1. The Data Link Layer Goal –As reliable as possible, efficient communication Point-to-Point –single connection –bits arrive in order sent Not necessarily reliable (unreliable)

2. Data Link Layer Problems Errors occur Finite data rate Propagation delays –Time required for signal to travel from source to destination

3. Data Link Functions Well-defined interface to the network layer Grouping the bits of the physical layer into frames Handling transmission errors Regulating the rate of data flow

4. Services Provided to the Data Link Layer Unacknowledged connectionless Acknowledged connectionless Acknowledged connection-oriented

5. Unacknowledged Connectionless Frames are independent No acknowledgment No connection Lost frames are not detected –may be detected in higher level layers Best when error rates are low Good when late data is not useful –speech, video Most LANs use this service

6. Acknowledged Connectionless (More Reliable) Each frame individually acknowledged Unacknowledged frames are eventually resent Good for unreliable channels –wireless for example

7. Acknowledged Connection-oriented Service Connection is established Each frame is numbered Every frame sent is guaranteed to be received eventually Frames guaranteed to arrive in correct order Provides a reliable bit stream

8. Framing Bit streams from physical layer are unreliable We would like to check for errors Sending a relatively small frame of bits, combined with some kind of data redundancy, allows us to provide some error checking

9. Methods that Break the Bitstream into Frames Character count Starting and ending characters with character stuffing Starting and ending flags with bit stuffing Physical layer coding violations

10. Big Problem: –How do we recover if we get out of sync? Character Count 5 T h i s 9 M e s s a g e 2 S Frames are prefixed by chars/frame ASCII 5 84 count What happens if we lose this character? 1041151011151059772831011151039732

11. Start and End Characters with Character Stuffing Special ASCII characters signal start and end of frame DLEData Link Escape (ASCII = 16) STXStart of text (ASCII = 2) DLEData Link Escape (ASCII = 16) ETXEnd of text (ASCII = 3) But what if these flags occur in the data? –Use two DLE characters to send one data DLE

12. Examples of Char Stuffing DLE STX T H I S M DLE ETX X Y DLE STX DLE ETX 5 DLE STX X Y DLE DLE STX DLE DLE ETX 5 DLE ETX TSH IM

13. Start and End Flags With Bit Stuffing Choose a bit sequence for start/end flag Example:01111110 Every time you see five 1’s in sequence in the data, stuff a zero into the stream

14. Final Framing Method - Physical Layer Coding Violations Start/End flag consists of sequence that is illegal in the data Example: 10 is 1 01 is 0 00 or 11 could be used as flags

15. Error Control Error Control provides feedback about the success of data transmission –frames may arrive correctly (receiver sends ack) –frames may arrive corrupted (receiver sends negative ack) –frames may be lost (no ack is ever received) acks may be lost (no ack is ever received) No error control in unacknowledged connectionless service

16. Flow Control What happens if the sender can send data faster than the receiver can handle it? –Receiver buffers overflow –Data is lost or requires retransmission Solution: Flow control –Receiver specifies the number of frames that may be transmitted (no more than can be held in its buffers). The sender cannot send more data until the receiver grants permission.

17. Error Detection and Correction Error rates are relatively low on digital networks and LAN’s Error rates are high on analog twisted pairs and wireless communication When errors occur, they tend to occur in bursts –More data blocks are correct –Errors are harder to detect

18. Definitions Error-correcting Codes –Codes that contain enough redundant information to correct errors Error-detecting Codes –Codes that contain enough redundant information to detect errors

19. Parity - a simple error detection mechanism The number of “1” bits in a fixed length bit sequence is either always even (even parity) or always odd (odd parity) Example (even parity, length = 8) parity bit 01010110 01100101 01011110 01000101 Any single bit errors are detected 11010111 01001101 Any even number of bit errors is not detected 11010111 01001111 Any odd number of bit errors is detected

20. Hamming Distance Frame codeword contains n = m + r total bits –where m is number of message bits r is number of redundant bits Hamming Distance –The number of bit positions in which two codewords differ Hamming Distance of Complete Code –The minimum Hamming Distance between codewords 01101000110 11010010100 Hamming distance = 6

21. Hamming Distance To detect d errors you need a Hamming distance (for the complete code) of d+1 To correct d errors you need a Hamming distance (for the complete code) of 2d+1

22. Error Correcting Codes Consider an encoding scheme where these are the only valid codewords 0000000000 0000011111 1111100000 1111111111 The Hamming distance of the complete code is 5 We can correct d-bit errors where 2d+1 = 5 We can detect d-bit errors where d+1 = 5 0000000000 with two errors 0000000101 (with < 3 errors must be 0) 0000000000 with three errors 0000011001 could be 0000000000 with three errors or 0000011111 with two errors.