Transmission Medium Limitations

Some of the limitations of transmission lines that reduce their ability to transfer analog and digital information include limited frequency response of the transmission lines, crosstalk, noise from external sources that cause distortion, non-terminated tap lines, and signal attenuation that results from line splices and line resistance.

Frequency Response
The twisting of copper wire pairs provides good frequency response for low frequency audio signals. Unfortunately, twisted copper wire pairs are not specifically designed for high frequency transmission. Analog signals have a frequency range of up to 3.4 kHz and most of the DSL technologies use frequencies up to 1.1 MHz. As the frequency applied to the copper wire pair increases, the attenuation of the line increases and signal energy leaks (emits) from the wire pair.

Figure 1 shows the typical frequency response of a twisted pair of copper wires. The frequency response depends on a variety of factors including the dimension of the copper wire (gauge), insulation type and installation environment (twisting or stapling of the wire).

Figure 1: Frequency Response of Copper and Coax Wire

Crosstalk (Signal Leakage)
Crosstalk is the undesired coupling of a signal from one communications channel to another. Crosstalk occurs when some of the transmission signal energy leaks from the cable. This leakage is called signal egress (emission from the line).

Crosstalk on communication systems can be divided into two categories: near end crosstalk (NEXT) and far end crosstalk (FEXT). Figure 2 shows two types of crosstalk. NEXT results when some of the energy that is transmitted in the desired direction seeps into one (or more) adjacent communication lines from the originating source. FEXT occurs when some of the digital signal energy leaks from one twisted pair and is coupled back to a communications line that is transferring a signal in the opposite direction. Generally, NEXT is more serious than FEXT as the signal interference levels from NEXT are higher.

Figure 2: FEXT and NEXT Crosstalk

Signal Ingress
Signal ingress occurs when electrical signals from other sources (such as radio or lightning spikes) enter into the transmission line. Figure 3 shows a source of signal ingress from a nearby radio tower that may occur in a transmission system. This diagram shows that a high power AM radio transmission tower that is located near a telephone line couples some of its energy onto the telephone line. This interference signal (radio ingress) usually reduces the data transmission capacity of a digital subscriber line (DSL).

Figure 3: Radio Signal Ingress

Bridge Tap Reflections
A bridge tap is an extension to a communication line that is used to attach two (or more) end points (user access lines) to a central office. Bridge taps provide connection options to the telephone company on connecting different communication lines to a central office without having to install new pairs of wires each time a customer requests a new telephone line.

The connection of one (or more) bridge taps on a communication line that is used for plain old telephone service (POTS) does not usually cause signal distortion. However, unterminated bridge taps that are installed on communication lines that transfer high frequency DSL signals can result in signal distortion. The signal distortion comes from the reflections of signal energy reflections off the bridge taps.

When an electrical signal is applied to the end of a copper wire, electrical energy begins to travel down the copper wire. Ideally, when the energy reaches the end of the copper wire, the signal is absorbed at the other end (called a matched line). If the end of the wire is not connected, some (or all) of the energy is reflected back to the beginning of the line.

Figure 4 shows how reflections from bridge line tap can cause distortion. This signal shows that some of the energy from the bridge tap is reflected back to the communications line. This reflected signal is a delayed representation of the original signal. Typically, bridge taps must be removed from communications lines that use DSL technology.

Figure 4: Bridge Tap Reflections

Loading Coils
Loading coils are sometimes used to adjust the frequency response of a communication line to better transfer audio signals. While these loading coils work well for specific types of signals (e.g., audio signals), they can disable the ability of the line to be used for other types of signals (e.g., high frequency DSL signals.)

Figure 5 shows that there may be several installed audio loading coils on a single local loop line. Although these loading coils improve the audio frequency response, they must be removed to allow for high-frequency transmission for systems such as DSL.

Figure 5: Audio Loading Coils

Line Splice Attenuation
Telephone cables usually come in 500 feet roles. Because most telephone lines are several thousand feet from the central office, several cable splices are required. Each line splice attenuates the signal and the amount of signal attenuation varies depending on the type of splice (solder, twist, or pegs) and the amount of corrosion inside the splice.

Because the average distance for local access lines is over 10,000 feet, there are more than 20 splices in the average local access loop. Each of these splices offers the potential for corrosion and increased resistance.

One method that is used to decrease the effects of corrosion (and reduce the attenuation) is to continuously run electric current through the copper wire pair. This “sealing current” is a small amount of direct current that is passed through a copper wire to reduce the corrosion effects of the splice points. The sealing current effectively maintains conductivity of mechanical splices that are not soldered. The direct current effectively punches holes in the corrosive oxide film that forms on the mechanical splices.

Line Resistance Attenuation

The copper cable also has resistance (impedance) that is dependent on the size (diameter) of the cable. The resistance of the copper wire increases as the diameter decreases (gauge number increases). The higher the line resistance, the more of the signal energy is dissipated by the line and less energy is transferred to the receiving device.

Figure 6 shows how line resistance attenuation and the wire size decreases. This diagram shows that cables with larger diameter copper wires are typically used to in the distribution system. As the distribution system nears its destination, the size of the wire often decreases.

Figure 6: Line Resistance Attenuation

Group Delay (Dispersion)
Group delay is the amount of delay a particular group of frequencies experience as they travel through a transmission medium. Because transmitted signals are composed of multiple frequency parts (e.g., high frequency components for rapid signal changes), the delay of some of the parts results in distortion of the transmitted signal.

Figure 7 shows how group delay can cause pulsed signals, such as in digital transmission system, can cause signal distortion. This diagram shows that a digital pulse signal is actually composed of many low, medium, and high frequency components. As the pulse is transmitted through the transmission line, some of the frequency components are delayed more than others. This results in a distorted pulse at the receiving end of the transmission line.

Figure 7: Group Delay (Dispersion)


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