Principles of Transmission
Baseband Analog
A baseband analog video signal is a continuous varying signal whose magnitude and frequency represent the video content (e.g., luminance, chrominance, synchronization). A baseband video signal contains all the necessary information to reproduce a picture, but it does not modulate a radio frequency (RF) carrier. Two terms commonly used to describe different types of baseband signaling are: • Composite. • Component.
Aluminum
A bluish silver-white malleable ductile light trivalent metallic element that has good electrical and thermal conductivity, high reflectivity, and resistance to oxidation. It has about 60% conductivity compared with copper and is lighter in weight than copper. Aluminum is most commonly used in electrical utility distribution lines.
Component Format
A color video picture is made up of three colors (red, green, and blue [RGB]), which are mixed in varying intensities to create a complex image. Component video, also called RGB video, keeps separate the three-color components of the image using three cables to carry the video signal. RGB signals separate the primary color information from the luminance signal, which minimizes crosstalk and permits higher resolutions. RGB signaling is typically used for highend graphic workstations where the need for higher-quality imaging is required.
Digital Signals
A digital signal changes from one state to another in discrete steps. Figures 1.9 through 1.13 show examples of digital signals. The most significant property of digital signals is that at any time they can take on only a value from a discrete set of values. For example, the digital signal in Figure 1.12 can have only one of four possible values. Analog signals can be converted to digital data using a process called A/D conversion, as explained below.
Digital Signal Level (DS1)
A form of TDM, the digital data from 24 speech channels is combined for transmission over a single transmission channel. The data from the 24 voice channels is arranged in a frame.
Solid Wall Metal Tubes
A low-resistance solid wall metal tube (conduit) is the best possible shield, displaying superior shielding properties at all frequencies. While solid metal tubes are used as shields in some specialized applications, their rigid nature makes them inappropriate for most normal cable applications.
High-strength
A mixture of copper and other metals to improve certain copper alloy properties and characteristics of copper. Alloys such as cadmium-chromium copper and zirconium copper offer important weight reductions or greater strength. These factors are especially important in aerospace and computer applications. However, the alloying of pure copper always has an adverse effect on its conductivity. The alloys mentioned above have 85% conductivity ratings.
Cable Shielding
A shield is a metallic covering or envelope enclosing the: • Insulated conductor. • Individual group of conductors within a core. • Cable core. Shields are made of foil or braided metal strands. They are usually tinned copper, bare copper, aluminum, or another electrically conductive material. When properly terminated, bonded, and grounded (earthed) cable shields can: • Reduce the radiated signal from the cable. • Reduce the effects of electrical hazards. • Minimize the effect of external EMI on the conductors within the shielded cable.
Nominal Velocity of Propagation (NVP)
A signal traveling from the input to the output is delayed in time by an amount equal to the length of cable divided by the velocity of propagation (υ) for the transmission medium. In the case of an ideal transmission line consisting of two conductors in free space, the velocity of propagation is equal to the velocity of light in a vacuum (c). For practical cables, the velocity of propagation depends on the properties of the dielectric materials surrounding the conductors. At very high frequencies, υ asymptote tends toward a constant value. Where: c = Velocity of light in a vacuum μ = Relative permeability of dielectric ε = Relative permittivity of dielectric
Insertion Loss Performance Limits
Channel insertion loss is equal to the sum of the attenuation of the various components in the test channel, plus all the mismatch losses at cable and connector interfaces, and the increase in attenuation adjusted for temperature. In the worst case, the channel shown in Figure 1.21 consists of ≈90 m (295 ft) of horizontal cable and up to a total of ≈10 m (33 ft) of equipment and patch cords combined. Generally, patch cords are of flexible stranded construction, thereby presenting higher losses per meter or foot than horizontal cables. All components must meet the minimum attenuation requirements of the appropriate standard for balanced twisted-pair category or class. NOTE: In many documents, the terms attenuation and insertion loss are used interchangeably. Strictly speaking, attenuation is a measure of the signal loss under ideal termination conditions where the load and source impedance matches the cable characteristic impedance and all components are exactly matched in impedance.
Polyvinyl Chloride (PVC)
Commonly used Inside Dielectric
Composite Conductor
Composite conductor is a term used to describe conductors constructed from nontraditional materials (e.g., metallic resins, graphite). This conducting substance is impregnated into, coated over, or between layers of polymer tape or other similar material. These types of conductors are often used in telephone receiver and mounting cords, inexpensive headsets, and other low-end audio devices. They also are used to embed audio devices into plastic shells such as helmets.
Conductive Nonmetallic Materials
Conductive nonmetallic materials (e.g., semi-conductive tapes made with high carbon content) are sometimes used at power and some low audio frequencies. These semiconductive shields are not normally used for applications at frequencies above 500 kilohertz (kHz).
Reflection Coefficient
Consider the case where the terminating impedance is not the same as the characteristic impedance of a cable (e.g., Zt ≠ Zo). In this case, a signal will be partly reflected at the cable/load junction. The magnitude of the reflection is given by the reflection coefficient (ρ). If Zt < Zo, then the polarity of the reflected wave is inverted; if Zt > Zo, then the polarity of the reflected wave is not inverted. Reflection coefficient (ρ) = (Zt - Zo)/(Zt + Zo)
Selecting a Cable Shield
Consider the following primary criteria when selecting a cable shield for a given application: • Nature of the signal to be transmitted—Frequency range affects the performance of most shields. • Magnitude of the EM fields through which the cable will run—EM fields are usually expressed in volts per meter (V/m) at a given frequency. • Electromagnetic compatibility (EMC) regulations—Any cable operating within a given system must be designed to conform to the EMC radiation limits of that system. NOTE: See Chapter 2: Electromagnetic Compatibility, Class A and Class B limits. • Physical environment and specific mechanical requirements—The shield may need to add support to the cable. NOTE: Overall cable size limitations also may affect this decision.
