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Transmission Characteristics

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

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Transmission Characteristics

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  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. 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 Transmission of data one bit at a time using only one single link Advantage: reduced cost Disadvantage: requires conversion devices Methods: Asynchronous Synchronous 1. Parallel Transmission and Serial Transmission

  7. 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 2. Asynchronous and Synchronous Transmissions

  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. N=6) • 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. Synchronous receiver clock Clock encoding: RZ, biphase, NRZ, AMI, differential manchester. Synchronous control scheme: Character-oriented Bit-oriented Difference in Frame synchronization Bit-oriented Preamble for bit synchronization Bit encoded violation for a start-of-frame delimiter JK0JK000 (start delimiter, JK1JK111 (end of frame) 2.2 Synchronous Transmission • 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. • Character-oriented inefficient for binary data because of • Additional DLE characters • Control characters varies for different character sets.

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