Data Communications-Analog signal
Unshielded vs shielded twisted pair cable. The most common twisted pair cable in communications is referred to as unshielded twisted pair (UTP). IBM has also produced a version of twisted pair cable for its use, called shielded twisted pair (STP). STP cable had a metal foil or braided mesh covering that encases each pair of insulated conductors. Although metal casing improves the quality of cable by preventing the penetration of noise or crosstalk, it is bulkier and more expensive. The figure shows the difference between UTP and STP.
Categories The electronic industries association(eia) has developed standards to classify unshielded twisted pair cable into seven categories. Categories a determined by cable quality, with 1 as the lowest and 7 ad the highest. Each EIA category is suitable for specific uses.
Coax Cable
Coaxial cable or coax carries signals of higher frequency ranges than those in twisted pair (higher frequency ranger- more cycles(crest and trough) per second. In part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.
Computers and other telecommunications devices use signals to represent data. These signals a transmitted from one device to another in the form of electromagnetic energy, which is propagated through transmission media.
Electromagnetic energy, a combination of electric and magnetic fields vibrating in relation to each other, includes power, radio waves, infrared light, visible light, ultraviolet light, and C, gamma, and cosmic rays. Each of these constitutes a portion of electromagnetic spectrum. Not all portions of the spectrum are currently usable for telecommunications, however. The media to harness those that a usable are also limited to a few types.
The use of long distance communication using electric signals started with the invention of the telegraph by Morse in the 19th century. Communication by telegraph was slow and dependent on a metallic medium.
Extending the range of the human voice became possible when the telephone was invented in 1869. Telephone communication at that time also needed a metallic medium to carry the electric signals that were the result of a conversion from the human voice. The communication was, however, unreliable due to the poor quality of the wires. The lines were often noisy and the technology was unsophisticated.
Category 1 - 1 pair - unshielded twisted pair used in telephone - data rate <0.1 Mbps, used for telephone Category 2 - 2 pairs - unshielded twisted pair originally used in T lines - 2Mbps - T-1 lines Category 3 - 4 pairs - Improved CAT 2 used in LANs - 10Mbps - LANs Category 4 - 4 pairs - Improved CAT 3 used in Tomen Ring networks - 20Mbps - Used in LANs Category 5 - 4 pairs - Cable wire is normally 24 AWG with a jacket and outside sheath - 100Mbps - used in LANs Category 5E - 4 pairs - an extension to category 5 that includes extra features to minimize the crosstalk and electromagnetic interference- 125mbps - used in LANs Category 6 - 4 pairs - a new category with matched components coming from the same manufacturer. The cable must be tested at a 200 Mbps data rate - 200Mbps - used in LANs Category 7 - sometimes called SSTP (shielded screen twisted pair). Each pair is individually wrapped in a helical metallic foil followed by a metallic foil shield in addition to the outside sheath. The shield decreases the effect of crosstalk and increases the data rate.
Gauge is a measure of the thickness of the wires
Coaxial cable connectors To connect coaxial cable to devices, we need coaxial connectors. The most common type of connector used today is the Bayonet Neil-Concelman connector The BNC connector is used to connect the end of the cable to a device, such as a TV set. The BNC T connector is used in Ethernet networks to branch out to a connection between to a computer or other device. The BNC terminator is used at the end of the cable to prevent the reflection of the signal.
Performance As we did with twisted pair, we can measure the performance of a coaxial cable. Attenuation is much higher in coaxial cable than in twisted pair cable. In other words, although coaxial cable has much higher bandwidth, the signal weakens rapidly and requires the frequent use of repeaters Applications Coaxial cable was widely used in analog telephone networks where a single coaxial network could carry 10,000 voice signals. Later it was used in digital telephone networks where a single coaxial cable could carry digital data up to 600 Mbps. However, coaxial cable in telephone networks has largely been replaced today with fiber optic cable.
Coaxial cable standards - coaxial cables a categorized by their Radio Government RG) ratings. Each RG number denotes a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and type of the inner insulator, the construction of the shield, and the size and type of the outer casing. Each cable defined by an RG rating is adapted for a specialized function, as shown in the table
Impedance is the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance Alternating current/direct current: Alternating current is an electrical current which periodically reversed direction, in contrast to direct current which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form in which electrical energy that consumers typically use when they plug in kitchen appliances, televisions, fans and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.
