How do we get our signal transmissions to their destinations without distortion or diminishment?
Before we get into the technical basics of data communications, we should review some definitions. Telecom is short for telecommunications—the transmission of telephone signals from place to place. Datacom is short for data communications—the transmission of data (computer) signals from place to place. We can usually interchange these terms. There is a good reason for this: Telephone systems are changing into data systems. Right now, Internet traffic over telephone lines is almost equal to voice traffic. Industry experts predict Internet traffic over the phone system will exceed voice traffic in 2000 or 2001. So, with the telephone system becoming a voice/data system, it’s difficult to say whether anything attached to a phone line is telecom or datacom.
Electronic communication systems operate under the same laws that govern power wiring. Ohm’s law, Watt’s law, Kirchoff’s theorems, and circuit calculations are all the same. The applications may be different, but what you learned in apprenticeship school still holds true.
Signal transmission. Our concern is to get signals to their destination without distortion or diminishment. Special types of conductors, cables, and hardware help us accomplish this task efficiently.
In general, there are two primary choices for signal transmission:
• Sending electrical signals over copper conductors.
• Sending pulses of light through optical cables.
There are, of course, a few other options, such as radio signals and microwave signals, but these are used primarily in special circumstances. For now, we will focus on copper and fiber transmission only.
Copper transmission. Most of you are familiar with the basics of copper-based data transmission: Electrical pulses pass through copper conductors from one location to another. In most cases, these signals are binary code (the presence of voltage corresponding to the digital designation “on” or “1,” and the absence of the voltage as “off” or “0”). The coding of these signals is what makes transmission of information possible.
We often classify data transmissions as simplex, half-duplex, or full-duplex in terms of their method of transmission. Simplex and half-duplex systems use only one pair of conductors to communicate, and are less expensive to build. The simplex method transmits in one direction, while the half-duplex system can send signals in both directions, but not at the same time. The full-duplex system uses two pairs to communicate. This way, one pair always transmits from point A to point B while the other pair is transmitting from B to A.
The main problem we have with power wiring is a loss of power, which we call voltage drop. We have the same problem with data signals, when we attempt to send them through conductors with too much resistance (usually due to distance). We call this attenuation.
Pulse spreading is the most common type of signal distortion. Notice the digital signal sent into the cable is square. As the signal travels down the cable, it distorts, and begins to spread. Pulse spreading is a distortion of the signal. If the pulses spread too much, they will be unintelligible to the receiver.
Copper cabling. The copper data cabling we use today is unshielded twisted-pair cable (commonly referred to as UTP), with four pairs (eight conductors). The conductors are 22- or 24-gage copper, insulated with some type of flouropolymer plastic, and cabled under some type of plastic outer jacket.
The characteristics of the outer jacket (and sometimes the insulation of the conductors) will vary from one cable rating to another. General duty, riser, and plenum cables will have different types of jackets, due to the fire risks they may encounter. Copper data cabling is inexpensive, but its capabilities are limited.
Optical transmission. You can send all types of data and communication signals with optical fiber systems. A fiber-optic system works on pulses of light, rather than pulses of electricity. Coded pulses of light are sent into one end of a fiber and received at the far end, where they are translated and used.
These systems use infrared light. One advantage of optical fiber is that it can handle huge amounts of data; many times as much as copper conductors. For example, the greatest amount of data sent over copper is usually about 155 Mb/s (megabits per second). New fiber systems are now sending 40 Gb/s (gigabits per second), and could theoretically go many times higher.
How fiber works. Some of the more common terms used to describe optical fiber are a light tube or conduit for light. These terms have led a lot of people to believe there is some type of hole in the middle of an optical fiber—this is not the case.
Nonetheless, optical fiber does function as a “tube” or “conduit” for light. Light flows down the center of a fiber like water flows through a pipe. We could even say the fiber is a “virtual” tube. The light stays in the center of the fiber, not because there is a physical opening there, but because the cladding (ultra-pure glass) reflects any escaping light back to the core.
There are three concentric layers to an optical fiber. Light flows only through the glass core of the fiber. The cladding (which is a different type of glass) serves as a barrier to keep the light within the core, functioning much like a mirrored surface. The buffer (also called coating) layer has nothing to do with light transmission, and is used only for mechanical strength and protection.
Optical cabling. There are many types of optical cables, each suited for different applications. Two common types are breakout cables and armored loose-tube cables. To protect the glass fibers in these cables, we package them in cabling. It’s a misconception that fiber cables are fragile. Fiber is several times as strong as steel; but because the cables are so thin, they sometimes break. They are often more durable than copper communication cables. Optical cables encase the glass fibers in several layers of protection.
Each layer protects the fiber, and in many cases, an additional stiffening member (typically Aramid yarn) also increase the cable’s strength and durability.