Chap 3: Physical Aspects of Data Communication: Data Transmission

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Chap 3 Physical Aspects of Data Communication
Data Transmission

• MIS 3523 Business Data Communications
• Dr. Segall
• Spring 2001

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Data Flow

• 2 Levels of Data Flow
• 1.) Contention control (discussed in later
  chapters!)
• which determines which stations may transmit
• conditions under which transmission of data is
  allowed
• pacing of data transmission
• 2.) Basic level which relates to transmission
  equipment used
• lines
• modems
• devices

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Data Flow

• 3 basic types
• 1. SIMPLEX TRANSMISSION
• data may flow in one direction only.
• One station is sender.
• One station is receiver.
• Examples Radio, TV, devices such as optical
  character recognition (OCR).
• 2. HALF-DUPLEX TRANSMISSION
• data may flow in both directions, but ONLY in one
  direction at a time.
• Example CB Radio, common method of flow control
  in LANs.

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Data Flow

• 3. FULL-DUPLEX TRANSMISSION
• data can be transmitted in both directions
  SIMULTANEOUSLY.
• Example Switched telephone connections.

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Data Codes

• Data in digital computers is stored as a
  combination of binary digits (bits) each with a
  value of 0 or 1.
• Common Data Codes
• 1. ASCII (American Standard Code for Information
  Interchange)
• 7-bit code
• See Table 3-2 on page 89.
• 2. EBCDIC (Extended Binary-Coded Decimal
  Interchange Code)
• 8-bit code
• Table 3-3(a) on page 89 and Table 3-3(b) on page
  90.
• No definition for square bracket symbols.

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Data Codes

• 3. Touch-tone Telephone
• 12 frequencies corresponding to 12 keys on
  key-pad.
• Allows customers to pay bills and transfer money
  with some banks.
• 4. BCD (Binary Coded Decimal)
• 4 bits yields 2416 characters representable
• 6 bits yields 2664 characters representable
• 5. Baudot telegraph code
• 5 bits yields 2532 characters representable
• 6. SBT (Six Bit Transcode)
• 26 64 characters representable

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Data Codes

• EXAMPLE OF ASCII EBCDIC Conversions
• Bob in ASCII using Table 3-2 on page 89
• High-Order Bits and then Low-Order Bits, so
• Bob in ASCII is 1000010 1101111 1100010
• Bob in EBCDIC using Table 3-3 on pg 89 90
• Bob in EBCDIC is 11000010 10010110 10000010

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Data Code Size

• Transition from 5-bit and 6-bit codes of Baudot
  and SBT and BCD to 7-bit and 8-bit codes of ASCII
  and EBCDIC was necessary to
• increase the number of unique code sequences that
  could be represented.

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Data Code Size

• Parity Bit is appended to 7-bit Standard ASCII to
  detect errors to create Extended ASCII of 8-bits.
• Example
• a.) Create an odd parity for Standard ASCII
  letter B
• sum of digits of 10000010 is 2 so must add
  binary digit of 1 to make sum of odd number of 3.
• So Extended ASCII for odd parity of B is
  10000101 .
• b.) Create an even parity for Standard ASCII
  letter B
• Similarly, Extended ASCII for even parity
  of B is 10000100 .

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Error Sources

• 1. Attenuation
• 2. Impulse Noise
• 3. Crosstalk
• 4. Echo
• 5. Phase Jitter
• 6. Envelope Delay Distortion
• 7. White Noise

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Error Sources

• 1. Attenuation weakening of a signal as a result
  of distance and characteristics of medium
• 2. Impulse Noise a noise characterized by signal
  spikes.
• 3. Crosstalk when the signals from one channel
  distort or interfere with the signals of a
  different channel.
• 4. Echo the reflection or reversal of the signal
  being transmitted.
• 5. Phase Jitter a variation in the phase of a
  continuous signal from cycle to cycle.