Transmission medium can be broadly defined as anything that can carry information from a source to a destination. For example, the transmission medium for two people having a dinner conversation is the air. The air can also be used to convey the mess in a smoke signal or semaphore. For a written message, the transmission medium might be a mail carrier, a truck, or an airplane.
In data communications, the definition of the information and the transmission medium is more specific. The transmission medium is usually free space, metallic cable, or fiber optic cable. The information is usually a signal that is the result of a conversion of data from another form.
Noiseless channel: nyquist bit rate For a noiseless channel, the nyquist bit rate formula defined the theoretical maximum bit rate Bitrate = 2 x bandwidth x log2L In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and bitrate is the bit rate in bits per second. According to the formula, we might think that, given a specific bandwidth, we can have any bit rate we want by increasing the number of signal levels. Although the idea is theoretically correct, practically there is a limit. When we increase the number of signal levels, we can impose a burden on the receiver. If the number of levels in a signal is just 2, the receiver can easily distinguish between a 0 and a 1. If the level of a signal is 64, the receiver must be very sophisticated to distinguish between 64 different levels. In other words, increasing the levels of a signal reduces the reliability of the system.
Increasing the levels of a signal may reduce the reliability of the system.
SC connector
SC connectors offer excellent packing density, and their push-pull design reduces the chance of fiber end face contact damage during connection; frequently found on the previous generation of corporate networking gear, using GBICs. The Subscriber Connector (SC) is a fiber optic connector with a push-pull latching mechanism that provides quick insertion and removal while ensuring a positive connection. The SC is also available in a duplex configuration. SC connectors, being square, have a mnemonic of "Square Connector", which some people believe to be the correct name, rather than the more official "Subscriber Connector".[6] Other terms often used for SC connectors are "Set and Click" or "Stab and Click"
SNR is actually the ratio of what is wanted (signal) to what is not wanted (noise). A high SNR means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by noise. Because SNR is the ratio of two powers, it is often described in decibel units, SNRdB, defined as: SNRdB= 10log10 SNR
SNR is often described in decibel units SNRdB = 10 log 10 (average signal power)/(average noise power)
MT-RJ (mechanical transfer registered jack) connector
MT-RJ (Mechanical Transfer Registered Jack) uses a form factor and latch similar to the 8P8C (RJ45) connectors. Two separate fibers are included in one unified connector. It is easier to terminate and install than ST or SC connectors. The smaller size allows twice the port density on a face plate than ST or SC connectors do. The MT-RJ connector was designed by AMP, but was later standardized as FOCIS 12 (Fiber Optic Connector Intermateability Standards) in EIA/TIA-604-12. There are two variations: pinned and no-pin. The pinned variety, which has two small stainless steel guide pins on the face of the connector, is used in patch panels to mate with the no-pin connectors on MT-RJ patch cords. An MTRJ connector is a type of fiber optic cable that has an endpoint similar to the RJ-45 connector endpoint found in Ethernet connections. MTRJ connectors are designed to snap into the Ethernet port of a computer, modem, or wireless router and provide that computer or network with extremely fast data transfer rates. MT-RJ connectors look like a miniature RJ-45 connector.
Hertz (Hz)
Ma'am/sir 1Hz = 1 completion of a crest & through per second 1 kHz = 10^3 completions of a crest & trough per second 1 MHz = 10^6 completions of a crest & trough per second 1 GHz = 10^9 completions of a crest & trough per second 1 THz (terahertz) = 10^12 completions of a crest & trough per second ma'am/sir
period
Ma'am/sir A periodic signal completes a pattern within a measurable time frame, called a period — basically a TIME FRAME. So, in a time frame (period) you can have 1.125 cycles sir/ma'am .
