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

2. 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. 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. 4 Topic 16 – Data link control Learning Objectives Describe flow control and error control

5. 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. 6 TCP Flow Control receive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate app process may be slow at reading from buffer sender won’t overflow receiver’s buffer by transmitting too much, too fast flow control

7. 7 TCP Flow control: how it works (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

8. 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. 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. 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. 11 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0

12. 12 Internet checksum (review) 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? Goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only) 0110011001100000 0101010101010101 1000111100001100 0110011001100000 0101010101010101 1011101110110101 1011101101010101 1000111100001100 0100101011000010 Complement: 1011010100111101 1111111111111111

13. 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. 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 – exactly divisible by G (modulo 2) –receiver knows G, divides 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. 15 CRC Example Want: D. 2 r XOR R = nG equivalently: D. 2 r = nG XOR R equivalently: if we divide D. 2 r by G, want remainder R R = remainder[ ] D.2rGD.2rG

16. 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