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Error Sources

• 6. Envelope Delay Distortion
• occurs when signals that have been weakened or
  subjected to outside interference by transmission
  over long distances are enhanced by being passed
  through filters.
• 7. White Noise
• also known as termal noise or Gaussian noise. The
  amount of white noise is directly proportional to
  the temperature of the medium. White Noise
  results from the normal movements of electrons
  and is present in all transmission media at
  temperatures above absolute zero.

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Error Prevention

• 1. Telephone Line Conditioning
• 2. Lower Transmission Speed
• 3. Shielding
• 4. Line Drivers (Repeaters)
• 5. Better Equipment

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Error Prevention

• 1. Telephone Line Conditioning
• Conditioning also called equalization.
• 2 classes of conditioning of C and D
• 4 levels of Class C of C1, C2, C4 and C5.
• C5 is the most error free.
• Class D conditioning has service that telephone
  company with select line with the least amount of
  noise.

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Error Prevention

• 2. Lower Transmission Speed
• Some modems automatically adjust their speed to
  accommodate noisy lines.
• For example, some modems can converts from 56Kbps
  modem to 28.8 Kbps if the quality of the line
  deteriorates.
• 3. Shielding
• reduces the amount the amount of crosstalk and
  impulse noise form the environment.

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Error Prevention

• 4. Line Drivers (Repeaters)
• function is to restore signals to their full
  strength and overcome signal loss due to
  attenuation.

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Error Detection

• 1. Parity Check
• 2. Longitudinal Redundancy Check (LRC)
• 3. Cyclic Redundancy Check (CRC)
• 4. Sequence CHECKS
• 5. Message Sequence Numbers
• 6. Packet Sequence Numbers
• 7. Error Correction Codes

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Error Detection

• 8. Miscellaneous Error Detection Methods
• (a.) Check Digits
• (b.) Hash Totals
• (c.) Byte Counts
• (d.) Character Echoing

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Error Detection

• In Telegraphy, error reduction is obtained by
  sending each message multiple times. However this
  reduces drastically the line capacity.
• 1. Parity Check
• also called vertical redundancy check (VRC)
• sum of bits is checked as being either even or
  odd
• Parity Bit is added as last bit in ASCII to
  create Extended ASCII with selected parity.

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Error Detection

• 2. Longitudinal Redundancy Check (LRC)
• Block Check Character (BCC) is appended to a
  block of transmitted characters.
• First bit of the BCC serves as a parity for all
  of the first bits of the characters in the block.
  
• Second bit of the BCC serves as a parity for all
  of the second bits in the block, etc.

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Error Detection

• 3. Cyclic Redundancy Check (CRC)
• can detect bit errors better than VRC or LRC or
  both.
• A very efficient error detection method.
• is computed for blocks of transmitted data.
• uses a polynomial function to generate the block
  check characters.
• Because of its reliability, CRC is becoming the
  standard method of error detection for block data
  transmission.

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Error Detection

• 4. Sequence Checks
• Sequence numbers are assigned to each block so
  that the ultimate receiver can determine that all
  the blocks have indeed arrived and can put the
  blocks back into proper perspective.
• 5. Message Sequence Numbers
• One technique is to append a message sequence
  number to each data block transmitted between two
  stations.
• If the received message number disagrees with the
  expected message number, and error condition is
  created and the receiver requests that the sender
  retransmit the missing messages.

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Error Detection

• 6. Packet Sequence Numbers
• packet sequence numbers are appended to each
  packet.
• transport layer is responsible for for sequence
  numbers between the source and destination nodes.
• If some of the blocks in a sequence have not been
  received, the recovery method is to ask the
  sending station to retransmit the erroneous or
  last data.

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Error Detection

• 7. Error Correction Codes
• allows the receiving station not only to detect
  errors but also to correct some of them.
• These are called forward error-correcting
  codes, the most common are Hamming codes
• effectiveness of forward error-correcting codes
  is reduced by transmission noise.

