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A Comeback for Coax

Long relegated to the sidelines in the enterprise network, coaxial cable remains primarily a medium for video transmission and an assortment of specialized uses. Yet it has recently found one new nich...


April 1, 2003  


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Long relegated to the sidelines in the enterprise network, coaxial cable remains primarily a medium for video transmission and an assortment of specialized uses. Yet it has recently found one new niche in the enterprise too — odd as it may sound, in wireless networks.

A 2002 report by British research firm Gobi International forecasts global demand for coaxial cable will grow to $7 billion (U.S.) by 2005.

While early local-area networks, including the first Ethernets, used coax extensively, it gave way to less costly twisted-pair cable as twisted pair’s ability to handle higher bandwidths improved. Coax is rarely used in LANs today.

Cable television, always a major market for coaxial cable, remains its stronghold, although even in this area coax has given some ground to fiber. Cable TV networks increasingly are moving to hybrid fiber coax (HFC) networks. In this model, cable operators run fiber from the head end to nodes, each of which serve several homes, and then coax from the node into each residence.

Rob Wessels, vice-president of engineering at CommScope Inc. in Hickory, N.C., explains that by eliminating the cascades of amplifiers used in all-coax cable networks, carriers have reduced maintenance and increased available bandwidth. A decline in the cost of fiber, of course, has helped make hybrid networks more practical.

Will fiber eventually reach right to the home and squeeze coax out of the cable networks entirely?

It is dangerous to say never, but the dream of fiber to the home has been around for some years without becoming reality. Fiber to the curb, in which optical fiber runs past every home and only the drop cable from the street into the building is coax, may be a bit more realistic but such installations are still fairly rare.

And improvements in coax technology are, to some extent, fighting back against fiber’s invasion. For instance Narad Networks Inc. in Westford, Mass., has developed technology that lets cable providers use frequencies from 860 Mhz up to 2.5 GHz — traditionally not available on coax — to offer high-speed data services to businesses over their existing hybrid networks.

Inside the enterprise, coax seemed for a time to be practically gone, but growth in wireless networking has helped it make a limited comeback. Wireless networks are, of course, not totally wireless.

Devices on the network communicate over radio frequencies with transceivers throughout a building or campus, but the transceivers themselves are connected by network cabling back to a central hub and to stationary devices such as servers.

Several wireless LAN manufacturers use coax for the connection from the transceiver back to the network backbone.

Kip Coates, marketing manager at Belden Inc. in Richmond, Ind., explains that twisted-pair cabling does not work well at the 2.4-gigahertz to 5.8-gigahertz frequencies used for wireless networks.

“Twisted pair can’t handle those frequencies – attenuation is very high and especially unshielded twisted pair doesn’t contain the signal inside the cable and is prone to any outside interference,” he says.

One option is to carry the signal over the twisted-pair cable at another frequency, then modulate it to the frequency required for wireless transmission at the transceiver. But rather than place the necessary hardware at each transceiver, a number of wireless LAN manufacturers choose to modulate the signal farther from the edge of the network, then use 50-ohm coaxial cable to carry the signal out to the transceivers.

Another niche for coax in the office is video applications, such as videoconference rooms and sophisticated presentation facilities.

For instance, Coates says, coax is the cabling of choice for connecting large plasma video screens. And of course there are vertical markets, notably the broadcast industry, where it is widely used.

Security systems also make considerable use of coaxial cable, says Wessels. It is used to connect security cameras to central monitoring stations, for instance. Since security has been a growing concern in North America in the past year and a half, that represents an important application for coax today.

There are assorted other applications, including cellular transmission towers, telecommunications carriers’ central offices and even the auto industry, which uses coax quite extensively inside cars.

The 50-ohm coax commonly used in wireless networks is not the same as the 75-ohm cable most often used in the cable-TV network and in other video applications. And broadcast studios use a high-precision video coax designed to minimize signal loss.

Lower-quality cable is good enough for applications like security cameras.

In short, using the right cable for the job is important when working with coax as it is with other types of cabling. “Make sure you match the cable with what you’re trying to accomplish,” Wessels advises.

The type of cable must also be appropriate to where it is being installed. For example, Coates says, cable installed in or around high-powered equipment or in plenums may need Teflon shielding to provide proper heat resistance.

And as with any sort of cable, proper installation is important. Wessels says it is particularly important to make sure coax is not bent too sharply, and to use the proper connectors for the type of cable.

The cable itself is changing, too. “If you were just to look at two pieces of cable from 10 years ago and today,” Wessels says, “they wouldn’t look necessarily very different.” But looks are deceiving. Cable is more stringently tested today, and the industry has improved its manufacturing techniques so that coax is more consistent and meets higher standards.

For example, Coates says, coaxial cable was once tested only up to frequencies of 450 megahertz. Today, most coax must meet tests at 3 Ghz, and in some applications 6 GHz is required. Moving from a standard braid shielding to a combination of foil and braid has improved shielding performance at high frequencies.

High-frequency signals can leak through the openings in a braid shield, Coates explains, and external signals can also cause interference.

Braid provides better insulation at lower frequencies, though, so combining the two gives the best all-round performance.

Manufacturers have also moved away from solid polyethylene shielding to foam polyethylenes, Coates says.

The air trapped in the foam lowers the dielectric constant of the insulating layer, which increases the speed at which a signal can travel through the cable (called the velocity of propagation).

Because air is a better insulating material, trapping air in the polyethylene foam means a thinner layer of insulation will do the same job, which means the cable can be made thinner or the conductor in the centre can be thicker — reducing signal attenuation — while the cable remains the same thickness on the outside. “Air is the perfect dielectric,” Coates notes. “If we could insulate cables with just pure air we would do it, but that’s kind of impossible.”

Twisted pair and fiber continue to dominate enterprise networks, leaving coax a few niches and of course its dominance in the cable-TV market. That will probably remain the case.

Still, some think coax is poised for growth. Maintaining that today’s networks are pushing the limits of twisted pair and “people have been talking about fiber being the end-all, be-all cable since the mid-seventies,” Coates predicts “down the road coax is going to be making a comeback, just because you can’t do it with twisted pair.”

Grant Buckler has written about information technology and telecommunications since 1980. He is now a freelance writer and editor living in Kingston, Ont.

Improvements in the technology are, to some extent, fighting back against fiber’s invasion.

Inside the enterprise, coax seemed for a time to be practically gone, but growth in wireless networking has helped it make a limited comeback.


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