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7. Revisiting copper cables
Contrary to popular belief copper cables are not dead. In fact, today, a far greater length of copper cable is installed throughout the world for communication purposes than of fiber optic cable.
The advantages and disadvantages of copper cables compared to fiber optic cables were discussed in Section 1. This section also provides a brief review of copper cables used for communication purposes. The author has included this section for the reasons of revision, reference, and completeness of this guide.
7.1 Cable Types
Two main types of copper cable used are:
• Coaxial cable, also called coax
• Twisted pair cable, which can be shielded (STP) or unshielded (UTP)
Each of the cable types is subdivided into more specialized categories and it has its own design and specifications, standards, advantages and disadvantages. Cable types differ in price, transmission speed, and recommended transmission distance. For example, twisted pair wiring is currently the cheapest but has the most limited performance. Coaxial cable lies between twisted pair and fiber optic cables on most of the performance and price features.
7.2 Cable Structure
All cable types have the following components in common:
• One or more conductors to provide a medium for the signal
• Insulation of some sort around the conductors to help keep the signal in and interference out
• An outer sheath, or jacket, to ensure the cable-elements. The sheath keeps the cable components together, and may also help protect the cable components from water, pressure, vibration or other external environmental factors.
Conductor
For copper cable, the conductor is known as the signal, or carrier wire, and it may consist of either solid or stranded wire. Solid wire is a single thick strand of conductive material, usually copper. Stranded wire consists of many thin strands of conductive material wound tightly together.
The signal wire is described in the following terms:
• The wire's conductive material (for example, copper)
• Whether the wire is stranded or solid
• The carrier wire's diameter, expressed directly (for example, in inches, centimeters or millimeters), or in terms of the wire's gauge, as specified in the AWG (American Wire Gauge).
• The total diameter of the strand determines some of the wire's electrical properties, such as resistance and impedance. These properties, in turn, help to determine the performance of the wire.
Insulation
The insulating layer keeps the transmission medium's signal from escaping and also helps to protect the signal from outside interference. For copper wires, the insulation is usually made of a dielectric such as polyethylene. Some types of coaxial cable have multiple protective layers around the signal wire. The size of the insulating layer determines the spacing between the conductors in a cable and therefore its capacitance and inductance.
Cable sheath
The outer casing, or sheath of the cable, provides a shell that keeps the cable's elements together. The sheath differs for indoor and outdoor exposure. Outdoor cable sheaths tend to be black or blue, with appropriate resistance to UV light, and have enhanced water resistance. Two main indoor classes of sheath are plenum and non-plenum.
Plenum cable sheath
In some countries, plenum cable is required to be used in certain environments by law. It would be required to be used where the cable is being run 'naked' (without being put in a conduit) inside walls. Plenum sheaths are made of less-flammable fluoropolymers such as Teflon or Jynar. They are highly fire-resistant and give out less toxic fumes when burning. They are also considerably more expensive (by a factor of 1.5 to 3) than cables with non-plenum sheaths. Studies have shown that copper cables with plenum sheaths have less signal loss than non-plenum cables.
Non-plenum cable sheath Non-plenum cable uses less expensive material for sheaths, so it is consequently less expensive than cables with plenum sheaths. Non-plenum cable sheaths are made of polyethylene (PE) or polyvinyl chloride (PVC), which has a greater tendency to burn and give off toxic fumes than Plenum cables.
Cable packaging
Cables can be packaged in different ways, depending on its application and the location where they are laid. For example, the older IBM token ring cable topology specifies a flat cable for use under carpets. The following types of cable packaging are available:
• Simplex cable - one cable within one sheath, which is the default configuration
• Duplex cable - two cables, or fibers, within a single sheath
• Multifiber core - multiple cables within a single sheath
8. Factors affecting copper cable performance
Copper cables are good media for signal transfer, but they are not perfect. Ideally, the signal at the end of a length of cable should be the same as at the beginning. Unfortunately, this will not be true in practical cables. All signals degrade when transmitted over a distance through any medium. This is due to a decrease in signal amplitude, as the medium resists the flow of energy. The signals become distorted, as the shape of the electrical signal changes over distance. Any transmission also consists of signal and noise components. Signal quality degrades for several reasons, including attenuation, crosstalk, and impedance mismatches.
