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Direct-link-lan

Direct-link-lan - Dire Link Ne ct tworks Encoding and Fram...

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Csci 232 Computer Networks Direct Link networks 1 Direct Link Networks Encoding and Framing Error Detection/Correction ReliableTransmission will becovered later (with TCP) Media Access Control (Wired) LAN Technologies Readings Chapter 2 except Section 2.5, which is delayed to the discusson on Transport layer.
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Csci 232 Computer Networks Direct Link networks 2 Encoding Signals propagateover a physical medium modulate electromagnetic waves (on amplitude, frequency, and phase): the process of encoding source data onto a carrier signal with frequency f_c e.g., vary voltage (amplitude) Encodebinary data onto signals e.g., 0 as low signal and 1 as high signal known as Non-Return to zero (NRZ) Bits NRZ 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
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Csci 232 Computer Networks Direct Link networks 3 Problems: Consecutive1s or 0s Low signal (0) may beinterpreted as no signal High signal (1) leads to baselinewander Receiver keeps an average of the signal it has seen so far and uses this average to distinguish between low and high signals Unableto recover clock Clocks at thesender and receiver use signal transition to synchronize each other
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Csci 232 Computer Networks Direct Link networks 4 Alternative Encodings Non-return to Zero Inverted (NRZI) make a transition from current signal to encode a one; stay at current signal to encode a zero solves the problem of consecutive ones Manchester transmit XOR of the NRZ encoded data and the clock only 50% efficient.
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Csci 232 Computer Networks Direct Link networks 5 Encodings (cont) 4B/5B every 4 bits of data encoded in a 5-bit code 5-bit codes selected to have no more than oneleading 0 and no more than two trailing 0s thus, never get more than three consecutive0s resulting 5-bit codes aretransmitted using NRZI achieves 80% efficiency
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Csci 232 Computer Networks Direct Link networks 6 Encodings (cont) Bits NRZ Clock Manchester NRZI 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
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Csci 232 Computer Networks Direct Link networks 7 Framing Break sequenceof bits into a frame– why? determine wherea framestarts and ends Typically implemented by network adaptor Frames Bits Adaptor Adaptor Node B Node A
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Csci 232 Computer Networks Direct Link networks 8 Approaches Sentinel-based delineate frame with special pattern: 01111110 e.g., HDLC, SDLC, PPP problem: special pattern appears in the payload solution: bit stuffing sender: insert 0 after five consecutive 1s receiver: delete 0 that follows fiveconsecutive 1s What if the receiver sees a 1? Header Body 8 16 16 8 CRC Beginning sequence Ending sequence
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Csci 232 Computer Networks Direct Link networks 9 Approaches (cont) Couter-based include payload length in header e.g., DDCMP problem: count field corrupted solution: catch when CRC fails How many frames are affected when counter or sentinel is corrupted?
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