Physical Layer

1
Physical Layer

• Fundamentals
• Transmissions factors
• Transmissions Media

2
Physical Layer

• The physical layer deals with transporting bits
  between two machines.
• How do we communicate 0's and 1's across a
  medium?
• By varying some sort of physical property such as
  voltage or current.

3
Physical Layer

• Our goal is to understand what happens to a
  signal as it travels across some physical media.
• That is, will the receiver see the exact same
  signal generated by the sender? Why or why not?

4
Theoretical Basis for Data Communication

• Fourier AnalysisFourier showed that a periodic
  function g(t) can be represented mathematically
  as an infinite series of sines and cosines
• f is the function's fundamental
  frequency
•  T1/f      is the function's period
•  an  and bn are the amplitudes of the nth
  harmonics

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Theoretical Basis for Data Communication

• The series representation of g(t) is called its
  Fourier series expansion.
• In communications, we can always represent a data
  signal using a Fourier series by imagining that
  the signal repeats the same pattern forever.

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Theoretical Basis for Data Communication

• We can compute the coefficients  an and bn
• Suppose we use voltages (on/off) to represent
  1''s and 0''s, and we transmit the bit string
  011000010'. The signal would look as follows

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Theoretical Basis for Data Communication
8
Theoretical Basis for Data Communication

• Points to note about the Fourier expansion
• The more terms in the expansion, the more exact
  our representation becomes.
• The expression represents the
  amplitude or energy of the signal (e.g., the
  harmonics contribution to the wave).

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Theoretical Basis for Data Communication

• In our example, the amplitude continually gets
  smaller
• The first harmonics are the most important ones
• So what does this have to do with data
  communication?
• The following facts are important

10
Theoretical Basis for Data Communication

• Signals attenuate (strength of signal falls off
  with distance) during transmission. How much
  attenuation occurs? The exact amount is dependent
  on physical properties of the medium.

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Theoretical Basis for Data Communication

• Distortion results because attenuation is
  non-uniform across the frequency spectrum some
  frequencies distort more than others. That is,
  the signal doesn't distort uniformly.
• If every component decreased by the same amount,
  the signal would be weaker, but not distorted,
  and amplifying the signal would restore it.
• Because the received signal is distorted,
  however, amplification simply magnifies the
  distortion and probably won't help.

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Theoretical Basis for Data Communication

• A transmission medium carries signals lying
  within a spectrum or range of frequencies the
  absolute width of the spectrum is called the
  bandwidth of the channel.
• Most channels completely attenuate (e.g. chop
  off) frequencies beyond some threshold value.
• What does this mean in terms of Fourier
  harmonics? In terms of fundamental frequencies of
  a Fourier representation, higher harmonics get
  completely chopped off during transmission and
  are not received at the receiving end!

13
Theoretical Basis for Data Communication

• Conclusion it's essentially impossible to
  receive the exact signal that was sent. The key
  is to receive enough of the signal so that the
  receiver can figure out what the original signal
  was.
• Note bandwidth'' is an overloaded term.
  Engineers tend to use bandwidth to refer to the
  spectrum of signals a channel carries. In
  contrast, the term bandwidth'' is often also
  used to refer to the data rate of the channel, in
  bps.

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Nyquist Theorem

• Noise-free channel
• Limiting factor on transmission is channel BW
• If bandwidth is B, highest signal rate is 2B
• Multi-level signaling
• C 2B log2 M where
• C is the data rate
• B is the bandwidth
• M is the number of levels
• For example, a noiseless 3-kHz channel cannot
  transmit binary signals at a rate exceeding 6000
  bps.

15
Shannons Theorem

• If random noise is present, the situation
  deteriorates rapidly. The amount of noise present
  is measured by the ratio of the signal power to
  the noise power, called the signal-to-noise ratio
  (S/N).
• Usually, the ratio itself is not quoted instead,
  the quantity 10 log10S/N is given. These units
  are called decibels (dB).
• Maximum number of bits/secHlog2(1S/N)
• For telephone line 3000log2(130dB)?30000bps.

