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TEL 415E Data Communica2ons
İstanbul Teknik Üniversitesi Elektrik Elektronik Fakültesi
Elektronik Haberleşme Mühendisliği Fall 2012
WEEK Course Outline DATE
1 Introduc2on 24/9
2 Protocol Architecture, TCP/IP, and Internet-‐Based Applica2ons 1/10
3 Data Transmission 8/10
4 Transmission Media (+ Project Proposals) 15/10
5 Signal Encoding Techniques 22/10
6 -‐ (Cumhuriyet Bayramı) 29/10
7 Midterm Exam 5/11
8 Digital Data Communica2ons Techniques 12/11
9 Data Link Control Protocols 19/11
10 Mul2plexing (+ Project Progress Reports) 26/11
11 Circuit Switching and Packet Switching 3/12
12 Midterm Exam 10/12
13 Rou2ng in Switched Data Networks 17/12
14 Project Workshop (+ Project Final Reports) 24/12
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Data and Computer Communica2ons
Ninth Edi2on by William Stallings
Chapter 3 – Data Transmission
Data and Computer Communica2ons, Ninth Edi2on by William Stallings, (c) Pearson
Educa2on -‐ Pren2ce Hall, 2011
Data Transmission What we've got here is failure to communicate. Paul Newman in Cool Hand Luke
Data Transmission
The successful transmission of data depends on two factors:
• quality of the signal being transmiced
• characteris2cs of the transmission medium
Transmission Terminology
Data transmission occurs between transmi<er and receiver over some transmission medium.
Communica@on is in the form of electromagne@c
waves.
Guided media
twisted pair, coaxial cable, op2cal fiber
Unguided media (wireless)
air, vacuum, seawater
Transmission Terminology
Transmission Terminology
• Simplex
– signals transmiced in one direc2on • eg. Television
• Half duplex – both sta2ons transmit, but only one at a 2me
• eg. police radio
• Full duplex – simultaneous transmissions
• eg. telephone
Frequency, Spectrum and Bandwidth
Ø analog signal • signal intensity varies smoothly with no breaks
Ø digital signal • signal intensity maintains a constant level and then abruptly changes to another level
Ø periodic signal • signal pacern repeats over 2me
Ø aperiodic signal • pacern not repeated over 2me
Time Domain Concepts
Analog and Digital Signals
Periodic Signals
Sine Wave
• peak amplitude (A) – maximum strength of signal – typically measured in volts
• frequency (f) – rate at which the signal repeats – Hertz (Hz) or cycles per second – period (T) is the amount of 2me for one repe22on – T = 1/f
• phase (φ) – rela2ve posi2on in 2me within a single period of signal
(periodic continuous signal)
Varying Sine Waves s(t) = A sin(2πi +Φ)
Wavelength (λ) the wavelength of a signal is the distance occupied by a single
cycle
can also be stated as the distance between two points of corresponding phase of two consecu@ve cycles
assuming signal velocity v, then the wavelength is
related to the period as λ = vT
or equivalently λf = v
especially when v=c • c = 3*108 ms-‐1 (speed of light in free space)
Frequency Domain Concepts
• signals are made up of many frequencies • components are sine waves • Fourier analysis can show that any signal is made up of components at various frequencies, in which each component is a sinusoid
• can plot frequency domain func2ons
Addi2on of Frequency Components
(T=1/f)
Frequency Domain
Representa2ons
• frequency domain func2on of Fig 3.4c
• frequency domain func2on of single square pulse
Spectrum & Bandwidth
Signal with dc Component
Data Rate and Bandwidth
any transmission system has a limited band of frequencies
this limits the data rate that can be carried on
the transmission medium
square waves have infinite components and hence an infinite
bandwidth
most energy in first few
components
limi@ng bandwidth creates
distor@ons
There is a direct relationship between data rate and bandwidth.
