Physical Layer

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

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.

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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.

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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.

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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).

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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(tT) s(t) -? lt t lt ?
• 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.

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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.

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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 A0.5
• (c) Increased frequency f 2, thus T ½
• (d) Phase shift ? ?/4 radians (45 degrees)
• note 2? radians 360 1 period

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Time-Domain View of Signals
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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

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Frequency-Domain Concepts
14
Frequency-Domain Concepts
15
Frequency-Domain Concepts
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Frequency-Domain Concepts

• Fundamental frequency - when all frequency
  components of a signal are integer multiples of
  one frequency, its 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-f2f
• Many signals have infinite bandwidth
• Effective bandwidth (or just bandwidth) - narrow
  band of frequencies that most of the signals
  energy is contained in

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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.

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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

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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.

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

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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).

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The Electromagnetic Spectrum

• The FCC is not bound by WARCs 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.

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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.

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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.

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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

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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).

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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.

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Satellites
37
Satellites

• Applications
• Television distribution
• Long-distance telephone transmission
• Private business networks

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Satellites

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

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Satellites

• Types of communication satellites
• Multiple MEOs and LEOs are needed to complete
  communications.
• LEOs must be replaced every few years because the
  Earths 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 Earths surface such as near the equator.

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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