lect-05 - Data Links The first building block ECE/CS 438...

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CS/ECE 438 1 Data Links: The first building block ECE/CS 438 Fall 2011 Slides from Peterson & Davie Text
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Outline ± Connecting nodes ± Encoding ± Framing ± Error Detection ± Reliable Transmission ± Ethernet and Multiple Access Networks ± Wireless Networks
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Connecting to the Internet An end-user’s view of the Internet
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Link Capacity: Shannon-Hartley Theorem ± Gives the upper bound to the capacity of a link in terms of bits per second (bps) as a function of signal-to-noise ratio (S/N) of the link measured in decibels (dB). ± C = B*log 2 (1+S/N) { Where B = 3300 – 300 = 3000Hz, S is the signal power, N the average noise. { The signal to noise ratio (S/N) is measured in decibels is related to dB = 10 x log 10 (S/N). If there is 30dB of noise then S/N = 1000. { Now C = 3000 x log 2 (1001) = 30kbps. { How can we get 56kbps?
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Links ± All practical links rely on some sort of electromagnetic radiation propagating through a medium or, in some cases, through free space ± One way to characterize links, then, is by the medium they use { Typically copper wire in some form (as in Digital Subscriber Line (DSL) and coaxial cable), { Optical fiber (as in both commercial fiber-to-the home services and many long-distance links in the Internet’s backbone), or { Air/free space (for wireless links)
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Links ± Another important link characteristic is the frequency { Measured in hertz, with which the electromagnetic waves oscillate ± Distance between the adjacent pair of maxima or minima of a wave measured in meters is called wavelength { Speed of light divided by frequency gives the wavelength. { Frequency on a copper cable range from 300Hz to 3300Hz; Wavelength for 300Hz wave through copper is speed of light on a copper / frequency { 2/3 x 3 x 10 8 /300 = 667 x 10 3 meters. ± Placing binary data on a signal is called encoding . ± Modulation involves modifying the signals in terms of their frequency, amplitude, and phase.
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Links Electromagnetic spectrum
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Links Common services available to connect your home
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Encoding Signals travel between signaling components; bits flow between adaptors NRZ encoding of a bit stream
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Encoding ± Problem with NRZ { Baseline wander ± The receiver keeps an average of the signals it has seen so far ± Uses the average to distinguish between low and high signal ± When a signal is significantly low than the average, it is 0, else it is 1 ± Too many consecutive 0’s and 1’s cause this average to change, making it difficult to detect
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Encoding ± Problem with NRZ { Clock recovery ± Frequent transition from high to low or vice versa are necessary to enable clock recovery ± Both the sending and decoding process is driven by a clock ± Every clock cycle, the sender transmits a bit and the receiver recovers a bit ± The sender and receiver have to be precisely synchronized
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Encoding ± NRZI { Non Return to Zero Inverted { Sender makes a transition from the current signal to encode 1 and stay at the current signal to encode 0 { Solves for consecutive 1’s
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This note was uploaded on 02/08/2012 for the course ECE 438 taught by Professor Luo during the Fall '08 term at University of Illinois, Urbana Champaign.

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lect-05 - Data Links The first building block ECE/CS 438...

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