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Error Detection

• 8. Miscellaneous Error Detection Methods
• 1. Check Digits appended to transmitted data
• 2. Hash Totals sum of group items
• used as check in credit card sum of total charges
• 3. Byte Counts Transmitting one of more
  characters that indicate the total number of
  characters in the message helps detect
  transmission errors in which entire characters
  may be lost.
• 4. Character Echoing characters transmitted are
  echoed to the user as a check.
• less often used because of of additional line
  time required.

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Error Correction

• Most common error correction method is to
  retransmit the data.
• This is called Automatic Request for
  Retransmission or Automatic Request for
  Repetition (ARQ).
• Retransmit individual characters in asynchronous
  transmission.
• Retransmit block(s) in synchronous transmission.

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Error Correction

• 1. Message Acknowledgement
• When station receives message, it computes the
  number of error detection bits or characters and
  compares the results with the check number
  received.
• If the two are equal, the message is assumed to
  be error-free and the receiver returns a positive
  acknowledgment to the sender.
• If the two are unequal, a negative
  acknowledgement is returned and the sending
  station retransmits the message.

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Error Correction

• 2. Retry Limit
• upper limit on number of continual
  retransmissions of a message.

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Digital Data Transmission

• Advantages of Digital Transmission
• 1. Lower Error Rates
• 2. Higher Transmission Rates
• 3. No Digital-Analog Conversion
• Theoretically digital transmission avoids the
  need for conversion.
• Unfortunately, not all locations are digital
  networks.
• Hence need for codec which is short for
  coder-decoder.
• 4. Security
• One method is encryption.
• Examples of encryption are voice scramblers
  used on secure telephone lines, digital
  encryption algorithms, etc.

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Digital Data Transmission

• Digital Voice Using Pulse Code Modulation
• Codec is used to convert to transform analog
  voice patterns into digital representation and
  then convert the digital patterns back into
  analogue format.
• Most common technique used is called Pulse Code
  Modulation (PCM).

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Interface

• 2 Classes of Equipment in Data Communications
• 1. data communications equipment (DCE)
• modems, media, media support facilities such as
  telephone switching equipment, microwave relay
  stations, and transponders.
• 2. data terminal equipment (DTE)
• terminals, computers, concentrators,
  multiplexers.
• Interface between DCE and DTE has4 aspects
• mechanical, electrical, functional and procedural.

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Interface

• 1. Mechanical
• type of connectors, number of pins connections in
  the connectors, and maximum allowable cable
  lengths.
• 2. Electrical
• allowable line voltages and the representations
  for the various voltage levels.
• 3. Functional
• specifies which signals, timing, control, data or
  ground leads are to be carried by each pin in the
  connector.
• See Table 3-8 lists the signals assigned to each
  of the 25 pins in an RS-232-C interface.

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Interface

• 4. Procedural
• define how signals are exchanged and delineated
  the environment necessary to transmit and receive
  the data.
• One pin or conducting wire in the connector might
  represent the ability of a terminal to accept a
  transmission.
• Table 3-9 shows a Procedural Interface to
  transmit from a processor to a terminal.

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Interface Standards

• 1. RS-232-C Standard
• 2. RS-449 Standard
• 3. RS-366 Standard
• 4. International Standards (ISO) and
  International Telecommunications Union (ITU)
  Standards
• See page 108 for a list of international
  standards ISO-2110, ITU V.10 and V.11, ITU V.24,
  ITU V.25, ITU V.28, ITU V.35, ITU X.20 and X.21
  and X.24.

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Interface Standards

• 5. Other Standards
• U.S. Government Interface Standards
• Ex. 1020 and 1030 for electrical interfaces
• U.S. Military Interface Standards
• Ex. MIL-STD-188-114

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Interface Standards

• 1. RS-232-C Standard
• encompasses serial binary data interchanges at
  rates up to 20,000 bps a recommended distance of
  up to 50 feet.
• 2. RS-449 Standard
• provides for a pin 37-pin connection, cable
  lengths up to 200 feet, and data transmission
  rates up to 2 Mbps.
• 3. RS-336 Standard
• 25 pin connection with enhanced capabilities for
  automatic calling equipment, and interface
  details for what signals must be present when
  dial tone is detected, etc.