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

This preview shows pages 1–13. Sign up to view the full content.

CS/ECE 438 1 Data Links: The first building block ECE/CS 438 Fall 2011 Slides from Peterson & Davie Text

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Outline ± Connecting nodes ± Encoding ± Framing ± Error Detection ± Reliable Transmission ± Ethernet and Multiple Access Networks ± Wireless Networks
Connecting to the Internet An end-user’s view of the Internet

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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?
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)

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Encoding Signals travel between signaling components; bits flow between adaptors NRZ encoding of a bit stream

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
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

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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
This is the end of the preview. Sign up to access the rest of the document.

## 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.

### Page1 / 139

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

This preview shows document pages 1 - 13. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online