n05DL-CRC2015.pdf - Data Link Layer The Data Link Layer...

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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 1 The Data Link Layer Error handling
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 2 Data Link Layer Functions Goal : Lossless and error-free transmission of data between adjacent machines Adjacent: Machines connected by a direct link Data: Blocks of bits created by the higher layer Functions : – Framing Error control Flow control
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 3 What Causes Transmission Errors? Impulse noise : arcing of relays; lightning; backfiring cars Distortion : amplitude, propagation speed, and phase are frequency-dependent Crosstalk between adjacent wires Fading in microwave links
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 4 Characterization of Errors Bit error rate (BER): Fraction of bits in error measured over a long period of time . Wide range: 10 -9 ( fiber optics links ) - 10 -3 ( wireless channels ) Bit error patterns: 1. bit errors uniformly spread across bits: Independent Error Model 2. bit errors are clustered : Burst Error Model
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 5 Independent Error Model The probability that a bit is in error is BER Bit errors are independent of each other The probability that a packet of N bits is in error is: PER = 1- (1 - p) N For p = 10 -5 and N = 1000, PER is about 10 -2 100,000 bits on average
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 6 Burst Error Model Characterized by: 1. Rate of bursts 2. Length of bursts burst length=16 burst length=5 time between consecutive bursts is long compared to burst lengths
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 7 Example of a Burst Error Model Average number of bits between bursts: 1,000,000 bits Average Rate: one burst every 10 6 bits Average duration of a burst: 20 bits Probability that a bit is in error during the burst: 0.5 Bit Error Rate: 10 -5 Packet Error Rate: 10 -3 - 2 x 10 -3 (1000 bits/packet ) ... packet
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Prof. F. Tobagi EE284, Introduction to Computer Networks Data Link Layer 8 Error Handling Forward Error Correction (FEC): Encode the data in a frame in such a way as to allow the receiver to recover the transmitted information.
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