Understanding Data Link Control: Flow Control, Error Detection, and Correction, Slides of Network Programming

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Data Link Control and Protocols

The Data Link Layer

Model of Frame Transmission

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 sending ACK Works well for a few large frames

Sliding Windows Flow Control

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

Sliding Window Diagram

Sliding Window 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)

Error Detection

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

Error Control

Detection and correction of errors Lost frames Damaged frames Automatic repeat request Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Back to Flow Control

Stop and Wait ;; Sliding Window. What can go wrong: Lost frame Lost Ack Frame in error – damaged Ack in error – damaged Duplicate frame Duplicate Ack

Stop and Wait

Source transmits single frame Wait for ACK If received frame damaged, discard it Transmitter has timeout If no ACK within timeout, retransmit If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK

Stop and Wait -

Diagram

High Level Data Link Control

HDLC

ISO 33009, ISO 4335

Other DLC Protocols (LLC)

Logical Link Control (LLC) IEEE 802 Different frame format Link control split between medium access layer (MAC) and LLC (on top of MAC) No primary and secondary - all stations are peers Two addresses needed Sender and receiver Error detection at MAC layer 32 bit CRC Destination and source access points (DSAP, SSAP)