TEC5313 Advanced Data Telecommunication Eastern Illinois University Introduction to Telecommunications and security policies By: Dr. Rigoberto Chinchilla Introduction • The basic communication LINK structure ALWAYS is composed of: – Emitter (Antenna, Light emitter/receptor, etc) – Path (medium) (Fiber Optic, Open Space, Copper, Coax Cable) – Receiver (Antenna, Light emitter/receptor, etc) Communications Channel Transmitter Transmitted Received Receiver Signal Signal Communication channel Signal Bandwidth • In order to transfer data faster, a signal has to vary more quickly. Channel Bandwidth • A channel or medium has an inherent limit on how fast the signals it passes can vary • Limits how tightly input pulses can be packed Transmission Impairments • Signal attenuation • Signal distortion • Spurious noise • Interference from other signals • Limits accuracy of measurements on received signal MODEMS – In order to transmit information over a medium it is necessary to change the frequency of the signal • Why? Why Do I need to Modify the signal? • Signals like voice ARE NOT compatible with the telecommunication media (Copper, coax, fiber, Open space) • Although the open space is pretty good for your voice, if you shout, it will not go much farther than maybe ¼ of mile in the best conditions. • So we need to ADAPT the signal to the media, the ADAPTER is called MODEM. MODEM • - MODULATION: Is the process, which change • • the shape and/or the frequency of the signal in the transmitter. - The signal change MUST be changed according to a specific pattern, which MUST be the same at reception. - DEMODULATION: Is the process of recuperating the original signal after a modulation process MODEMS • Typically a MODEM (modulator/demodulator) is used in the transmitter and in the receiver. • The MODEM at each side traditionally are named as part of the TRANSMITTER and RECEIVER What is a “carrier” • Every time the MODEM adapts the signal to the media somehow it changes the original frequency of the signal (i.e. voice signal) to a NEW frequency. • This NEW frequency (called carrier) is capable to move perfectly to the media “Carrying” the original signal within – NOTE: Sometimes we call “carriers” to those companies (VERIZON, AT&T) who carry the signals of the phones, ultimately that is exactly what they are doing : !providing a carrier” for our communications needs Attenuation • As an electrical signal travels through a cable, it becomes weaker due to the resistance offered by the cable to the flow of electrons or light.(remember other electrons are “living there!” ) • The “weakness” of the signal is called attenuation and it refers to the reduction in POWER (This translates in amplitude or height loss) of a transmitted signal. Attenuation • A second source of attenuation which is not related with the loss of power due to cable is related with the signal frequency. • Not all frequencies can be transported at the same level within a medium, some frequencies are more attenuated than others due to bandwidth limitations. • When signals are close to the bandwidth limits tend to have more attenuation respect to signals close to the bandwidth center Attenuation • The attenuation effect in general can be corrected by amplifying the signal, these amplifiers are called “repeaters” • Most of the time is not practical to insert amplifiers in the transmission line ( Although, if you see bulky boxes hanging there in the cables , these are amplifiers and equalizers.) • The power of the signal has to be enough to reach the other side and high enough to be recognized Distortion • Different to attenuation, distortion is the effect of changing the shape of the signal by a random pattern. • The changes in the shape of the signal are due mainly to “noise”. (noise is random). – Noise can come from internal sources like electrons moving randomly while the signals is passing the media or EXTERNAL natural or man-made sources • Noise and distortion are the variables ALWAYS present in ANY telecommunication application, they can only be reduced not eliminated. • Emerging technologies can ONLY diminish the effect of these effects. Equalization • Equalization is the process by which the effect of distortion is compensated. – Now a days it is relatively easier because we have only to reconstruct the shape of a “1” (i.e. +5 volts) or a “zero” (i.e. -5 volts) An Equalizer is the cevice used to do