wirelesstech1.ppt

1. Physical Layer

2. 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

3. Network protocol stack
 application: supporting network
applications
 FTP, SMTP, STTP
 transport: host-host data transfer
 TCP, UDP
 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”
application
transport
network
link
physical

4. Transformation of Information to
Signals
 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
data.
 A signal is what travels on a communication
medium.
 A signal can be viewed as a function of time (time-
domain) or a function of its frequencies
(frequency-domain). More on this later.

5. Analog and Digital Data
Transmission
 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.

6. Analog and Digital Data
 Analog data takes on continuous values in
some interval.
 Examples: voice, video
 Digital data takes on discrete values.
 Examples: text,integers
 Analog data can be encoded using either
analog or digital signals.
 Digital data can be encoded using either
analog or digital signals.

7. 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).

8. 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
sent.

9. 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.

10. 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) 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)
 note: 2 radians = 360° = 1 period

11. Time-Domain View of Signals

12. 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

13. Frequency-Domain Concepts

14. Frequency-Domain Concepts

15. Frequency-Domain Concepts

16. 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

17. 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.

18. 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

19. 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.

20. The Electromagnetic Spectrum
The electromagnetic spectrum and its uses
for communication.

21. 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
(WARC).

22. The Electromagnetic Spectrum
 The FCC is not bound by WARC’s
recommendations.
 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.

23. The Electromagnetic Spectrum
 The FCC (Federal Communications Commission)
sells segments of the spectrum to wireless
communications companies and other
organizations.
 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.

24. 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
pairs.
 For each transmitter-receiver pair, the bit is sent
by propagating electromagnetic waves across a
physical medium.

25. 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

26. Radio
 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.

27. Radio
 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.

28. Radio
 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.

29. Radio
 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

30. Radio
 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
floors.
 Can reach data rates of a little more than
20Mbps.

31. Radio
 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.

32. Microwave
 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.

33. Microwave
 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
obstacles.
 Disadvantages
 Must have a clear path for transmission since
microwaves cannot penetrate metal structures.

34. Microwave
 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).

35. Satellites
 A satellite is in effect a microwave relay
station.
 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
frequency.

36. Satellites

37. Satellites
 Applications
 Television distribution
 Long-distance telephone transmission
 Private business networks

38. Satellites
 Types of communication satellites
• Geostationary Earth Orbit (GEO) –
22,282 miles above the Earth’s
surface.
• Medium Earth Orbit (MEO) - 6000 to
12000 miles.
• Low Earth Orbit (LEO) - 200 - 400
miles.

39. Satellites
 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.

40. Infrared
 Infrared is limited to a small area (e.g., a single
room)
 Transmitter should be pointed toward the
receiver
 Commonly used for wireless remote
 Advantages
 Inexpensive
 No antenna required
 Disadvantages
 Transmission limited to line of sight
 Limited to a room with all the computers visible