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What goes around comes around

Mon, 04/30/2001 - 8:00pm
James Careless, Contributing Editor

Return path maintenance: it's perhaps the most important, yet least understood, aspect of today's cable TV networks.

Small wonder: until a few years ago, cable TV engineers only had to pump video out to end users, with no regard about what happened at the other end. Granted, it was possible for defects at the user's end to generate network interference, but that was nothing compared to the havoc a single humming fridge can do to a two-way network today.

So what should cable TV engineers do to keep their return paths in tip-top condition? When it comes to maintaining a pristine, interference-free return path, it's all about "ingress" and keeping signals from leaking into or out of the cable plant.

To put it mildly, "signal leakage is the biggest issue for the cable TV industry," says Joseph Van Loan, Mediacom's senior vice president of technology. That's because ingress from a single source can screw up the entire return path, resulting not just in stalled traffic, but also steamed subscribers.

But what causes ingress? That's a two-part problem. The first part is access: there has to be a cracked cable or loose connection somewhere, to provide a point of entry into the network. The second part is the actual noise source. According to Paul Janson, president and CEO of the third-party network installer Worldbridge Broadband Services, ingress noise can come from anywhere. It could be caused by a loose coil on the back of a subscriber's refrigerator, a faulty electronic device emitting an RF field, or virtually anything that can give off a spark.

Sometimes the causes of ingress are truly strange. A case in point: One return path that Worldbridge activated for Comcast Cable kept coming up with ingress problems. No one knew why. "We couldn't find a problem," says Janson, "and we do this for a living." In desperation, Janson posted a crew round-the-clock near the troublesome node. It was located next to a dirt lot that had been empty during the daytime.

At 8 p.m., the lot started to fill up with heavy construction equipment. "Over the next three hours, the ground was pounded as the trucks and earth movers came in," Janson recalls. "Turns out that this was a parking lot for a very large construction company. We'd never known this, because it was always empty when we were there."

It was the pounding of the equipment that was shaking the ground, and cracking Comcast's lines in the process. "This was the strangest cause of ingress that we'd ever seen," Janson says. "We still don't know precisely how a multitude of construction equipment could damage the network, but it did."

Batten down the hatches

Fighting ingress successfully is similar to keeping a boat afloat. In each case, you have to patch the leaks so nothing gets in. In the case of cable TV return paths–and the entire network, in fact–this means making sure everything's in good shape and tightly connected.

"Fundamentally, a cable TV plant is a mechanical system," says Wayne Hall, Comcast Cable Communication's vice president of engineering. "To work, it has to be tightly connected with all its connections properly shielded."

Sometimes, this tightening is done as a matter of course. For instance, even before Time Warner Cable began its extensive two-way upgrade program, "We were cleaning up our signal paths and our drops," says Paul Gemme, the company's vice president of plant engineering. "We did this not just in the field, but every time we visited a customer's home. As a result, when we started to build the return path, we already had ingress noise well under control."

Other times, the tightening came as a result of the MSO upgrading to two-way. "We started by finding and fixing all leaks that were more than 4 or 5 microvolts per meter in the system," says Mediacom's Van Loan. "We then changed all our old-style connectors to high integrity Snap-N-Seal connectors. Then we equipped our technicians with meters to check each and every drop for ingress."

When it comes to finding ingress, what's required is "a bit of detective work," adds Comcast's Wayne Hall. "It's not just a matter of having the right equipment to do the job, like spectrum analyzers. You also need people who like solving problems."

To trap or not to trap

Just knowing which subscriber location is generating ingress interference is only half the battle. Once you've found the source, you then have to get inside, find the exact offending piece of equipment, and fix it.

Unfortunately, many subscribers really don't appreciate having their homes scoured by cable techs. This is why some MSOs prefer to eliminate interference by installing traps on the offending cable drops, thus avoiding trips into subscribers' homes.

But is trapping out ingress really such a good idea? Not necessarily. In fact, most engineers don't like doing it. They reason that trapping is only a Band-Aid solution to ingress: one that not only doesn't solve the problem now, but leaves it lurking to screw up further deployments later.