that • The transmitter is the one which typically perform this task. (although the bulky boxes hanging lines between poles also due amplification/equalization) • Note that equalization (fixing the shape) perform a different task that amplification the power) (increasing Bit Error Rate (BER) • Is the measurement of how many bits are not recognized as they were sent • The main sources of error are Noise and Distortion (always present !) • BER = (Number of errored bits received) / (Number of total bits transmitted) Carrier to Noise Ratio (C/N) (Sometimes called Signal to Noise ratio S/N) • Under noise conditions (always!) you need the signal level much more higher than the noise level • The “carrier” is the frequency that modulates the signal or the signal within the medium • If the carrier (Signal) level is high and the noise level is low the C/N(S/N) ratio is high which is the ideal condition Equalization • Equalization is the process by which the effect of distortion is compensated • The transmitter is the one which typically perform this task • Note that equalization (fixing the shape) perform a different task that amplification the power) (increasing Modulation Techniques • Amplitude Modulation: (AM) – The binary signal (“0” or “1”) • A “1” typically means presence of signal in a time interval (i.e. +5 Volts) • A “0” Typically means no signal present in a time interval (i.e. = zero volts – The binary Signal (“0” or “1”) • Sometimes a “1” means a positive signal and a “0” means a negative signal (i.e. +5volts =1 and 5volts =“0”) – The binary Signal (“0” or “1”) • Sometimes “1” = zero volts and “-1” = - 5volts Modulation Techniques • Amplitude Modulation: (AM) – ENCODING: Is to decide how a “1” and a “0” will be represented in terms of voltage levels • Each modem is designed with a particular way to “encode” logical zeroes and logical “ones” in terms of voltage. As far as we are CONSISTENT in both ends it should not be a problem – AMPLITUDE SHIFT KEYING (ASK): Is the name of Amplitude Modulation in the context of digital technologies. Modulation techniques • Frequency Modulation: – Here instead of sending a “1” or a zero as in AM modulation (amplitude of voltage) we send a constant AMPLITUDE signal but TWO different frequencies – One frequency (f1) is present when a “one” is transmitted and another frequency (f2) is present when a “zero” is transmitted. • FSK: Frequency Shift Keying is called in the digital technologies context Modems and Routers • Routers are devices used to route information over the internet, however routers need the modems to transport the information over the lines. A router is considered a computer (in the big picture) or a Data Transmission Equipment (DTE) a modem is considered a Data Communication equipment (DCE) Routers and Modems • At your home the Cable Company typically provides the MODEM (DCE) and you buy the router (i.e. At Walmart) and you connect them to have an internet connection • Many times Routers and modems are incorporated in just one box…here is how it works Frequencies within a Digital signal • A digital Signal looks like a “train” of “ones” and “zeroes” as follows • Although they look like just positive voltage (“1”) and no voltage (“0”) every digital signal is composed of many frequencies Fundamentals of “1” and “0” • A “1” typically is a • • positive voltage signal A “0” in this picture is a negative voltage Notice that the bottom signals begin to look like a squares when the two signal on the top are added , showing the concept of frequency components in a “square” signal. Frequency Concepts In practice, an electromagnetic signal is made up of many frequencies (has sinus components; one is the fundamental frequency, others are multiples) Spectrum – range of frequencies a signal contains. Bandwidth – signal’s width of the spectrum. Any media has a limited bandwidth => limited data rate! Frequency Concept In practice, an electromagnetic signal is made up of many frequencies (has sinus components; one is the fundamental frequency, others are multiples. Spectrum – range of frequencies a signal contains. Bandwidth – signal’s width of the spectrum. dc Component (continuous component) – component with zero frequency. Any media has a limited bandwidth => limited data rate!!! How the frequency of a signal is calculated? • Frequency : Is the number of signals that can be • transmitted in ONE second The more