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

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Data Link Control and Protocols

The Data Link Layer

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Flow Control

Ensuring the sending entity does not overwhelm the receiving entity ִ Preventing buffer overflow
Transmission time ִ Time taken to emit all bits into medium
Propagation time ִ Time for a bit to traverse the link

Model of Frame Transmission

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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

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Example Sliding Window

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)

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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

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Automatic Repeat Request

(ARQ)

Stop and wait
Go back N
Selective reject (selective retransmission)

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

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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)

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SLIP

Serial Line IP
Uses character stuffing ִ Flag used 0xC ִ 0xDB, 0xDC replaces 0xC0 data ִ 0xDB in data is also stuffed
No Error Detection
Supports only IP
Both sides should already know the other’s IP
No authentication process

PPP

Point to Point Protocol

Based largely on HDLC ִ But uses character stuffing instead of bit

Improves on SLIP ִ Error detection ִ Supports multiple protocols ִ IP negotiation at connection time ִ Permits authentication