Metallic Transmission Media
This section presents some physical characteristics of metallic wiring familiar to consumers, specifically phone wire and coaxial cable used in cable TV. Fiber transmission and wireless media will be discussed later in this chapter.
Copper wiring is a ubiquitous, well-understood medium. It comes in two broad classes, for this discussion: phone wire and coaxial cable. Both classes have variants, but each has been standardized on a few options. Phone wire consists of two thin copper wires, which are twisted in a helical pattern around each other. The twisted pair is wrapped in an insulating cover of plastic, rubber, or lacquer. At its limit, phone wire can transmit 1 MHz for a distance of roughly 3 miles, or 30 MHz for a distance of 200 yards. The distance that electricity passes through phone wire is resistance (measured in ohms), which in turn is partly a function of the wire's thickness, or gauge. Table 2-4 lists various wiring gauges with their diameters and electrical resistance.
| Gauge | Diameter (inches) | Feet/Ohm |
|---|---|---|
| 18 | 0.0403 | 156.5 |
| 20 | 0.0320 | 98.5 |
| 22 | 0.0253 | 62.0 |
| 24 | 0.0201 | 39.0 |
| 26 | 0.0159 | 24.5 |
Note that the higher the gauge number, the smaller the diameter and the shorter the distance traveled.
Coaxial cable consists of a single, thicker length of copper with stronger, metallic shielding. It can transmit up to 750 MHz for a distance of a mile or so.
Table 2-5 summarizes several important data services in terms of bandwidth and type of metal wire used.
| Service | Upper Limit Bandwidth | Type of Wire |
|---|---|---|
| Voice service | 3.4 kHz | Phone |
| Alarm service | 8 kHz | Phone |
| ISDN | 80 kHz | Phone |
| T1 using 2B1Q | 250 kHz | Phone |
| Cable TV | 350–750 MHz | Coaxial cable |
The distance and signal integrity obtainable from phone wire or coaxial cable is limited by certain local factors associated with general problems of high frequency on wires.
Problems Associated with High Frequency on Wires
The types of wire listed provide well-known, stable services for the indicated frequencies. But RBB intends to push frequency used on phone wire and on cable to their limits. This will be difficult due to impairments associated with high-frequency transmission on metal wire. Some of these impairments are listed here:
More attenuation
More crosstalk
More resistance
More phase error
Attenuation
Signal loss, or attenuation, is a function of frequency, distance, and temperature, As frequency increases, the distance the signal can travel decreases by the square root of the frequency. So a 40 MHz signal will travel half as far as a 10 MHz signal. Also, the velocity of signal through a wire is a function of frequency. The higher the frequency, the slower the signal. Signal strength also drops with distance. Finally, signal strength drops with temperature. The warmer the ambient temperature, the greater the signal loss. That's why in the case of extreme cold, superconductors work extremely efficiently in conducting electricity.
Because optical signals through glass fiber retain signal strength better than electronic signals through metallic wires, they provide improvements required for RBB services.
Crosstalk
When two adjacent wires are carrying signals, there is the possibility that signals from one wire will enter the other wire as a result of electromagnetic coupling. Crosstalk increases with increasing frequency. At low frequencies used by voice, crosstalk is not noticeable. However, at frequencies required by high-speed services, this can be a principle cause of signal degradation.
Resistance
As signals are transmitted through wires at very high frequencies, a phenomenon called the skin effect occurs. This is the behavior whereby electricity migrates to the outside wall of the wire, leaving little conductivity in the middle of the wire. In fact, in extreme cases, it is possible to replace the core of the wire with cheaper nonconductive material such as plastic or wood. As electricity migrates to the skin, resistance increases because less of the wire is used. Increased resistance weakens signals.
The skin effect explains why there are no services above 1 GHz over wired media, whereas over-the-air spectrum can be used for frequencies in the range of 20 to 30 GHz.
Phase Error
Higher frequency signals not only weaken more quickly when compared with lower frequencies, but they are also a little slower, which causes a phase error. For modulation techniques dependent on phase, this can introduce bit errors.