signals (up and down changes) are transmitted the higher the frequency. In the following picture if the TOTAL time shown is ONE second, the highest frequency signal will be the one at the bottom. How the frequency of a signal is calculated? • To measure how many signals are transmitted, the easiest way is to count either the TOTAL number of peaks in the whole one second snapshot or the number of valleys in the whole one second snapshot. As an Example the GREEN signal has approximately 6 signals transmitted in a second (assume the snapshot is for exactly ONE second). : 6 signals / 1 sec • EACH of the SIX SIGNAL take a period ( “T”) of 0.166666 seconds • Frequency in HERTZ is simply = 1/T = 6 HERTZ !!! , So HERTZ is the number of signals (BAUDS) transmitted in ONE second ! What is a phase ? • Phase is just a delay of a wave (i.e. sinusoidal) respect to a reference wave. • As an Example , we can use then red sinusoid as an indication of a “1” and the blue one as an indication of a “zero” , based on the delay (phase shift). Modulation Techniques • Phase Modulation: • A phase “p1” is present when a “one” is transmitted and a phase “p2” is present when a “zero” is transmitted (see previous slide) • PSK: Phase shift keying is called within digital technologies Transmission Modes • SIMPLEX: The communication goes ALWAYS in one direction. – T.V. Broadcast – Radio Broadcast • HALF DUPLEX: The channel can transmit OR receive but not at the same time – Typical Telephone conversation – Internet “chatting” Transmission Modes • FULL DUPLEX: Information can travel in both directions simultaneously – INTERNET – Most High-speed links between machines – Most equipment in the data telecomm field now a days is full duplex Advanced Modulation techniques • There are more advanced techniques for modulating (adapting a signal to the media) a signal • These advanced techniques use a combination of the previous basic techniques in order to put more information in a signal and optimize medium capacity Bits/sec vs. Bauds/sec • The amount of bits transmitted in a second is the instant speed of data (bits/second) • When we transmit with ASK, FSK, or PSK we can transmit one or more bits per symbol • When a symbol implies many bits is called a baud Symbols (bauds)/second • Capacity is defined by how many SYMBOLS per second a media can transmit under PERFECT conditions • Each symbol may represent different amount of bits – 1 Symbol  2 bits – 1 symbol  4 bits – 1 symbol  8 bits • The important point is we can “pack” many bits in one symbol BAUD/second versus BITS/second • If I agree with you that every time I tell you by phone • “YES” this means = “11111111” and • “NO” means “00000000” THEN – I am transmitting MANY bits (8) with shorter SYMBOLS (YES and NO) – How about is we agree that “y” = “11111111” and “n” = “00000000” ? – In this case I am transmitting the SAME 8 bits with less symbols (Y and N) Bits/sec vs Baud/sec Symbols (bauds) per second vs. Bits/second • One symbol may be transmitted over a line that may represent more than one bit • Example: – 1 volt – 1.5 volts – 2 volts – 2.5 volts – etc = = = = 0000 0001 0010 0011 Decibel Concept • In telecommunications, signal are attenuated by noise and • • other factors. The decibel represents the attenuation of a signal Attenuation = 10 *Log10 (Power received/Power transmitted) As an Example if a Signal was sent with a power of 5 watts and was received with a power of 3 wats then the attenuation is Attenuation = 10* log 10 (3/5) = 10 log = 10* (-0.22) 10 (0.6) = -2.2 Decibels The negative sign means a loss of power Notice : When a Signal does not lose power the result is zero decibels , zero decibels means no Loss of power in telecommunications Attenuation = 10* log = 10* 0 10 (5/5) = 10 log = 0 Decibels 10 (1) Bandwidth Concept • Any media is limited in its capacity to transmit frequencies • BANDWIDTH DEFINITION/CALCULATION: (Minimum Frequency allowed by the media) subtracted from (Max. Frequency allowed by the media ) Or BW = fmax-fmin • Bandwidth is in HERTZ (Hertz is represented as number of variations/second or BAUDS/sec) BW Illustration The following figure illustrates how BW is defined (Hertz) , in the peak the signals are not attenuated (central frequency) however near the borders (- 3 DB line) the signal are attenuated to a point that might become indistinguishable. The peak is ZERO decibels More formal definition of BW • “ Is the difference between frequencies at the extremes of the central frequency , when the signal has been attenuated 3 decibels” • HOWEVER for this course , we will assume that the frequencies outside f1 and f2 will be completely attenuated and the frequencies inside fi and f2 will be passed without attenuation (see previous figures) and BW = f2-f1 A little bit of history of the “bandwidth” concept and current misconception • Originally (in the old modem technologies one symbol (hertz) per second was one bit per second, so it was no NUMERICAL difference between Bandwidth and the number of bits per second transmitted of the 60’s and 70’s) 1 analog symbol sent = 1 bit send (“0” or “1”) so ORIGINALLY (50’s , 60’s and even 70’s) if you say that a BW was 1000 Hertz it meant 1000 bits per second A little bit of history of the “bandwidth” concept and current misconception • However with the improvement of coding/modulation many bits begun to be packed in ONE symbol , then the BW in hertz was NOT the SAME than the number of bits send – THE BW (in HERTZ) can’t change …is an inherent property of the media , every media has a CONSTANT BW . For example Copper has a BW in Hertz t5hta could not change, Coaxial Cable has a BW (in hertz_ that you can’t change • What it can be done is to pack more bits in each symbol so NOW BW (in Hertz) is NOT equal to the number of bits per second we send. CAPACITY DEFINITION • Capacity is a measurement derived from complex formulas (theoretical out of the scope of this course) that translates the particular coding/modulation technique and the BW available in a THEORETICAL value how many bits per second can be transmitted • Example MEDIA BW (fmax-fmin) CAPACITY (from formula) COAX 3,000,000 Hertz 50,000 bits/second COPPER 2200 Hertz 8000 bits/second BW (in Hertz) NOT equal to capacity (bits/second) • BUT many technicians kept calling (70’s 80’s) “digital bandwidth” to the capacity (Bits per second) then in the daily activities they dropped the word “digital” and kept calling BW (erroneously) to CAPACITY • That is why this term is NOT properly used today CAPACITY DEFINITION (con’t) • Therefore, CAPACITY is a theoretical, formula derived, amount of bits per second that a media can theoretically transport based on an SPECIFIC combination of modulation/coding • If we change the modulation/coding technique the CAPACITY (bits/second) might change but NOT the BW (in hertz) • The formula for deriving capacity assumes IDEAL conditions: NO NOISE, PERFECT COMPUTERS, PERFECT SERVERS etc. Bandwidth Concept • Generally ,the more frequency a media can handle the more SYMBOLS per second is possible to transmit over the media. - Therefore if a BAUD (Symbol) represents many bits , the more BW , the more number of bits can be transmitted per second • The higher the BW (HERTZ) of a media the more changes per second is capable to transmit • This implies more bauds (symbols) per second can be transmitted or more bits per second (depends on the transmission technique used) Summary: Bandwidth vs Capacity • Bandwidth: is in frequency Units (HERTZ) • Capacity: is in bits/second. (Theoretical maximum) • Bandwidth NOT EQUAL to Capacity, and terms are used WRONGLY interchangeable today (after this course you will not do that..hopefully ) Transfer rate or speed concept • Speed (transfer rate): In bits/second is a fraction of the total capacity and it is the REAL amount of bits/second we receive in a connection and it is an instantaneous value (can’t be predicted precisely) • Transfer rate (speed) is what we got in real life and it will ALWAYS be less than the Capacity , the transfer rate includes – Noise losses/retransmissions – Loaded servers not ready for the demands of our connection – Your computer limitations etc. Transfer rate (“speed”) • Transfer rate is an instant measurement of the amount of information transmitted in bits/sec , for example you might be paying an INTERNET service of 15 Mbits/sec (theoretical capacity) – However when you download anything from the internet you probably will never be even close to half of what you are paying • “popular culture” confuses all these terms • The ideal setting is to achieve a transfer rate as close to the capacity Bandwidth vs. Capacity • Bandwidth: is in frequency (hertz) Units – (Freqmax-Freqmin): hertz • Capacity: is in bits/second. (Ideal maximum) • Speed or transfer rate (bits/second): Always less