8.1 Attenuation
Discussed in Section 3.2.
8.2 Crosstalk
Crosstalk is interference in the form of a signal from a neighboring cable or circuit; for example, signals on different pairs of twisted wire in a twisted pair cable may interfere with each other. A commonly used measure of this interference in twisted pair cable is near-end crosstalk (NEXT), which is represented in decibels. The higher the decibel value, the less crosstalk and the better the cable. Additional shielding between the carrier wire and the outside world is the most common way to decrease the effects of crosstalk.
8.3 Charac. Impedance
The impedance of a cable is defined as the opposition to the flow of electrical energy at a particular frequency. The characteristic impedance of a cable is the value of impedance, which is characteristic of that particular cable. The characteristic impedance is the input impedance of the cable seen when it is terminated to a load impedance equal to the characteristic impedance, as shown in FIG. 18. Such a cable then appears electrically to be infinitely long and has no signals reflected from the termination. If one cable is connected to another of differing characteristic impedance, then signals are reflected back from their interface. These reflections cause interference with the data signals and must be avoided by using cables of the same characteristic impedance.
FIG. 18 Characteristic Impedance
9. Coaxial cable
Coaxial cable, often called coax, is used for radio applications and data transmission. The cable is remarkably stable in terms of its electrical properties at frequencies below 4 GHz (Gigahertz). This makes the cable popular for radio and microwave systems, cable television (CATV) distribution, as well as for creating local area networks (LANs). The telephone companies also make use of coaxial cable to route long distance calls.
9.1 Coaxial cable Components
A coaxial cable consists of the following layers (moving outward from the center) as shown in FIG. 19.
FIG. 19 Coaxial cable
Carrier wire
A conductor wire or signal wire is in the center. This wire is usually made of copper and may be solid or stranded. There are restrictions regarding the material composition for certain applications. The diameter of the signal wire and the number of strands in a multistrand conductor affect the signal attenuation.
Insulation An insulation layer consists of a dielectric around the carrier wire. This dielectric is usually made of some form of polyethylene or Teflon.
Foil shield
A thin foil forms a shield around the dielectric. This foil shield usually consists of aluminum bonded to both sides of a tape. Not all coaxial cables have foil shielding. Some have two foil shield layers, interspersed with braid shield layers.
Braid shield
A braid, or mesh, conductor, made of copper or aluminum, that surrounds the insulation and foil shield. This conductor can serve as the ground for the carrier wire. Together with the insulation and any foil shield, the braid shield protects the carrier wire from electromagnetic interference (EMI) and radio frequency interference (RFI). Note that the braid and foil shields provide good protection against electrostatic interference, but little protection against magnetic interference.
Sheath
The outer protection cover that can either be plenum or non-plenum. The layers surrounding the carrier wire also help prevent signal loss due to radiation from the carrier wire. The signal and shield wires are concentric, or coaxial. Common coaxial cable impedances are 50, 75, 93 and 120 ohms.
9.2 Coaxial cable connectors
A segment of coaxial cable has an end connector at each end. Connectors differ in their attachment mechanism and components. Different size coaxial cables require different sized connectors, matched to the characteristic impedance of the cable, so that the introduction of connectors cause minimal reflection of the transmitted signals.
For coaxial cable, the following types of connectors are available:
• BNC (bayonet nut connector)
This connector is used for radio and data.
• N-series connector
This connector is used for radio and data.
• UHF series
This connector is used for radio.
• TNC (threaded nut connector)
This connector may be used in the same situations as a BNC, provided the other connector is also using TNC.
• J-type This connector is used for radio.
It is recommended that connectors for coaxial cable should be silver plated rather than tinned. This improves the contact and the durability of the connector.
