Rural Outside Plant

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Another section of this web site talked about outside plant, which is the telephone company's term for the cabling and other equipment which connects your home telephone to their Central Office.

This diagram shows an overview, and below are some pictures and descriptions of how outside plant is different for rural areas.

The main differences are that the Central Offices are typically smaller, and the cable distances are much greater.

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Here is a typical Central Office. Note it is only one storey, but is still well-kept, and mostly windowless.

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Around back are some air conditioners, technician's trucks, and even a small aluminum motor boat (the technicians working in this area often have to run telephone lines to islands). Also, at the right are some Jumpered Wire Interfaces (JWI), which split the high pair count cables from the Central Office (such as 1,200 pairs) to smaller cables (such as 200 pairs) for distribution to terminals and people's houses.

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Here's another Central Office. This one has a few meters of unused 1,200-pair feeder cable in front (much more attractive than plastic pink flamingos).

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Around back, no security cameras, but there is good lighting, and of course, air conditioners.

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The front door has a mechanical push-button lock, and a discrete sign.

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Here's a close-up of the 1,200 pair cable. It is about 4 cm in diameter.

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The feeder cables coming out of the Central Office are all buried, but within a few meters of the building, they surface to come be run up a utility pole. Note the ancient lead-covered splice enclosure.

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Here's the top of that utility pole. These cables carry everything that this Central Office does (this is what is called a “single point of failure” – one car accident, and there's a lot of people with no phone service).

There's another lead closure at the top-left.

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As shown in the close-up of the 1,200-pair feeder cable above, cable construction is to have twisted pairs covered by a thick plastic protective jacket. After enough sun and flexing in the wind, the cable jacket can develop leaks, which could allow water to seep in between the cable pairs. This would change the electrical characteristics of the cable, and cause many problems (such as cross-talk between telephone lines and distorted signals). Therefore paper insulated cable (these cables will all be more than 40 years old) and sometimes newer cable) are pressurized with air, typically less than 9 pounds per square inch (for comparison, automobile tires typically have a pressure of more than 30 pounds per square inch). This provides several benefits:

Rather than water seeping into the cable, air will leak out, and this keeps the cable dry.
The flow of air through the cable will dry and remove any moisture that does get into the cable.
By measuring the pressure along test points in the cable, and graphing this, a technician can determine approximately where the cable jacket is bad (the pressure will be lower where the air leaks out).

I'm sure you've seen the little packages of dessicant (which look like restaurant coffee sugar packets) of that often come (along with a warning not to eat it) with electronic and other products which have been shipped from overseas. This stuff is a chemical that likes to absorb water (from the damp ocean crossing), so the moisture won't damage the goods. Similarly, there is a need to ensure that the air used to pressurize the cable is dry, and this blue crystalline material is dessicant, and does just that. The air compressor (or “bottled gas”, where the air comes from a compressed air tank) would be in the Central Office, and feeds the black hose, so the air passes through the dessicant, and the hose continues up the utility pole to pressurize the cables father along.

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Here's a close-up of the dessicant canister. It reminds technicians to:

Change the desiccant when it turns pink.
That the maximum pressure allowed is 25 pounds per square inch (automobile tires typically have a pressure of 28 to 40 pounds per square inch).
And to be gentle with it.

This device is made by Jameson Corporation, and is model J-100-8.

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Here is a new air pipe (in an urban location) from a central office, waiting to be connected through to a cable vault farther down the road.

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This air pipe is made by Superior Essex. Their catalogue page is here. It shows that this type of pipe is typically installed in ducts (underground plastic conduits), the pipe has an outside diameter of 18 mm, and has a 4 mil (about the thickness of paper) aluminum tape inside the pipe (since the plastic used would otherwise allow water vapour to pass through the pipe wall (it is important to keep the air dry).

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To split a feeder cable into smaller cables (to each go in different directions) a splice enclosure is needed. Here is a really old lead one (on top), and a newer plastic (and openable) one below it.

Cables with a lead splice closure are old enough that the wires are likely paper insulated (as the bundle of spliced wires is covered in a protective and insulating cloth before the lead closure is sealed with solder, plastic insulated wires can also be enclosed in lead closures, without damage when the lead is sealed with solder).

Due to the difficulty of opening and resealing the lead closures, and the health concerns for technicians working with the lead, telephone companies are working at removing this type of closure.

Modern buried telephone cables are generally grease filled: when the cable is manufactured, a gel is injected to fill the interstitial spaces between the twisted pairs, so water cannot seep in. In contrast, aerial cables are generally dry, and do not have this gunk.

Attached to the lead splice cover is a pressure transducer to enable remote monitoring of the pressure in the cable.

