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Point-to-Point Links: Framing

Section 2.3

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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?

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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

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Framing• Approaches

– Sentinel– Length-based – Clock based

• Characteristics– Bit- or byte-oriented– Fixed or variable length– Data-dependent or data-independent length

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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

01111110 01111110Frame contents

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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

• 0 END of Frame marker• 1 Error , frame is discarded; receiver waits for next 0111 1110

to start receiving next frame

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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)

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Framing:Byte Oriented Protocols/ Sentinel-based

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

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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

Header Body

8 8 4214 168

CRCCount

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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

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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

………………………

Overhead Payload

9 ro

ws

90 columns

STS-1 frame

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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

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SONET Multiplexing (continued)• A single SONET frame can contain subframes for lower-

rate channels

• STS-N signal can be used to multiplex the bytes of N 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)

HDR HDR HDR

HDR

STS-1 STS-1 STS-1

STS-3