Transcript DCN-5-Data_Transmission

Data Transmission
• Data Transmission:
– Question:
• How does data (such as voice, speech etc.) travel from
source to destination,
– Over a long distance?
– Answer:
• Data (such as voice, speech etc.) is put over,
– An electronic signal called,
» Carrier signal which can,
– Travel and take the data over a long distance from,
– Source to destination.
• This process of putting/coding the data on the carrier signal
is called:
– Modulation/Encoding
» Modulation means ‘to change/vary something’.
Data Transmission
• Modulation:
– Types of Data:
• Analog Data:
– Voice, Speech etc.
• Digital Data:
– Computer data.
– Types of Signals:
• Analog Signal:
– Wave like signal having properties such as:
» Amplitude, Wavelength, Frequency, Oscillations etc.
• Digital Signal:
– Step like signal having properties such as:
» Bit rate, Baud rate etc.
Data Transmission
•
Categories of Modulation:
– ‘Analog Signal’ used to transmit ‘Analog Data’.
• Put Analog data on Analog signal.
• Example:
– Telephone conversation.
– Radio broadcast stations.
– ‘Analog Signal’ used to transmit ‘Digital Data’.
• Put Digital data on Analog signal.
• Example:
– Computer communication over Telephone lines using Modems.
– ‘Digital Signal’ used to transmit ‘Analog Data’.
• Put Analog data on Digital signal.
• Example:
– Voice communication over computer network.
– Voice communication through Fiber-optic cable.
– ‘Digital Signal’ used to transmit ‘Digital Data’.
• Put Digital data on Digital signal.
• Example:
– Computer network communication.
Analog Signal to transmit Analog Data
Amplitude Modulation (AM)
Analog Data
(Voice)
Analog
Carrier Signal
Output
(Modulated Signal)
Analog Signal to transmit Analog Data
Amplitude Modulation (AM)
Analog Data
(Voice)
Analog
Carrier Signal
Output
(Modulated Signal)
Analog Signal to transmit Analog Data
Frequency Modulation (FM)
Analog Data
(Voice)
Analog
Carrier Signal
Output
(Modulated Signal)
Analog Signal to transmit Analog Data
• Amplitude Modulation (AM):
– Refers to a change in the,
• Height of the carrier signal.
– When data is put/encoded on the carrier signal,
• The height/amplitude of the signal is changed to indicate
appropriate data.
• Example:
– High peaks in the wave might produce louder sounds (or quieter
sounds), while,
– Low peaks on the wave might produce a quieter sound (or louder
sound).
– Used in:
• Radio transmission for AM radio stations.
– Disadvantage:
• Highly susceptible (prone/vulnerable) to noise and interference from
external source such as ‘Thunderstorm’.
Amplitude Modulation (AM)
Analog Signal to transmit Analog Data
• Frequency Modulation (FM):
– Refers to a change in the,
• Number of waves of the carrier signal.
– When data is put/encoded on the carrier signal,
• The frequency of the signal is changed to indicate appropriate data.
• Example:
– More waves per unit of time might indicate a higher tone (or lower tone)
and,
– Fewer waves per unit of time might indicate a lower tone (or higher
tone).
– Used in:
• FM radio stations.
– Advantage:
• More resistant to external interference than Amplitude Modulation
(AM).
– Unlike an AM radio signal, an FM radio signal would not be interrupted
by a passing thunderstorm.
Frequency Modulation (FM)
Analog Signal to transmit Digital Data
Amplitude Modulation (AM) / Amplitude Shift Keying (ASK)
Digital Data
Analog
Carrier Signal
Output
(Modulated Signal)
0
1
1
0
0
0
1
0
1
1
1
Analog Signal to transmit Digital Data
Frequency Modulation (FM) / Frequency Shift Keying (FSK)
Digital Data
Analog
Carrier Signal
Output
(Modulated Signal)
0
1
1
0
0
0
1
0
1
1
1
Analog Signal to transmit Digital Data
Phase Modulation (PM) / Phase Shift Keying (PSK)
Digital Data
Analog
Carrier Signal
Output
(Modulated Signal)
0
1
1
0
0
0
1
0
1
1
1
Analog Signal to transmit Digital Data
Differential Phase Shift Keying (DPSK)
Digital Data
Analog
Carrier Signal
Output
(Modulated Signal)
0
1
1
0
0
0
1
0
1
1
1
Analog Signal to transmit Digital Data
Summary
AM / ASK
PM / PSK
FM / FSK
DPSK
Analog Signal to transmit Digital Data
• Amplitude Modulation (AM) / Amplitude
Shift Keying (ASK) :
– Refers to a change in the,
• Height/Amplitude of waves of the carrier signal.
