Data Link and Flow Control Networks and Protocols Prepared by: TGK First Prepared on: Last Modified on: Quality checked by: Copyright 2009 Asia Pacific Institute of Information Technology

Networks and Protocols Data Link and Flow Control Slide 2 of 27 Topic & Structure of the lesson Overview of Data Link Control and Flow Control Mechanisms Flow Control Techniques Error Detection Techniques Error Control Techniques

Networks and Protocols Data Link and Flow Control Slide 3 of 27 Learning Outcomes At the end of this module, YOU should be able to: Discuss Asynchronous and Synchronous communications Discuss Stop-and-Wait and Sliding Windows flow-control techniques Discuss the Parity check and Cyclic Redundancy Check approaches for error detection Discuss error control techniques of Automatic Repeat Request - Stop-and-Wait, Sliding Window and Selective Reject

Networks and Protocols Data Link and Flow Control Slide 4 of 27 Key Terms you must be able to use If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams: Flow Control Error Detection Error Control

Networks and Protocols Data Link and Flow Control Slide 5 of 27 Main Teaching Points SYNC characters START and STOP Bits ACK status Sliding Windows ODD and EVEN parity Automatic Repeat Request Stop-and-Wait ARQ Sliding Window Selective Reject

Networks and Protocols Data Link and Flow Control Slide 6 of 27 Overview of Data Link Control and Flow Control Mechanisms Synchronous Communications Receiver clock operates in synchronism with the received signal The clocking signal is embedded into the transmitted bit stream and subsequently extracted by the receiver (SYN characters) Asynchronous Communications The receiver’s clock runs asynchronously with respect to incoming signal A Byte is framed by a START and STOP bit

Networks and Protocols Data Link and Flow Control Slide 7 of 27 The START bit changes the signal to Non-Idle state, this warns receiver a byte is arriving Eight (8) data bits follow The STOP bit follows the data bits and returns the signal to the Idle state Overview of Data Link Control and Flow Control Mechanisms

Networks and Protocols Data Link and Flow Control Slide 8 of 27 Flow Control Techniques Stop-and-Wait Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not send ACK Works well for a few large frames Sliding Windows Allow multiple frames to be in transit Receiver has buffer of W long/length Transmitter can send up to W frames without ACK

Networks and Protocols Data Link and Flow Control Slide 9 of 27 Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k) –Frames are numbered modulo 2 k Following figure shows the sliding-windows technique operation: Flow Control Techniques

Networks and Protocols Data Link and Flow Control Slide 10 of 27 Flow Control Techniques

Networks and Protocols Data Link and Flow Control Slide 11 of 27 Sliding Windows Enhancements Receiver can acknowledge frames without permitting further transmission (Receive Not Ready) Must send a normal acknowledge to resume If duplex, use piggybacking  If no data to send, use acknowledgement frame  If data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP) Flow Control Techniques

Networks and Protocols Data Link and Flow Control Error Detection Techniques Parity Check Additional bits added by transmitter for error detection code Parity Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones Even number of bit errors goes undetected Example : Odd parity Check  (data will be accepted)  (reject because single bit error)

Networks and Protocols Data Link and Flow Control Slide 13 of 27 Disadvantage:  (data will be accepted) but the message has burst error Cyclic Redundancy Check For a block of k bits transmitter generates n bit sequence Transmit k+n bits which is exactly divisible by some number Receive divides frame by that number If no remainder, assume no error Error Detection Techniques

Networks and Protocols Data Link and Flow Control Slide 14 of 27 Error Control Techniques Automatic Repeat Request Detection and correction of errors Lost frames Damaged frames Features: Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission Techniques adopted: Stop and Wait ARQ Sliding window Selective reject (selective retransmission)

Networks and Protocols Data Link and Flow Control Error Control Techniques Stop and Wait ARQ Source transmits single frame Wait for ACK If received frame damaged, discard it Transmitter has timeout If no ACK within timeout, retransmit

Networks and Protocols Data Link and Flow Control Slide 16 of 27 If ACK damaged transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1 Go-Back-N ARQ Based on sliding window If no error, ACK as usual with next frame expected Use window to control number of outstanding frames If error, reply with rejection Discard that frame and all future frames until error frame received correctly Error Control Techniques

Networks and Protocols Data Link and Flow Control Slide 17 of 27 Transmitter must go back and retransmit that frame and all subsequent frames Go-Back-N ARQ - Damaged Frame Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all subsequent Go-Back-N ARQ - Lost Frame Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of sequence Error Control Techniques

Networks and Protocols Data Link and Flow Control Slide 18 of 27 Error Control Techniques Receiver send reject i Transmitter goes back to frame i and retransmits Selective Reject ARQ Also called selective retransmission Only rejected frames are retransmitted Subsequent frames are accepted by the receiver and buffered Minimizes retransmission Receiver must maintain large enough buffer More complex logic in transmitter

Networks and Protocols Data Link and Flow Control Slide 19 of 27 Quick Review Question

Networks and Protocols Data Link and Flow Control Slide 20 of 27 Follow Up Assignment

Networks and Protocols Data Link and Flow Control Slide 21 of 27 Summary of Main Teaching Points

Networks and Protocols Data Link and Flow Control Slide 22 of 27 Q & A Question and Answer Session

Networks and Protocols Data Link and Flow Control Slide 23 of 27 Topic and Structure of next session Routing Routing characteristics Routing strategies Routing algorithms Next Session