Transcript Document

The Physical Layer
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Highlights of this chapter
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Theoretical Basis for Data Communication
The transmission medias (Guided and Unguided)
The Public Switched Telephone Network
(PSTN)
The Mobile Telephone System
The Physical Layer
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The Objectives
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Understand the theoretical basis for data
communication, know how to computer the
data rate for specific media.
Understand the principles of data
communication for guided and unguided
media.
Study the popular telephony technologies.
The Physical Layer (Cont’d)
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Theoretical Basis for Data
Communication
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Information can be transmitted on wires by
varying some physical property such as
voltage or current.
Fourier Analysis, where f = 1/T is the
fundamental frequency, an and bn are the
sine and cosine amplitudes of the nth
harmonics (terms), and c is a constant.
The Physical Layer (Cont’d)
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Bandwidth-Limited Signals
An example: the transmission
of the ASCII character “b”
encoded in an 8-bit byte. The
bit pattern that is to be
transmitted is 01100010.
The Physical Layer (Cont’d)
According to (a), we have the coefficients:
But, HOW?
root-mean-square amplitudes:
Note: RMS varies according to the harmonics number. (see above figure)
Basic conclusions:
1. (a) is the most ideal, however, it has the most harmonics;
2. (b) is the most simple, and the lest harmonics, but it is difficult to
reconstruct the data;
3. (c) – (e) are trade-offs.
The Physical Layer (Cont’d)
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The definition of Bandwidth:
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The range of frequencies transmitted without
being strongly attenuated is called the bandwidth.
The bandwidth is a physical property of the
transmission medium and usually depends on the
construction, thickness, and length of the medium.
For example, a telephone wire may have a
bandwidth of 1 MHz for short distances, but
telephone companies add a filter restricting each
customer to about 3100 Hz. This bandwidth is
adequate for intelligible speech and improves
system-wide efficiency by limiting resource usage
by customers.
The Physical Layer (Cont’d)
Given a bit rate of b bits/sec, the time required to send 8 bits is
8/b sec, so the frequency of the first harmonic is b/8 Hz. For
voice-grade line, whose bandwidth is 3000 Hz, the number of
the highest harmonic passed through is roughly 3000/(b/8) or
24,000/b .
Conclusions:
1. It should be obvious that at data rates much higher than 38.4
kbps, there is no hope at all for binary signals, even if the
transmission facility is completely noiseless.
2. limiting the bandwidth limits the data rate, even for perfect
channels.
The Physical Layer (Cont’d)
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The Maximum Data Rate of a Channel
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If an arbitrary signal has been run through a low-pass filter
of bandwidth H, the filtered signal can be completely
reconstructed by making only 2H (exact) samples per second.
Sampling the line faster than 2H times per second is
pointless (Nyquist's theorem). If the signal consists of V
discrete levels, we have:
The maximum data rate of a noisy channel whose bandwidth
is H Hz, and whose signal-to-noise ratio is S/N, is given by
(Shannon's theorem):
The Physical Layer (Cont’d)
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Guided Media —— the Twisted Pair
Figure 2-3. (a) Category 3 UTP. (b) Category 5 UTP.
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The most common application of the twisted pair is
the telephone system.
Twisted pairs can be used for transmitting either
analog or digital signals. The bandwidth depends on
the thickness of the wire and the distance
traveled, but several megabits/sec can be achieved
for a few kilometers in many cases.
The Physical Layer (Cont’d)
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Guided Media —— the Coaxial Cable
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Two kinds of coaxial cable are widely used. One kind, 50-ohm
cable, is commonly used when it is intended for digital
transmission from the start. The other kind, 75-ohm cable, is
commonly used for analog transmission and cable television but
is becoming more important with the advent of Internet over
cable.
The bandwidth possible depends on the cable quality, length,
and signal-to-noise ratio of the data signal. Modern cables have
a bandwidth of close to 1 GHz.
The Physical Layer (Cont’d)
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Guided Media —— the Fiber Optics
(a) Three examples of a light ray from inside a silica fiber impinging on the
air/silica boundary at different angles. (b) Light trapped by total internal
reflection
 With current fiber technology, the achievable bandwidth is
certainly in excess of 50,000 Gbps (50 Tbps) and many people are
looking very hard for better technologies and materials.
 An optical transmission system has three key components: the light
source, the transmission medium, and the detector.
 Currently available single-mode fibers can transmit data at 50 Gbps
for 100 km without amplification. Even higher data rates have been
achieved in the laboratory for shorter distances.
The Physical Layer (Cont’d)
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Wireless Transmission —— The
Electromagnetic Spectrum
The electromagnetic spectrum and its uses for
communication.
The wider the band, the higher the data rate.
The Physical Layer (Cont’d)
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The Public Switched Telephone Network
(PSTN)
Computer Networks
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PSTN
Structure of the Telephone System
(a) Fully-interconnected network. (b) Centralized switch. (c)
Two-level hierarchy.
The Physical Layer (Cont’d)
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PSTN Structure
A typical circuit route for a medium-distance call.
A variety of transmission media are used for telecommunication. Local loops
consist of category 3 twisted pairs nowadays. Between switching offices,
coaxial cables, microwaves, and especially fiber optics are widely used.
In the past, transmission throughout the telephone system was analog, with
the actual voice signal being transmitted as an electrical voltage from source
to destination. With the advent of fiber optics, digital electronics, and
computers, all the trunks and switches are now digital, leaving the local loop
as the last piece of analog technology in the system.
The Physical Layer (Cont’d)
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Transmit digital signals on PSTN (Local
Loops)
Modem
ISDN
ADSL (Asymmetric Digital
Subscriber Line)
The Physical Layer (Cont’d)
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Switching
(a) Circuit switching. (b) Packet switching.
The Physical Layer (Cont’d)
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Switching
Timing of events in (a) circuit switching, (b) message switching,
(c) packet switching.
The Physical Layer (Cont’d)
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Switching
A comparison of circuit-switched and packet-switched networks.
The Physical Layer (Cont’d)
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The Mobile Telephone System
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three distinct generations
1 G: Analog voice.
 2 G: Digital voice. (D-AMPS, GSM, CDMA, and
PDC.)
 3 G: Digital voice and data.
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The Physical Layer (Cont’d)
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3G and the future mobile network
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Digital Voice and Data
2 Mbps for stationary indoor users (which will
compete head-on with ADSL), 384 kbps for people
walking, and 144 kbps for connections in cars.
3G will give us:
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Proposed Standards
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High-quality voice transmission.
Messaging (replacing e-mail, fax, SMS, chat, etc.).
Multimedia (playing music, viewing videos, films, TV, etc.)
Internet access (Web surfing, pages with audio and video).
W-CDMA (Wideband CDMA), by Ericsson.
CDMA2000, by Qualcomm.
Both standards are not compatible with GSM.
Stepwise Solutions (GPRS, i.e., 2.5G)
The Physical Layer
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Exercises
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Please try to solve these problems:
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1-4, 7-9, 18-20.