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Transmission Characteristics

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EBCDIC (Extended Binary Coded Decimal Interchange code) 8 bit, proprietary (equipment ... Printable characters. Control characters (Non-printable characters) ... – PowerPoint PPT presentation

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Title: Transmission Characteristics


1
Transmission Characteristics
  • Introduction (Information Interchange codes)
  • Asynchronous and Synchronous Transmissions
  • Error detection (bit errors)
  • Data compression

2
Introduction
3
1. Introduction (Information Interchange codes)
  • Input encodes 7 (128 different element) or 8 bits
    a codeword, output decodes.
  • Codes
  • EBCDIC (Extended Binary Coded Decimal Interchange
    code)
  • 8 bit, proprietary (equipment manufactured by
    IBM)
  • ASCII (American Standard Committee for
    Information Interchange)
  • 7 bit, ISO 645
  • Printable characters
  • Control characters (Non-printable characters),
  • Format control BS, CR, SP, DEL, ESC
  • Information Separator FS (file), RS (record)
  • Transmission Characters
  • SOH (start-of-heading), STX (Start of Text), ETX,
    ACK,NACK, SYN
  • Word several bytes.

4
1. Introduction (characteristics)
  • Parallel transfer mode, bit-serial mode
  • Communication modes (simplex,half duplex, full
    duplex)
  • Transmission modes
  • Receiver DTE must detect
  • Start of each bit cell period (bit or clock sync)
  • Start or end of each element (character or byte)
    char or byte sync
  • Start or end of each message block (frame) frame
    sync.
  • Asynchronous transmission (for a long time in
    idle state, or low data rate)
  • (resynchronize at the start of each character
    received)
  • Uses start bit, stop bit (or 2 stop bits)
  • Synchronous transmission (at high data rate)
  • Bit stream suitable encoded
  • Frames preceded by (one or more) reserved bytes,
    character synchronization
  • Frame content encapsulated between a pair of
    reserved characters

5
1. Introduction (cont.)
  • Error Control
  • Asynchronous Tx Parity bit for each transmitted
    character.
  • Synchronous Tx Possible errors on the complete
    frame
  • Based on frame content transmitted, sequence of
    error check digits.
  • Receiver detects error
  • Need a scheme to get correct frame.
  • Flow control
  • Avoid congestion in network communication devices
  • Control of flow between two DTE
  • Data Link Protocols
  • Protocol set of convention rules between two
    communicating entities
  • Allows Error control, Flow control
  • Can also define the format of data being
    exchanged and type of encoding scheme
  • The type and order of messages that are to be
    exchanged (Sender DTE may set up a connection to
    Receiver DTE)

6
1. Parallel Transmission and Serial Transmission
  • Transmission of data one bit at a time using only
    one single link
  • Advantage reduced cost
  • Disadvantage requires conversion devices
  • Methods
  • Asynchronous
  • Synchronous
  • Bits in a group are sent simultaneously, each
    using a separate link
  • n wires are used to send n bits at one time
  • Advantage speed
  • Disadvantage cost limited to short distances

7
2. Asynchronous and Synchronous Transmissions
  • Transfer of data with start and stop bits and a
    variable time interval between data units
  • Timing is unimportant
  • Start bit alerts receiver that new group of data
    is arriving
  • Stop bit alerts receiver that byte is finished
  • Synchronization achieved through start/stop bits
    with each byte received
  • Requires additional overhead (start/stop bits)
  • Slower but Cheap and effective
  • Ideal for low-speed communication when gaps may
    occur during transmission (ex keyboard)
  • Requires constant timing relationship
  • Bit stream is combined into longer frames,
    possibly containing multiple bytes
  • Any gaps between bursts are filled in with a
    special sequence of 0s and 1s indicating idle
  • Advantage speed, no gaps or extra bits
  • Byte synchronization accomplished by data link
    layer

8
2.1 Asynchronous Transmission
  • Transmitting and receiving clocks assumed out of
    sync
  • Within each DTE
  • Parallel-to-serial conversion of each character
    (byte) in preparation for its transmission.
  • Serial-to-parallel conversion of each received
    character in preparation of its processing and
    storage
  • Receiver must have a mean to achieve bit,
    character, and frame synchronization
  • A generation of suitable error checks for error
    detection
  • Bit synchronization
  • Receiving clock samples N times faster (ex. N6)
  • Character Synchronization
  • After received start bit, simple count to detect
    the character
  • Use a buffer register.
  • Frame Synchronization
  • Use STX and ETX
  • Use DLE-STX, DLE-ETX, DLE-DLE (byte stuffing) for
    pure binary data (ex. Compiled program)

9
2.2 Synchronous Transmission
  • Synchronous receiver clock
  • Clock encoding
  • RZ, biphase, NRZ, AMI, differential manchester.
  • Synchronous control scheme
  • Character-oriented
  • Bit-oriented
  • Difference in Frame synchronization
  • Character-oriented control
  • Uses a synchronous idle (SYN)
  • Helps in bit synchronization
  • And frame synchronization
  • Uses DLE-STX, DLE-ETX, DLE-DLE for pure binary
    data.
  • Bit-oriented
  • Preamble for bit synchronization
  • Bit encoded violation for a start-of-frame
    delimiter JK0JK000 (start delimiter, JK1JK111
    (end of frame)
  • Character-oriented inefficient for binary data
    because of
  • Additional DLE characters
  • Control characters varies for different character
    sets.
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