than the capacity (the real transmission which can vary every instant upon different conditions) • Bandwidth NOT EQUAL to Capacity • NOTICE that even your textbook does not explain well this (see figure 4.3, page 114) in which is NOT well explained and wrong , Please do NOT use the textbook for this purposes) – Indeed I am a consultant for this book and this are one of the things we will correct in the next edition (see bottom right in page Xiii in the preface for my name) BW vs. Capacity vs. Transfer rate • Why these terms are confused ? – Bandwidth is typically directly proportional to the capacity: The more Bandwidth IN HERTZ the media has, in general, the more Capacity in “bits per second” can be achieved. – It is easier for the common public to confuse all these terms as one. (Although they are very different!) Bandwidth concept Misunderstood The expression: “We offer you a bandwidth of 15 Mbps” It is NOT technically correct but it might be acceptable as far you understand the proportionality” Another reason is that non-technical people seems to understand better the concept of “bits/second” that the concept of “HERTZ”. Ultimately people is more interested in “bits/second” and not in “Hertz” Bandwidth is NOT a Bit accelerator! • The fact that a company offers an specific capacity (for example 1 Gbps) DOES NOT mean that we will reach that speed every time the media is used. • What means is : – You have a “pipe” theoretically capable of transport 1 Gigabps. Good luck! – you will be achieving some partial “transfer rate”. Bandwidth is NOT a bit accelerator! • The transfer rate (“speed”) you can reach over the media depends mainly of: – The capacity of the media – The capacity of the transmitter to “fill” the media (the “pipe”) at its maximum – The capacity of the receiver to “accept” these amount of bits per second – The amount of other users using the same connection (i.e. Satellite or intercontinental optical fiber) Take a closer look at this table and be sure you understand the difference among BW, Capacity and Transfer rate MEDIA BW (HERTZ) Fmax-Fmin CAN’T CHANGE CODING Capacity : /MODULATIO form formula N TECHNIQUE that uses the two previous columns Transfer rate Can’t be guaranteed or predicted , COAX 3,000,000 HZ Technique “A” 30,000,0000 bits/second Variable upon Instant conditions COAX 3,000,000 HZ Technique “B” 40,000,000 bits/second Variable upon Instant conditions COPPER 2200 HZ Technique “C” 5000 bits/second Variable upon Instant conditions COPPER 2200 HZ Technique “D” 7500 bits/second Variable upon Instant conditions FIBER OPTIC 50,000,000 HZ Technique “F” 40*109 bits /second Variable upon Instant conditions FIBER OPTIC 50, 000,000 HZ Technique “G” 60*109 bits /second Variable upon Instant conditions More about Capacity vs Transfer rate (I) • EVERY internet provider Typically sells CAPACITY when • announcing their product .(TV, Internet etc.) or as an example they might say “with speeds up to 15 Gbps” meaning the transfer rate MIGHT reach the capacity of 15 Gbps in theory In other words your internet provider sells the MAXIMUM theoretical amount of BITS/SECOND that can be transferred if ALL is perfect like: – NO NOISE – Capacity o the Internet server you are visiting : Always available and as fast as the capacity of your computer – Capacity of your computer to fill the pipe: As fast as the capacity of your internet provider. – THE provider gives you that capacity ONLY between your computer and your internet provider main equipment, after that your internet provider may share “BW pipes” differently. More about Capacity vs Transfer rate (II) As you might suspect THESE factors, are not always are like that, this makes IMPOSSIBLE to reach the capacity (bits/sec) of what they are selling you . • This is a COMMERCIAL TRICK allowed by the government, basically you will have to connect at 3 a.m.