The Most Common Electrical conductors that ITS wire and cable are made of are?
Copper, Copper Covered Steel, High Strength Copper Alloys, Aluminum
Crosstalk
Crosstalk is signal interference between cable pairs, which may be caused by a pair picking up unwanted signals from either: • Adjacent pairs of conductors. • Nearby cables. For example, this interference can result from the magnetic field that surrounds any currentcarrying conductor. The crosstalk interference can be intelligible or unintelligible, depending on the coupling modes. Refer to the Glossary at the end of this manual for specific types of crosstalk, including near-end crosstalk (NEXT), far-end crosstalk (FEXT), equal level far-end crosstalk (ELFEXT), power sum near-end crosstalk (PSNEXT), and power sum equal level far-end crosstalk (PSELFEXT)
Discrete Multitone
DMT uses multicarrier modulation. A frequency band is sliced into several hundred (typically 256) sub-bands, each of which carries a signal modulated with part of the data stream. Data rates can be adjusted with DMT by increasing the number of sub-bands and by varying the number of bits carried in each sub-band.
Delay Skew
Delay skew is the difference in propagation delay between any pairs within the same cable sheath. The delay skew between the fastest and slowest pairs in category 6/class E and category 5e/class D cabling shall not exceed 45 nanoseconds (ns) at ≈100 m (328 ft [see Table 1.14]).
Composite Conductor (Disadvantages)
Disadvantages of composite conductors include the following: • Poor analog transmission characteristics, including high attenuation, especially above 4000 hertz (Hz) • Poor digital transmission characteristics • Easily damaged unless encased in a rigid material • Inconsistent quality Cables with these types of conductors are not recommended for use with modern telecommunications networks. If equipment is shipped with this type of cable, discard and replace the cable with the proper structured cabling patch cord for the project.
Drain Wires
Drain wires are used: • With foil, nonmetallic, and hybrid shields. • Occasionally with braided shields to make it easier to terminate the shield ground. Drain wires are usually: • Applied longitudinally next to the metallic part of the shield for the length of the cable. • Made of solid or stranded copper conductors, which may be bare or tinned. The type of drain wire must be specified when selecting the type of cable. The termination requirements of the application determine whether the drain wire should be made of bare or tinned copper in stranded or solid construction.
Shielding Effectiveness
EM waves are attenuated and reflected by a shield. Consequently, the effectiveness of a shield depends on such factors as the: • Type and thickness of the shield material. • Number and size of openings in the shield. • Effectiveness of the bonding connection to ground. The shielding effectiveness of a cable shield is determined by measuring the surface transfer impedance. The surface transfer impedance is the ratio of the conductor-to-shield voltage per unit length to the shield current. Surface transfer impedance is usually measured in milliohms per meter or ohms per foot. Cable shields are usually connected in such a way that they may be called upon to carry relatively large currents that are induced from an external field. The current flowing in the shield results in a voltage drop along the shield because of the shield resistance. As a result, there is a voltage gradient between the conductors inside the shielded cable and the shield itself.
Electromagnetic Interference (EMI)
Electromagnetic interference (EMI) is stray electrical energy radiated from electronic equipment and electronic systems (including cables). EMI can cause distortion or interference to signals in other nearby cables or systems.
Channel Model
Figure 1.21 shows a channel and the cabling components that determine the channel performance. The components that may make up the channel consist of a: • Telecommunications outlet/connector. • Balanced twisted-pair cable of ≈90 m (295 ft). • Cross-connect system. • Equipment and patch cords. • Consolidation point (CP). • Horizontal connection point (HCP). • Transition point (TP). • Multiuser telecommunications outlet assembly (MUTOA).
sinusoid
Is an oscillating, periodic signal that is completely described by three parameters: • Amplitude • Frequency • Phase
Dielectric strength
Measures the maximum voltage that an insulation can withstand without breakdown. Dielectric strength is recorded in breakdown tests in which the voltage is increased at a controlled rate until the insulation fails. The voltage at that time, divided by the thickness of the insulation, equals the dielectric strength. Dielectric strength is expressed in volts (V) per millimeter (or V per mil, where 1 mil equals 0.001 inch). A high value is preferred (to withstand voltage stress). Insulated conductors in telecommunications applications have a typical dielectric strength of between 7500 and 30,000 V per millimeter (300 and 1200 V per mil).
Symmetrical Digital Subscriber Line (SDSL)
SDSL is a single-pair version of HDSL, transmitting up to DS1 rate signals over a single balanced twisted-pair. SDSL has an important advantage compared with HDSL. SDSL suits the market for individual subscriber premises that are often equipped with only a single telephone line. SDSL is desired for any application needing symmetrical access (e.g., servers, power remote LAN users) and therefore complements ADSL (see the following section on ADSL Technologies). It should be noted, however, that SDSL would not reach much beyond ≈3000 m (9850 ft), a distance over which ADSL achieves rates up to 6 Mb/s.
Copper
Sets the conductor value of all other conductors Copper = 100%
Digital Subscriber Line (DSL)
Several related telecommunications technologies fall under the broad category of digital subscriber line (DSL) solutions (also referred to as xDSL). Variants of DSL technology include: • HDSL. • Symmetrical digital subscriber line (SDSL). • Asymmetric digital subscriber line (ADSL, ADSL2, ADSL2+). • Rate-adaptive digital subscriber line (RADSL). • Very high bit-rate digital subscriber line (VDSL). In general terms, all of these solutions are oriented toward providing high-speed, high-quality transmission of data, voice, and video over existing balanced twisted-pair telephone lines.
