Use+of+Copper+Plant+for+Digital+Subscriber+Loops

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looptelecom.com 1 March 5, 2002

Use of Copper Plant for Digital Subscriber Loops (DSL)

By Dr. John W. Pan

Introduction

Traditional copper pairs installed for voice telephone service use can also be used for digital subscriber loopapplications (DSL). Advantages of reusing this copper plant (installed wires) include:

Rapid response to customer need for digital services Savings due to the avoidance of installing new transmission plant

However, coppers pairs in use for voice services for many years are not always suitable for digital services.This paper discusses various issues regarding the use of copper plant for digital services such as:

Wire Gauge Insulation Splicing Installation

Wire Gauges

The diameter of the copper wires, wire gauge, affects both voice signal transmission and digital signaltransmission. In the early days of telephone development, in order to save on copper cost, very fine copperwires were used for subscribers near the central office. Thicker wires are used for subscribers far away. Thisway the shorter/finer wire will have the same signal attenuation as the longer/thicker wire. Therefore, allsubscribers would experience the same voice volume when talking.

However, starting at around 1950, a policy of "uni-gauge" was recommended, meaning that all subscriberwire pairs were required to use the same gauge. The advantage of unigauge is ease of installation andadministration. At the same time, newer telephone sets were equipped with automatic volume controls, sothat the nearby customers were still able to maintain the same sound volume as faraway customers.

For the digital transmission network since the 1960s and 70s, unigauge is preferred over mixed gauge. Thisis because of the higher signal losses over copper. DSL installation for copper plant older than 1950 may

have wire gauge problems.

Insulation

Early wire pairs use paper insulation, starting with paper tape wound around the wire and progressing topaper pulp directly extruded onto the wire. Pulp insulated cable, as they are called suffer from two majorproblems. First, when wet, the paper loses its insulating qualities. Second, after many years in service, thepaper will pulverize and become powder. These pulp cables are in use even today, but providing poorservice even for voice signals, let alone digital signals.

Starting in 1940, plastic was used for insulation, which is a significant improvement over pulp. The mostsignificant improvement over pulp is the resistance to moisture. Although longer lasting than pulp, even

plastic will eventually harden and break into pieces.


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Splices

Splices are the weakest part of a copper run. Except for subscribers very near a central office, most copperpairs to subscribers will have one or more splices. This is because copper cables are supplied in reels oflimited size. Splices can be of two types, simple twist and gas tight.

Simple twist splices are used for most old copper plant. Electrical conductivity depends on copper-to-coppercontact between the two wires. This type of splice deteriorates quickly with temperature swings and moisture.Temperature swings cause expansion and contraction which can break the contact. Moisture will causeoxidation of the copper which can also cause break of contact. Unreliable twist splices can be detected byusing micro-interruption testing equipment.

To increase the reliability of twist splices, a common industry practice is to use "wetting current", a directcurrent of about 20 ma always present in the wires in addition to the signal current.

A recent development is gas-tight splices, which do not suffer from the problems of twist splices. In a gastight splice, the copper wires are pressed into clamps. Copper to copper connections are not just contacts,but more like welds, which as implied prevent moisture from affecting the splices. In addition, the springnature of the clamps adjusts to temperature changes to maintain the pressure on the wires. Copper wireplant using gas-tight splices do not need wetting current and have greater reliability for digital transmission.


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Installation

Depending on the local practice and local laws, wire cables are installed either on poles overhead or buriedunderground. In rural countryside, overhead is cheaper. In some congested cities, local laws requireunderground installation. In either case, the performance and reliability are different.

Overhead installations are exposed to more environmental danger, such as wind, rain, and extremetemperature variations. Telephone poles are also subject to damage from collision by vehicles.

Underground installations, while protected from the above dangers, are prone to flooding and constructionaccidents from people digging into the ground.

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