(when NOBODY else is connected) PLUS you have to have NO noise + your computer should be so fast to fill the CAPACITY they are selling to you+ The server you are downloading should serve a very few people (or no one) and be fast enough to fill the capacity promised More about Capacity vs Transfer rate (III) • EVEN if the internet provider mention "Bandwidth " or "speed " they are wrong !!!! Transfer rate (speed) can’t be guaranteed because it depends on the conditions of the line in that very moment. They do this for commercial purposes only • Speed could not be sold or guaranteed because the speed depends of so many factors (NOISE, Speed of your computer, speed of the server you are visiting etc.) . This is easy to test from your home, for example try to download a huge file and see if the file downloads at a constant speed. What you will see that the speed (transfer rate) is changing by the second (constantly) meaning the conditions in the connection are constantly changing , also you will see in your download that you will NEVER reach the capacity they offered you when you contract your internet provider • Bandwidth is in Hertz (although DIGITALBANDWIDTH might be applied, the expression , DIGITAL BANDWIDTH means capacity in Bits/sec ) Analogy…When you buy a new car …. • You buy MANY things in the market the same way , for example when you buy a new car, the dealer tells you the IDEAL CONSUMPTION (miles/per gallon)…meaning consumption in the car LABORATORY testing facility. • ONCE you buy a car , you have to drive the car in traffic, with loopholes, you might not be a good driver etc. etc. making your car almost always make LESS consumption (miles per gallon) than the ideal label of the car , but they sell to you the ideal consumption (if everything is perfect) • Capacity (bits/second) is the same (is the IDEAL label for your connection) however REALITY makes your connection less than the ideal they have sold to you.(Transfer rate in bits/second !!) Capacity Concept Analogy Bandwidth Concept Analogy • We can think of a “Capacity” as a pipe with an hydraulic capacity of “X” liters/sec. • If you input “Y” liter/sec you will have “Y” liter/sec as an output. (YX) liters/sec you will likely have problems! Bandwidth “pipe” Transmission methods • Asynchronous serial data transmission: Each character is transmitted individually and consists of four parts: – One or more start bits – The data bits – A parity bit – One or more stop bits Asynchronous transmission Data are transmitted one character at a time, where each character is five to eight bits in length (utile data). See ASCII code… Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character Idle state Start bit 5 to 8 data bits (+ 1 parity bit – eventually) 1-2 Stop bits Idle or next Start bit Asynchronous Transmission The receiving interface must know how many bits there are to a character in order to determine where the current character ends and when to start looking for the next start bit. Asynchronous transmission • The best way to think “asynchronous” is to understand that there is NO TIME pattern BETWEEN each transmitted character • In other words you can send a character the next second and the second character in 10 seconds and the third character in 3 seconds and so on • The receiver will be “patiently” collecting the information until the whole word is transmitted Asynchronous transmission • It does not matter if you send characters not synchronized with an specific time, the “start” bit, the “stop” bit will always indicate the receiver when to start to identify the information • Asynchronous therefore is associated with unpredictability in time between information. Asynchronous transmission • Disadvantages: Because the unpredictability, the need of extra bits between small pieces of information give the method a typical 70% of efficiency – Example 1: 1 stop bit, 1 start bit, 1 parity bit, 7 bits of information. (7/10) – Example 2: 1 start bit, 2 stop bits,1 parity bit, 8 bits of information (8/12) Advantages of Async Transmission • Pretty simple • Async: Used when devices are close or for short messages. – A Router that is being programmed in a room – A Switch – Our Routers in 4411 will be accessed initially using async lines through the “console port” – A printer connected to a home computer Synchronous Transmission • A serial bit stream is sent over the line without start or stop bits to synchronize the bits in each character • The characters follow one another immediately rather than coming at random intervals as in asynchronous transmission. Synchronous transmission ♦ Works with blocks of bits (characters) ♦ Inter-clock synchronization:  auxiliary clock line  synchronization at the block level => extra flag and control fields => data structure of frame ♦ Flag fields (synchronization) fields: special bit sequences or sync characters; denoted as preamble-header and trailer Flag Control Field field Data field Control Flag