Signal-to-Noise Ratio (SNR)
Signal-to-noise ratio (SNR) is the ratio of the level of the received signal at the receiver-end and the level of the transmitted signal. The level of the received signal must significantly exceed the level of the received noise for a feasible communication condition. SNR can be determined by the following expression. Where, Vnoise = Level of the noise voltage at the receiver-end (in volts, V) Vsignal = Level of the transmitted signal (in volts, V)
Simplex
Simplex is a term used to describe the transmission of signals in one direction only. A simple, but familiar, example of simplex transmission is a public address system without twoway speakers. The signal, which represents the speaker's voice, is carried to a number of loudspeakers. There is no path for listeners to respond.
Solid Conductors
Solid conductors consist of a single piece of metal wire. Stranded conductors bundle together a number of small-gauge solid conductors to create a single, larger conductor. Advantages of solid conductors include the following: • Less costly • Less complex termination systems • Better transmission performance at high frequencies • Less resistance
Synchronous Transmission
Synchronous transmission is performed by synchronizing the data bits in phase or in unison with equally spaced clock signals or pulses. Both the sender and the receiver must have timing and synchronizing capabilities. The clocking pulses prevent confusion of the characters in the data stream. Synchronous transmission is more efficient than asynchronous transmission because no start and stop bits are required. It is used with digital baseband transmission systems.
Time Division Multiplexing (TDM)
Telecommunications systems typically combine binary data from several different sources (e.g., voice channels) into a single composite bit stream. This process is called time division multiplexing (TDM). TDM is one means of increasing the information-carrying capacity of a digital telecommunications channel.
8B/1Q4 PAM5 Encoding
The 8B/1Q4 PAM5 encoding scheme is specified in IEEE 802.3ab for use with 1000BASE-T, which uses all four cable pairs for simultaneous transmission in both directions. This is accomplished through the use of echo cancellation and 8B/1Q4 PAM5 encoding. Each group of eight bits (8B) is converted to one transmission of four quinary symbols (1Q4) across four balanced twisted-pairs. Each symbol represents two binary bits using PAM5 modulation.
Near-End Crosstalk (NEXT) Loss Limits
The NEXT loss in the channel is the vector sum of crosstalk induced in the cable, connectors, and patch cords. NEXT loss is dominated by components in the near zone (less than ≈20 m [66 ft]). To verify performance, measure NEXT loss from both the TR and the telecommunications outlet/connector. All components must meet the minimum NEXT requirements for the appropriate standard for balanced twisted-pair category or class.
Attenuation-to-Crosstalk Ratio (ACR)
The attenuation-to-crosstalk ratio (ACR) is a ratio obtained by subtracting the attenuation (dB) from NEXT (dB). ACR is normally stated at a given frequency. It can be calculated as follows: ACR = Minimum NEXT loss - maximum attenuation
Return Loss
The power of the reflected signal is called the return loss (RL) in. The better the impedance matching, the lower the reflected energy and the higher the return loss. Return loss can be determined as follows: Where, P reflected = Signal power of the reflected signal (in watts, W) P in = Signal power of the injected signal (in watts, W) Return loss is an important parameter for gigabit networks that employ parallel, full-duplex transmission over all four pairs because each pair will carry information in both directions, the same as an analog telephone line. Any impedance mismatch between components will result in signal reflections (echoes) that appear as noise at the receiver. Although this noise is partially canceled in the equipment, it can be a significant contributor to the overall noise budget.
Power Sum Attenuation-to-Alien-Crosstalk Ratio at the Far End (PSAACRF)
The power sum attenuation-to-alien-crosstalk ratio at the far end (PSAACRF) is a ratio in decibels determined by subtracting the attenuation from the power sum alien far-end crosstalk (PSAFEXT) loss between cables or channels in close proximity. It can be calculated as follows: PSAACRF = Minimum PSAFEXT loss - maximum attenuation
Power Sum Attenuation-to-Alien-Crosstalk Ratio at the Near End (PSAACRN)
The power sum attenuation-to-alien-crosstalk ratio at the near end (PSAACRN) is a ratio in decibels determined by subtracting the attenuation from the power sum alien near-end crosstalk (PSANEXT) loss between cables or channels in close proximity. It can be calculated as follows: PSAACRN = Minimum PSANEXT loss - maximum attenuation
Power Sum Attenuation-to-Crosstalk Ratio (PSACR)
The power sum attenuation-to-crosstalk ratio (PSACR) is a ratio in decibels determined by subtracting the attenuation from PSNEXT loss. It can be calculated as follows: PSACR = Minimum PSNEXT loss - maximum attenuation
Dielectric Constant
The ratio of the capacitance of an insulated conductor to the capacitance of the same conductor uninsulated in the air. Air is the reference with a dielectric constant of 1.0. Generally, a low dielectric constant is desirable. The dielectric constant changes with temperature, frequency, and other factors.
Dissipation factor
The relative power loss in the insulation is due to molecular excitement and subsequent kinetic and thermal energy losses. This is of primary concern in the high-frequency megahertz ranges where signal loss increases because of the structure of the insulating material. For example, polar molecules, such as water, absorb energy in an EM field. This effect is best understood in terms of microwave heating. A low dissipation factor is preferable.