– When data is put/encoded on the carrier
signal,
• The height of the signal is changed to indicate
appropriate data.
• Example:
– Highest peak might represent a ‘1’ bit and,
– Lowest peak might represent a ‘0’ bit.
Analog Signal to transmit Digital Data
• Frequency Modulation (FM) / Frequency Shift
Keying (FSK) :
– Refers to a change in the,
• Number of waves of the carrier signal.
– When data is put/encoded on the carrier signal,
• The frequency of the signal is changed to indicate
appropriate data.
• Example:
– ‘1’ might be represented by more waves per unit time and,
– ‘0’ might be represented by less waves per unit time.
Analog Signal to transmit Digital Data
• Phase Modulation (PM) / Phase Shift Keying
(PSK) :
– Refers to a change in the,
• Phase/Direction of the wave of the carrier signal.
– When data is put/encoded on the carrier signal,
• The phase/direction of the signal is changed to indicate
appropriate data.
– Neither the amplitude nor the frequency are changed.
• Example:
– To indicate a ‘1’ bit, wave moves in a specific direction and,
– To indicate a ‘0’ bit, the direction of the wave changes to the
opposite direction.
Analog Signal to transmit Digital Data
• Differential Phase Shift Keying (DPSK) :
– Similar to Phase Shift Keying (PSK) except for
the fact that,
• Signal changes direction only when a ‘1’ bit is
detected and,
• ‘0’ bit causes no change in the wave.
Digital Signal to transmit Analog Data
Analog Data (Speech)
Digital Carrier Signal
Conclusion: Difficult to directly represent Analog Data using Digital Signal.
Analog data can have more values than what Digital can represent.
Digital Signal to transmit Analog Data
Pulse Code Modulation (PCM)
0 0 0 0 0 0 1 0…
Analog Data
(Speech)
Output
(Modulated Digital
Signal)
Sampling
Sampled
Data
Modulation/Encoding
10
5
Quantization
2
Quantized
Data
2 5 10 5 3 -2 …
Binary
Encoding
00000010 00000101
00001010 00000101
00000011 10000010
……
Binary Data
Digital Signal to transmit Analog Data
• Pulse Code Modulation (PCM):
– Steps:
• Sampling:
– Continuous analog signal is converted to,
» Non-continuous (discrete) signal by,
» Taking samples (values) at small time intervals.
– To reproduce/rebuild the exact analog signal, number of samples taken should be:
» Twice the maximum frequency. (Nyquist Theorem)
– Example:
» For a telephone call of 4,000 Hz frequency, sampling should occur at:
» 4000 x 2 = 8000 samples per second.
• Quantization:
– Integer values (Digits / Quantities) are assigned to the samples depending on the
amplitude of the signal at the instant that it is sampled.
• Binary Encoding / Digital Encoding:
– Values are converted to binary equivalents.
• Modulation:
– Binary digits are represented using Digital Signal.
Digital Signal to transmit Analog Data
Some other techniques used in the past.
Analog Data
Pulse Amplitude Modulation (PAM)
Pulse Width Modulation (PWM)
Pulse Duration Modulation (PDM)
Pulse Position Modulation (PPM)
Common
Do not produce a ‘Pure Digital Signal’
disadvantage: (having only 2 unique states).
