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Physical Layer
Useful References
 Wireless Communications and Networks by
William Stallings
 Computer Networks (third edition) by
Andrew Tanenbaum
 Computer Networking (second edition) by
J. Kurose and K. Ross
Network protocol stack
 application: supporting network
 transport: host-host data transfer
 network: routing of datagrams from
source to destination
IP, routing protocols
 link: data transfer between
neighboring network elements
PPP, Ethernet
 physical: bits “on the wire”
Transformation of Information to
 Information like text, voice, pictures can go
through an encoder.
 The encoder can transform the information to
either an analog or digital signal. This encodes the
 A signal is what travels on a communication
 A signal can be viewed as a function of time (timedomain) or a function of its frequencies
(frequency-domain). More on this later.
Analog and Digital Data
 An analog signal is one in which the signal intensity
varies in a smooth fashion over time
 A digital signal is one in which the signal intensity
maintains a constant level for some period of time
and then changes to another constant level.
Analog and Digital Data
 Analog data takes on continuous values in
some interval.
Examples: voice, video
 Digital data takes on discrete values.
 Examples:
 Analog data can be encoded using either
analog or digital signals.
 Digital data can be encoded using either
analog or digital signals.
Analog and Digital Data
 Digital signals are less susceptible to noise
interference, but suffer more from
attenuation than do
 Analog signals can be propagated over a
variety of media including copper wire,
twisted pair, coaxial cable; and atmosphere
or space propagation (wireless).
Time-Domain View of Signals
 Some signals repeat themselves over fixed intervals of time.
Such signals are said to be periodic.
 A signal s(t) is periodic if and only if:
s(t+T) = s(t) - < t < +
where the constant T is the period.
 A periodic signal is one where the same signal
pattern repeats over time.
 The sine wave is the fundamental analog signal.
 We study periodic signals since measuring how
fast a communications medium is done by
measuring how quickly an oscillating signal can be
Time-Domain View of Signals
 A generic sine wave
 Amplitude A: Peak value of a signal at any time.
 Frequency f: Inverse of the period (f = 1/T) represents
number of cycles per second (measured in Hertz (Hz)) i.e.,
this is the rate at which the signal repeats.
 Phase : Relative position within a signal period.
Time-Domain View of Signals
 General sine wave
s(t ) = A sin(2ft + )
 The figure on the next pages shows the effect of
varying each of the three parameters
A = 1, f = 1 Hz,  = 0; thus T = 1s
 (b) Reduced peak amplitude; A=0.5
 (c) Increased frequency; f = 2, thus T = ½
 (d) Phase shift;  = /4 radians (45 degrees)
 (a)
 note: 2 radians = 360° = 1 period
Time-Domain View of Signals
Frequency Domain Concepts
 In practice, an electromagnetic signal will be made
up of many frequencies. For example,
s(t) = (4/) x (sin(2ft) + (1/3) sin(2(3f) t)
 The components of this signal are just sine waves of
frequencies f and 3f
Frequency-Domain Concepts
Frequency-Domain Concepts
Frequency-Domain Concepts
Frequency-Domain Concepts
 Fundamental frequency - when all frequency components
of a signal are integer multiples of one frequency, it’s
referred to as the fundamental frequency.
The period of the total signal is equal to the period of the
fundamental frequency.
The spectrum of a signal is the range of frequencies that a
signal contains (measured in Hz)
Absolute bandwidth - width of the spectrum of a signal; for
out example the spectrum is 3f-f=2f
Many signals have infinite bandwidth
Effective bandwidth (or just bandwidth) - narrow band of
frequencies that most of the signal’s energy is contained in
Frequency-Domain Concepts
 Any periodic signal can be expressed as a sum of
sine waves using “Fourier Analysis”.
 This includes a square wave.
The square wave has an infinite bandwidth.
Relationship between Data Rate
and Bandwidth
 Suppose we let a positive pulse represent a
zero and a negative pulse represents a one.
The following represents 01010
The Electromagnetic Spectrum
 The amount of information that an
electromagnetic wave can carry is related
to its bandwidth.
 Lower frequencies implies fewer bits can
be transmitted per second.
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses
for communication.
The Electromagnetic Spectrum
 To prevent chaos, there are national and
international agreements about who gets to
use which frequencies.
 The FCC in the US and the CRTC in Canada
allocate spectrum for AM/FM radio,
television and cellular phones as well as for
telephone companies, police, military, etc
 Worldwide is done by an agency of ITU-R
The Electromagnetic Spectrum
 The FCC is not bound by WARC’s
 For example,
The FCC chose a different piece of the
bandwidth from what WARC recommended for
personal communications.
 Why? The people who “owned” the WARC
recommended bandwidth had the political clout.
