The world of low-voltage cabling for voice and data can seem rather dark and mysterious for the uninitiated. With this series of posts, I hope to shed some light on the dark art. In Part I (this post), I will cover the basics of copper cabling for voice and data. In Part II, I will cover cable categories, certification and ratings. In Part III, we will go into optical fiber for communications cabling and in Part IV I will discuss Structured Cabling Systems.
Traveling in Pairs
To start with, why does cable come in pairs? We’ve all seen 4-pair or 25-pair cables. Why is that? In a word, (or two) Noise Reduction. Communications engineers figured out a long time ago that using two wires to connect circuits together allowed for much better noise reduction. Think of noise as something that’s just floating around in the air. It dirties up any electrical wire that passes near it. So a single wire going from point A to point B will end up at B with the original signal from A plus some induced noise. Let’s call that (+A+n). The signal-to-noise ratio (SNR) here is approximately A/n. The larger the SNR, the better.
Knock Off The Noise!!
Now, think what happens when we run a pair of wires side-by side, one of them with the desired signal from A and the other with say, it’s opposite (-A). Wire 1 of the pair arrives as before with the signal from A and some induced noise (+A+n). Wire 2 of the pair arrives with -A plus the same induced noise (-A+n). By using a circuit called a ‘difference amp‘, circuit designers can subtract Wire 2’s signal from Wire 1’s signal and end up with (+A+n)-(-A+n)=2A. We’ve doubled our signal level and completely eliminated the noise. Pretty cool, eh? In real life, the noise is not completely eliminated. If it is 90% eliminated, our SNR becomes 2A/.1n, a 20x improvement in SNR!
That’s Just Twisted!
So your next question might be, “OK, I get the pairs thing, but why are the pairs twisted?” Good question. I glossed over a few details in the previous section. (You gotta watch me, I’ll do that!) The quality of noise reduction achieved in a difference circuit depends on how closely the noise induced on each wire matches. The match will be more exact the closer together the two wires are. There is some pretty sophisticated physics and electromagnetics involved, but the upshot is that wires twisted together will match much better than untwisted wires. A twisted pair makes the loop area between the conductors as small as possible, and ensures that a magnetic field will pass equally through adjacent loops and will induce equal levels of noise on both lines, which is canceled out by the differential amplifier. So this gets us to our first vocabulary lesson. Low voltage cables (for voice and data) come in two major classifications:
- Shielded Twisted Pair (STP) and
- Unshielded Twisted Pair (UTP)
You may have seen these abbreviations before. They are pretty common in the cabling world. We won’t go into much detail about STP here, as we rarely use it for voice or data cabling these days. STP is often used for higher-voltage signals such as paging and intercom speakers. Shielding just provides an additional barrier to induced noise. UTP is most common for voice and data cabling and we will concentrate on it here.
Please come back for Part II of this series: Copper Cabling Categories, Certification ad Ratings. Share your thoughts in the comments.