sine wave
Ma'am/sir A simple periodic analog signal which cannot be decomposed into simpler signals-smooth and consistent, continous, rolling flow sir/ma'am
Analog phone lines
Ma'am/sir An example of broadband transmission using modulation is the sending of computer data through a telephone subscriber line, the line connecting a resident to the central telephone office. These lines, installed many years ago, a designed to carry voice (analog signal) with a limited bandwidth (frequencies between 0 and 4kHz). Although this channel can be used as a low-pass channel, it is normally considered a bandpass channel. One reason is that the bandwidth is so narrow 4kHz) that if we treat the channel as low-pass and use it for baseband transmission, the maximum bit rate can be only 8kbps. The solution is to consider the channel a bandpass channel, convert digital signal from the computer to an analog signal, and send the analog signal. We can install two converters to change the digital signal to analog signal and vice versa at the receiving end. The converter is called a modem. Ma'am/sir
Broadband transmission (using modulation)
Ma'am/sir Broadband transmission or modulation means changing the digital signal to an analog signal for transmission. Modulation allows us to use a band pass channel— a channel with a bandwidth that does not start from zero. This type of channel is more available than a low pass channel. Sir/ma'am
Wavelength binds the period or the frequency of a simple wave to the propagation speed of the medium.
Ma'am/sir Frequency is independent of the medium, but wavelength depends on both frequency and the medium. Sir/ma'am
Period
Ma'am/sir Period is formally expressed in seconds (T) sir/ma'am
Period
Ma'am/sir Refers to the amount of time, in seconds, a signal needs to complete 1 cycle (the completion of one full pattern of a crest or trough) sir/ma'am
Period (T) is the inverse (1/x) of frequency, ie, period is 1/f
Ma'am/sir Since a period is a wavelength, a wavelength must also be the inverse (1/x) of frequency l, ie, frequency is 1/f sir/ma'am
The amplitude of the frequencies at the border of the bandwidth
Ma'am/sir The border of the bandwidth — the bandwidth is the difference between the uppermost frequency and lowest frequency The border of the bandwidth could be the frequency approaching the two extremes, such as the uppermost or lowermost frequency. The amplitude is the measure between the height of the crests and bottom trench of the troughs. This means the amplitude of the frequencies at the upper and lower frequency extremes within the bandwidth I think ma'am/sir
Change
Ma'am/sir They are sneaky. They like to talk about 'change', but all they mean by 'change' is crests and troughs! The number of crests and troughs per second is the frequency. Change (many crests and troughs) in a short time equals high frequency. Change (few crests and troughs) in a long span equals low frequency. If a signal has no crests and troughs (does not change) then the frequency is zero If a signal has infinite crests and troughs all the sudden (changes instantaneously) then the frequency is infinity sir/ma'am
Wavelength = (propagation speed or c) x period = (propagation speed c)/frequency
Ma'am/sir Wavelength lambda = c/f, or lambda = c propagation speed x period sir/ma'am
Low-pass channel
Ma'am/sir a channel that starts from zero. This is the case if we have a dedicated medium with a bandwidth constituting only one channel. For example, the entire bandwidth (whereas bandwidth is the range of frequencies, the difference between highest frequency and lowest frequency) the entire bandwidth of a cable connecting two computers is one single channel. As another example, we may connect several computers to a bus, but not allow more than two stations to communicate at a time. Sir/ma'am
Propagation speed depends on medium and on frequency of the signal.
Ma'am/sir for example, in a vacuum, light is propagated with a speed of 3x(10^8) m/s which is c(propagation speed). That speed is lower in air and even lower in cable. The wavelength is normally measured in micrometers instead of meters. The wavelength of red light is 4x(10^6). So, lambda =c/f, or lambda equals the propagation speed 3x(10^8) over the frequency which is 4x(10^14). Sir/ma'am
Hz
Ma'am/sir frequency is formally expressed in Hertz(Hz) which means cycles per second (cycle=completion of one full pattern of a crest and a through) sir/ma'am
fiber-optic cable
A fiber optic cable is made of glass or plastic and transmits signals in the form of light. To understand optical fiber, we first need to explore several aspects of the nature of light. Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another substance (of a different density), the ray changes direction. The figure shows how a ray of light changes direction when going from a more dense to a less dense substance. As the figure shows, if the angle of incidence I (the angle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser substance. Note that the critical angle is s property of the substance, and its value differs from one substance to another. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding or less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it.
