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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
MechanismsFlow Control TechniquesError Detection TechniquesError Control Techniques
Networks and Protocols
Data Link and Flow Control
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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
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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
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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
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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
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• 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
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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
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• 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
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• 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 Check1+ 11011 111011(data will be accepted)1+ 11011 110011(reject because single bit error)
Networks and Protocols
Data Link and Flow Control
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• Disadvantage:
• 1+11011 111000(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
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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
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• 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
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• 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
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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