 As a result, personal communications built
for the US market will not work in Europe
or Asia, and vice-versa.
The Electromagnetic Spectrum
 The FCC (Federal Communications Commission)
sells segments of the spectrum to wireless
communications companies and other
Usually, a certain range of hertz is auctioned when
the need for more space becomes apparent.
Selling is done through an auction with about 4 to
6 months of warning.
There can be multiple bidding rounds.
How to winning bidders pay for this? Higher costs
to customers.
Physical Medium
 When a bit is transferred from source to
destination, it is being transmitted from one end
system, through a series of links and routers, to
another end system.
 The source end system first transmits the bit; the
first router transmits the bit, etc
 A bit, when traveling from source to destination,
passes through a series of transmitter-receiver
 For each transmitter-receiver pair, the bit is sent
by propagating electromagnetic waves across a
physical medium.
Physical Medium
 The physical medium can take many shapes and
forms and does not have to be of the same type
for each transmitter-receiver pair;
 Two Categories:
Guided Media
• Waves are guided along a solid medium.
• Examples: twisted pair, coaxial cable, fiber optics
Unguided Media
• Waves propagate in the atmosphere and in outer space
• Examples: radio, infrared, microwave, satellite
 By attaching an antenna of the appropriate size to
an electrical circuit, the electromagnetic waves
can be broadcast efficiently and received by a
receiver some distance away.
 A network that uses electromagnetic radio waves
is said to operate at radio frequency.
 The antennas used with RF networks may be large
or small depending on the range designed.
 Example:
 An antenna designed to propagate signals
several miles across town may consist of a
metal pole approximately two meters long that
is mounted vertically on a building.
 An antenna design to permit communication
within a building may be small enough to fit
inside a portable computer.
 Radio waves are easy to generate, can travel long
distances and penetrate buildings easily.
 Radio waves are omnidirectional, meaning that
they travel in all directions from the source. This
means that the transmitter and receiver do not
have to be carefully aligned.
 Disadvantages
Since radio may go a long distance, interference
is possible. Thus, governments tightly license
the user of radio transmitters.
 May require a license
 More expensive than copper wire and glass
fiber (used in our wired networks)
 High maintenance costs
 Radio frequency transmission is used in
multiple areas of wireless communications.
 HomeRF was designed specifically for home
and small offices.
HomeRF operates on a variety of data and voice
products, providing data networking among PCs,
printers and cordless phones.
 HomeRF has a range of up to 150 feet and can
send and receive signals through walls anf
 Can reach data rates of a little more than
 Wireless Fidelity (Wi-Fi)
Part of the 802.11b standard
 Deployed in airports, restaurants, buildings
 Most laptops manufactured by Dell, Apple, IBM
and Toshiba have Wi-Fi technology built into
their devices.
 Wi-Fi offers speeds of up to 12 Mbps and
covers 30 precent more area than HomeRF.
 A microwave antenna is like a dish.
 The antenna is fixed rigidly and focuses a
narrow beam to achieve line-of-sight
transmission to the receiving antenna.
 To achieve long-distance transmission, a
series of microwave relay towers is used.
 Microwaves are a higher frequency version of
radio and thus can carry more information then
lower frequency RF transmissions.
 Single direction transmission
 Often placed at substantial heights above ground
level so that they can transmit over intervening
 Disadvantages
 Must have a clear path for transmission since
microwaves cannot penetrate metal structures.
 Primarily used in long-haul
telecommunications as an alternative to
coaxial cable or optical fiber.
 Another application is for short point-to-
point links between buildings. This can be
used for closed-circuit TV or as a data link
between local area networks.
 Covers a substantial portion of the
spectrum (from 2 to 40).
 A satellite is in effect a microwave relay
 It is used to link two or more ground-based
microwave transmitter/receivers known as
ground stations.
 The satellite receives transmissions on one
frequency band, amplifies or repeats the
signal, and transmits it on another
 Applications
Television distribution
 Long-distance telephone transmission
 Private business networks
 Types of communication satellites
• Geostationary Earth Orbit (GEO) –
22,282 miles above the Earth’s
• Medium Earth Orbit (MEO) - 6000 to
12000 miles.
• Low Earth Orbit (LEO) - 200 - 400
 Types of communication satellites:
Multiple MEOs and LEOs are needed to
complete communications.
 LEOs must be replaced every few years
because the Earth’s gravitational pull drags the
satellites down from their original orbit.
 GEOs need to replaced less often than LEOs or
MEOs, but they encounter problems with
certain areas of Earth’s surface such as near
the equator.
 Infrared is limited to a small area (e.g., a single
 Transmitter should be pointed toward the
 Commonly used for wireless remote
 Advantages
No antenna required
 Disadvantages
 Transmission limited to line of sight
 Limited to a room with all the computers visible