field field Synchronous transmission • The entire block of data is synchronized with a unique code which, when recognized, tells the receiver where a character begins • As in the asynchronous technique, the receiver must know the number of bits of a character Synchronous transmission • A clock in each side must be synchronized in order to understand when each bit starts and stops • The synchronization is done with an initial bit stream like “110011001010101010” • The efficiency in this case reach typical values up to 95% to 98 %. Synchronous transmission • Example: “11001100101010101010” to synchronize and to indicate the end of transmission of a “chunk” of 10,000 bits of information “00110011010101010101” • Efficiency of 10,000/10,040 which is approximately 99% • Routers use Synchronous transmission in their serial ports Advantages/Disadvantages • IS pretty clear that Synchronous transmission is more efficient – Is the most used method now a days • Disadvantages: – Difficult to achieve in noisy environments – Expensive equipment – Expensive software routines – Synchronization may be a serious problem Transmission Modes • SIMPLEX: The communication goes ALWAYS in one direction. – T.V. Broadcast – Radio Broadcast • HALF DUPLEX: The channel can transmit OR receive but not at the same time – Telephone – Internet “chatting” Transmission Modes • FULL DUPLEX: Information can travel in both directions simultaneously – INTERNET – Most High-speed links between machines are full duplex Multiplexing • Multiplexing means the use of one facility to handle several separate but similar operation simultaneously • In Telecommunications it means the use of one telecommunication link to handle several channels of voice, data or video. Multiplexing • To multiplex means to divide the link into “slots”, with each division containing information from a separate source • The slots can be divided in time (TDM) or frequency (FDM). Time Division Multiplexing (TDM) • TDM can be compared to a switch that rapidly samples a number of lines • These samples are sent across the data link, then routed back to their original sequence by another switch at the receiver • “Baseband” technologies use TDM as a main technique to transmit data, voice and video TDM multiplexer Frequency Division Multiplexing (FDM) • In FDM Each transmission is assigned its own individual frequency, allowing simultaneous transmission of many data streams over a single line • The receiver contains demodulators, each looking for a particular frequency • Cable companies use FDM to transmit the video channels and offer other services as INTERNET and telephone (BROADBAND) FDM Multiplexer FDM multiplexer Analog and Digital signals Why digital signals are so popular now ? • It is generally less expensive to make digital equipment • Digital signals are less vulnerable to errors caused by interference (noise) • We know that there are only TWO choices A “1” or a “0” (analog signals have infinite number of choices !) Advantages of Analog signals • Analog signals can be easily multiplexed • Analog signals are less vulnerable to the attenuation problem due to distance can travel farther without becoming too weak for reliable transmission) (they Baseband Vs. Broadband • Baseband transmission means that the signal to be transmitted has ALL the BW of the media (only transmit one signal at a time in any direction over the cable!) • In general the base band signal is not modulated (modified) in certain protocols like the Ethernet protocol • Baseband is used mostly to transmit digital signals and is used by most computer networks • So baseband can transmit only ONE signal at a time Broadband • Broadband divides the capacity of a link in two or more channels, each of which can carry a different signal • All channels can send simultaneously • Satellite, DSL, Wireless technologies, cable TV etc are examples of applications using broadband media How good is a channel? • Performance: What is the maximum reliable transmission speed? – Speed: Bit rate, R bps – Reliability: Bit error rate, BER=10-k – Where “k” is an indication of how many times one bit is in error per amount of bits transmitted • i.e. 1 errored Bit every 1000000  10-6 • Cost: What is the cost of alternatives at a given level of performance? – Wired vs. wireless? – Electronic vs. optical? – Standard A vs. standard B?
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