New numbering rules for phones in Australia [telecom]

Obviously fibre has advantages in capacity over copper, but does > that mean existing copper plant should be abandoned? > Why is fibre cable cheaper to _maintain_ than existing copper > cable? I would guess that the big maintenance expense of outdoor > physical plant would physical protection against weather and > injury and access for maintenance. Wouldn't physical protection > costs, such as durable outer shells, be the same for copper as > fibre? > If say a car knocks down a pole carry > H*LL no!! When the pole got knocked down, the lines -broke-. The > labor cost for splicing umpty-hundred pairs -- probably _twice_ > (since you probably have to insert a _replacement_ section of > cable as opposed to just re-connecting the broken ends to each > other) -- probably swamps the cost of re-setting the pole.

Depends on which cables break, where they break, and where the pole itself breaks.

Except for drops, every aerial communications cable of any description (copper telephone, CATV coax, or fiber) is almost always supported by a strand, a grounded 1/4- or 3/8-inch stranded steel cable placed under tension. The strand provides mechanical support for the communications cable and prevents the cable from sagging. The communications cable is lashed to the strand by lashing wire. Examples:

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Note that the strand is not a single wire; it consists of several (typically seven) individual wires twisted together.

Open-wire transmission and primary electric power conductors are also tensioned. Tension is maintained by downguys at end poles and corner poles.

If a car "knocks down a pole", there are two possible failure scenarios:

- SCENARIO: The pole breaks off near ground level -- where the car hits it, or at ground level -- but the upper part of the pole remains intact, suspended in midair by tensioned stands and power conductors. In this case, the pole owner can indeed just "replace the pole and remount the lines" -- more precisely, its own lines. Other pole occupants replace their own cables. The power utility always goes first; other pole occupants must wait until the power utility has repaired its facilities and "cleared" the area before they can work on their own facilities.

- SCENARIO: The pole is physically displaced to the extent that strands are broken. In this case, the damage to communications conductors may be extensive. A strand doesn't break cleanly; it stretches, then individual wires break one-at-a-time, the broken wires untwist and flail around, until the last wire finally breaks.

When the strand breaks, the communications conductor itself is subjected to severe tension. But it doesn't break cleanly either -- in most cases, it will be so badly damaged that it will be necessary to replace an entire section of cable, requiring two splices. As Bonomi noted, making two splices in a multiconductor telco cable is a tedious process that can take many hours to complete.

CATV coax conductors are easier to splice, but restoration still requires a lot of time. CATV coax stretches before it breaks, damaging large sections. It may be necessary to replace as much as several hundred feet of coax.

Fiber cables can be damaged as well. Fiber cable usually contains a steel or nylon "strength member" to prevent stretching, but if it's subjected to sufficient tension (which it surely would be if its supporting strand breaks) even the strength member will stretch and/or break. If the strength member breaks, everything else in the cable -- jacket, buffer tubes, filler -- will be stretched and break as well. Furthermore, the fiber itself won't necessarily break at the same place that strength member breaks; it will break at its weakest point. If the fiber is in a loose-buffered tube, the actual break point may be several poles away.

Further exacerbating all of these problems is possible damage from falling electric power conductors. If an energized transmission or primary conductor breaks and falls, it may land on a communications cable or strand. A 13-Kv energized wire landing on a grounded strand can have interesting results: sparks, smoke, noise, awful smells, melting insulation. I'm not sure what it would to do a multiconductor telco cable, the possibilities are interesting to contemplate....

Bonomi continued:

Needless to say, you can make a few fibre splices in _far_ > less time than it takes to splice the equivalent umpty-hundred > copper pairs.

Well, I'm not sure about that.

Even under the best of conditions, splicing a fiber is a tedious time-consuming process: the cut ends of the fiber must be cut at precise angles, cleaned with alcohol, aligned perfectly, then sealed with transparent cement having the same index-of-refraction as the glass itself. A source of ultraviolet light is necessary to cure the cement. Fiber splicing jigs usually include a microscope or video camera to facilitate the alignment process. This is not something that can be done from a bucket truck during a snowstorm.

And, of course, two splices will be required if there isn't enough slack in the fiber cable. Slack is necessary not only to replace the damaged section of fiber cable, but to provide enough fiber cable that the cut ends ("tails") can be spliced under controlled conditions, usually inside a vehicle or a tent. Most companies that own fiber networks either own a vehicle dedicated to fiber maintenance, or have contracts with fiber-maintenance companies.

(I'm aware that newer splicing techniques, involving fusing the glass, are available, although I've never used one. So maybe splicing isn't so time-consuming now, but I suspect it still has to be done under controlled conditions.)

In addition to the physical difficulties described above, fiber splices also introduce signal attenuation. One of fiber's big advantages is low attenuation, allowing for design budgets of only a few decibels of overhead. But even the best splice can introduce as much as one decibel of attenuation. Two splices can eat up most of the overhead -- more than hundreds of feet of unspliced fiber.

Further complicating all this are public works projects such as road and bridge construction. These projects often require moving entire polelines. Power, telco, and CATV cables can be spliced -- with difficulty to be sure, but the work can be scheduled to minimize service disruptions and signal attenuation.

But if fiber cables must be spliced, we encounter the same problems mentioned above. For this reason, fiber networks are usually designed with excess slack. The slack is introduced in "loopbacks" every thousand feet or so. In a loopback, the fiber loops back for a hundred feet or so, then loops forward again. These pairs of loops are clearly evident on roadside polelines, either as loops hanging below the cable, or supported by "snowshoes". Example:

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I agree with other posters who assert that fiber has enormous advantages over copper or coax because of its wider bandwidth and lower attentuation. But I'm not convinced that splicing time is one of them. In the case of CATV, splicing good old aluminum-sheathed coax is far easier than splicing fiber. As for telco networks, Bonomi may be right, I'll leave that for the rest of you.

Neal McLain

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Neal McLain
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