"You just mask your ingress problems by putting trap after trap on the system," explains Worldbridge's Janson. "To truly solve ingress, you've got to actively eliminate the problem, and traps don't do this." Mediacom's Van Loan agrees. That's why his company "only uses traps when (it) absolutely (has) to."

Keeping the return path in shape

Many cable TV engineers believe that effective return path maintenance begins at the construction phase. If everything is done properly from the outset, they say, then keeping the plant in shape really isn't that difficult.

Still, even the best-built return paths require upkeep. So what do the experts do?

"We routinely check our return paths using spectrum analyzers," says Gemme. The idea is to check out each network component on a regular basis, so that problem areas can be identified and isolated as soon as possible.

But that's not all: effective return path maintenance also requires regular physical inspections as well. Is all the cabling still in good shape, or has an earthmover damaged the shielding since it was installed? Are all the connections still tight? Are all the back-up power supply batteries still up to snuff? These are questions that should be answered before problems occur, not after.

Network monitoring

It goes without saying that the best form of return path maintenance is prevention: solving problems before they start. But what's the best way to do this? Most argue that the key to prevention is data–knowing just how well the return path is functioning, so that it can literally be fixed before anyone else knows it's broken. With the advent of digital networks and the placement of an increased amount of customer premise equipment, operators are able to poll that equipment on a regular basis and determine the health of the network at its end point.

As two-way units each have their own IP addresses, set-tops and cable modems are ideal network watchdogs. Just by detecting which ones are out of tolerance and why, you can pinpoint network problems with ease.

But what about preventive maintenance? How can a system adequately monitor a huge network? To answer this question, Time Warner and equipment manufacturer C-COR.net have joined to test an integrated network monitoring system in Tampa Bay, Fla. The goal is to create a software product that uses data from Time Warner's set-tops and modems to automatically monitor the network in both directions.

"As designed, this software will collect data from all of our two-way units, and correlate it against their addresses as stored in Time Warner's billing system," explains Gemme. "After all, each unit's unique IP address is already stored in the billing database, along with their user's home address and location.

"Once these linkages are in place, it's easy to ask the database which units are experiencing low RF readings, for instance. In response, the software generates an on-screen map showing exactly where the set-tops/modems are, and where the likely problem areas are as well.

"C-COR.net's software uses the exact same maps we use for our plant design," Gemme adds. "This means the trouble maps include the necessary equipment information, making it easy for our technicians to know beforehand what they're up against, and what parts to take with them."

A year in the making, this automated monitoring system is just "three or four months" from being finalized, says Gemme. When it is, Time Warner–and by extension, the entire cable TV industry–will have access to a monitoring product that could prove invaluable to proactive preventive maintenance and all the ramifications it has on keeping a network up and running.

Intelligent monitoring

Clearly, a preventive IT-driven monitoring trial is the next step for return path maintenance. By using two-way set-tops and cable modems to pinpoint data sources, cable TV engineers can keep their return paths as pristine as the day they were installed.

Of course, this data will only supplement, not replace, the physical labor that keeps today's plants ingress-free. That's because nothing can substitute for tight connections and high-integrity cables.

In turn, both automated monitoring and field work will only succeed if there's the right attitude driving return path maintenance. That attitude consists of "never letting the job get away from you," says Gemme. "To keep your return path clean, you have to stay on top of it at all times."

So how often is that? Mediacom's Van Loan recommends checking the return path "every day."

What this means is that the days of letting the end user be the network monitor–by only responding when people complain of service outages–are long gone for the cable TV industry. To make return path maintenance successful, it must be an ongoing commitment. There's just no two ways about it.

email: james@tjtdesign.com

 

Principles of effective return path maintenance

  1. Tighten all physical network connections.
  2. Check all cables for damage; replace where needed.
  3. Include the drops in your survey; don't just focus on the core network.
  4. Your spectrum analyzer is your best friend: use it constantly to search for ingress noise.
  5. Trapping is for fur-traders. Cable TV engineers should eliminate ingress noise, rather than just masking it.
  6. Integrate set-tops and cable modems into your monitoring system.
  7. When in doubt, automate your network monitoring.
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