Communications Cables Revised: 08/22/99 18:42 Residential Phone Wire Basics Reason for Revision: Add header
If you haven't reviewed the pages on Color Codes: Pair Identification by Conductor Color and Categories of Cable Transmission Performance, you may wish to do so before proceeding with the information on this page.
There is a good chance that your home has one or both of the following conditions in its phone wiring:
1. Wire made of individual conductors and not pairs.
2. Some combination of bridge taps and daisy chains.
Both conditions are widely encountered in residential installations. Until recently, when data connections and multiple lines became common in the home, they were acceptable. Unfortunately, they both contribute significantly to poor transmission quality and may make installation of a second phone line or data communication via fax or modem a near impossibility.
Conductors vs Pairs
[The term "pair" is used generically to describe a telephone circuit. For this discussion, however, when I use the word "pair" I mean the physical twisting of one conductor with a mate to form a balanced circuit. As I use the word circuit, it means the electrical loop formed by either two parallel conductors or a twisted pair.]
A phone circuit consists of an electrical loop. Two wires laying parallel are more than adequate to accomodate voice communications. In fact, the earliest telephone signals were sent using one metallic conductor and the earth itself as the other conductor in the loop!
Much residential phone wiring (called Station Wiring) is made with either 2, 3, or 4 insulated conductors bunched together and jacketed. The bunch may or may not be twisted along its length, but the point is that any two conductors connected to form a circuit are essentially parallel to each other. I will refer to this as conductorized Station Wire, or simply SW.
In this design signal loss due to crosstalk noise is very likely. If you've ever had a phone call in which you could hear (but couldn't pariticpate in) another conversation, you've experienced crosstalk. It is the partial transfer of the signal on one circuit to a nearby circuit in the cable.
The best circuit for transmission is a well balanced pair of insulated conductors, twisted together at regular intervals. In a cable with multiple pairs, this helps isolate the signals being sent along a pair from other signals being sent on neighboring, parallel pairs. In order to further isolate the signals, the interval of twists is different pair to pair. Isolating the signals in this way helps prevent crosstalk.
The lay length or twist length of a pair is the distance from one twist to another. Its complement is referred to as the number of twists per foot (TPF). For example, a pair with a lay length of 3 inches has 4 TPF (12 inches per foot/3 inches per twist.) There is a correlation between the lay length of the twists and reduction in crosstalk. In general, the shorter the twist length, the better the crosstalk.
A typical exchange, distribution, or drop cable in the Outside Plant, or Category 3 or lower Premises cable, has twist lengths which range from around 2 inches (6 TPF) to a maximum of 6 inches (2 TPF). In a typical 25 pair unit (remember the color code scheme?), each pair has its own, unique, lay length.
Categories 4 and 5 have much more stringent crosstalk requirements than Category 3 and so the twist lengths are much shorter or "tighter". Category 5 cables typically have twist lengths less than 1.5 inch (more than 8 TPF).
Getting back to the residential installation, millions of feet per year of conductorized SW is put into homes. Noisy, poor quality circuits result when second phone lines are added. This becomes critical when fax machines or modems are expected to operate over one of the lines.
How can you tell what you've got?
SW is color coded according the the alternate system of red/green, yellow/black for pairs one and two. When you strip back the jacket for a foot or more, it should be evident that no two conductors particularly belong to each other. When you look at the cable in cross section, the jacket should have filled in the spaces around the conductors, although this may not always be the case. Four conductor wire is commonly referred to as "quad" SW.
SWT (Station WIre, Twisted pairs) usually has the standard color code of Blue/White, Orange/White for pairs one and two. When you strip back the jacket for a foot or so, at least some of the pairs should be obvious. (SWT may meet, or comes close to, Category 3 performance. Pair twist lengths are usually in the 2 to 6 inch range.) In cross section, the jacket will look like a tube, or straw, with the pairs laying in the tube. CAUTION: Some paired designs do use the alternate color code system.
If you are fortunate enough to have 2, 3, or 4 pair wire already in your home, adding a second phone line or getting good data transmission has been made easier for you.
Bridge Taps and Daisy Chains
A bridge tap is a way of creating an extension by splicing into a main wire which leads back to where the telephone service enters your house. In a bridge tap scenario, your wiring ends up looking like a fishbone diagram.
Daisy chains are similar. In a daisy chain, each extension is wired in series with the next one down the line.
Bridge Tap/Daisy Chain
What's wrong with these two methods? Each connection splice and termination weakens the signal. Each one becomes a point of potential noise in the circuit or causes attenuation, mainly through impedance mismatches.
Installers compensate for these practices, and for the potential poor signal quality in a conductorized cable, by using 22 AWG wire. The larger copper conductor in a 22 AWG cable compared to a 24 AWG product means lower resistance, which translates directly into less inherent attenuation of the signal. Category 3 cable in 24 AWG is capable of 16 MHz of bandwidth over a 100 meter (328 foot) distance. Category 5 in 24 AWG has 100 MHz of bandwidth for the same distance. Clearly, 22 AWG conductors are not necessary for the residence. The irony is, for not much more cost than a 4 conductor 22 AWG Station Wire, an installer could purchase 2 pair, or even 4 pair, 24 AWG Station Wire Twisted.
What is becoming a widespread recommendation by communications professionals is to star-wire your home in a manner similar to the structured wiring schemes being implemented in commercial installations. In such a scheme, each extension is "home-run" or "star" wired with a single cable being installed from each wall plate back to a central point where the telephone service enters the home. Such a scheme provides maximum flexibility for assignment of separate voice and data outlets and also allows for a future home LAN.
There are ways to compensate for your home's existing wiring. For example, my old house was wired with 3 pair SWT, which is good, but it was daisy-chained, which is not-so-good. A second phone line installed by the phone company works fine for both voice and fax. For my modem connection, however, I ran a new Category 3 cable from the wall outlet in my office directly back to the Network Interface Unit on the outside of the house.
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