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Chap 3: Physical Aspects of Data Communication: Data Transmission

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... OF ASCII & EBCDIC Conversions: 'Bob' in ASCII using Table 3-2 ... 'Bob' in ASCII is: 1000010 1101111 1100010 'Bob' in EBCDIC using Table 3-3 on pg 89 & 90: ... – PowerPoint PPT presentation

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Title: Chap 3: Physical Aspects of Data Communication: Data Transmission


1
Chap 3 Physical Aspects of Data Communication
Data Transmission
  • MIS 3523 Business Data Communications
  • Dr. Segall
  • Spring 2001

2
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

3
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.

4
Data Flow
  • 3. FULL-DUPLEX TRANSMISSION
  • data can be transmitted in both directions
    SIMULTANEOUSLY.
  • Example Switched telephone connections.

5
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.

6
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

7
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

8
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.

9
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 .

10
Error Sources
  • 1. Attenuation
  • 2. Impulse Noise
  • 3. Crosstalk
  • 4. Echo
  • 5. Phase Jitter
  • 6. Envelope Delay Distortion
  • 7. White Noise

11
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.

12
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.

13
Error Prevention
  • 1. Telephone Line Conditioning
  • 2. Lower Transmission Speed
  • 3. Shielding
  • 4. Line Drivers (Repeaters)
  • 5. Better Equipment

14
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.

15
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.

16
Error Prevention
  • 4. Line Drivers (Repeaters)
  • function is to restore signals to their full
    strength and overcome signal loss due to
    attenuation.

17
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

18
Error Detection
  • 8. Miscellaneous Error Detection Methods
  • (a.) Check Digits
  • (b.) Hash Totals
  • (c.) Byte Counts
  • (d.) Character Echoing

19
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.

20
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.

21
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.

22
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.

23
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.

24
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.

25
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.

26
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.

27
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.

28
Error Correction
  • 2. Retry Limit
  • upper limit on number of continual
    retransmissions of a message.

29
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.

30
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).

31
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.

32
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.

33
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.

34
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.

35
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

36
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.
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