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Introduction_TO_TELECOMMUNICATIONS.ppt
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TEC5313
Advanced Data Telecommunication
Eastern Illinois University

Introduction to Telecommunications

By: Dr. Rigoberto Chinchilla

(Updated JAN 2018)

Introduction

  • The basic telecommunication LINK structure Typically 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

Signal Bandwidth

  • In order to
  • 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 in frequency
  • 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

Transmitted Signal

Received Signal

Receiver

Communication channel

Transmitter

*

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?

MODEMS

In order to transmit information over a medium for long distances, it is typically necessary to change/adapt the frequency of the signal.

For short distances (i.e. Computer to printer) sometimes the above is not necessary as far as the coding (how ones and zeros are represented electrically or optically) is consistent on both sides.

  • Why?

Why Do we need to Modify the signal?

  • Voice frequency signals (even if they are transformed to electrical/optical pulses) ARE NOT compatible with the frequencies the telecommunication media (Copper, coax, fiber, Open space) can transport.

  • 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, therefore we need frequencies that can travel over long distances.

  • Therefore, we need to ADAPT the signal to the media, the ADAPTERS are called MODEMS.

MODEM

  • MODULATION: Is the process that changes the shape and/or the frequency of the signal in the transmitter.

The signal MUST be changed according to a specific pattern (frequency adaptation) that should be able to

Travel over the media

Be understood 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 so the transmission can go in both directions

  • The MODEMS at each side traditionally are named as part of the TRANSMITTER and RECEIVER

Meaning…Sometimes you don’t see them depicted in the diagrams

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 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 typically amplifiers and equalizers.)

  • The power of the signal has to be high enough to be recognized at the receiver.

3.*

Attenuation

  • A loss of energy through a medium due to the resistance of the medium.
  • Amplifiers are used to amplify the signal to compensate the loss.

*

3.*

Decibel

  • To measure the relative strengths

Of two signals, or

One signal at two different points

  • P1 and P2 are the powers of a signal at point 1 and 2, respectively.
  • Note:

The decibel is

negative if a signal is attenuated and

Positive if a signal is amplified

Example

  • Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 is (1/2)P1. In this case, the attenuation (loss of power) can be calculated as
  • A loss of 3 dB (–3 dB) is equivalent to losing one-half the power.

3.*

*

Decibel Concept

  • In telecommunications, signals 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 (0.6)

= 10* (-0.22) = -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 (5/5) = 10 log 10 (1)

= 10* 0 = 0 Decibels

Example

  • A signal travels through an amplifier, and its power is increased 10 times. This means that P2 = 10P1 . In this case, the amplification (gain of power) can be calculated as

3.*

*

  • One reason that engineers use the decibel to measure the changes in the strength of a signal is that decibel numbers can be added (or subtracted) when we are measuring several points (cascading) instead of just two. In Figure 3.27 a signal travels from point 1 to point 4. In this case, the decibel value can be calculated as

3.*

*

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 (Amplifiers, equalizers and modern coding techniques) can ONLY diminish the noise. However, the noise can be reduced to a level that it is difficult to detect.

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 device used to do that

  • The transmitter modem shapes the original form of 1’s and 0’s . (Although the bulky boxes hanging lines between poles (equalizers/amplifiers) reshape the distorted /diminished original signals.

  • Note that equalization (fixing the shape) perform a different task that amplification (increasing the power)

Bit Error Rate (BER)

  • Is the measurement of how many bits are not recognized as they were sent.

Meaning a zero was recognized as a one and vice versa

  • The main sources of error are Noise and Distortion (always present !)

  • BER =

[(Number bits received with error) / (Number of bits transmitted)] *100

The result will be a percentage

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 (The ideal condition)

Modulation Techniques

  • Amplitude Modulation: (AM) : Can be done is several ways as far as it is consistent on both sides

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

Modern modulators

  • Modern modulators (AM, FM, PHASE etc.) make modulation and encoding a unified process

However we should not forget the fact that encoding is to decide how to represent a one and a zero and modulation is the process of adapting the signal frequency to the media.

Encoding

Modulation Techniques

  • Amplitude Modulation: (AM)

AM refers to the fact that we will differentiate a “1” from a “0” exclusively on the signal amplitude (level) only

For example a “1” = +5 volts and a “0” = +3 Volts

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) wemight 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 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 or the telecom company might provide both upon your contract with them.

  • 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!

*

Wavelength

Distance occupied by one cycle

Distance between two points of corresponding phase in two consecutive cycles

Assuming signal velocity v

 = vT

f = v

c = 3*108 ms-1 (speed of light in free space)

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!!!

*

Wavelength

Distance occupied by one cycle

Distance between two points of corresponding phase in two consecutive cycles

Assuming signal velocity v

 = vT

f = v

c = 3*108 ms-1 (speed of light in free space)

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 (they can travel farther without becoming too weak for reliable transmission)

Do not confuse the Bandwidth(BW) of the signal with the Bandwidth of the media

  • Every SIGNAL to be transmitted over the media has a BW (range of frequency of the SIGNAL)
  • Every Media has a BW or Range of frequencies that can pass (or transported over the media)
  • The BW of the MEDIA should be COMPATIBLE with the BW of the SIGNAL otherwise the signal will NOT pass over the media (it will be severely attenuated)

Or at LEAST MOST of the MAIN frequency components (The components who allow to identify the shape of the signal properly) of the SIGNAL should be able to pass over the BW of the media

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 or SYMBOLS (BAUDS) transmitted in ONE second !
  • Can you see why the bottom (pink) signal has a frequency of 15 hertz and a period of T = 0.067 seconds (approx.)

