CS 635 Lecture 3, Part 3 Logical Link Control

Error Control

Go-Back-N ARQ

• Packets transmitted continuously (when available) without waiting for ACK, up to N outstanding, unACK’ed packets
• A logically different sender timer association with each unACK’ed packet: extension of AB protocol
• Receiver:
• ACK packet if correctly received and in-order, pass to higher layer
• NACK or ignore (possibly discard) corrupt or out-of-order packet
• Sender
• If NACK received, or timeout, for packet n, begin resending from n all aver again
• Cumulative ACK: ACK of n implicitly acknowledges up through n

Go-Back-N continued

• No receiver transport buffering with discard
• Saves resources at receiver
• Avoids large bursts of packet delivery to higher layers
• Simplicity in buffering and protocol processing at sender and receiver

• Tradeoff between buffering/processing complexity and bandwidth

Go Back N: Example

Selective Repeat ARQ

As in go back-N:

• packet transmitted when available, up to limit
• timer associated with each unACK’ed packet
• receiver NACK’s or ignores corrupted packets

• out-of-order (but otherwise correct) packet is ACK’ed
• receiver: buffers out-of-order packets

• sender: on timeout or NACK of packet n, just retransmit n

Notes:

• more receiver buffering than Go_back-N
• more complicated buffer management by both sides
• saves bandwidth: no need to retransmit correctly received packets

Selective Repeat ARQ: example:

How Big Can a Window Be?

Suppose sequence number space size is N

• Question: How big can window be and have SR still work?
• Partial Answer: N-1 won’t work:

Fundamental problem: sender and receiver

• Do not have synchronized info
• Can not know the exact same information

Detecting Errors: checksums

Approach: add extra bits to packets that will aloow us to detect (possibly correct) bit errors

Simple example: parity

• Given n-1 bit packet, add nth bit, choosing value so that total number of 1 bits in packets (including nth bit) is even (even parity).
• Example packet:

• At receiver:
• count # 1’s in packet, if odd, then error!
• What if even number of bit flips?
• What if odd number of bit flips?
• Note:
• Many codes with more powerful error detection capabilities
• Packet header itself often separately checksummed
• Checksumming also done at data link layer
• Hardware support for transport-level checksum: SGI

Forward Error Correction: FEC

ARQ protocols operate by detecting errors and retransmitting

• Retranmission needs round-trip delay to recover
• May be too long for deep space, or high-speed, real-time applications
• FEC: key idea is to transmit enough redundant data to allow receiver to recover from errors itself! (no sender transmission required)

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