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Networking: Challenges in Encoding, Framing, Error Detection, Error Correction, and Media Access

This text discusses the five key problems in networking: encoding/decoding, framing, error detection, error correction, and media access. Learn about the challenges and solutions in these areas.

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Networking: Challenges in Encoding, Framing, Error Detection, Error Correction, and Media Access

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  1. Getting Connected(Chapter 2 Part 1) Networking CS 3470, Section 1 Sarah Diesburg

  2. Five Problems • Encoding/decoding • Framing • Error Detection • Error Correction • Media Access

  3. Five Problems of Chapter 2 • How do we turn signals into bits that are recognized at the receiver? • This is known as the encoding problem R S 

  4. Five Problems of Chapter 2 • Delineating the sequence of bits into complete messages is called framing. • When does a frame start? • Byte-oriented • Bit-oriented • Clock-based

  5. Five Problems of Chapter 2 • Data verification • Has the data been corrupted? • If data has been corrupted, can we take the appropriate action? • This is the error detection problem. • CRC • 2-D parity • Checksums

  6. Five Problems of Chapter 2 • Error Recovery • Frames that are damaged will need to be retransmitted. • This is the reliability problem • ARQ • Stop and wait • Sliding window • Concurrent channels

  7. Five Problems of Chapter 2 • How do you arbitrate, or self-regulate access to a shared link? • This is the media-access problem • Ethernet • Token Ring • Wireless

  8. Perspectives on Connecting

  9. Perspectives on Connecting • All the links seem the same, yet can be characterized in different ways • We can characterize them by the services and bandwidth they provide

  10. Perspectives on Connecting • Another way to characterize links is by their physical makeup • Copper – DSL, coaxial, cat5e/cat6 cables • Optical fiber – FTTH • Air – Wireless

  11. Links • We care about electromagnetic waves • These links provide the foundation to propagate binary information/bits (0’s and 1’s) – otherwise known as encoding

  12. Visualizing Links • Network adaptors connect nodes to links • Why abbreviated NIC? • Signals travel between signaling components

  13. Data Encoding (signals)

  14. Tick, tock! Let's synchronize the clock • Encoding and decoding processes are driven by a clock • Every clock cycle the sender transmits a bit and the receiver recovers a bit 1 0 0 1 Clock

  15. Encoding • Encoding schemes • NRZ (Non-return to zero) • NRZI (Non-return to zero inverted) • Manchester • 4B/5B

  16. NRZ Non-Return to Zero • The simplest thing to do is to map “1” onto the high signal and “0” onto the low signal • There are a few challenges to this approach 1 0 0 1 NRZ Clock

  17. NRZ Non-Return to Zero • Baseline Wandercaused by signal averaging • Receiver keeps average of signal it has received so far and uses average to distinguish highs and lows • Problem occurs when to many consecutive 1’s or 0’s cause average to change

  18. NRZ Non-Return to Zero • Clock recovery required when signal remains constant too long • Receiver uses high-low transitions to mark the clock boundaries • What happens when we send a lot of consecutive 1’s or 0’s?

  19. NRZI Non-Return to Zero Inverted • Transition on the half-cycles • 1's indicated by a transition (low-to-high, high-to-low) • 0's are where there is no transition • Takes care of problem of consecutive 1’s • Still a problem for consecutive zeros 0 0 1 1 NRZI Clock

  20. Manchester Encoding • Transition on the half-cycles • low-to-high indicates a zero • high-to-low indicates a one • Receiver is able to synchronize clock every cycle 1 1 0 0 Manchester Clock

  21. Baud rate • The baud rate is the rate at which the signal changes • The bit rate is the rate at which you can transmit information • For the same baud rate, NRZ and NRZI have twice the bit rate as Manchester.

  22. Different Encoding Strategies So Far…

  23. 4B/5B encoding • 4-bit payload in a 5-bit gift box • Goal is to improve upon Manchester (50% efficiency), but to avoid baseline wander and clock drift • Insert extra bits into bit stream to break up long sequences of 0’s and 1’s • 5-bit codes selected such that there are never more than three consecutive zero's. • Resulting codes transmitted through NRZI encoding

  24. 4B/5B • 5 bits (32 patterns) to represent 4 bits (16 patterns) • 5-bit patterns with no more than 1 leading zero • 5-bit patterns with no more than 2 trailing zero's • 16 leftovers • 7 not valid • Others control signals • 11111 (idle) • 00000 (dead) • 00100 (bad)

  25. Framing • We know how to transmit bits on a link between two nodes • Now we need to figure out how to send distinct messages in frames • (Think packets at the link layer) • Why would we want to break up messages into frames instead of just a bit stream?

  26. Framing • Framing Protocols • Bi-sync • HDLC • PPP • SONET • Framing Approaches • Sentinel Approach • Byte-counting approach

  27. Sentinel Approach • Use sentinel characters to designate where frames start and end • Bi-sync frame format (IBM and mainframes) HEADER BODY SOH SYN SYN ETX CRC STX

  28. Sentinel Approach • SYN • Synchronization • STX • Start of Text • ETX • End of Text • SOH • Start of Header (Why no EOH?) • CRC • Cyclic Redundancy Check

  29. Character Stuffing • How do you handle the situation where the body contains STX, ETX, SOH, etc? • Escape out ETX with at Data Link Escape Character (DLE) • Now, how do you deal with a body that has a DLE in it? • Also known as character stuffing • Examples in programming

  30. Byte-counting Protocols • Just like with C strings, we can detect the end of the string in two ways • Special character • An extra length field • Same is true in framing • In the byte-counting approach, we detect the end of the frame with an extra “Count” field

  31. Byte-counting Protocols • DECNET DDCMP • SYN: 8 • CLASS: 8 • Count:14 • Header: 42 • What happens if the count field gets corrupted? HEADER COUNT CLASS BODY SYN SYN CRC

  32. Bit-oriented Protocols • Unlike byte-oriented protocols, these protocols don’t care about bytes • Could be transmitting • ASCII (7-bits) • Pixel values in an image • …

  33. Bit-oriented Protocols • High-Level Data Link Control (HDLC) protocol • Denotes beginning and end of a frame with the delimiter: 0 1 1 1 1 1 1 0 • Also transmitted anytime link is idle to keep clocks synchronized • Still has bit stuffing problem if special delimiter occurs in body • 5 1's; zero ALWAYS follows in the body.

  34. ...and then there's Sonet • Synchronous Optical Network standard • Dominant standard for long-distance transmission of data over optical networks • Every frame is exactly the same size! • Has some special bit pattern to tell receiver where frame starts and ends, with no bit stuffing

  35. Sonet • How does the receiver know where each frame starts and ends? • Receiver looks for it consistently (once every fixed number of bytes) • Encoded using NRZ • To combat NRZ clock recovery problem, XORs data to be transmitted to a well-known bit patten • Can XOR encoded data with well-known bit pattern to decode

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