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Chapter 10

Chapter 10. Data Link Control Protocols. Agenda. Definition Functions Attributes Concepts Types Conversion. Definition. Protocols: set of rules or guidelines for communication between DTEs Data link protocol: rules for operating the circuit and sending message over it. Functions.

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Chapter 10

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  1. Chapter 10 Data Link Control Protocols

  2. Agenda • Definition • Functions • Attributes • Concepts • Types • Conversion

  3. Definition • Protocols: set of rules or guidelines for communication between DTEs • Data link protocol: rules for operating the circuit and sending message over it

  4. Functions • Communication startup • Character identification and framing • Message identification • Line control • Error control • Termination

  5. Data Link Protocols Attributes • Transparency (DTE – any bit pattern as data) • Code independence • Efficiency (control codes)

  6. Protocol Concepts • Line Access • Contention • Polling • Token passing • Message format

  7. Contention • Carrier sense multiple access with collision detection (CSMA/CD) by XEROX • For: • Point-to-point circuits • Traffic light for multipoint circuits • Fast speed of circuit • Office application & LAN • Not for • Manufacturing due to inconsistent response time

  8. Polling • Characteristics • One master and slavers • Special polling character • Types • Roll call polling (polling list) • Fast select polling (several terminals) • Hub polling (station to station) • Token passing (token) • Token (particular sequence of bits, Free or busy)

  9. Message Format • Start of header (SOH) • Header (Address, sequence, date and time) • Start of text (STX) • Text • End of text (ETX) or text length • Trailer • Block check character (BCC) • Synchronization character (SYN)

  10. Asynchronous Data Link Control Protocols - Characteristics • Start/stop bits • Parity checking • Character oriented • Less expensive and less complicated equipment • For personal computers

  11. Asynchronous Data Link Control Protocols - I • XMODEM (simple, less reliable error checking) • XMODEM-CRC (more reliable) • XMODEM-1K (more efficient) • YMODEM (reliable, multiple files transfer) • YMODEM-G (fast) • ZMODEM (fast, good failure recovery) • X.PC (pack switching network, multiple sessions on one circuit)

  12. Asynchronous Data Link Control Protocols - II • KERMIT (reliable, fast file transfer, PC & mainframe) • Serial Line Internet Protocol (SLIP) • Full-duplex • IP over asynchronous dial-up or leased lines • No error correction • Point-to-point Protocol (PPP) • PC to a TCP/IP network • Full-duplex for synchronous and asynchronous transmission • Authentication, compression, error correction, & packet sequencing

  13. Synchronous Data Link Protocols - Classification • Character-oriented protocols • Special character for start and end of message • Binary Synchronous Communication Protocol (BSC or BISYNC) • Byte-count-oriented protocols • Special character for start of the header, count field, message, block check character (BCC) • DEC’s Digital Data Communication Message Protocol (DDCMP) • Bit-oriented protocols • Use flag character for start and end of message • IBM’s Synchronous Data Link Control (SDLC) • ISO’s High-Level Data Link control (HDLC)

  14. Binary Synchronous Communications • By IBM • For 6-bit transcode (SBT), ASCII, EBCDIC • SYN at start and middle of transmission • Point to point and multipoint (polling) • ARQ approach for error checking (ACK1, ACK0, NAK) • Pros: • Transparency and non-transparency modes • Efficient, understandable, and widely used • Point-to-point & multipoint operations • Cons: • Code dependent • Half-duplex protocol • Cumbersome for transparency mode

  15. Digital Data Communications Message Protocol (DDCMP) • Usage • Digital Network Architecture (DNA) • High-bandwidth and high-latency (satellite) links • Data message and control message • Error checking • Use block check character • Unique and increasing sequence number for sending message • ACK indicates the last message received • Pro: • Efficient at transparency mode • Variable length of data • Only one control character: SOH • Easy implement for full-duplex and message sequence

  16. High-level Data Link Control (HDLC) - I • Operating modes • Normal response mode (NRM) for a primary node and one or more secondary modes on a circuit (polling) • Asynchronous balanced mode (ABM) for nodes with peers (most used for efficiency with no polling in full duplex) • Asynchronous response mode (ARM) for primary and secondary nodes with same transmission right (rarely used) • Frames • Supervisory or S format: control acknowledge • Unnumbered or U format: (operation mode, start, termination) • Information or I format: (data)

  17. High-level Data Link Control (HDLC) - I • Flag • 01111110 • Bit stuffing 0 after five 1s by hardware • For synchronization • Other fields • Address field • Control field: type of frame, sequence number for information frames • Information field: multiple of 8 bits • Frame check sequence (FCS) field: error checking • Frame flow • Initialization phase, data transfer phase, and disconnect phase

  18. Other Data Link Control Protocols • Link access procedure, balanced (LAPB) • Full-duplex, point-to-point mode • For X.25 DTE and packet switching network • Link access procedure, D-channel (LAPD) • ISDN network • Link access procedure for frame –mode bearer service (LAPF) • High-speed packet switching network on low error rate digital circuits • Asynchronous transfer mode • Data transfer on high s-speed, digital, error free network

  19. Protocol Conversion • Methods • Dedicated hardware protocol converters • Add-in circuit board protocol converters • Software protocol converters • Gateway • Functions • Code translation • Control characters, format, and error checking

  20. Points to Remember • Definition • Functions • Attributes • Concepts • Types • Conversion

  21. Discussion • Select proper protocol for the network of a bank state you reasons

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