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 the 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

Modern Modulation Techniques

  • The above three modulation techniques are the BASIS for modern modulation techniques.
  • Now a days a combination of Amplitude, Phase and frequency is used when modulating a signal

The objective is to pack more ones and zeros in every BAUD. (symbol transmitted)

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

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

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

  • A BAUD (symbol) CAN pack many bits on it !!!

Symbols (bauds)/second

  • CAPACITY: is defined by how many bits 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 or BAUD

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 = 0000

1.5 volts = 0001

2 volts = 0010

2.5 volts = 0011

etc

Media 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 as we will visualize it in this course

  • In this course we will assume that the BW of a media can be visualized as a RECTANGLE limited by a minimum frequency and a maximum frequency.

  • In other words we will visualize BW (for exam and Homework purposes) as a SQUARE (blue)

frequencies INSIDE the square will NOT be attenuated at all

Frequenc8es OUTSIDE the square will be completely attenuated

BW …REAL picture

  • In reality the BW of a media is not a “perfect Square” is more like bell shaped and it attenuates some frequencies more than others.
  • The frequencies close to the center of the shape are lightly attenuated and the frequencies close to the borders of the shape are greatly attenuated.
  • BW is defined (in reality) as the signals that can pass over the media with less than 3 DB of attenuation (-3DB) or signal frequency components that have not loss 50% of their power.
  • HOWEVER , for THIS course purposes we will imagine the BW of the MEDIA as a perfect SQUARE FILTER , that attenuates completely the frequencies of the signal when the frequencies are outside the box and let pass the signals when its frequencies are inside the box defined by fmax and fmin

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 f1 and f2 will be passed without attenuation (see previous figures) and BW = f2-f1 for a perfect square filter.


A little bit of history of the “bandwidth” concept and current misconception

  • Originally (in the old modem technologies of the 60’s and 70’s) 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

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 that 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 saying “Bandwidth” (erroneously) instead of CAPACITY

That is why this term is NOT properly used today by the general public.

BW (in Hertz) NOT equal to capacity (bits/second)

  • HOWEVER, if you are well trained in telecommunications, every time you mention “BANDWIDTH” you should think and speak!!! In terms of frequencies and every time you mention “bits/second” you should think in terms of CAPACITY.

CAPACITY DEFINITION (con’t)

  • Therefore, CAPACITY is a theoretical VALUE, formula derived and is amount of bits per second that a media can theoretically transport based on an SPECIFIC modulation/coding technique.
  • 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 )
  • CAPACITY (in Bits /second) is directly proportional to BW (in HERTZ) , meaning the MORE BW (in Hertz) the MORE CAPACITY a media possess for a PARTICULAR coding/modulation technique.

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 is less than the capacity because

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 sometimes does not uses this concept properly.

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”

How companies advertise the above concepts?

  • Typically a company will NEVER mention Hertz when selling a service (for Obvious reasons)
  • Internet providers typically use the Phrase

“Speeds up to 10 Megabits/second” (or 100 Megabits per second etc.)

  • What this means is: “Your transfer rate will vary from nothing to a maximum of 10 Megabits/second” (Meaning “The Capacity is 10 Megabits/second if you are lucky” or “Under perfect conditions (never!) you will reach a transfer rate equal to the capacity”

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 necessarily 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)

Noise conditions (or quality of the connection)

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 /MODULATION TECHNIQUE Capacity : form formula 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 perfect, 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

  • THEN ….you might reach the capacity advertised !

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.
  • 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

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…including the driver!)

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. (Y<X)

  • If you try to put “Z” (Z>X) liters/sec you will likely have problems!

Bandwidth “pipe”

SIGNAL Bandwidth

  • Each SIGNAL also has its own BW, however the BW of the SIGNAL is not necessarily the (and generally is not !) similar to the BW of the Media.
  • Theoretically the Signal BW is infinite, although is typically defined by a subtraction of frequencies as the BW of the media

Meaning the higher and lower components are not quite essential to distinguish the signal

  • The Key is to match the SIGNAL(S) BW with the MEDIA BW we will put the signals through

Modem helps with that !!!

  • In this course, unless specified when we mention “bandwidth” we will be referring to the MEDIA BW

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 1-2 Stop bits Idle or next

(+ 1 parity bit – eventually) Start bit

*

Synchronous

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

What is parity?

  • It is a SIMPLE or SIMPLISTIC way to track errors in a transmission of small amounts of bits.
  • Parity can be “ODD” or “EVEN” which might mean the quantity of ones(“1’s) transmitted.

I.E. If we transmit 5 ones and 2 zeros of information, then the parity bit will indicate let’s say “ODD” number of ones in the transmission side. Meaning the receiver must receive and odd number of ones.

However , if TWO “1’s” are in error (and are interpreted as zeros) no error will be caught ! That is why is simplistic! However, it works fine most of the time because it is done with very short messages and at very short distances (i.e. Computer to printer)

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 Data field Control Flag

Field field 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 “00110011010101010101” to indicate the end of transmission of a “chunk” of 10,000 bits of information

  • 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

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

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

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