9.3 Baseband vs. broadband over coax cable
Functionally, coaxial cable is grouped into baseband and broadband varieties. To understand these terms, you need to understand the difference between baseband and broadband signaling. A baseband system is one in which the whole bandwidth of the transmission medium (the cable in this situation), is taken up by a single signal. The information may be digital or analog but the signals are impressed directly on to the cable and transmitted using the natural frequency components. The alternative is broadband signaling. In this system, each signal is modulated with a separate carrier frequency (fc). By using a series of carrier frequencies that are separated by a sufficiently wide guard band, simultaneous transmission of multiple data streams can be supported on one cable.
9.4 Advantages of Coaxial cable
Coaxial cable has the following general advantages over other types of cable. These advantages may change or disappear over time, as technology advances or products improve.
• Broadband coaxial can be used to transmit voice, data, radio, TV, and video
• The cable is relatively easy to install
• Coaxial cable is reasonably priced compared to other cable types
• High frequency applications (up to 4 GHz for distances up to several hundred metres)
• Wide bandwidth of operation
• Stable characteristics over wide operating range of frequencies
• Relatively low attenuation.
9.5 Disadvantages of Coaxial cable
Coaxial cable has the following disadvantages when used for a network:
• It is easily damaged and sometimes difficult to work with, especially in the case of thicker coaxial cable.
• Coaxial cable is more difficult to work with than twisted pair cable.
• Some thicker coaxial cable can be expensive to install, especially if it needs to be pulled through existing cable conduits.
• Connectors can be expensive.
• Connectors are difficult to install.
• Coaxial cable provides limited bandwidth compared to fiber.
10. Twisted pair cable
Twisted pair cable is widely used, inexpensive, and easy to install. Twisted pair cable comes in two main varieties:
• Shielded (STP)
• Unshielded (UTP)
This cable can transmit data at an acceptable rate -- up to 1 Gbps in some local area network architectures. Several companies are in the process of developing 10 Gbps STP systems. The most common twisted pair wiring is telephone cable, which is unshielded and is usually voice-grade, rather than the higher-quality data-grade cable used for local area networks. In a twisted pair cable, two conductor wires are wrapped around each other. A signal is transmitted as a differential between the two conductor wires. The current flows in opposite directions in each wire of the active circuit, as shown in FIG. 20.
FIG. 20 Current flow in a twisted pair cable
Since these currents are equal and opposite, their magnetic fields cancel each other, and cancel any magnetic interference caused by outside noise sources. This type of cable is therefore self-shielding and is less prone to interference.
Current flowing out of this conductor
Current flowing into this conductor X Magnetic Fields Cancel Insulation Conductor
FIG. 21 Magnetic shielding of twisted pair cables
Twisting within a pair minimizes crosstalk between pairs. The twists also help deal with electromagnetic interference (EMI) and radio frequency interference (RFI), as well as balancing the mutual capacitance of the cable pair. The performance of a twisted pair cable can be improved by increasing the number of twists per meter in a wire pair. Each of the pairs in a 2-pair cable will have a different twist rate to reduce the crosstalk between them.
10.1 Components of twisted pair cables
A twisted pair cable has the following components:
Conductor wires
The signal wires for this cable come in pairs that are wrapped around each other. The conductor wires are usually made of copper. They may be solid (consisting of a single wire) or stranded (consisting of many thin wires wrapped tightly together). A twisted pair cable usually contains multiple twisted pairs: 2, 4, 6, 8, 25, 50, or 100 twisted pair bundles are common. For local area network applications, 2- and 4-pair cables are most commonly used.
Shield
Shielded twisted pair (STP) cable includes a foil shield around each pair of conductors.
Sheath
The wire bundles are encased in a sheath made of polyvinyl chloride (PVC) or, in plenum cables, of a fire-resistant material, such as Teflon or Knar. Twisted pair cable comes in two main varieties: shielded (STP) and unshielded (UTP). STP contains an extra shield or protective screen around each of the wire pairs to cut down on external interference signals. This added protection also makes STP more expensive than UTP. (The price of thin coaxial cable generally lies between UTP and STP prices.)
