Part 2: What's In the Last Mile?

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Last time, we reviewed the ample amount of stuff
needed to build what's commonly known as "the last mile" of the cable network — the section of plant that starts where the fiber
part of hybrid fiber coaxial (HFC) adjoins to the coaxial
part, and ends a few hundred feet later at subscribing homes.

The last mile, as previously described, isn't usually a linear mile. It's a more of collection of lengths of plant of several hundred feet, stemming out from the node in a way that blends with neighborhood geographies.

Summed together, those lengths of coax emanating from the node can equal a mile.

Building the last mile involves six distinct and mostly sequential steps, at least in this (very) simplified overview of aerial plant construction: Making maps, hanging strand, placing cable, cutting things in, testing and dropping off to subscribing homes.

Building the last mile starts with a map.

These days, there are companies that make a tidy business selling digitized maps. In the early days of cable, though, maps obviously didn't exist and had to be drafted the hard way: On foot, pushing one of those pizza-pie shaped contraptions that ticks off linear feet. (This is why they're called "walkout maps.")

Making a walkout map means walking the streets, measuring the footage from telephone pole to telephone pole. Any identifying characteristics — a transformer, an anchor wire — are jotted down to help the construction effort later.

(When climbing poles and hanging plant, line technicians say, it's fairly easy to lose track of which pole you're on. Alleys, easements and backyards with barking dogs can all start to look alike after a while.)

From the maps, a topology
— the blueprint of the network — is created. (Note: Some engineers get persnickety about the difference between an architecture
and a topology. A topology, they say, is only the physical layout, and as such is a sub-set of an architecture. An architecture is all of it.)

Next comes the strand — heavy lengths of steel wire, affixed with three-bolt clamps to the poles. At the last pole, a different type of clamp — with a ring on it — attaches the hung strand to a guy wire and to the ground.

Electrical bonds, installed to protect the strand from lightning damage, are typically done "first, last and 10," meaning every first, last and 10th pole.

"Bond" means the strand is bonded to someone else's ground: Usually the public utility company that owns the pole.

Once the strand is up, the cable crew starts its work.

First, they hang the cable. They next plunk a lashing machine onto the strand to do its dizzying work, circumnavigating the strand and the coax with lashing wire. The lashing machine has a hook on the bottom so someone on the ground can pull it along.

At the end of the line, another connector, sometimes called a bug nut, clamps everything down.Spacers
make sure nothing can rub the jacket off the coax. (It is proper bug-nut installation that is a source of great pride for some technicians. Craftsmanship matters, especially when clamping off.)

One of the most critical aspects of stringing cable, aficionados say — other than making sure it doesn't get kinked or crimped — is the expansion loop.
Expansion loops are needed because strand wire contracts and expands differently than coaxial cable.

(One of the most awkward terms in cable technology — the suckout
— is rooted in the expansion loop: If it isn't big enough, the cable can pop out of its connectors. This usually happens in winter, when temperatures tend to make things taut.)

Step four in last-mile construction is the cutting in of things like amplifiers and taps. This is another critical craftsmanship area, particularly when coring and prepping the hung cable.

Forget to put on a length of heat-shrink tubing before adjoining the cable into the amplifier tap? Start over. Scratch or damage the center conductor? Start over. Do too much of either? Use up expansion loops.

Next comes turn-up and testing. The idea is to first check for shorts in the installed plant, then to stuff modules into amplifiers, then to check signal integrity — although procedures vary widely in this phase.

The installer, who runs the last bit of coaxial cable (usually somewhat less thick than what's on the poles) to subscribing homes, handles the last job.

That's the unsung story of last-mile plant construction. It's not easy work. It's not glamorous work. But this industry wouldn't exist without it. Maybe today's the day, then, to give your colleagues in plant construction a cold beer and a clap on the shoulder, in appreciation.

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