Telephone Line Impedance
The telephone from which the voice signal originates can be considered a signal generator that is connected to a load, which is a combination of the connecting cables and the other telephone. The connecting cables make up a transmission line. The maximum transmission of electrical power occurs when a transmitting device and a receiving device have the same load resistance or, more specifically, the same impedance. Impedance is a parameter that applies to alternating current (ac) signals. Like resistance, impedance is expressed in ohms, but it has both a magnitude and a phase component. It is important to ensure that the source or load impedance connected to a line is matched in the best way possible for maximum efficiency. Telephone cables have a characteristic impedance that depends on frequency. In voice-band applications, a typical impedance of either 600 or 900 ohms is used to match the cable pairs. The 600 ohms impedance is preferred for private line circuits and trunks while 900 ohms is used in CO switching system line circuits.
Broadband Video
The term broadband video refers to composite baseband video and audio signals that are amplitude and frequency modulated, respectively, with an RF carrier in accordance with the video and audio information that need to be conveyed (e.g., CATV). Each RF carrier represents a TV channel. RF carriers are separated by 6 to 8 MHz.
Balanced Twisted-Pair Media Implementation
Video signals traditionally have been transported using coaxial and optical fiber cables. Because of increased requirements for the transmission of video signals in commercial applications, support for analog video transmission, along with the associated audio component, using structured balanced twisted-pair cabling systems has been developed. Baseband composite signaling can be supported over category 3/class C or higher cabling in excess of ≈100 m (328 ft). RGB component signals are supported with category 3/class C or higher cabling for a minimum of ≈100 m (328 ft) using passive media adapters. Broadband analog CATV signaling can be implemented on category 5e/class D or higher balanced twisted-pair cabling. For example, category 5e/class D cabling can support CATV downstream delivery between 55 MHz and 550 MHz over limited distances. Category 6/class E cabling provides better performance because of lower signal loss, higher SNR, and higher noise immunity. It can support more broadband channels or longer distances than category 5e/class D.
telecommunications transmission system
consists of three basic components: • Source of energy • Medium to carry the energy • Receiving device In telephony, the three basic components of the transmission system are: • Source of energy—The acoustic energy of speech is converted to an equivalent electrical signal at the transmitting handset by a microphone. • Medium to carry the energy—A balanced twisted-pair cable is commonly used as the transmission medium. • Receiving device—The transducer in the receiving handset acts like a small loudspeaker and converts the electrical energy back to sound energy for the ear.
idealized transmission line
consists of two conductors that are separated by a dielectric material uniformly spaced over its length.
Ethylene chlorotrifluoroethylene (ECTFE)
e.g., Halar NEW Higher Transmission Performance Dielectric.
balanced twisted-pair construction
involves twisting individual pairs and grouping those twisted pairs to form either a cable or a unit for larger cable
Pair-to-pair capacitance unbalance
is a measure of the electric field coupling between two pairs if a differential voltage is applied on one pair and a differential noise voltage is measured on another pair in close proximity.
Mutual inductance
is a measure of the magnetic field coupling between two pairs if a differential current is applied on one pair and a differential noise current is measured on another pair in close proximity. the ability of one circuit to induce an emf in a nearby circuit in the presence of a changing current
Insulation (Dielectric)
is used to isolate the flow of current by preventing direct contact between: • Conductors. • A conductor and its environment
Asymmetric Digital Subscriber Line (ADSL) Technologies
ADSL technology is asymmetric—it allows more bandwidth downstream (server to client) than upstream (client to server). An ADSL circuit connects an ADSL modem on each end of a single balanced twisted-pair telephone line, creating three information channels—a high-speed downstream channel, a medium-speed duplex channel, and a plain old telephone service (POTS) channel. The POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS, even if ADSL fails. The high-speed downstream channel ranges from 1.5 to 8 Mb/s, while the upstream rate for ADSL varies from about 128 kb/s to just over 1 Mb/s. Good Internet performance requires a down-to-upstream ratio of at least 10:1. ADSL is ideal for Internet connections, video on demand, and remote LAN access—typical applications that are found in the home. Several ADSL technologies have been defined in standards (see Table 1.11).
Composite Conductor (Advantages)
Advantages of composite conductors include the following: • Flexible • Lightweight • Inexpensive and easy to produce • Easily embedded into other materials • Low coefficient of expansion
Stranded Conductors
Advantages of stranded conductors include the following: • More flexible • Longer flex life • Less susceptible to damage during crimp termination processes
Copper Covered
Also known as copper-clad steel, it combines the conductivity of steel copper with the strength of steel. It is typically used as a conductor for aerial, self-supporting drop wire. In the production of this type of conductor, a copper layer is bonded to a steel core.
Decibel
An important property of a signal is its strength (power), which is often expressed in decibels. The decibel is a measure that compares two power levels. The decibel is defined as: dB = 10 log (P1/P2) It is critical to observe that the dB is a relative power measurement. The power of a signal (P1) stated in decibels indicates the power of that signal relative to some reference power (P2). It is often convenient to define a signal power to be used as a reference. For instance, 1 milliwatt (mW) is frequently used as a reference power in telephony. The power of a 50.1 mW signal would be expressed as 17 decibel milliwatt (dBm). Notice that m is added to dB to indicate that the reference power is 1 mW. If a reference power of 1 W is used, the signal power is expressed as dBW. Decibel levels are used to express power ratios of all types of analog and digital signals, regardless of the medium.
Echo and Delay
Another phenomenon that occurs in signal transmission is echo. When a signal encounters a discontinuity in the impedance of the medium carrying the signal, some of the signal power is reflected back to the transmitter. The reflected signal appears as a delayed version (e.g., echo) of the original signal. A familiar, but extreme, example of this phenomenon is when a sound wave encounters a rock wall. Echoes of voice are occurring at all times, but they usually return so fast that they cannot be distinguished from the original sound. For an echo to be experienced, there must be enough delay for it to be distinguishable from the original source of the sound. In telephony, delays greater than 50 milliseconds (ms) are perceptible if they are of sufficient strength.