ST Connectors- Straight Tip
ST connectors have a key which prevents rotation of the ceramic ferrule, and a bayonet lock similar to a BNC shell. The single index tab must be properly aligned with a slot on the mating receptacle before insertion; then the bayonet interlock can be engaged, by pushing and twisting, locking at the end of travel which maintains spring-loaded engagement force on the core optical junction. A straight tip connector (ST connector) is a connector used in fiber-optic cables that utilizes a bayonet-style plug and socket. It has become the de facto standard for commercial wirings. The ST connector setup allows for unidirectional communication, so two ST connectors and two fiber cables are used for bidirectional communication. ST connectors refer to having a "straight tip", as the sides of the ceramic (which has a lower temperature coefficient of expansion than metal) tip are parallel—as opposed to the predecessor bi-conic connector which aligned as two nesting ice cream cones would. Other mnemonics include "Set and Twist", "Stab and Twist", and "Single Twist",[citation needed] referring to how it is inserted (the cable is pushed into the receiver, and the outer barrel is twisted to lock it into place). Also they are known as "Square Top" due to the flat end face
The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a cable with -0.3 dB/km has a power of 2mW, what is the power of the signal at 5km
Sir/ma'am The loss in the cable in decibels is 5x(-0.3) = -1.5 dB ......, so, if we have the loss formula it's dB/km times the km, so likely at 10 km it's -3 dB=10log10(P2/P1) = -1.5 -> same old eq (P2/P1) = 10^-0.15. —-> log properties P2=0.71P1 = .7 x 2 mW = 1.4 mW. —> algebra ma'am/sir
Period
Sir/ma'am 1second - 1 second (T) required to complete one cycle (one complete crest & trough) 1 millisecond - 1 milliseconds (T) required to complete one cycle (one complete crest & trough) 1 microsecond - 1 microseconds (T) required to complete one cycle (one complete crest & trough) 1 nanosecond - 1 nanosecond (T) required to complete one cycle (one complete crest and through) 1 picosecond - 1 picosecond (T) required to complete one cycle (one complete crest & trough) ma'am/sir
ω=2πfT This is a relationship worth permanently etching in your brain! Let's run through a few examples. Suppose you have a continuous-time sinusoid with frequency 1000 Hz and sample it with a sampling frequency of 5000 Hz (so T=15000 seconds). The frequency of the discrete-time signal is thus ω=2π10005000=2π5 radians. If we change the sampling frequency to 4000 Hz, then the frequency of the discrete-time signal is ω=2π10004000=π2 radians. The discrete-time frequency of the sinusoid depends on both the frequency of the corresponding continuous-time sinusoid and the sampling interval T . Of course we can also use this relationship to convert from discrete-time to continuous-time frequency given the sampling interval T . If the sampling frequency is 100 Hz (T=0.01 seconds), then a discrete-time sinusoid with frequency ω=π/5 radians corresponds to a continuous-time sinusoid of frequency f=10 Hz.
Sir/ma'am ? Crazy relationship between continuous time frequency and discrete time frequency ma'am/sir
A single-frequency sine wave us not useful in data communications; we need to send a composite signal, a signal made of many simple sine waves
Sir/ma'am A single sine wave to convey a telephone conversation would just be a buzz, we need to send a composite signal. Ma'am/sir
BECAUSE the amplitudes if the frequencies at the border of the bandwidth can be ignored, this means that if we have a medium, such as coaxial or fiber optic cable, with a very wide bandwidth, two stations can communicate by using digital signals with very good accuracy, as shown in the figure of
Sir/ma'am Although the output signal is not an exact replica of the original signal, the data can still be deduced from the received signal. Note that although some frequencies a blocked by the medium, they are not critical. Ma'am/sir
Baseband transmission of a digital signal that preserves the shape of the digital signal is possible only if we have a low pass channel with an infinite or very wide bandwidth
Sir/ma'am An example of a dedicated channel where the entire bandwidth of the medium is used as one single channel is a LAN. Almost every LAN today uses a dedicated channel for two stations communicating with each other. In a bus topology (couldn't find an example of this on a quick search, old vampire clamps cone to mind— this book is written in 2013) LAN with multipoint connections, only two stations can communicate with each other a a time (half duplex? CSMA/CD)?) at each moment in time(timesharing); the other stations need to refrain from sending data. In a star topology LAN, the entire channel between each station and the ?hub? (Switch) is used for communication between these two entities. Ma'am/sir
Fourier analysis
Sir/ma'am Any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. Ma'am/sir
Attenuation
Sir/ma'am Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the signal. The figure shows the effect of attenuation and amplification ma'am/sir
Important to note and master this simple designation
Sir/ma'am Bandwidth is the difference between highest and lowest frequencies, not amplitudes. FREQUENCIES!! Ma'am/sir
Baseband transmission
Sir/ma'am Baseband transmission means sending a digital signal over a channel without changing the digital signal to an analog signal ma'am/sir
Bit length - we discussed the concept of the wavelength for an analog signal: the distance one cycle occupies on the transmission medium. We can define something similar for a digital signal: the bit length. The bit length is the distance one bit occupies on the transmission medium.