10.2 Shielded twisted-pair cable STP
STP cable has pairs of conductors twisted around each other. Each pair is covered with a foil shield to reduce interference and minimize crosstalk between wire pairs. It can handle high-speed transmissions, but the cable itself is relatively expensive. It can be quite bulky and heavy, and is rather difficult to work with.
10.3 Unshielded twisted-pair cable
UTP cable does not include any extra shielding around the wire pairs. This type of cable is used in some slower speed data networks (in particular, 10Base-T, 10 Mbps) and for voice applications.
10.4 Cable categories
To distinguish varieties of UTP, the United States Electronic Industries Association/ Telecommunications Industries Association (EIA/TIA) has formulated five categories.
The electrical specifications for these cables are detailed in the following: EIA/TIA 568A, TSB-36, TSB-40 documents and their successor document SP2840. These categories are:
• Category 1: voice-grade UTP telephone cable Voice-grade, UTP telephone cable describes the cable that has been used for years in North America for telephone communications. Officially, such cable is not considered suitable for data-grade transmissions. In practice, however, it works fine over short distances and under ordinary working conditions. You should be aware that other national telecommunications providers have often used cable that does not even come up to this minimum standard, and is totally unacceptable for data transmission.
• Category 2: voice-grade UTP cable
This cable is capable of supporting transmission rates of up to 4 Mbps. IBM type 3 cable falls in to this category.
• Category 3: data-grade UTP cable
This cable is used extensively for supporting data transmission rates of up to 10 Mbps. An Ethernet network cabled with twisted pair requires at least this category of cable.
• Category 4: data-grade UTP cable
This cable is capable of supporting transmission rates of up to 16 Mbps. An IBM token ring network transmitting at 16 Mbps requires this type of cable.
• Category 5: data-grade UTP cable
This cable is capable of supporting transmission rates of up to 155 Mbps (but officially only up to 100 Mbps). The proposed CDDI (copper distributed data interface) networks and 100Base/TX network architecture require such cable.
• Category 6: data-grade UTP cable This cable is specified to operate at frequencies up to 250 MHz but manufacturers are obtaining operating frequencies of 550 MHz. It is specified to support IEEE 802.3 specifications for 1000base-T (gigabit Ethernet).
• Category 7: data grade STP This cable has just been commercially released. Each pair is shielded and the whole cable is shielded too. Manufacturers are quoting operating frequencies of up to 1.2 GHz.
10.5 Performance requirements
Twisted pair cable is categorized in terms of its electrical performance properties. The features that characterize the data grades of UTP cable are defined in EIA/TIA 568 as follows:
Attenuation This value indicates how much power the signal loses and is dependent on the frequency of the transmission. The maximum attenuation per 1000 feet of UTP cable at 20 degrees
Celcius at various frequencies is specified as follows:
Table 2 Maximum attenuation in dBs per 1000 ft @ 20°C
Mutual capacitance Cable capacitance is measured in capacitance per unit length, for example, pF/ft, and lower values indicate better performance. The standards equate to mutual capacitance (measured at 1 kHz and 20°C) for category 3 cable not exceeding 20 pF/ft and for categories 4 and 5 cables 17 pF/ft.
Characteristic impedance All UTP cable should have a characteristic impedance of 100 ohms over the frequency range from 1 MHz to the cables highest frequency rating. Note these measurements need to be made on a cable of length, at least one-eighth of a wavelength.
NEXT The near end crosstalk (NEXT) indicates the degree of interference from neighboring wire pairs. This is measured by applying a balanced signal to one pair of wires and measuring its disturbing effect on another pair, both of which are terminated in their nominal characteristic impedance of 100 ohms. This is shown in FIG. 22.
