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)

Transmission Mediums : Surface Acoustic Wave (SAW)

A surface acoustic wave (SAW) involves the mechanical transmission of a wave that travels on the surface of a material. The wave is created by a piezoelectric transducer that converts electrical to acoustic energy into mechanical vibrations that are coupled (connected) to a material that allows the wave to propagate across the surface of the material. A second transducer (e.g., a pattern of metal fingers) converts the acoustic energy back into an electrical form. Depending on the shape and type of material for which the SAW travels, the signal characteristics can be changed (such as the passing or rejecting of particular frequency bands).

A fundamental use of a SAW device is to act as a signal delay line. The relatively slow propagation velocity of the surface acoustic waves of typically 3500 m/s allows delays of several microseconds on a small chip[1]. There are many variations of SAW technology including delay lines, filters, resonators, pulse compressors, convolvers, and many more.

The first SAW devices were introduced to the marketplace in the mid-1960’s. Before the year 2000, more than 5 million surface acoustic wave (SAW) devices were being installed in electronics assemblies each day[2].

Figure below shows a transmission system that uses surface acoustic wave (SAW) technology. This device is a radio frequency filter that uses SAW technology. The radio signal is applied to an IDT. The IDT converts the electrical signal to an acoustic (mechanical) wave that moves across the surface of the SAW filter substrate. When the acoustic wave reaches the destination IDT, it is converted back into an electrical signal.


Surface Acoustic Wave (SAW) Transmission

Transmission Mediums : Fiber Optic Cable

Fiber optic cable is a strand of glass or plastic that is used to transfer optical energy between points. The size of most fibers is from 10 to 200 microns (1/100th to 1/5th of a mm). Optical fibers are typically used in a unidirectional mode (e.g., data moves in only one direction). Because of this, every transmission system requires at least two fibers (one for transmission and one for reception).

For most fiber systems, the transmitting end-node uses a light amplification through stimulated emission of radiation (LASER) device to convert digital information into pulsed light signals (amplitude modulation). The light signals travel down the fiber strand by bouncing (reflecting) off the sides of the fiber (called the cladding) until they reach the end of the fiber. The end of the fiber is connected to a photo-detector that converts these light pulses back into their electrical signal form.

Optical fibers are often characterized by either single mode or multimode transmission. Single mode of fiber transmission only allows a specific narrow wavelength of light to pass through the fiber. Multimode fiber transmission allows a much wider wavelength of light to pass through the fiber by gradually bending different wavelengths back towards the center of the fiber. Single mode fiber strands are very narrow with a fiber diameter of 9-10 microns (1/100 of a mm). Multimode fibers are much wider as they can have a fiber diameter of 50-125 microns).

Figure below shows single mode and multimode fiber lines. This diagram shows that multimode fibers have a relatively wide transmission channel that allows signals with different wavelengths to bend back into the center of the fiber strand as they propagate down the fiber. The diagram also shows that single mode fiber has a much small transmission channel that only allows a specific wavelength to transfer down the fiber strand.


Single and Multimode Fiber Lines

Because of the relatively wide frequency bandwidth (and high data-transmission rate), multimode fibers are predominantly used for high-speed short runs such as those occurring within a building or around a campus.

Typical multimode fiber runs are less than a mile, but can be several miles (2 – 5). Because of this, it is often referred to a “short haul” fiber solution. Single mode is a “long haul” fiber solution (50 – 75 mile runs). Single-mode fiber has been used by telephone companies and long distance carriers for several years to off-load expanding requirements from traditional terrestrial microwave.

Single-mode fiber can transmit much further than multimode fibers. Single-mode applications use a diode laser as the light source, while multimode uses a light emitting diode (LED). Use of a LASER ensures a high energy at a very narrow optical bandwidth as compared to the LED that has optical energy distributed over a wide optical bandwidth. Single mode fibers use a narrow glass filament with a diameter of approximately 10 microns compared to multimode where the diameter ranges from 50 to 125 microns. The narrow channel of the single mode fiber minimizes the bending of the wave and this results in less dispersion (less smearing of the pulses) over distance.

Figure below shows a fiberoptic communication system that is composed of two end-nodes and a fiber optic cable transmission medium. This diagram shows two optical network units (ONUs) that connect data networks together using fiber cable. This diagram shows that two fiber strands are needed: one for transmitting and one for receiving.


Fiber-Optic Cable Transmission System

To increase the capacity of fiber systems, multiple optical signals of different wavelengths are combined on a single strand of fiber. This is called wave division multiplexing (WDM). When 40 (or more) optical signals are combined on a single fiber strand, this is called dense wave division multiplexing (DWDM).