Sir/ma'am Bit length = propagation speed x bit duration ma'am/sir
Baseband transmission requires that we have a low pass channel, a channel with a bandwidth that starts from zero
Sir/ma'am Broadband transmission or modulation means changing the digital signal to an analog signal for transmission. Modulation allows us to use a bandpass channel— a channel with a bandwidth that does not start from zero. This type of channel is more available than a low pass channel. Ma'am/sir
Discrete data contains finite values that have nothing in between
Sir/ma'am Continuous data contains data that can be measured, that includes fractions and decimals ma'am/sir
If give a bandwidth if 200kHz, with a middle frequency of 140 kHz and peak amplitude of 30V. The two extremes frequencies have an amplitude of 0. Draw the frequency domain of the signal.
Sir/ma'am Define what is a time domain Define what is a frequency domain. Realize what is bandwidth and what can be derived by knowing the bandwidth and middle frequency. Ma'am/sir
phase
Sir/ma'am Describes position of the waveform relative to zero. A sine wave with a phase of 0 starts at time 0 with 0 amplitude. A sine wave with phase 90* starts at time zero, shifted 1/4 (90* is 1/4 of 360*) shifted 1/4 of the period. And so on, a sine wave of phase 180* shifts 1/2 the period, so that lays the period at a zero amplitude as its transitioning from crest to trough at 1/2 way point) ma'am/sir
Based on Fourier analysis a digital signal is a composite analog signal. The bandwidth is infinite. A digital signal, in the time domain, comprises connected vertical and horizontal line segments. A vertical line in the time domain means a frequency of infinity(sudden change in time); a horizontal line in the time domain means a frequency of zero(no change in time). Going from a frequency of zero to a frequency of infinity (and vice versa) implies all frequencies in between a part of the domain.
Sir/ma'am Fourier analysis can be used to decompose a digital signal. If the digital signal is periodic, which is rare in data communications, the decomposed signal has a frequency domain representation with an infinite bandwidth and discrete frequencies. If the digital signal is nonperiodic, the decomposed signal still has an infinite bandwidth, but the frequencies are continuous. Ma'am/sir
Frequency is period per time or frequency is wavelength per time
Sir/ma'am Frequency is period per second, likewise since a period is equal to a wavelength, frequency is wavelength per second ma'am/sir
Amplitude measured in meters (SI unit) — the measure from the top of the crest to the bottom of the through Frequency measured in Hertz (SI unit) —the measure of the cycles per unit time (second)
Sir/ma'am Frequency sub high minus frequency sub low equals bandwidth ma'am/sir
A composite signal can be periodic or nonperiodic
Sir/ma'am If a composite signal is periodic, the decomposition gives a series of signals with discrete frequencies; if the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies. Ma'am/sir
Low-Pass Channel with Wide Bandwidth
Sir/ma'am If we want to preserve the exact form of a nonperiodic digital signal with vertical segments (infinite frequency) and horizontal segments(zero frequency), we need to send the entire spectrum, the continuous range of frequencies between zero and infinity. This is possible if we have a dedicated medium with an infinite bandwidth between the sender and the receiver that preserves the exact amplitude of each component of the composite signal. Although this (a dedicated medium which can preserve the exact amplitude of each component) may be possible inside a computer(eg between CPU and ram), it is not possible between two devices. Fortunately, the amplitude of the frequencies at the border of the bandwidth are do small that they can be ignored. Ma'am/sir
Low pass channel with limited bandwidth
Sir/ma'am In a low pass channel with limited bandwidth, we approximate the digital signal with an analog signal. The level of approximation depends on the bandwidth available. Ma'am/sir
Non periodic digital signal is composed of non periodic composite analog signals with infinite bandwidth with continuous frequencies
Sir/ma'am Non periodic digital signal - this is hard to describe mathematically since it's not periodic. Basically this would be something like a digital bit stream that you are sending to someone Composed of nonperiodic analog signal with infinite bandwidth with continuous frequencies - because the signal is not periodic you can't reproduce it with only discrete frequencies, instead you have to use other frequencies. This is why they say continuous frequencies. Ma'am/sir
The figure shows the modulation of a digital signal. In the figure, a digital signal is converted to a composite analog signal. We have used a single-frequency analog signal (called a carrier); the amplitude of the carrier has been changed to look like a digital signal. The result, however, is not a single-frequency signal; it is a composite signal. More in this in chapter 5. At the receiver, the received analog signal is converted to digital, and the result is a replica of what has been sent.
