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ITC242 – Introduction to Data Communications Week 10 Topic 16 Data link control

Learn about flow control and error control in data link protocols. Understand how flow control prevents buffer overflow and how error control detects and corrects transmission errors.

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ITC242 – Introduction to Data Communications Week 10 Topic 16 Data link control

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  1. ITC242 – Introduction to Data CommunicationsWeek 10Topic 16 Data link control

  2. Last Week Data Transmission • Describe the difference between analogue and digital signals • Transmission impairments – attenuation and noise affect signal quality • Shannon’s formula provides a theoretical estimate of maximum channel capacity

  3. Last Week Data Communication Fundamentals • The difference between analogue and digital transmission • Digital and analogue encoding techniques: • ASK, FSK, PSK • NRZ-L, NRZI, Manchester, Differential Manchester • Asynchronous transmission • Synchronous transmission

  4. Topic 16 – Data link control Learning Objectives • Describe flow control and error control

  5. Flow Control • Necessary when data is being sent faster than it can be processed by receiver • Computer to printer is typical setting • Can also be from computer to computer, when a processing program is limited in capacity

  6. receive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast TCP Flow Control • app process may be slow at reading from buffer

  7. (Suppose TCP receiver discards out-of-order segments) spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unACKed data to RcvWindow guarantees receive buffer doesn’t overflow TCP Flow control: how it works

  8. Error Control Process • All transmission media have potential for introduction of errors • All data link layer protocols must provide method for controlling errors • Error control process has two components • Error detection • Error correction

  9. Error Detection: Parity Bits • Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) • Good for detecting single-bit errors only • High overhead (one extra bit per 7-bit character=12.5%)

  10. Error Detection • EDC= Error Detection and Correction bits (redundancy) • D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction otherwise

  11. Parity Checking Two Dimensional Bit Parity: Detect and correct single bit errors Single Bit Parity: Detect single bit errors 0 0

  12. Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? Internet checksum (review) Goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only) Complement: 1011010100111101 1011101101010101 1000111100001100 0100101011000010 0110011001100000 0101010101010101 1011101110110101 0110011001100000 0101010101010101 1000111100001100 1111111111111111

  13. Error Detection: Cyclic Redundancy Check (CRC) • Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame • 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder • For a CRC of length N, errors undetected are 2-N • Overhead is low (1-3%)

  14. Checksumming: Cyclic Redundancy Check • view data bits, D, as a binary number • choose r+1 bit pattern (generator), G • goal: choose r CRC bits, R, such that • <D,R> exactly divisible by G (modulo 2) • receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! • can detect all burst errors less than r+1 bits • widely used in practice (802.11 WiFi, ATM)

  15. CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R D.2r G R = remainder[ ]

  16. Error Correction • Two types of errors • Lost frame • Damaged frame • Automatic Repeat reQuest (ARQ) • Error detection • Positive acknowledgment • Retransmission after time-out • Negative acknowledgment and retransmission

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