Jan 082011
twisted pair utp cable
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THE torrential rains that battered California last month (parts of Los Angeles got a year’s supply in ten days) have left a trail of telecoms troubles.

In the hillside community where your correspondent resides, the local phone company has still not fixed all the water-logged cables—most of which are buried underground, alongside the electricity lines and storm drains. With torrents gushing down the hillside, the telephone conduits have become more akin to plumbing than wired connections to the world.

Incoming calls have rung once and then rung off. For a while, outgoing calls were impossible. The source of the problem seems to have been the pulp used in older connections to sheath individual phone lines within the cable. Paper sheathing was introduced in the 1940s to help reduce the relative humidity inside telephone cables. Being a natural desiccant, the pulp is supposed to absorb any moisture passing through the cable’s outer polyvinyl chloride casing. That it does rather well, up to a point. Unfortunately, when the pulp becomes sodden, it can become more of a hindrance than a help.

The telephone line connecting a residence to a local exchange (called “central office” in America) consists of a pair of plastic-coated copper wires that are twisted together along their length. The idea of the twisted-pair was invented by Alexander Graham Bell in 1881 to cancel out any electromagnetic interference and crosstalk between adjacent lines. A typical cable comprises hundreds, even thousands, of twisted-pairs divided into bundles, which are themselves twisted around one another to form the cable. If too much moisture gets into the cable, the electrical properties of entire bundles, let alone individual twisted-pairs, can be affected, resulting in widespread interference and dropped calls.

It is an old problem. Some early paper-sheathed cables were capable of drying themselves out. A blast of air would be sent along the cable and an electric current applied to various twisted-pairs to provide enough heat to evaporate the moisture within. By all accounts, the arrangement worked reasonably well but was considered too cumbersome for everyday use. Besides, moisture was less of a problem in the overhead lines more typical of the times. Though an eyesore, telephone lines strung on poles dry out fast enough naturally once the rain ceases.

With the seasonal rains far from over, then, your correspondent is praying that his local carrier (Verizon) becomes tired of having to deal with all the complaints from neighbours about poor, or no, telephone connections—and pulls the 60-year-old copper cables out of the ground in order to replace them with optical fibre. The benefits of doing so would be far greater than just delivering a reliable service when it rains.

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