Digital Signal to transmit Digital Data
Unipolar Coding
Digital Data
Digital Carrier Signal
Output
(Modulated Signal)
0 1 0 1 0 0 1 1 0 1 1 1 0
Digital Signal to transmit Digital Data
Polar Coding (NRZ Coding)
Digital Data
Digital Carrier Signal
Output
(Modulated Signal)
1 0 0 1 0 1 0 0 0
Digital Signal to transmit Digital Data
Bipolar Coding
Digital Data
Digital Carrier Signal
Output
(Modulated Signal)
0 0 1 1 0 1 0
Digital Signal to transmit Digital Data
• Unipolar Encoding:
– Polar refers to,
• The polarity of a pulse (signal) on the
communications line indicating,
– Whether it is positive or negative.
– So unipolar encoding uses,
• Only one voltage level indicating that,
– A binary ‘1’ is present and,
• Absence of voltage pulse indicates,
– A binary ‘0’ bit.
Digital Signal to transmit Digital Data
• Polar Encoding:
– Uses two voltage levels where,
• Positive voltage might represent a ‘1’ bit and,
• Negative voltage might represent a ‘0’ bit.
– Because the voltage never goes to 0 value, it
is also called:
• NRZ Encoding.
– Non-return to zero
Digital Signal to transmit Digital Data
• Bipolar Encoding:
– Uses three voltage levels where,
• Binary ‘0’ is indicated by zero voltage and,
• Binary ‘1’ is indicated by alternative (changing)
positive and negative voltages.
Modulation Summary
•
‘Analog Signal’ used to transmit ‘Analog Data’.
– Amplitude Modulation (AM)
– Frequency Modulation (FM)
•
‘Analog Signal’ used to transmit ‘Digital Data’.
– Amplitude Modulation (AM) / Amplitude-shift keying (ASK)
– Frequency Modulation (FM) / Frequency-shift keying (FSK)
– Phase Modulation (PM) / Phase-shift keying (PSK)
• Differential Phase-shift keying (DPSK)
•
‘Digital Signal’ used to transmit ‘Analog Data’.
–
–
–
–
•
Pulse Code Modulation (PCM)
Pulse Amplitude Modulation (PAM)
Pulse Width Modulation (PWM) / Pulse Duration Modulation (PDM)
Pulse Position Modulation (PPM)
‘Digital Signal’ used to transmit ‘Digital Data’.
– Unipolar encoding
– Polar encoding (NRZ)
– Bipolar encoding
Data Transmission
• Transmission Direction:
– Path/Direction in which data travels or flows
on the network.
– Types of data flow:
• Simplex.
• Duplex.
– Half-Duplex
– Full-Duplex
Transmission Flow/Direction
Simplex Transmission
Half-Duplex Transmission
Full-Duplex Transmission
Transmission Flow/Direction
• Simplex Transmission:
– Occurs in only one direction and so,
• Person/Entity behaving as a source always behaves as a source
and,
• Person/Entity behaving as a destination always behaves as a
destination and hence,
• There is no role reversal.
– Examples:
• Flow of water from a Tap into the Bucket / Rain.
– Tap always acts as sender and Bucket always acts as receiver.
• Communication from Keyboard to CPU.
– Keyboard always behaves as a sender and CPU as a receiver.
• Communication from CPU to Monitor.
– CPU acts as sender and Monitor acts as receiver.
• Radio broadcast from a radio station.
Simplex Transmission
Transmission Flow/Direction
• Half-Duplex Transmission:
– Data can go in either direction, but,
• Only one way at a time.
– As a result,
• Sender can behave as a receiver and,
• Receiver can behave as a sender but,
– Not at the same time.
– Example:
• Very narrow bridge over a river.
• Walkie-Talkie / Push-To-Talk systems.
– Both users can talk on the radio, but only one at a time.
• Computer communication using Modems.
– Delay that occurs when the transmission switches from one direction to the other
is called,
» Modem turnaround time.
– In half-duplex transmission, the switch/changeover between the
transmissions is so fast that,
• It is transparent (not noticeable) to the user.
Half-Duplex Transmission
800MHz
800MHz
Transmission Flow/Direction
• Full-Duplex Transmission:
– Data can travel in both directions on the communications line,
• Simultaneously / At the same time.