Sir/ma'am Of the available channel is a bandpass channel, we cannot send the digital signal directly to the channel; we need to convert the digital signal to an analog signal. Ma'am/sir
A signal travels through an amplifier, and it's power is increased 10 times 10log10(10p1/p1) = 10log10(10) = 10(10 = 10 dB
Sir/ma'am One reason that engineers use the decibel to measure the changes in strength of a signal is that decibel numbers can be added (or subtracted) when we a measuring several points (cascading) instead of just two. In the figure a signal travels from point 1 to point 4. The signal is attenuated by the time it reaches point 2. Between points 2 and 3, a signal is amplified. Again, between pints 3 and 4, the signal is attenuated. We can find the resultant decibel value for the signal just by adding the decibel measurements between each set of points. Simply add and subtract decibels ma'am/sir
Periodic digital signal is composed of periodic composite analog signal with infinite bandwidth with discrete frequencies
Sir/ma'am Periodic digital signal - a specific example of this would be a simple square wave with a 50% duty cycle Composed of periodic composite analog signal with infinite bandwidth with discrete frequencies - in order to reproduce a square wave fully, you need an infinite amount of sine waves that are at discrete frequencies ma'am/sir
Frequency
Sir/ma'am Refers to the number of periods(time in second to complete 1 cycle) in one second... if it takes .25 seconds to complete a cycle, the period is 4, therefore the frequency is 4 cycles per second ..... f =1/T (whereas T is time in seconds(to complete 1 cycle))...... T=1/f (whereas, obviously, f is frequency) so period is the inverse of frequency and frequency is the inverse of period ma'am/sir
Transmission impairment
Sir/ma'am Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment a attenuation, distortion, and noise. Ma'am/sir
The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is normally a difference between two numbers. For example, if a composite signal contains frequencies between 1000 and 5000, it's bandwidth is 5000 - 1000, or 4000
Sir/ma'am The bandwidth of a composite signal is the difference between the highest and lowest frequencies contained in that signal. Ma'am/sir
cycle
Sir/ma'am The completion of one full pattern of a crest and a trough. Ma'am/sir
Composite periodic signal and decomposition of a composite periodic signal in the time and frequency domains
Sir/ma'am The frequency of the sine wave with frequency f is the same as the frequency of the composite signal; it is called the fundamental frequency, or first harmonic. The sine wave with frequency 3f has a frequency of 3 times the fundamental frequency; it is called the third harmonic, and so on the 9th harmonic. Note that frequency decomposition of the signal is discrete; it has frequencies f, 3f, and 9f. Because f is an integral number, 3g and 9f are also integral numbers. There a no frequencies 1.2f or 2.6f. The frequency domain of a periodic composite signal is always made of discrete spikes. Ma'am/sir
peak amplitude
Sir/ma'am The height (positive or negative) of a crest or through. For electrical signals, peak amplitude is normally measured in volts ma'am/sir
Bit rate - most digital signals are nonperiodic, and thus period and frequency a not appropriate characteristics.