FIG. 22 Near end crosstalk NEXT is expressed in decibels, in accordance
with the following formula:
NEXT = dB 10
where:
Pd is the power of the disturbing signal
Px is the power of the crosstalk signal
NEXT depends on the signal frequency and cable category. Performance is better at lower frequencies and for cables in the higher categories. Higher NEXT values indicate small crosstalk interference.
The standard specifies minimum values for NEXT for the fixed 10Base-T cables, known as horizontal UTP cable and also for the connecting hardware. The following tables show these values for the differing categories of cable at various frequencies.
Table 3
Minimum NEXT for horizontal UTP @ 20°C
It should be noted that the twists in the UTP cable, which enhance its crosstalk performance, need to be removed to align the conductors in the connector. To maintain adequate NEXT performance the amount of untwisted wire and the separation between the conductor pairs should be minimized. The amount of untwisting should not exceed 13 mm (0.5 inch) for category 5 cables and 25 mm (1 inch) for category 4 cables.
Structural return loss
The structural return loss (SRL) is a measure of the degree of mismatch between the characteristic impedance of the cable and the connector. This is measured as the ratio of the input power to the reflected power. SRL = 10 log (Input Power/Reflected Power) dB Higher values are better implying less reflection. For example 23 dB SRL corresponds to a reflected signal of seven per cent of the input signal.
Table 4 Minimum structural return loss (SRL) @ 20°C
Direct current resistance
The DC resistance is an indicator of the ability of the connectors to transmit DC and low frequency currents. The maximum resistance between the input and output connectors, excluding the cable, is specified as 0.3 ohm for category 3, 4 and 5 UTP cables.
Ground plane effects
It should be noted that if cables are installed on a conductive ground plane, such as a metal cable tray or in a metal conduit, the transmission line properties of mutual capacitance, characteristic impedance, return loss and attenuation can become two or three per cent worse. This is not normally a problem in practice.
10.6 Advantages of twisted pair cable
Twisted pair cable has the following advantages over other types of cables for networks:
• It is easy to connect devices to twisted pair cable.
• If an already installed cable system, such as telephone cable, has extra unused wires, you may be able to use a pair of wires from that system -- BUT see the warnings on this above.
• STP significantly reduces external interference.
• UTP is quite inexpensive.
• UTP is very easy to install.
• UTP may already be installed (but make sure it all works properly and that it meets the performance specifications your network requires).
10.7 Disadvantages of twisted pair cable
Twisted pair cable has the following disadvantages:
• STP is bulky and difficult to work with.
• UTP is more susceptible to noise and interference than coaxial or fiber optic cable.
• UTP has higher signal attenuation than other cables.
• Skin effect can increase attenuation. This occurs when transmitting data at a fast rate over twisted pair wire. Under these conditions, the current tends to flow mostly on the outside surface of the wire. This greatly decreases the cross- section of the wire being used, and thereby increases resistance. This, in turn, increases signal attenuation.
• Narrower bandwidth of operation than coaxial cables.
11. Sources of interference and noise on cables
Noise is normally introduced into cable circuits through electrostatic (capacitive) coupling, magnetic (inductive) coupling, and resistive coupling. The reduction of these noise signals takes the form of shielding and twisting of signal leads, proper grounding, separation and good insulation.
Shielding is the protection of the signal wires from noise or unwanted signals.
FIG. 23 Typical shield
The purpose of the shield is to reduce the magnitude of the noise coupled into the low level signal circuits by electrostatic or magnetic coupling. The shield may be considered an envelope that surrounds a circuit to protect the cable from the coupling.
11.1 Electrostatic coupling
Electrostatic or capacitive coupling of external noise is illustrated in FIG. 24. The external noise source couples the noise into the signal wires through capacitors C1 and C2 and the resulting flow of current produces an error voltage signal across R1, R2 (the cable resistance) and RL. The error signal is proportional to the length of the cable leads, the resistance of the cable leads, the amplitude and the frequency of the noise signal and the relative distance of the cable leads from the noise source.
