net midterms mod 3

3.2 Error Detection and Correction

• Error detection is crucial in communication systems to ensure data integrity.

• A burst error of length ( r + 1 ) can be detected if it matches the polynomial ( G(x) ).

• Probability of an incorrect frame being accepted as valid is ( \frac{1}{2^{r}} < 1 ).

• If errors are longer than ( r + 1 ) or there are multiple errors, the probability of undetected corruption is ( \frac{1}{2^{r}} ).

• IEEE 802 polynomials are standard examples; one notable polynomial is: ( x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} + x^{11} + x^{10} + x^{8} + x^{7} + x^{5} + x^{4} + x^{2} + x^{1} + 1 )

• This polynomial detects:

  • All bursts of length 32 or less.

  • All errors affecting an odd number of bits.

• Castagnoli et al. (1993) and Koopman (2002) conducted extensive work to find optimal CRCs with higher reliability.

• A notable finding is that CRC-32 has a Hamming distance of 4, while some discovered CRCs have a distance of 6.

• Hardware implementations make CRC calculations efficient and straightforward.

3.3 Elementary Data Link Protocols

3.3.1 Initial Simplifying Assumptions

• Independent Processes: The physical, data link, and network layers are generally treated as independent entities.

• The physical layer and some data link processes run on Network Interface Cards (NICs), with other processes on the main CPU.

• The independence helps clarify discussions and allows each layer to evolve separately.

Key Assumptions

• Unidirectional Communication: Focus on a situation where one machine (A) sends a large amount of data to another (B) without interruption.

• Reliable Machines: Assumed that machines do not crash or fail during operations, avoiding complexities from such events.

3.3.2 Basic Transmission and Receipt

• Frame Structure: When A's data link layer receives a packet, it encapsulates it within a frame by adding header and trailer components.

• Procedures handle frame transmission and checksum (often computed in hardware).

• The receiver waits for events, processing occurs only upon receiving an incoming frame.

• Correct handling of checksums is mandatory; frames with errors are discarded.

3.3.3 Simplex Link-Layer Protocols

• Introduces three protocols of increasing complexity:

  1. Utopia Protocol:

     • Simplistic; assumes no errors or flow control.

     • Unidirectional and lossless communication without feedback to sender.

  2. Stop-and-Wait Protocol:

     • Relieves receiver overload by requiring confirmation (acknowledgement) after each frame.

     • The sender must wait for Acknowledgement (ACK) before sending another frame.

  3. Automatic Repeat reQuest (ARQ):

     • Introduces sequence numbers for each frame.

     • Ensures frames are delivered in order, using timeouts and requiring retransmissions.

• Describes the significance of timers for tracking frame acknowledgements and retransmissions in protocols.

3.4 Improving Efficiency

3.4.1 Goal: Bidirectional Transmission

• Piggybacking: A method to combine data and acknowledgement frames to reduce overhead and improve efficiency.

• Introduces a sliding window mechanism allowing for multiple frames in transmission before requiring acknowledgements.

3.4.2 Full-Duplex, Sliding Window Protocols

• Go-Back-N Protocol:

  • Allows the sender to transmit multiple frames without waiting for ACKs.

  • Affects bandwidth efficiency, especially over long distances with high round-trip delays.

• Selective Repeat Protocol:

  • Accepts frames out of order and recognizes which frames require retransmission after an error.

  • Uses NAKs to request retransmission of specific lost frames, optimizing recovery.

3.5 Data Link Protocols in Practice

Packet over SONET

• SONET provides the backbone for wide area networks, offering byte payload transport.

• PPP (Point-to-Point Protocol) frames packets and handles multiple network layer protocols.

• It manages error detection, line testing, and configuration, supporting multiple network layers interchangeably.

ADSL Protocol Stacks

• ADSL: Incorporates DSL modems carrying packets over telephone infrastructures with layers including AAL5, ATM, and PPP.

• AAL5 frames packets efficiently, allowing packets of variable lengths to be encapsulated in fixed-size cells for transport.

DOCSIS Protocols

• Handles data transmission over cable networks, incorporating both physical and MAC layers for bandwidth management.

• Supports multiple traffic types and frames while defining bandwidth allocation and error correction processes.

Conclusion

• The data link layer turns raw bit streams into reliable data frames that can be passed to the network layer, adjusting for reliability and flow control as necessary. Various methods of error detection, flow control, and frame structure are essential for maintaining effective communication.