Figure below shows how a wave division multiplexing over fiber operates. This diagram shows that there are several lasers operating at different optical wavelengths (different colors/frequencies). Each laser converts an electrical signal into a pulsed light signal. These optical signals (optical carriers) are combined by an optical multiplexer (lens) for transmission through the optical fiber. At the receiving end, the different optical carriers are separated by an optical demultiplexer (lens) and each optical carrier is sent to a photo-detector. The photo-detector converts the optical signal back into its original electrical form.


Wave Division Multiplexing

Transmission Mediums : Free Space/Air

Transmission in free space and air can be accomplished by radio or light signals. Free space/air transmission is the transfer of signal energy through an unobstructed medium. Free space transmission occurs in an ideal medium (vacuum) that is free of objects or particles that may disrupt the transmission of signal energy. Transmission of signals in air has similar characteristics as free space transmission. However, particles in the air result in signal scattering and absorption during transmission.

Free space transmission systems require a transducer to convert signal energy of one form into electromagnetic or optical energy for transmission. The transducer must also focus the energy so it may launch the energy in the desired direction. When air is the medium, particles in the air (such as water) may absorb or redirect (scatter) the transmitted signal.

Figure below shows two types of free space transmission systems: radio and optical. The microwave transmission system shows that some of the electromagnetic energy is absorbed by the water particles in the air. The optical transmission system uses a laser and photo-detector. The optical transmission system shows that some of the optical energy is scattered in other directions as it passes through smog and water particles.


Free Space Transmission System

In 1951, microwave radio transmission through free space became the backbone of the telecommunications infrastructure. Point-to-point microwave transmission systems have the data transmission capacity of hundreds of megabits per second. Although the extensive deployment of fiber optic cable has removed some of need for microwave radio systems, microwave radio is still used in places that are hard to reach or not cost effectively served by fiber cable.

In addition, microwave radio free space technology is the basis for satellite communications. In the commercial broadcast industry satellites are fed from terrestrial sites called “mother stations.” The mother station transmits up to the satellite on multiple frequencies called “uplinks”. Transceivers on the satellite, referred to as “transponders”, retransmit the signals back down to earth. These signals are known as “downlinks”. On the ground, satellite dishes (focusing antennas) receive the downlink signals and a radio receiver converts these signals back into television images and sound. Home Box Office (HBO) and ShowTime are examples of commercial broadcast companies that use satellite almost exclusively for distribution.

In developing countries and countries where the telecommunications infrastructure is of poor quality, radio, microwave, and satellite have been used to solve connectivity requirements in short order. Data radio and very small aperture satellite (VSAT) systems have allowed banks and other information-dependent companies to reliably connect to branch offices for the online exchange of information.

Since the mid-1980’s data radio has played a major role in the telecommunications industry in developing countries where the copper wire infrastructure is generally of substandard quality.

Modern optical transmission systems use infrared, and other laser optical signals to carry large amounts of information. Free space infrared and laser communication systems have been limited to span small distances of a few miles due to interference of the particles in air. Infrared systems have gained popularity because of their high bandwidth and ease of installation. Optical systems usually do not require government licenses or other authorization to use. These optical systems can be found connecting buildings on a campus and as supplements to wired LAN’s within an office or plant.

Transmission Mediums : Coaxial Cable (Coax)

Coaxial cable is a transmission line that is constructed from a center conductor that is completely surrounded by shield conductor. The center conductor and the shield conductor are separated by an insulation material. The shield can be inter-woven strands of wire or metal foil. Because the center conductor is complete surrounded by the shield, in an ideal coaxial cable, all the transmitted energy is contained within the cable.

Figure below shows a cross sectional view of a coaxial cable. This diagram shows a center conductor that is surrounded by an insulator (dielectric). The insulator is surrounded by the shield. This diagram shows that during transmission, electric fields extend perpendicular from the center conductor to the shield and magnetic fields form a circular pattern around the center conductor.


Cross Sectional View of Coax Cable

Coaxial cable is best known as the medium for cable television. It was primarily chosen because of its durability, wide frequency bandwidth capacity (often up to 1 GHz bandwidth), and less rigid length restrictions. Coax (as it is normally called) is often used in local area networks (LAN) to transport high-speed data signals with relatively high security (low signal leakage).

Twinax is a derivative of coax and is constructed as noted above with the exception that twinax uses two center conductors. Each center conductor is individually insulated, but, as with coax, each references the single shield for ground.

The first local area networks (LAN’s) were almost exclusively centered around coax as the network medium of choice. It became the standard for the early LAN’s. There are three common types of coax that are, or have been, in use extensively with many computer systems: thicknet, thinnet, and twinax.

Thicknet is often associated with the first Ethernet LAN’s and with high-speed bus cables used between mainframe computers and their peripherals. It is bulky and difficult to install but provides high speed and capacity where it is most critical. With respect to LAN’s, use of thicknet provides extra protection from electrical interference that may be encountered (e.g., such as on factory floors near assembly equipment).