Sir/ma'am The term bit rate is used to describe digital signals. I'm he bit rate us the number of bits sent in 1's, expressed in bits per second(bps) ma'am/sir
Sir/ma'am *rough approximation* Focusing on the first box in the image; digital:bit rate N The sequence is 000 Analog: f=0, p=180 (phase = 180) Focusing on the second box in the image; Digital:bit rate N The sequence is 001. Analog: f=N/4, p=180 (phase = 180) Focusing on the third box in the image; Digital: bit rate N The sequence is 010 Analog: f=N/2, p=180 (phase = 180) Focusing on the fourth box in the image; Digital: bit rate N The sequence is 011 Analog: f=N/4, p=270 (phase = 270) Focusing on the fifth box in the image; Digital: bit rate N The sequence is 100 Analog: f=N/4, p=90 (phase = 90) Focusing on the sixth box in the image; Digital: bit rate N The sequence is 101 Analog: f=N/2, p=0 (phase = 0) Focusing on the seventh box in the image; Digital: bit rate N The sequence is 110 Analog: f=N/4, p=0 (phase = 0) Focusing on the eight box in the image; Digital: bit rate N The sequence is 111 Analog: f=0, p=0 Ma'am/sir
Sir/ma'am The two similar cases 000 and 111 a simulated with a signal with frequency f = 0 and a phase of 180 for 000 and a phase of 0 for 111. The two worst cases (010 and 101) are simulated with an analog signal with frequency f= N/2 and phases of 180 and 0. The other four cases can only be simulated with an analog signal with f = N/4 and phases of 180, 270, 90, and 0. In other words, we need a channel that can handle frequencies 0, N/4, and N/2. This rough approximation is referred to as using the first harmonic (N/2) frequency. The required bandwidth is: bandwidth = N/2 - 0 , which comes outs to N/2 ma'am/sir
The time and frequency domains of a nonperiodic signal
Sir/ma'am This figure shows a nonperiodic composite signal. It can be the signal created by a microphone or telephone set when a word or two is pronounced. In this case the composite signal cannot be periodic because that implies that we are repeating the same word or words with exactly the same tone. In a time-domain representation of this composite signal, there are an infinite number of simple sine frequencies. Although the number of frequencies in a human voice is infinite the, the range is limited. A normal human being can create a continuous range of frequencies between 0Hz and 4kHz. Ma'am/sir
Decibel (dB)
Sir/ma'am To show that a signal has lost or gained strength, engineers use the unit of the decibel. The decibel (dB) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if the signal is amplified. dB = 10log10 (P2/P1) Variables P1 and P2 are the power of a signal at points 1 and 2, respectively. Note that some engineering books define the decibel in terms of voltage instead of power. In this case, because power is proportional to the square of the voltage, the formula is dB = 20log10(V2/V1). Ma'am/sir
Suppose a signal travels through a transmission medium and its power is reduced to one half. We use the formula 10log10 (P2/P1). And if the power is reduced by half the second variable must be 1/2 the first, therefore p2=1/2p1. Plug it in: 10log10(p2/p1) = 10log10(.5p1/p1)= 10log10(.5) = 10(-0.3) = -3dB
Sir/ma'am a loss of 3dB (-3dB) is equivalent to one half the power. Ma'am/sir
broadband
Sir/ma'am a second example is the digital cellular telephone. For better reception, digital cellular phones, convert the analog voice signal to a digital signal. Although the bandwidth allocated to a company providing digital cellular phone service is very wide, we still cannot send the digital signal without conversion. The reason is that we only have a bandpass channel available between caller and caller. For example, if the available bandwidth is W and we allow 1000 couples to talk simultaneously, this means the available channel is W/1000, just part of the entire bandwidth. We need to convert the digitized voice to a composite analog signal before sending. The digital cellular phones convert the analog audio signal to digital and then convert the digital (not voice) to analog signal for transmission over a bandpass channel. Ma'am/sir
The power of a signal is 10 mW and the power of the noise is 1 micro W, what are the values of SNRdB
The values of SNR and SNRdB can be calculated as follows: SNR=(avg signal power/avg noise power) = (10,000 micro watts (10 mW converted to micro) / (1 micro W) = 10,000 SNRdB = 10log10 SNR SNRdB = 10log10 10,000 = 10 log10 10^4 = 40
Noise is another cause of impairment. Several types of noise, such as thermal noise, induced noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise is the random motion of electrons in a wire, which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving. Crosstalk is the effect of one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna. Impulse noise is a spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on.
Thermal noise - Johnson-nyquist noise is the electronic noise generated by the thermal agitation of the charge carriers(usually electrons) inside an electrical conductor at equilibrium. Which happens regardless of any applied voltage. Induced noise - induced noise is the noise generated in a circuit by a varying magnetic or electrostatic field produced by another circuit. Induced noise just like any other noise, degraded the useful signal and may lead to equipment errors, shutdown, or malfunction. This is more critical in digital circuits, signal transmission circuits and other sensitive circuits. Crosstalk: in electronics, crosstalk is a phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another or circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, channel, to another. Impulse noise: impulse noise is a category of (acoustic) noise which includes unwanted, almost instantaneous (this impulse like) sharp sounds (like clicks and pops). Noises of the kind are usually caused by electromagnetic interference, scratches on the recording disks, gunfire, explosions and ill synchronization in digital recording and communication. Ma'am/sir
Wireless communications started in 1895 when Hertz was able to send high frequency signals. Later, Marconi devised a method to send telegraph type messages over the Atlantic Ocean.
