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Network Technologies essentials Week 2: Physical layer & direct links Compilation made by Tim Moors, UNSW Australia Original slides by David Wetherall, University of Washington ISBN-10: 0133766624 © 2014 For the book A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011A. Tanenbaum and D. Wetherall: Computer Networks, 5 th ed, Prentice-Hall, 2011 Tim added green graffiti

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Message Latency (2) Latency is the delay to send a message over a link – Transmission delay: time to put M-bit message “on the wire” T-delay = M (bits) / Rate (bits/sec) = M/R seconds – Propagation delay: time for bits to propagate across the wire P-delay = Length / speed of signals = Length / ⅔c = D seconds – Combining the two terms we have: L = M/R + D Computer Networks2 2-1.7

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Computer Networks3 Bandwidth-Delay Product Messages take space on the wire! The amount of data in flight is the bandwidth-delay (BD) product BD = R x D – Measure in bits, or in messages – Small for LANs, big for “long fat” pipes 2-1.11

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Signals over a Wire What happens to a signal as it passes over a wire? 1.The signal is delayed (propagates at ⅔c) 2.The signal is attenuated (goes for m to km) 3.Frequencies above a cutoff are highly attenuated 4.Noise is added to the signal (later, causes errors) Computer Networks4 EE: Bandwidth = width of frequency band, measured in Hz CS: Bandwidth = information carrying capacity, in bits/sec 2-3.5

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Passband Modulation (3) Computer Networks5 NRZ signal of bits Amplitude shift keying Frequency shift keying Phase shift keying 2-4.12

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Computer Networks6 Shannon Capacity (2) Shannon limit is for capacity (C), the maximum information carrying rate of the channel: C = B log 2 (1 + S/N) bits/sec 2-5.7

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Bit Stuffing (3) So how does it compare with byte stuffing? Computer Networks7 Transmitted bits with stuffing Data bits 111111 Tim added green graffiti 3a-2.12 Coursera 2-7

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Computer Networks8 Approach – Add Redundancy Error detection codes – Add check bits to the message bits to let some errors be detected Error correction codes – Add more check bits to let some errors be corrected Key issue is now to structure the code to detect many errors with few check bits and modest computation 3a-3.4 Coursera 2-8

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Computer Networks9 Using Error Codes (2) Receiver: – Receive D+R bits with unknown errors – Recompute R check bits based on the D data bits; error if R doesn’t match R’ DR’ Data bits Check bits R=fn(D) =? 3a-3.8 Coursera 2-8

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Computer Networks10 Simple Error Detection – Parity Bit Take D data bits, add 1 check bit that is the sum of the D bits – Sum is modulo 2 or XOR 3a-4.3 Coursera 2-9

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Computer Networks11 Internet Checksum (3) Sending : 1.Arrange data in 16-bit words 2.Put zero in checksum position, add 3.Add any carryover back to get 16 bits 4.Negate (complement) to get sum 0001 f203 f4f5 f6f7 +(0000) ------ 2ddf0 ddf0 + 2 ------ ddf2 220d 3a-4.8 Coursera 2-9

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Computer Networks12 CRCs (3) Send Procedure: 1.Extend the n data bits with k zeros 2.Divide by the generator value C 3.Keep remainder, ignore quotient 4.Adjust k check bits by remainder Receive Procedure: 1.Divide and check for zero remainder 3a-4.14 Coursera 2-9

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Computer Networks13 Hamming Code (3) Example: data=0101, 3 check bits – 7 bit code, check bit positions 1, 2, 4 – Check 1 covers positions 1, 3, 5, 7 – Check 2 covers positions 2, 3, 6, 7 – Check 4 covers positions 4, 5, 6, 7 0 1 0 0 1 0 1 p 1 = 0+1+1 = 0, p 2 = 0+0+1 = 1, p 4 = 1+0+1 = 0 1 2 3 4 5 6 7 3a-5.9 Coursera 2-10

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Computer Networks14 Detection vs. Correction (4) Error correction: – Needed when errors are expected – Or when no time for retransmission Error detection: – More efficient when errors are not expected – And when errors are large when they do occur 3a-5.21 Coursera 2-10

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END © 2013 D. Wetherall Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, © 2011. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Computer Networks15

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