External Impairments
Apart from inherent problems associated with high frequencies, metallic networks encounter external impairments as well. Many of these impairments have to do with noise—that is, disturbances that reduce the clarity of the signal being sent. A challenge for RBB engineers is to characterize noise for specific networks so that proper encoding and noise-mitigation techniques can be used. Noise characterization involves determining whether impulse noise or narrowband noise is the major cause of imperfections, at what frequency the noise occurs, and what the cause of the noise is. Noise characterization of cable TV networks continues to be the subject of research by CableLabs, a research consortium for North American cable operators located in Louisville, Colorado. Telcordia Technologies (once known as Bellcore, then controlled by the major Bell Operating companies, and now owned by SAIC Corporation) does the same for telephone companies.
Leakage
Outdoor insulation and the conductive outer shield of coaxial cable suffer wear and tear. Eventually, lesions form in the insulation, allowing radiation from the conducting material to leak through the perforation to the outside. This is a source of signal loss. Moreover, the radiated signal that emanates from the wire can cause interference with wireless services, such as radio transmission. This is a reason why the FCC has rules limiting signal leakage.
Another reason for leakage is poorly fitted couplings at the customer site, such as F connectors for cable TV not tightly joined to the cable box. This presents a system management problem because such leakage is outside the control of the carrier.
Impulse Noise
Impulse noise (or burst noise) occurs when one wire picks up an unintended signal that lasts for a very short period of time (a few microseconds) but that interferes with a wide frequency range. Sources of impulse noise include other wires, motors, and electronic devices. This is often caused by imperfections in the wire, such as corrosion or malfunctioning amplifiers. Lightning or any other source of strong electric sparks also cause impulse noise.
Spread spectrum is useful for mitigating impulse noise because the intended signals are spread out over longer periods of time, long enough to persist through the impulse noise.
Narrowband Interference
Whereas impulse noise affects a wide range of frequencies for a short period of time, narrowband interference adversely affects a small number of frequencies over a long period of time. Amateur radio, or AM or FM radio interference, are types of narrowband interference. Consider AM transmission occurring at 1070 kHz. If there is a signal at the same frequency in a wire, and if there is a lesion in the wire or a loose fitting, then receivers at the end of the wire will pick up the signal at 1070 kHz.
One way to avoid the interference is to identify the frequency range of the noise in advance and simply not transmit on that frequency. This is a technique that is under consideration by some proponents of ADSL, which is discussed in Chapter 4. If you don't know the interfering frequency, you can use Frequency Hopping Spread Spectrum (FHSS), which was discussed earlier in this chapter.
Loading Coils
Loading coils are passive (unpowered) devices that lengthen the distance voice can travel over phone wire. The positive effect is better voice fidelity over longer distance. The negative effect is that higher frequencies needed for any digital service, such as ISDN, ADSL, or leased line T1 service, are filtered out. Loading coils were devised decades ago, before thoughts of high-speed services.
Loading coils are needed on the roughly 15 percent of loops of the United States that are longer than 18,000 feet. But the way provisioning works, if one line in a small binder (50 lines) requires loading, then all lines are loaded for ease of installation. The result is that no one knows how many loaded loops exist; still, it is on the order of 20 percent.
The problem of loading coils is a big one, but it is slowly resolving itself because the coils are removed one by one with the introduction of new services.
Thermal Noise
Differences in ambient temperatures change the resistive quality of a wire. Lowering temperatures lowers resistance, thereby increasing signal quality. Warm temperatures tend to lengthen wires, which affects timing, an important part of synchronization, as we shall see in Chapter 3. Time differences can be significant over a 3-mile radius for phone wire or a 50-mile radius for coaxial cable.
Bridged Taps
Telephone circuits in their cleanest form are a pair of copper wires, which connects your handset directly to the telephone company office.
Bridged taps are extra copper wires that tap into the original wire for possible connection to another device. These were originally installed for flexibility in field installations. If the phone company was stringing wire to your house and there was another house along the way that may need a line in the future, a wire was branched off.
This was good for field installation operations but it causes echoes (signals echo off the end of the branch into the mainstream network) and changes timing on the circuit. If lots of taps are present on the line, the echoes can become serious enough to impair high-speed transmission.
Additionally, some taps may be unterminated. Unterminated wires cause the additional problem of leakage, which causes a loss of signal strength.
Locating bridged taps to correct them can be difficult because network providers often have incomplete records of where they occur. Without the provider's knowledge, the consumer can instigate bridged taps with do-it-yourself phone wire installation. In both telco and cable networks, bridged taps must be addressed through some noise-mitigation technique.