Phase and Delay
As previously mentioned, one of the three defining parameters of a sinusoidal signal is phase. The two sinusoids shown in Figures 1.4 and 1.5 differ only in phase. Note that the signal in Figure 1.4 is simply a delayed (in time) version of the signal in Figure 1.5. Thus, the delay of a sinusoidal signal can be equally well expressed as either a phase shift or a time delay. It should be observed that the result of adding the two sinusoids of the same frequency would depend on their phase difference. For example, if the phase difference is zero, the sum will be a single sinusoid with amplitude 2A. However, if the phase difference is exactly one-half of the period, the sum will be zero. Two sinusoids whose sum is zero are considered 180 degrees "out of phase." For more complex signals that are composed of many sinusoidal components, a delay is expressed only in time (seconds), not in phase.
Asynchronous Transmission
Asynchronous transmission occurs without a precise time relationship in the signal characters or the bits that represent them. Each character of the information: • Is sent without a precise time relationship between it and any other character of information. • Carries with it start and stop signals. Asynchronous transmission is a popular method of telecommunications among microcomputer users because of a common standardized interface and protocol between machines. Asynchronous transmission is less efficient than synchronous transmission because it requires the addition of some combination of start and stop bits to the data stream, but it is not difficult to implement in systems at speeds less than 20 kb/s.
Attenuation
Attenuation corresponds to the ratio in decibels of the output power (or voltage) to the input power (or voltage) when the load and source impedance are matched to the characteristic impedance of the cable. Where the terminations are perfectly matched, the ratio of output to input power (or voltage) is called attenuation. Practical attenuation measurements yield values that are higher than the attenuation, depending on the degree of mismatch. When evaluated in terms of voltage ratio, attenuation can be determined according the expression below. Where: Vin = Input voltage (in volts, V) Vout = Output voltage (in volts, V)
Performance Parameters
The most important parameters that affect performance are insertion loss, PSNEXT loss, and return loss in the case of bidirectional transmission. Other parameters (e.g., velocity of propagation, delay skew, longitudinal conversion loss, attenuation deviation, PSELFEXT [also called PSACRF]) are also important for certain higher speed applications where more complex encoding schemes and duplex balanced twisted-pair transmissions are implemented. For 10GBASE-T applications (IEEE 802.3an standard), alien crosstalk parameters, including PSANEXT loss and PSAACRF, are specified.
Tight Twisting
The option of tight twisting, where pair twist lengths are less than ≈12.7 mm (0.50 in), is used particularly within and between computers and other data processing equipment. Category 5e, category 6, category 6A, and higher category cables employ tight twisting for optimum transmission performance. Tight twists tend to preserve their shape better in a cable. Longer twists tend to nest together as they are packed in a cable, whereas shorter, tighter twists are less likely to deform.
Balanced Twisted-Pair Performance
Balanced twisted-pair cables are commonly used for data telecommunications in buildings. Successful implementation of the balanced twisted-pair approach for LAN installations requires proper design, installation, and testing to ensure that channel performance requirements are met. A channel, as defined in the cabling standards, includes all cables, cords, and connectors from an equipment connection at one end to the equipment connection at the other end. The transmission characteristics of telecommunications cables, cords, and connectors depend on the frequency of the applied signal. These differences are most apparent at frequencies above one MHz. It is important for the ITS distribution designer to be able to assess the capabilities of different transmission media for a given application. The transmission parameters of greatest importance include the: • Signal attenuation as a function of frequency. • Signal reflections at terminations. • Amount of noise relative to the received signal. The noise can be coupled into the cable from adjacent circuits sharing the same sheath (crosstalk coupling) or from external influences. Balanced twisted-pair cables have a nominal characteristic impedance of 100 ohms at 100 MHz. The improvement in attenuation for high-performance cables is realized through improved design and materials. Likewise, an improvement of upward of 10 dB in crosstalk performance is attained through better balance and pair-twist optimization. These balanced twisted-pair cables provide increased signal-to-noise margins, which equate to higher data throughput (fewer bit errors), a longer reach, or higher transmission rate capability
Temperature Effects
Balanced twisted-pair cables used in premises applications are expected to operate under a variety of environmental conditions. One concern is the attenuation increase at higher cable temperatures (above 20 °C [68 °F]). High temperatures can be routinely encountered in: • Exterior building walls. • Ceiling spaces, including plenums. • Mechanical rooms. Intermittent failures have been reported in LANs as a result of solar heating of walls and the cabling inside them. To avoid such problems, the attenuation at the highest expected temperature must be used in the premises cabling design process. Attenuation increases with temperature because of increased: • Conductor resistance. • Insulation dielectric constant. • Dissipation factor. The attenuation of some cables may exhibit significant variations because of temperature dependence of the material. All twisted-pair cables are referenced in the cabling standards at 20 °C +/- 3 °C (68 °F +/- 5.4 °F). For adjustment purposes, the attenuation increase is 0.2 percent per degree Celsius for temperatures above 20 °C (68 °F) for screened cables, 0.4 percent per degree Celsius for all frequencies and for all temperatures up to 40 °C (104 °F), and 0.6 percent per degree Celsius for all frequencies and for all temperatures from 40 °C to 60 °C (104 °F to 140 °F) for all unscreened cables. A temperature coefficient of 1.5 percent per degree Celsius is not uncommon for some category 3 cables. NOTE: Consult the manufacturer's specifications on the cable insertion loss margin compared with the maximum insertion loss that is specified in the standard Reference should be made to the relevant cabling component standard for the authority having jurisdiction (AHJ) because the attenuation requirements for each cable type and category or class vary. Some insulation performs better than others under high temperature conditions. Figures 1.1, 1.2, and 1.3 show a comparison of attenuation and frequency at various temperatures for: • FEP (e.g., Teflon®, NEOFLON™ FEP). • ECTFE (e.g., Halar®). • PVC. NOTE: Teflon is a trademark of E.I. du Pont de Nemours & Company, Inc.; NEOFLON FEP is a trademark of Daikin America, Inc.; and Halar is a trademark of Solvay Solexis. For more information on Figures 1.1, 1.2, and 1.3, refer to "Temperature-Related Changes in Dielectric Constant and Dissipation Factor of Insulations Increase Attenuation in Data Cables Used in Building Plenums," by C.Y.O. Lin and J.P. Curilla, which is available from IEEE®.