We have come a long way. Better metallic media have been invented (twisted pair and coaxial cables, for example). The use of optical fibers has increased the data rate incredibly. Free space (air, vacuum, and water) is used more efficiently, in part due to the technologies (such as modulation and multiplexing) discussed in the previous chapters.
As we will see later, to find the theoretical bit rate limit, we need to know the radio of the signal power to the noise power. The signal to noise ratio is defined as SNR SNR =(average signal power/average noise power)
What is signal power? What is noise power? High SNR vs Low SNR The first box shows a signal at the source, a digital signal, the horizontal line represents a frequency of zero, whereas the vertical line represents a frequency of infinity; presumably this illustration is graphed against time. The second box shows a random noise which can be from thermal noise, induced noise, crosstalk, or impulse noise. The third box shows the effects of the noise in the digital signal. Looks like the vertical lines of infinite frequency remain, but the horizontal lines of zero frequency are skewed. This has been a high SNR show. The bottom leftmost image show another digital signal which has vertical lines of infinity which seem to be presented as amplitudes. And if they a amplitudes, they are much smaller. The second box shows the noise as in the first row. Could be thermal noise, induced noise, crosstalk or impulse noise. The final box shows the effects of the noise on the digital signal, which seem contained but more dramatic per smaller quantity.
Does the nyquist theorem agree with the intuitive bit rate described in baseband transmission? They match when we have only two levels. We said, in baseband transmission, the bit rate is 2 times the bandwidth if we use only the first harmonic in the worst case. However, the nyquist formula is more general than we derived intuitively; it can be applied to baseband transmission and modulation. Also, it can be applied when we have two or more levels of signals.
What us the 'intuitive' bit rate?
Optical fibers Propagation modes
current technology supports two modes (multimode and single mode) for propagating light along optical channels, each requiring fiber with different physical characteristics. Multimode can be implemented in two forms: step-index or graded-index.
multimode graded index fiber
decreases the distortion of the signal through the cable as compared to step index fiber. the word index here refers to the index of refraction. as we saw in step index fiber, the index of refraction is related to density. a graded index fiber, therefore, is one with varying densities. density is highest at the center of the core and decreases gradually to its lowest edge the figure shows the impact of this variable density on the propagation of light beams
multimode step index fiber
multimode is so named because multiple beams from a light source move through the core in different paths. how these beams move within the cable depends on the structure of the core. in multimode step-index fiber, the density of the core remains constant from the center to the edges. a beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. at the interface, there is an abrupt change due to a lower density; this alters the angle of the beam's motion. the term step-index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber
fiber sizes
optical fibers are defined by the ratio of the diameter of their core to the diameter of their cladding, both expresses in micrometers. the common sizes are : type Core Cladding mode 50/125 50.0 mic 125 mic multi,graded 62.5/125 62.5 mic 125 mic multi,graded 100/125 100.0mic 100 mic multi, graded 7/125 7.0 mic 7.0 single, graded
single mode fiber
single mode fiber uses step index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. the single mode fiber itself is manufactured with a a much smaller diameter than that of the multimode fiber, and with substantially lower density (index of refraction). The decrease in density results in a critical angle that is close enough to 90* to make the propagation of beams almost horizontal. in this case, propagation of different beams is almost identical, and delays are negligible. all the beams arrive at the destination 'together' and can be recombined with little distortion to the signal
cable composition
the outer jacket is made of either PVC or Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Kevlar is a strong material used in the fabrication of bulletproof vests. below the kevlar is another plastic coating to cushion the fiber. the fiber is at the center of the cable, ans it consists of cladding and core
performance
the plot of attenuation versus waveslength shows a very
Fiber Optic Connectors
there are three types of connectors fo rfiber optic cables. the subscriber channel (SC) connector is used for cable TV. it sues a push/pull locking system. the straight-tip (ST) connector is used for connecting cable to networking devices. it uses a bayonet locking system and is more reliable than SC. MT-RJ is a connector that is the same size as RJ45