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Analog and Digital Data Transmission • data
– en22es that convey informa2on
• signals – electric or electromagne2c representa2ons of data
Ø signaling – physically propagates along a medium
• transmission – communica2on of data by propaga2on and processing of signals
Acous2c Spectrum (Analog)
Digital Data
Examples:
Text
Character strings
IRA: International Reference Alphabet (7 Bits => 128 Patterns)
Advantages & Disadvantages of Digital Signals
Audio Signals • frequency range of typical speech is 100Hz-‐7kHz • easily converted into electromagne2c signals • varying volume converted to varying voltage • can limit frequency range for voice channel to 300-‐3400Hz
Video Signals
• to produce a video signal a TV camera is used
• USA standard is 483 lines per frame, at a rate of 30 complete frames per second – actual standard is 525 lines but 42 lost during ver2cal retrace
• horizontal scanning frequency is 525 lines x 30 scans = 15750 lines per second
• max frequency if line alternates black and white
• max frequency of 4.2MHz
Conversion of PC Input to Digital Signal
-‐
Analog Signals
Digital Signals
Analog and Digital
Transmission
Transmission Impairments
• signal received may differ from signal transmiced causing: – analog -‐ degrada2on of signal quality – digital -‐ bit errors
• most significant impairments are – acenua2on and acenua2on distor2on – delay distor2on – noise
ATTENUATION
Received signal strength must be: • strong enough to be detected • sufficiently higher than noise to be received without error
Strength can be increased using amplifiers or repeaters.
Equalize acenua2on across
the band of frequencies used by using loading coils or amplifiers.
Ø signal strength falls off with distance over any transmission medium
Ø varies with frequency
Acenua2on Distor2on
Delay Distor2on • occurs because propaga2on velocity of a signal through a guided medium varies with frequency
• various frequency components arrive at different 2mes resul2ng in phase shiis between the frequencies
• par2cularly cri2cal for digital data since parts of one bit spill over into others causing intersymbol interference
Noise unwanted signals inserted between transmicer and receiver
is the major limi2ng factor in communica2ons system performance
No = kTB
Categories of Noise
Intermodula@on noise
• produced by nonlineari2es in the transmicer, receiver, and/or intervening transmission medium
• effect is to produce signals at a frequency that is the sum or difference of the two original frequencies
Categories of Noise Crosstalk:
– a signal from one line is picked up by another
– can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals Impulse Noise:
– caused by external electromagne2c interferences
– noncon2nuous, consis2ng of irregular pulses or spikes
– short dura2on and high amplitude – minor annoyance for analog signals
but a major source of error in digital data
Channel Capacity
Maximum rate at which data can be transmi<ed over a given communica@ons channel under given condi@ons
data rate
in bits per second
bandwidth
in cycles per second or Hertz
noise
average noise level over path
error rate
rate of corrupted
bits
limita@ons due to physical proper@es
main constraint
on achieving efficiency is noise
Nyquist Bandwidth In the case of a channel that is noise free: • if rate of signal transmission is 2B then can carry signal with frequencies no greater than B – given bandwidth B, highest signal rate is 2B
• for binary signals, 2B bps needs bandwidth B Hz • can increase rate by using M signal levels
– C= 2Blog2M (Hartley Law)
• data rate can be increased by increasing signals – however this increases burden on receiver – noise & other impairments limit the value of M
Shannon Capacity Formula • considering the rela2on of data rate, noise and error rate: – faster data rate shortens each bit so bursts of noise corrupts more bits
– given noise level, higher rates mean higher errors • Shannon developed formula rela2ng these to signal to noise ra2o (in decibels)
• SNRdB=10 log10 (signal/noise) • capacity C = B log2(1+SNR)
– theore2cal maximum capacity – get much lower rates in prac2ce
• SNR = Eb/No * Rb/B
Summary • transmission concepts and terminology
– guided/unguided media
• frequency, spectrum and bandwidth • analog vs. digital signals • data rate and bandwidth rela2onship • transmission impairments
– acenua2on/delay distor2on/noise • channel capacity
– Nyquist/Shannon
QUESTIONS ? REMINDER: PROJECT PROPOSALS ARE DUE OCTOBER 10
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