Pair Twists
Both mutual inductance and capacitance unbalance are affected by the relative length and uniformity of pair twists. To minimize crosstalk within a multipair cable, each pair is given a different twist length within a standard range. Generally, a counterclockwise twist length between ≈50 mm and ≈150 mm (2 in and 6 in) is used for voice and low-frequency data cables. Adjacent pairs are generally designed to have twist length differences of at least ≈12.7 mm (0.50 in). These specifications vary according to the manufacturer.
T1 Lines
For telephone quality, speech is sampled at a rate of 8000 samples/s. Thus, an 8-bit speech sample is generated every 1/8000 = 62.5 microsecond (μs) for each speech channel. Consequently, transmission of the digital speech data requires sending one 8-bit sample every 62.5 μs for each channel, as shown in Figure 1.7. One bit is added to each frame for control purposes. The data rate for this format is: (8 b/s channel × 24 channels + 1 framing bit) × 8000 frames/s = 1.544 kb/s T1 lines are designed to carry DS1 frames TDM also is used to multiplex signals from a lower level in the digital hierarchy to a higherlevel signal. For example, four T1 (DS1) signals can be combined to form a T2 (DS2), or 28 T1 can be combined to form a T3 (DS3) signal. In Europe, four E1 signals can be combined to form an E2 (8 Mb/s), four E2 can be combined to form an E3 (34 Mb/s), or four E3 can be combined to form an E4 (140 Mb/s) signal. The process of reconstituting the individual channels from the composite signal is called demultiplexing. The multiplexing and demultiplexing equipment is commonly called a channel bank. Modern units also are called intelligent multiplex terminals. It is important to remember that only the first order multiplexing stage (T1 or E1) contains any A/D conversion. From the second order upward, the system only deals with digital frames. It is not possible to extract a single channel from the digital stream without demultiplexing back to the first order stage.
Full-Duplex
Full-duplex is a term used to describe the transmission of signals in both directions at the same time. All telephone lines are full-duplex, allowing both parties to talk simultaneously.
High Bit-Rate Digital Subscriber Line (HDSL)
HDSL is a way of transmitting DS1 rate signals over balanced twisted-pair cable. HDSL requires no repeaters on lines less than ≈3600 m (11,811 ft) for 24 AWG [0.51 mm (0.020 in)]. Using advanced modulation techniques, HDSL transmits 1.544 Mb/s (DS1) or 2.048 Mb/s (E1) in bandwidths of less than 500 kHz, both upstream and downstream. Depending upon the specific technique, HDSL requires two twisted-pairs for DS1 and three twisted-pairs for E1, each operating at half or third speed. HDSL has effectively been replaced by SDSL and other xDSL technologies.
Half-Duplex
Half-duplex is a term used to describe the transmission of signals in either direction, but only in one direction at a time. This requires agreement between stations and typically employs a: • Push-to-talk switch arrangement on voice circuits. • Signaling protocol. A home intercom is a familiar example of a half-duplex operation.
How Does Temperature affect Dielectric Properties?
Higher temperature increase dielectric properties.
Composite Format
In the composite format, the analog signal contains all the components necessary to construct a monochrome or color picture, but it contains no audio information. This type of signal is typically found as the output from a digital recording device, TV monitor, camera, or camcorder. Figure 1.14 illustrates the composition of the signal.
Integrated Services Digital Network (ISDN)
Integrated services digital network (ISDN) uses digital transmission at a basic or primary rate, depending upon the application. Basic rate ISDN: • Is intended for residential and small business users. • Uses a digital signal consisting of two 64 kb/s B channels (assigned for voice and data) and one 16 kb/s D channel (assigned for signaling and packet data). • Has a total information capacity of 144 kb/s (line rate = 160 kb/s). Primary rate ISDN North America: • Is intended for large business users. • Has a total information capacity of 1.536 Mb/s (line rate = 1.544 Mb/s). • Uses a digital signal consisting of 23 B channels and one D channel, each operating at 64 kb/s. Primary rate ISDN can be implemented over repeated T1 carrier or high bit-rate digital subscriber line (HDSL) facilities. It also may be embedded in the higher rate transmission systems. Primary rate ISDN Europe: • Is intended for large business users. • Has a total information capacity of 1.92 Mb/s (line rate = 2.048 Mb/s). • Uses a digital signal consisting of 30 B channels and one D channel, each operating at 64 kb/s. Primary rate ISDN can be implemented over repeated E1 carrier or HDSL facilities. It also may be embedded in the higher rate transmission systems.
