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Section 2.3

Point-to-Point Links: Framing. Section 2.3. Framing. Demarcates (Separates) units of transfer Goal Enable nodes to exchange blocks of data Challenge How can we determine exactly what set of bits constitute a frame? How do we determine the beginning and end of a frame?. Why Framing?.

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Section 2.3

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  1. Point-to-Point Links: Framing Section 2.3

  2. Framing • Demarcates (Separates) units of transfer • Goal • Enable nodes to exchange blocks of data • Challenge • How can we determine exactly what set of bits constitute a frame? • How do we determine the beginning and end of a frame?

  3. Why Framing? • Synchronization recovery • Breaks up continuous streams of unframed bytes • Link multiplexing • Multiple hosts on shared medium • Simplifies multiplexing of logical channels • Efficient error detection • Per-frame error checking and recovery

  4. Framing • Approaches • Sentinel • Length-based • Clock based • Characteristics • Bit- or byte-oriented • Fixed or variable length • Data-dependent or data-independent length

  5. 01111110 Frame contents 01111110 Framing:Bit Oriented Protocols/ Sentinel-based • Beginning and End of Frame • Marked with a special bit pattern: 0111 1110 • Frame length is data-dependent • Problem: what if the special pattern 0111 1110 occurs within frame? • Solution: escaping the special characters • E.g., sender always inserts a 0 after five 1s • … and receiver always removes a 0 appearing after five 1s

  6. Bit Oriented Protocols/ Sentinel-based:bit stuffing After receiving 5 1s • next bit • 0>> stuffed bit >> removed Example • bits received 0111 1101 010; bits retained (data): 0111 1110 10 • bits received 0111 1100 010; bits retained (data): 0111 1100 10 • 1 >> bits received 0111 1110 or 0111 1111 next bit • 0END of Frame marker • 1Error , frame is discarded; receiver waits for next 0111 1110 to start receiving next frame

  7. Example Bit Oriented Protocol • ISO High-Level Data Link Control, HDLC protocol • Cyclic Redundancy Check (CRC) field: used to detect transmission errors (Section 2.4)

  8. Framing:Byte Oriented Protocols/ Sentinel-based • e.g. Point-to-Point, PPP protocol is similar in concepts as “Bit Oriented/ Sentinel-based” ( p. 84)

  9. Framing:Byte Oriented Protocols/ Counter-based • include payload length in header • COUNT: specifies how many bytes are contained in the frame’s body • e.g. Digital Data Communication Message Protocol, DDCMP • Problem: what if the count field gets corrupted? • Causes receiver to think the frame ends at a different place • Solution: catch when CRC fails • Receiver accumulates bytes numbered by bad COUNT field • Recognizes that frame is bad using the error detection field • And wait for the next sentinel (SYN) for the start of a new frame

  10. Clock-Based Framing • Continuous stream of fixed “time-length” frames • Example: • Synchronous Optical Network (SONET) • Dominant for long-distance transmission over optical networks • STS-1: lowest-speed SONET link (51.84 Mbps) • STS-1 frames - 125 µsec long • All SONET frames are 125 125 µsec long • Problems: • Frame synchronization • Clock synchronization

  11. Overhead Payload … … … … … 9 rows … … … … 90 columns STS-1 frame SONET • Frame synchronization • 2-byte synchronization pattern starts each frame (unlikely to occur in data) • Wait until pattern appears in same place repeatedly; once every 810 bytes • Clock synchronization • NRZ encoding, payload scrambled (XOR’d) with well-known 127-bit pattern • Ensures that there are plenty of transitions to allow the receiver to recover the sender’s clock

  12. SONET Multiplexing • STS-n • STS-1: lowest speed SONET, runs at 51.84 Mbps • STS-N viewed as N STS-1 sharing a fiber • STS-48 runs at 2488.32 Mbps…you can go faster • Since each frame is 125µs long • STS-1 frame is 810 bytes • STS-3 frame is 2430 bytes

  13. HDR STS-1 HDR STS-1 HDR STS-1 HDR STS-3 SONET Multiplexing (continued) • A single SONET frame can contain subframes for lower-rate channels • STS-N signal can be used to multiplexthe bytes ofN STS-1 frames • STS-3 thought of as consisting of 3 51.84 Mbps links sharing a fiber (bytes interleaved; STS-1 merged byte wise round-robin into STS-3) OR • Unmerged (single-source) Payload from N STS-1 frames can be linked to form a larger STS-N payload. The link is called an STS-Nc {concatenated} link (e.g. STS-3c is viewed as a single 155.52 Mbps pipe)

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