.

Users with large and complex requirements--including Fortune 1,000 firms and major property developers--have contracted with CATV operators for construction of local private coaxial networks, leaving overall maintenance to the CATV operator. In principal, this practice is identical to the construction of a private microwave network for local distribution of voice and data. The differences involve significantly lower costs, video capability and greater operating flexibility with coaxial broadband technology.

A variant of dedicated coaxial networks calls for CATV operators to construct institutional networks on speculation and market T1 circuits to multiple users in high-density business districts. Although this is occurring in several metropolitan areas, the practice hasn't yet become widespread. Such shared networks will multiply as demand increases. Vendor Marketing Strategies

While the first major force shaping today's cable-broadband marketplace is being driven by direct user demand, the second is the result of relatively new marketing strategies by equipment vendors and suppliers of long-haul broadband transmission services.

On the equipment side, manufacturers and interconnect firms are actively contacting CATV operators with the idea of structuring joint ventures designed to provide turnkey systems and local telecommunications facilities to large organizations. The bundling of hardware and cable-broadband transmission services can provide pricing advantages to users, and will likely become an increasingly common marketing practice by large manufacturers and systems integrators.

A similar type of approach is being explored by major nonregulated carriers. Just as large users are seeking to bypass high local-line charges in getting to the long-distance carrier of their choice, carriers are vitally interested in offering more cost-competitive services to potential customers. Carriers are approaching CATV operators in metropolitan areas throughout the nation, developing creative, cost-effective strategies for meeting users' turnkey requirements. Over the long term, the building of local and long-distance services by these groups may be among the most significant developments to emerge from deregulation of the broadband telecommunications marketplace.

As the future unfolds, we can expect to see a "mixing and merging" of various transmission technologies into hybrid networks that provide a very wide range of services and capabilities. Telecommunications services will be provided by a growing number of firms within a range of industries, including those discussed in this article. It's also likely that the telephone operating companies will tailor their technologies and service offerings more closely to real market requirements than is now the case. It will be more than surprising if broadband coaxial-cable technology is not a major part of those companies' standard offerings over the relatively near term.
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Author:Jordon, T.
Publication:Communications News
Date:May 1, 1984
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