Telephony Echo
Occasionally, users might encounter echoes on long-distance calls. For an echo to be perceived, part of the transmitted signal must be sent or reflected back to the originating end. Part of a transmitted signal is sent back to the transmitter (reflected) when the impedances of the transmission line and the receiver are not matched. In this case, the maximum power is not transmitted. Matching these impedances improves transmission efficiency and minimizes the echo. Many of us have been given the impression that electricity always travels at the speed of light (≈300,000 km/s [186,000 mi/s]). Since light is an EM wave, this speed applies to all EM radiation in free space. The speed of light in free space is usually represented by the symbol c. EM radiation travels slower than c in any physical medium. For example, signals travel slower in cables—about .56 c to 74 c. Longer circuits will have proportionately longer delays. The signal propagation speed in twisted-pairs will depend on the type of insulation used and its thickness, among other factors. Although satellite signals travel at velocity c, geostationary satellites are such a great distance away (≈35,786 km [22,236 mi]) that the round-trip delay is close to 1/4 s and is quite perceptible when holding a telephone conversation.
polyethylene (PE)
PE-insulated conductors display better transmission performance. However, they are unsuitable for indoor use unless they are encased in a suitable fire-retardant jacket material
Power Sum Equal Level Far-End Crosstalk (PSELFEXT) Loss Limits
PSELFEXT is a computation of the unwanted signal coupling from multiple transmitters at the near end into a pair measured at the far end. PSELFEXT is calculated in accordance with the power sum algorithm. All components must meet the minimum PSELFEXT requirements for the appropriate standard for balanced twisted-pair category or class.
Quadrature Amplitude Modulation
Quadrature amplitude modulation (QAM) is a widely used modulation technique for sending digital data. QAM and its derivatives are used in both mobile radio and satellite telecommunication systems. It is the basis for discrete multitone (DMT) and similar schemes used in x digital subscriber line (xDSL) systems. A QAM signal is composed of two sinusoidal carriers, each having the same frequency but differing in phase by one quarter of a cycle (hence the term quadrature). One sinusoid is called the I signal, and the other is called the Q signal. Mathematically, these two signals are equivalent to a sine wave and a cosine wave. At the transmitter, the I and Q carriers are amplitude modulated by bits selected from the data. The two amplitude modulated carriers are then combined for transmission. The combined signal is both amplitude and phase modulated by the data bits (e.g., data bits determine both the amplitude and the phase of the transmitted signal). At the destination, the carriers are separated; the data is extracted from each; and the data is converted into the original modulating digital data.
Return Loss Limits
Return loss is a measure of the reflected energy caused by impedance mismatches in the cabling system. All components must meet the minimum return loss requirements for the appropriate standard for balanced twisted-pair category or class.
Power Sum Attenuation-to-Crosstalk Ratio (PSACR)
The balanced twisted-pair channel performance specified previously is determined from transmission measurements on cables and termination hardware. These measurements are performed in the frequency domain. The range of frequencies that can be successfully transmitted for a given distance determines the available channel bandwidth in MHz for a specified channel. Different criteria can be used to determine the available bandwidth. One such criterion is the minimum signal level at the output of a channel relative to the peak NEXT noise level. This criterion is defined as PSACR. To ensure an acceptable bit error rate (BER), the signal should be a reasonable replica of the transmitted signal. Attenuation is a decrease in signal magnitude. Higher frequency components of the digital signal incur more attenuation over a given balanced twisted-pair channel. The net effect is not only a reduction in amplitude, but also a change in the shape of the transmitted signal as it appears at the receiver. Additionally, NEXT noise adds abrupt variations in the signal magnitude. The reliability of the receiver to detect changes in the signal waveform is affected by these signal impairments.
Propagation Delay
The development of new high-speed applications using multiple pairs for parallel transmission has shown the need for additional transmission specifications (e.g., propagation delay, delay skew) for 100 ohm, 4-pair cabling systems. The following equation is used to compute the maximum allowable propagation delay between 1 MHz to the highest referenced frequency for a given category of cable. μ = c Delay (ns/100 m) = 534 + 36/ freq MHz
Insulation resistance (IR)
The insulation's ability to resist the flow of current through it. For inside conductors, IR is typically expressed in megohm•kilometer or megohm•1000 feet. NOTE: There is an inverse relationship between insulation resistance and cable length (i.e., as the cable length increases, the insulation resistance becomes smaller).
Telephony Distortion
The transmission characteristics of conductor pairs vary with frequency. Consequently, the various sinusoidal frequency components of a signal that are sent over a transmission line will emerge in a somewhat different form—each signal component will experience a signal loss and a phase shift that is frequency dependent. At voice frequencies, the principal elements contributing to loss and phase distortion are the conductor resistance and the mutual capacitance of the cable pair. Increasing the frequency increases the speed of transmission through cable pairs. Using the example of 19 AWG [0.91 mm (0.036 in)] balanced twisted-pair cable, the velocity of transmission is 37,000 kilometers per second (km/s [23,000 miles per second (mi/s)]) at 300 Hz. At 3400 Hz, the velocity of transmission is ≈125,529 km/s (78,000 mi/s). This frequency-dependent transmission speed variation does not noticeably affect speech intelligibility, but it can have a great effect on data transmission. The application of inductors, called loading coils, placed at intervals along a cable improves speech transmission quality. Loading coils: • Compensate for the capacitance of a cable pair. • Reduce the capacitive current loading in the range of audio frequencies. The most common distances between loading points are ≈1.37 km (4495 ft) for D loading and ≈1.83 km (6004 ft) for H loading. NOTE: Load coils, by their design, will cut off frequencies above the voice range. Because of this, load coils will block analog high fidelity and digital signals. Although loading coils improve speech, they adversely affect data transmission. While loading improves the loss versus frequency characteristics, it causes severe delay problems. The delay of the higher frequencies is far greater on loaded facilities than nonloaded facilities. Loading coils also limit the frequency at which information can be transmitted. The loading coil spacing determines the upper cutoff frequency. The shorter the spacing is between loading points, the higher the cutoff frequency.
