Must market drivers keep the residential cable TV market perpetually unprepared to handle Internet and HDTV data rates?
By Bill Spink
The competition in Internet broadband capacity between the telco service providers like Verizon and SBC versus the cable-television service providers like Adelphia and Comcast is not news. Most people understand the tradeoffs: the telco providers offer digital subscriber line (DSL) service that delivers desirable broadband data rates up to a limited distance from the central office (18,000 ft. without remotely deployed DSL area multiplexers). The cable television (CATV) providers offer cable modems with higher data rates and no distance limitation, but with lower reputable quality of service (QoS). Meanwhile, services are converging-the telco guys are adding video services to their legacy voice and DSL offerings, while the CATV folks are moving into voice-over-Internet protocol (VoIP) on top of their entertainment delivery model.
The telco PSTN
The telco service providers come from a legacy of the AT&T telephone monopoly. The original telephone handsets that came from this segment were built to last by Western Electric (formerly a department of AT&T, then Lucent, then Avaya, now CommScope). Today, they are boat anchors, wiped away by companies like Panasonic and Sony who market $29 desk models that don’t work when the power goes out.
Regardless of the phone monopoly, the network that they designed, built, and supported worked extremely well, and in the U.S., we benefit from excellent QoS as a result. The Bell network uptime and reliability are the envy of the world, increasingly important features as the content being moved over the network expands from voice to data and services.
The selection of the coax in-station connector by the standards engineering personnel is an example of this early QoS excellence. Coax is used in the public-switched-telephone network (PSTN) central switching office between the optical infrastructure, the twisted pair T1, and digital voice lines. The coax line is called DS3, which has a data rate of 44 Mbit/s, equal to 28 T1 lines. The DS3 data rate is the fundamental building block of the North American telco central office. An OC48 optical routing device, in comparison, has the capacity of 48 DS3 lines.
The telco industry deliberately specified interconnects according to performance criteria and long life rather than cost (easy to do when you’re a telephone monopoly). The North American connector of choice for the coax assignment is the telco-grade bayonet nut coupling (BNC) connector. This radio-frequency (RF) connector features a thick-gold-plated pin, a polytetra-fluoro-ethelene (PTFE) dielectric for superior signal propagation velocity, a positive locking mechanism for port engagement, a fairly labor-intensive installation process (three pieces and crimp/crimp), and it requires expensive installation hand tools, as well as associated test equipment. This connector was designed to perform in place for 30 years. The result of applying this long-haul philosophy to component (in this case, connector) selection is that the plain old telephone service became a world standard for robust networks. This elaborate process involved extensive testing and engineering down-selection by components engineers who used data, life-testing, and other strict criteria such as Telcordia network-equipment building-standards testing that is familiar to PSTN equipment suppliers like Lucent, Nortel, and Cisco.
The CATV network
The commercial antenna television (CATV) transmission model is a hybrid-fiber-coax (HFC) network that uses a 75-Ω coaxial wire in the residential portion. This is the “edge” of the CATV network, where most of the interconnections involve mating the coax to the network appliances (such as televisions, satellite radio, and cable modems). The legacy in-home interconnection of choice for CATV is the commercial, pinless F-connector, which uses the center conductor of the 75-Ω cable as the “pin” of the connector itself (see Fig. 1). This product features low cost ($0.20 each). As you would expect, this selection was driven by price point. In stark contrast to the telco-grade BNC connector, the life expectancy for signal integrity does not measure up.
 FIGURE 1. The center conductor of a tri-shield RG-6 coax (foil, braid, foil) is always solid copper or copper-clad steel. |
Given the history of CATV, the F-connector was a rational choice. Originally designed to deliver one-way video signal to entertainment equipment, this network vision was dramatically different than the public voice network. But technology and market competition are drivers of change, and today that same installed network is a two-way digital transmission line that operates at elevated frequencies to accommodate the additional data involved with high-definition television (HDTV). Today’s HFC network supports bank transfers of cash and “shopping cart” traffic on the Internet, a far departure from analog television delivery.