FIG. 24 Electrostatic coupled noise
FIG. 25 Use of shield to reduce electrostatic noise
The noise due to electrostatic coupling can be reduced by the use of shielded wire, by separation and by twisting of the leads. As the separation between the noise source and the signal wires is increased, the noise coupling is thereby reduced. Twisting of the leads provides a balanced capacitive coupling which tends to make C1 = C2. Therefore, the induced voltages at the load will be equal in magnitude.
The use of a shield to reduce electrostatic noise is illustrated in FIG. 25. The noise induced currents now flow through the shield and return to ground instead of flowing through the signal wires. With the shield and signal wire tied to ground at one end, a zero potential difference would exist between the wires and the shield. Hence, no signal current flows between wire and shield.
The quality of the shield will determine how large C1 and C2 are compared to C3 and C4. The better quality shield (possibly 2 or 3 shields on one cable) will have the higher value of C1 and C2 and the lower value of C3 and C4. The lower the value of C3 and C4 the less noise value induced into the signal cables.
11.2 Magnetic coupling
Magnetic coupling is the electrical property that exists between two or more conductors: when there is a current change in one, there will be a resultant induced voltage in the other conductor. FIG. 26 illustrates a disturbing wire (noise source) magnetically coupling a voltage into the signal circuit.
The alternating magnetic flux from the disturbing wire induces a voltage in the signal loop, which is proportional to the frequency of the disturbing current, the magnitude of the disturbing current and the area enclosed by the signal loop and is inversely proportional to the square of the distance from the disturbing wire to the signal circuit.
FIG. 26 Magnetic noise coupling
FIG. 27 Reducing magnetic noise by twisting of wires
FIG. 26 illustrates all of the factors necessary to introduce an error voltage: rate of change of current, a signal loop with a given area and a separation of the conductors from the disturbing signal (D1 and D2). A common method of reducing the effect of magnetic coupling is the use of twisted conductors in the signal circuits, as illustrated in FIG. 27. The distance of these two signal wires in respect of the disturbing wire is approximately equal and the area of the circuit loop is almost zero. Reducing this area to practically zero will reduce the voltage induced by the magnetic field to almost zero due to the equal magnitude of current induced in each lead that will result in a near zero net circulating current. (The currents will induce voltages in the load that are equal and opposite in magnitude and will therefore cancel.) Employing a shield made of a high ferrous content material around the signal wires can also reduce magnetic coupling. This shield is effective because the magnetic field produces eddy currents in the shield that will produce magnetic flux in the opposite direction to the inducing flux and will oppose the original magnetic field. A sketch of this type of noise reduction is illustrated in FIG. 28. This type of shield is very rare and would have to be specially manufactured upon request. High ferrous content conduits are used sometimes but these are subject to corrosion problems.
FIG. 28 Effects of shield in reducing magnetic coupling
11.3 Impedance coupling
Impedance coupling (as illustrated in FIG. 29) is the electrical property that exists when two or more signal wires share the same common return signal wire. If there is any resistance in the common return wire, then the signal current from any of the loads will cause the voltage to rise at all the loads. In addition, noise that induces current flow into the common return will cause noise voltage at all the loads.
FIG. 29 Resistance or impedance coupling
To avoid impedance coupling in signal circuits, either of the following can be carried out:
• Employ a low-resistance wire or bus for the common return when a common return cannot be avoided. (For critical applications both the resistance and the inductance of the bus should be minimized.)
• Wherever possible, employ separate signal return leads.
A few alternative solutions to the problem of impedance coupling are indicated in the following diagrams. FIG. 30 indicates the ideal approach of separate signal returns.
Here, the common return conductor is reduced to a single terminal point on one side of the links. FIG. 31 illustrates the use of a large low impedance return bus. Note that the individual returns from each transmitter and receiver should also be as low impedance as possible.
FIG. 30 Cabling system illustrating individual signal returns
FIG. 31 Cabling system illustrating common signal returns
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