For most LAN’s installed in the early 2000’s, UTP or STP is used. Coax is used in cable television systems and for interconnection trunks at telephone company switching centers. This provides for relatively high data transmission capacity (e.g., for DS3 45 Mbps transmission). Figure below lists the most frequently encountered types of copper and coax line and their approximate information transmission capabilities.


Copper and Coax Cable Information Capacity

Transmission Mediums

A transmission medium guides signal energy in a particular direction. The key different types of transmission mediums include copper wire, coax (a form of copper wire), free space (air), glass fiber, and mechanical (acoustic wave). There are also other types of transmission mediums such as radio waveguides and stripline that use the fundamental principles of the other types of transmission mediums to help direct transmitted energy in a particular direction.

The propagation in free space (a vacuum) is the speed of light 300 million meters per second. In free space, the propagation delay (time from entry to exit) is exactly 11.7 nanoseconds per foot (a nanosecond is one Billionth of a second). Any other medium other than free space will slow the transmission speed (introduce additional delay).

When transmitting energy through a transmission medium, some of the transmitted signal will be absorbed of some of the material. This absorption converts some of the transmitted signal into heat.

As signals transfer through a transmission system, some of the energy may leak out. This leakage (egress) results in a decrease the transmitted signal energy as it propagates from end to end.

Transmission systems have a limit on the frequency range that can be transferred through the transmission channel. Signals that are applied to the transmission medium that are above or below the maximum frequency range are rejected, severely attenuated, or leaked out of the transmission medium.

Copper Wire
Twisted pair copper wire is the most utilized telecommunications medium and thus has the largest installed base in the worldwide telecommunications infrastructure by far. It is relatively inexpensive, easy to install, and, locally available in quantities. Practically every residential telephone worldwide connects to the local telephone company via a twisted pair jack or block located on an inside wall or baseboard in the residence. Because of these factors, intensive research and development (R&D) funding continues to be allocated for the purpose of extending the usefulness of twisted pair copper wire. By enhancing its ability to carry information faster and farther, R&D will continue as long as the resulting enhancements meet, or exceed, many of the requirements placed on the industry by customer demands. Indications are that twisted pair copper wire will remain a logical, cost-effective medium for providing many commercial services for some time to come.

Figure below illustrates the transmission of electrical energy (electricity) via copper wire. Note that the electricity is conducted via the outer surface of the wire, not the inner. Consequently the greater the outer surface the more electricity that can be conducted. In other words, the larger the wire diameter the more outer surface there is to conduct electricity.


Electrical Transmission through Copper Wire

In the telecommunications industry copper wire is normally referred to as twisted pair. Through twisting wire into pairs electrical radiation (eddy fields or cross talk) is reduced. This reduction is both radiation off the pair and the susceptibility to radiation from other pairs and sources. It has been found that the tighter the twist (this is still in R&D) the less interference (cross talk) and the higher the speed at given error rate.

Twisted pair cables come in a variety of sizes and jackets (outer covering). Twisted pair cables are jacketed and may contain from 2-pair to several hundred pairs. Twisted pair wire comes jacketed in PVC or plenum-rated. “Plenum” is the name given to the non-toxic PVC-like jacket that is authorized by local fire ordinances for use in ceilings and walls considered to be “air-return”.

Twisted pair cables are produced as either shielded twisted pair (STP) or unshielded twisted pair (UTP). When a twisted pair installation is to occur in an area that has abnormally high levels of electromagnetic energy, STP is recommended. In most office settings UTP is the standard; however, even in such seemingly neutral environments there can be problems such as fluorescent lighting fixtures and parallel runs with electrical wiring. A good installation plan prepared by a certified wiring engineer reduces the likelihood of such problems and also enhances the probability that the final infrastructure will operate at the desired performance (to allow the desired data transmission speed).

In office or campus environments UTP and/or STP provide the wiring infrastructure for LAN’s, some host-based data applications, video, and voice. Central office lines are normally delivered as twisted pairs to the client telephone or computer systems. Such lines range from single station analog lines up to and including T-1 (US, 1.544Mbps) or E-1 (Europe, 2.048Mbps). Of course this includes the intermediate digital service known as digital subscriber line (DSL).

In addition, twisted pair cable that is to be directly buried in the ground has a special construction to prevent water seepage. Cables to be run overhead outside are constructed with an extra steel cable known as a strength member. The strength member supports the weight of the cable between the poles from which the cable is suspended. This prevents the cable from sagging and ultimately rendering it useless. Finally, the US government contracts for “tap-proof” cable that has a special outer conductor, located under the outer covering, that is capable of conducting as much as 2,000 volts.

Figure below lists the types of twisted pair cables and their rated capacities and maximum links. Copper wire cable is “Category” rated. This table shows that the category varies based on the design of the cable (shielded or unshielded), size (gauge) of the wire, and the types of insulation material used.


Copper Wire Pair Information Capacity