How did this happen? In the PSTN model, the standards engineers dictate connector component choices to the installers, usually after years of environmental and rigorous life-testing over a range of conditions. The vision for the PSTN was that in the event of natural disasters or harsh environmental conditions, you could count on your telephone to work. In CATV, the residential installers, who often do not have a stake in customer satisfaction, make the choice. The connectors used in the CATV space are biased toward rapid installation and low price. As a result, CATV has a nefarious reputation for network dependability: Would anyone want to trust their dial tone to the “cable guy”?
Actually, the HFC network operated by CATV service providers is superb until the signal gets to the home and the F connector. The technical downside to the F connector is that the cable-center conductor was not engineered to withstand the environment outside the cable dielectric, so it corrodes very quickly. The business reality is that 65% of CATV customer service “truck rolls” are directly attributable to failure of these pinless F-connectors. The lowly F-connector can provide a troublesome, unreliable and, from a maintenance perspective, costly RF interface, and still is the cause of most drop-related service calls.1
The coaxial connector specification in the CATV sector contains little evidence of engineering rigor. Clearly, the weak link for the high-bandwidth carrier-class assignment in the CATV hybrid fiber/coax network is the pinless F-connector. The performance limitation of the pinless F-connector is technical.
For example, the center conductor of the cable that acts as a mating pin is vulnerable over time to fretting and oxygen-related corrosion when exposed to air and encapsulated in the mating socket. Also, installers routinely fail to tighten the threaded coupling sleeve on the plug to the proper specification (¼ turn with a wrench after finger tightening). Furthermore, the value of return loss over elevated frequency (2.2 GHz) is marginal, particularly compared to the carrier-class connector for 75-Ω coax in the public network. Finally, the socket in the jack must accommodate too wide a range of wire centers as contacts.
The variation in male pin size (from 0.020" to 0.064" per IEC standard 169.24) also makes it virtually impossible to design an instrument-grade Type-F connector with reasonable electrical repeatability. The ANSI/EIA-550 Type-FD specification and the specifications from the Society of Cable Television Engineers are attempts to narrow the variation of the Type-F connector.
The fix
Can we correct what is fundamentally wrong with the pinless F-connector? Yes. Some companies are working hard now to put a better solution in place. One solution is to bypass the F series altogether as rapidly as possible by using an adapter in the F ports on most residential appliances, simply converting that port to another series. One such adapter converts the F port to a BNC port (see Fig. 2).
 FIGURE 2. The telco-grade high-frequency HDTV-rated adapter is used to convert an F-connector to a BNC jack. |
null
 FIGURE 3. Three-piece, compression-style, fixed-pin F-connectors offer the quality of telco coax connectors, but find resistance from installers. |
Another approach is to design or deploy a better F-connector. Three-piece F-connectors offer all the performance and reliability attributes of the telco-grade BNC, in terms of installation techniques and tooling-strip dimensions, installer training, and tooling (see Fig. 3). Problem solved, except that the CATV installers enjoy a one-piece connector and a rapid installation process called “compression,” and they don’t want to change.
Just as there are telco-grade BNC connectors and hobby-grade BNC connectors, the industry could benefit from a proven line of “network-grade” F-compression connectors with properties that include a fixed pin with 30 millionths of an inch of plated gold, an under-plate of nickel to halt copper piping/migration over time, and fully enclosed high-frequency dielectrics like PTFE. More than one company is developing such a connector now.
Installers and user groups must be educated about CATV network technology issues so that we can elevate the QoS performance of the hybrid fiber-coax network. The CATV segment is not known for rigorous specifications and test plans. The skill set and desire to learn may be there, but the historical market drivers have been price, ease of installation, and the “curb appeal” of appearance. We must move the point of determination from the installer to the network-engineer. In spite of the relatively significant cost impact-the new connectors may cost as much as 10× the price of the pinless F-it is minor compared to the QoS benefits of carrier-class performance for the CATV delivery network.
REFERENCES
- R. Hranac, Communications Technology magazine (March 2000).
BILL SPINK is connector design manager of Sterling, 5550 East McDowell Road, Mesa, AZ 85215. He can be reached at Tel: (480) 985-9000; Email: bill.spink@trompeter.com.