Types of Shields
There are many types of shields, including: • Braided wire. • Spiral-wrapped wire. • Reverse spiral-wrapped wire. • Metal foils, either helically or longitudinally wrapped. • Hybrids, combining other types. • Metal tubes. • Conductive nonmetallic materials.
Internet Protocol (IP) Telephony Architecture
There are three common implementation options for IP telephony architecture (see Figure 1.6): • Separate lines—one for the IP telephone and one for the computer • One line for everything using a dual-port IP telephone or softphone • Wireless connection using access points (APs) to connect the IP telephone Deciding to install a dual-port IP telephone or softphone, using one line for everything, may seem an attractive option since it requires just one data cable for two devices—the IP telephone and computer. However attractive that option sounds at first, a single cable carrying all information reduces flexibility and redundancy. For example, the need for additional or upgraded services (e.g., 1000BASE-T) may require pulling new cables, which means disrupting services and additional costs. Two telecommunications outlets or connectors are recommended for each individual work area—one may be associated with voice and the other with data. Since IP telephony is now being added to the data network, both horizontal cables at the work area location should be considered cables that support data applications
Concept of Bandwidth
There is a fundamental relationship between the bandwidth of a channel expressed in Hz and the data rate expressed in b/s. The traffic flow on a major highway provides a good analogy to illustrate the concept of bandwidth versus data rate. The bandwidth is similar to the width of the highway and the number of lanes of traffic. The data rate is similar to the traffic flow or the number of vehicle crossings per hour. One way to increase the traffic flow is to widen the highway. Another way is to improve the road surface and eliminate bottlenecks. Similarly, it is possible to support a higher data rate for any channel by using a more elaborate line-encoding scheme to pack more bits of information per Hz of available bandwidth. More elaborate line encoding requires a higher SNR, which is like a smoother road surface in this analogy. The available bandwidth is commonly determined as the frequency range where the SNR is positive. For most LAN systems today, the dominant noise source is NEXT interference between all transmit pairs and a receive pair. If all four pairs are employed for parallel transmission, then the total NEXT noise is PSNEXT. In this case: • SNR is the PSACR when other noise sources are negligible and where PSACR = PSNEXT - attenuation. • Bandwidth is the frequency range where PSACR > 0.
Very High Bit-Rate Digital Subscriber Line (VDSL)
While VDSL has not achieved the same degree of definition as ADSL, it has advanced enough to discuss realizable goals, beginning with data rate and range. Downstream rates derive from submultiples of the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) canonical speed of 155.52 Mb/s, namely 51.84 Mb/s, 25.92 Mb/s, and 12.96 Mb/s. Each rate has a corresponding target range over existing outside plant (OSP [see Table 1.13]). Currently, VDSL targets only ATM network architectures, obviating channelization, and packet-handling requirements imposed on ADSL. VDSL is planned to use passive network terminations, enabling more than one VDSL modem to be connected to the same line at customer premises in much the same way as extension telephones connect to home cabling for POTS. VDSL was called VASDL or BDSL or even ADSL prior to June 1995 when VDSL was chosen as the official title. The other terms still linger in technical documents created before that time and in media presentations unaware of the convergence. The European counterpart to VDSL has temporarily appended a lowercase "e" to indicate that the European version of VDSL may be slightly different from the U.S. version. This is the case with both HDSL and ADSL although there is no convention for reflecting the differences in the name. The differences are sufficiently small (mostly concerning data rates) that silicon technology accommodates both.
Fluorinated ethylene propylene (FEP)
[e.g., Teflon®, NEOFLON FEP™ NEW Higher Transmission Performance Dielectric.
Characteristic Impedance
of infinite length: Zin = Vi/Ii = Zo, t→ ∞ It also corresponds to the input impedance of a transmission line of finite length that is terminated in its own characteristic impedance. In general, the characteristic impedance has both a resistive and reactive component. Characteristic impedance is a function of the frequency of the applied signal, but it is unrelated to the cable length. Maximum power is transferred from the source to the load when the source impedance (Zs) and the terminating impedance (Zt) are equal to the complex conjugate of the transmission line characteristic impedance (Zo). NOTE: Two impedances are complex conjugates if they have the same resistive component and their reactive components have opposite signs. Under these conditions, all the energy is transmitted and none of the energy is reflected back at the cable termination. At very high frequencies, the characteristic impedance asymptote leads to a fixed value that is resistive. For example, coaxial cables have an impedance of 50 or 75 ohms at high frequency. Typically, balanced twisted-pair telephone cables have an impedance of 100 ohms above 1 MHz.
American Wire Gauge (AWG)
sizing system has become generally accepted in North America. The AWG system is important because it provides a standard reference for comparing various conductor materials.
Rate-Adaptive Digital Subscriber Line (RADSL)
the length and signal quality of the line. RADSL products have the option to select the highest practical operating speed automatically or as specified by the access provider (AP). RADSL allows the AP to adjust the bandwidth of the DSL link to fit the need of the application and to account for the length and quality of the line. Additionally, RADSL extends the possible distance from the subscriber to the AP facility, thus increasing the percentage of users served by DSL services.
Reason for twisting pairs of conductors
to minimize crosstalk and noise by decreasing capacitance unbalance and mutual inductance coupling between pairs. Twisting conductors also improves the balance (physical symmetry) between conductors of a pair and reduces noise coupling from external noise sources.
key performance drivers of balanced twisted-pair channels are:
• Insertion loss. • PSNEXT loss. • PSELFEXT loss. • Return loss for bidirectional applications. NEXT and PSNEXT are of particular concern in network configurations of balanced twistedpair cables. When measuring insertion loss, the ITS distribution designer needs to know that the cable length and signal frequency affect the amount of loss. However, NEXT and PSNEXT occur at the beginning of the channel and do not change appreciably as the cable gets longer.