– In order to accomplish this type of transmission,
• Either communications line must have 2 separate transmission paths or,
– Physical division.
• The entire transmission capacity of the line must be divided between 2
signals that are traveling in opposite directions.
– Logical division using the concepts of FDM, TDM etc.
– Examples:
• 2-lane highway.
• Cell-phone transmission.
– Disadvantage:
• Echo can sometimes be heard in full-duplex transmission and is more
common in,
– Long-distance calls.
– To suppress echo, Telephone companies install devices called:
» Echo suppressors.
Full-Duplex Transmission
800MHz
900MHz
Transmission Flow/Direction
• Transmission Flow/Direction:
– Common example of Simplex, Half-Duplex and FullDuplex communication is:
• Lecture.
– Simplex
» Only from lecturer to student.
» If the lecturer is very strict.
– Half-Duplex
» From lecturer to student and vice-versa but not at the
same time.
» If the lecturer wants discipline but not at the cost of liberty.
– Full-Duplex
» From lecturer to student and vice-versa at the same time.
» If the lecturer does not believe in anything called discipline.
Data Transmission
• Transmission Modes:
– Way/Manner in which the network sends the
data.
– Types:
• Serial Transmission
• Parallel Transmission
Transmission Modes
A
01000001
A
LSB
MSB
1 0 0 0 0 0 1 0
01000001
Serial Transmission
Serial Port / Interface
Sender
Parallel Port / Interface
Receiver
Parallel Transmission
B
01000010
0 MSB
1
0
0
0
0
1
0 LSB
B
01000010
Serial Interface/Port
USB Interface/Port
Parallel Interface/Port
Transmission Modes
• Serial Transmission:
– Transmits data as individual bits,
• With a single bit following another on the transmission line and
hence,
• Uses a single communications line to transmit the entire data in the
form of,
– One bit at a time.
– At the receiver,
• Bits are reassembled to get the data back.
– Port/Interface that allows Serial Transmission:
• Serial Port
• USB (Universal Serial Bus)
– Use:
• Protocols such as:
– SLIP: Serial Line Internet Protocol
– PPP: Point-to-Point Protocol
Transmission Modes
• Parallel Transmission:
– Transmits an entire word/character/byte (composed of
several bits) at one time and hence,
• Uses multiple lines to send the number of bits required to
transmit the entire word at one time.
– Ports/Interfaces that allow Parallel Transmission:
• Parallel Port
– Use:
• Different parts inside a computer normally communicate
using parallel transmission.
– Disadvantage:
• More expensive for transmission of data over long distances.
Parallel Transmission
Intel 8085 Processor (8-bit)
8 pins
Data Transmission
• Synchronization:
– Refers to,
• The status of sender and receiver during
transmission.
– Types:
• Asynchronous Transmission
• Synchronous Transmission
Data Transmission
• Asynchronous Transmission:
– Sender and Receiver are not in sync during
transmission of data.
– Sender sends the data without checking the status of
the Receiver as to,
• Whether the Receiver is online or not.
– Also called:
• Different-time, Different-place mode.
– Examples:
•
•
•
•
Letter
Email
SMS
Discussion Boards
Data Transmission
• Synchronous Transmission:
– Sender and Receiver are in sync during transmission
of data.
– Sender sends the data only after checking the status
of the Receiver when,
• The Receiver is online.
– Also called:
• Same-time, Different-place mode.
– Examples:
• Phone call
• Online IM Chat
Data Transmission
Error
A
1 1
0 0 0 0 0 1
C
1000001
1000011
Sender
Receiver
Data Transmission
• Error:
– Process that,
• Changes the data that was originally sent so that,
• Receiver does not get the same data bits that were
in the original message.
Errors
• Sources of Errors:
– Impulse Noise:
• Caused by external electrical interference such as,
– Lightning or Thunderstorm.
• When a bolt of lightning strikes near a copper wire,
– A large amount of electricity travels through the wire which changes,
– The characteristics of the electrical signal traveling on that wire.
– White Noise / Thermal Noise:
• Every medium is composed of atoms which contain ‘Electrons’.