A web page for a typical outside plant pressure transducer made by TX Technology Corporation is here. This points out that the transducer is designed to measure pressures from 0 to 9.5 pounds per square inch, and has a corresponding output from 100 kΩ to 3,820 kΩ. More detailed information is here. It points out that the transducer has a water-tight screw cap (with a knurled edge for finger tightening, and a little ball-chain so it won't be lost if dropped) a which can be removed to set the zero-pressure point for the transducer, according to the altitude at which it is installed.

The flexible cable hanging down is the electrical signal from the transducer, and is connected to a pair of wires in the lower cable so the pressure at this location can be remotely monitored at the central office. Because most central office activities (such as changing the telephone line features which subscribers receive and trouble-shooting problems) can now be done remotely, most central offices are not normally staffed. Therefore, the system (at the nearest central office) which monitors all the remote pressure transducers will usually be itself monitored remotely.

When pressurized cable is spliced to dry or greased-filled cable, a plug must be poured into the end of the pressurized cable, so the air does not leak out.

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Now we get to a really interesting part. As you drive along rural roads, you'll see what appear to be paint cans, each with a this cable coming out the top. If you're really bored, you may also notice that they are exactly 6,000 feet apart (that would be 1.14 miles, and 1.83 km).

Just as you can walk slowly through water, but cannot easily run (since water is more viscous than air), higher-frequency electrical signals are attenuated (made quieter) more than lower-frequency signals.

In urban areas, there are generally enough customers within a 5 km radius of Central Offices to justify building a Central Office, and in 5 km of cable, the signals are not distorted too much. In rural areas, customers are farther apart, and the cable runs to customers are longer. The capacitance of the cable (which is analogous to the greater viscosity of water) would attenuate the high-frequency signals (distorting the sound), so inductors (which counter-act the capacitance) are connected to each pair of wires.

Since the capacitance is spread-out along the length of the cable, the inductance also has to be distributed along the cable. A popular scheme is called H88: the “H” refers to a distance of 6,000 feet between inductors, and the “88” refers to the inductance value of 88 mH (milliHenrys).

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As you might have guessed, these therefore are not paint cans, but are the enclosures for the loading coils. If there is a 600-pair cable, then you need 600 loading coils (these look something like little spools of thread), and you need to attach each to the feeder cable.

Here there are three old loading coils (two large, and one smaller metal enclosure mounted on the utility pole), and one newer black plastic loading coil enclosure attached (with stainless steel straps) to the cable at the left. A splice enclosure allows the cable from the loading coil enclosures to be spliced to the feeder cable

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Here's a close-up of those big old loading coil enclosures.

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As more cables are installed, more loading coils are needed. Here's the newest loading coil (cute, isn't it).

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That's it for the loading coils, but on the very next utility pole (and this does generally happen, it will be on a directly adjacent utility pole, as the spacing is also 6,000 feet, likely to simplify installation), there's something else. Rather than being up high, these are generally a waist-height.

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So we have the feeder cables running along, and a 3M splice enclosure bringing a few of the twisted pairs in the cable down the utility pole ...

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Down to these, which are T1 repeaters. Rather than the analogue signals carried by telephone lines, a T1 is a digital signal which carries data at 1,544,000 bits/s (on the exact same type of twisted pairs as the voice telephone lines). There are many advantages to T1 signals, including being able to carry 24 conversations over two twisted pairs, not gathering cumulative noise along the length of the cable run, and being able to carry (somewhat) high-speed data. But, the problem is that the signal needs to be amplified periodically – every 6,000 feet in fact. The amplifier is actually called a repeater, since it outputs a binary signal (which has only two states, on or off).

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This is the older of the two. The feeder cable is pressurized, and so is the cable coming down to this repeater enclosure, and in fact, the repeater enclosure is pressurized as well. At the base of the white enclosure, you can see the clamp (with padlock) which provides the air-tight seal. Since there are active electronics in this enclosure, and they can't have cooling fans (since it is an air-tight enclosure), these enclosures are always white to reflect the sunlight (and some of the heat) as these bake in the summer sun.

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Here's the newer enclosure, it is an HRE-458 HiGain Remote Therm-O-Nator Enclosure from ADC DSL Systems, Inc. Some documentation for it is here, and it can hold up to ten cards, such as this HDSL line extender which extends the range of two full-duplex 784 kbits/s DSL connections by up to 12,000' using two 24 gauge copper twisted pairs.

The enclosure has heat sink fins on top (since heat rises, this will be where it gets the hottest). These help cool the enclosure by providing more surface area, so it can radiate more heat.

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Here's a look at the bottom, showing the cable entry, and also a pressure relief valve to release the pressure before opening the enclosure. Another opening allows the pressure to be measures. The thinner black wire is to ground the enclosure, so goes to a metal stake driven into the ground.