• Movement of electrons in the wire causes a hissing sound / noise
and,
– The amount of noise is directly proportional to the temperature of the
medium.
• If the medium is very hot,
– The electrons move faster and the amount of noise increases.
• Some amount of this type of noise is always present in every
medium.
Errors
• Sources of Errors:
– Attenuation:
• As a signal travels along a transmission line,
– It loses some of its strength.
• It leads to errors because,
– A weak signal is more vulnerable to errors than a strong signal.
– Crosstalk:
• Occurs when the signals from,
– Two transmission lines interfere with one another.
• Normally caused by,
– Broken shielding on a cable which can result in,
– A copper wire coming too close to another copper wire and cause
interference.
• Example:
– Crosstalk during a telephone conversation.
Errors
• Errors:
– Steps to solve the errors:
• Error Prevention:
– Take precautions so that errors do not come in.
• Error Detection:
– If errors do come in, identify/detect the errors.
• Error Correction:
– Try to generate the original data from erroneous data.
Errors
• Error Prevention:
– Should be the ultimate goal for any kind of
data transmission.
– Although eliminating/removing noise
completely from the communications line is
nearly impossible,
• Steps must be taken to reduce the amount of noise
and the resulting interference from the line.
Errors
• Error Prevention:
– Steps:
• To reduce ‘Impulse Noise’,
– Improve shielding around the cables.
• To reduce ‘White Noise’,
– Make sure that temperature around the cables does not rise beyond a
certain level.
• To reduce ‘Attenuation’,
– Place special devices such as:
» Amplifiers (for Analog transmission) or
» Repeaters (for Digital transmission) at,
– Specific intervals to boost the strength of the signal.
• To reduce ‘Crosstalk’,
– Use techniques such as,
» ‘Twisting’ of the cables to minimize Electromagnetic Interference.
Errors
• Error Detection:
– Even after taking precautionary/prevention
steps,
• Some or the other errors might creep (come) in
because,
• So many sources of errors exist.
– Any data transmission that contains errors
must be,
• Corrected before it can be used in processing but,
• Error can only be corrected if,
– It is detected/identified at the first place.
Error Detection
Sender
Receiver
D
1000100
1000110
F
E
1000101
1000111
G
F
1000110
1000110
F
G
1000111
1000111
G
Question: How can a receiver identify/detect (come to know) that
there is some error?
Answer: Both Sender and Receiver have to follow a common
‘Rule’ / ‘Protocol’ for data transmission.
Error Detection
Sender
Parity Checking / VRC
(Even Parity)
Rule: Even number of 1’s.
Parity Bit
Odd number of 1’s.
Some Error
Receiver
D
1000100
0
1000110
0
F
E
1000101
1
1000111
1
G
F
1000110
1
1000110
1
F
G
1000111
0
1000111
0
G
1000111
0
G
Even number of 1’s.
Correct
Problem?
D
1000100
0
50% effective.
Even number of 1’s.
Correct / No Error.
Error Detection
• Parity Checking:
– One of the earliest forms of error detection.
– Also called:
• VRC: Vertical Redundancy Check.
– Used to determine if the bits have been changed on the transmission
line.
– Used with 7-bit codes or characters by adding an extra bit called,
• A parity bit to provide the parity checking.
– 2 types:
• Even Parity:
– Uses an even number of 1 bits as the measure of valid data.
• Odd Parity:
– Uses an odd number of 1 bits as the measure of valid data.
– Disadvantage:
• Only 50% effective because,
– It can detect changes only to odd number of bits.
Errors
• Error Detection:
– Advanced form of error detection is called:
• CRC (Cyclic Redundancy Check)
CRC Error
Errors
• Error Correction:
– When an error is detected on a network,
• It must be corrected before doing any further
processing on the data.
– 2 ways:
• Forward Error Correction:
– Corrects errors without retransmission of the data.
• Error Correction with Retransmission:
– Sender must retransmit the data if the receiver detects
an error.
References
Digital Signal to transmit Analog Data
ASK Example
http://www.asaecenter.org/Resources/articledetail.cfm?itemnumber=13572