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Transmission Media

• The purpose of the physical layer is to transport
  a raw bit stream from one machine to another.
• Various physical media can be used for the actual
  transmission.
• Each one has its own niche in terms of bandwidth,
  delay, cost, and ease of installation and
  maintanence.
• Media are roughly grouped into guided media, such
  as copper wire and fiber optics, and unguided
  media such as radio and lasers through air.

17
Transmission Media Magnetic Tape

• One of the most common ways to transport data
  from one computer to another is to write them
  onto magnetic tape or floppy disks, physically
  transport the tape or disks to the destination
  machine, and read them back again.
• This type of data transportation uses magnetic
  media.
• In this type transportation the feasibility of
  the transport depends on bandwidth of the
  transportation or cost per bit transported

18
Transmission Media Magnetic Tape

• Example 8-mm video tape can hold 7 GB
• A box 50x50x50 cm can hold about 1000 8-mm video
  tape
• That is totaly 7000 GB. (in USA)
  in 24 hours this box can be trasported to
  anywhere
• ?the effective bandwidth of this transmission is

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Transmission Media Magnetic Tape

• 56000 GB/86400 seconds -gt 648 Gbps. Which is 1000
  times better than an high speed version of ATM
  (622 Mbps). But if the destination is very near
  then then bandwidth of the transmission increases
  rapidly.
• The cost of this transmission is roughly 700
  for 7000 GB. That is about 10 cent per GB!!!
  There is no network carrier on earth can compete
  with that. But the possible delays is the
  weakness of this type of tranmission

20
Transmission Media Twisted Pair

• Twisted Pair
• For MOST applications an on-line connection is
  needed.
• For this, the oldest and still very common
  transmission medium is twisted pair.
• A twisted pair consists of two insulated copper
  wires, about 1 mm thick. The wires are twisted
  together in a helical form.
• .

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Transmission Media Twisted Pair

• What is the purpose of twisting the wires? To
  reduce electrical interference from similar pairs
  close by
• The most common application of twisted pair is
  the telephone system. Twisted pairs can run
  several kileometers without amplification, but
  for longer distances the repaters are needed.
• They can be used for either analog or digital
  transmission. The bandwidth depends on the
  thickness of the wire and the distance traveled.
• Data rates of several Mbps common.
• Spans distances of several kilometers.
• Good, low-cost communication

22
Transmission Media Coaxial Cable

• Another common trasmission medium is the coaxial
  cable (baseband) (known as coax).
• It can be used for longer distances at higher
  speeds - for either digital or analog
  transmission
• Data rate depends on physical properties of
  cable, but 10 Mbps is typical.

23
Transmission Media Coaxial Cable

• Broadband Coaxial Cable
• The other kind of coaxial cable system uses
  analog transmission on standard television
  cabling and it is called broadband.
• TV, audio, and data can be mixed on one cable.
• Typically bandwidth of 300 MHz, total data rate
  of about 150 Mbps.
• Operates at distances up to 100 km (metropolitan
  area!).
• Technology used in cable television - it is
  already available at sites such as universities
  that may have TV classes.

24
Transmission Media

• Broadband vs Baseband Coaxial Cable
• Which is better, broadband or baseband?
• There is rarely a simple answer to such
  questions. Baseband is simple to install,
  interfaces are inexpensive, but doesn't have the
  same range.
• Broadband is more complicated, more expensive,
  and requires regular adjustment by a trained
  technician, but offers more services (e.g., it
  carries audio and video too).

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Transmission Media Fiber Optics

• Fiber Optics
• An optical trasmission system has three
  components the light source, the transmission
  medium and the detector.
• A pulse of light indicates a 1 bit and the
  absence of a light indicates a 0 bit.
• The trasmission medium is an ultra-thin fiber of
  glass. The detector generates an electrical pulse
  when light falls on it.
• By attaching a light source to one end of an
  optical fiber and a detector to the other, we
  have a unidirectional data trasmission system
  that accepts an electrical signal, converts and
  trasmits it by light pulses, and then reconverts
  the output to an electrical signal by the
  receiving end.

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Transmission Media Optical Fiber
Single mode fiber
Multimode fiber
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Transmission Media Optical Fiber

• Fiber Optics
• Advantages
• Tremendously high data rate, low error rate. gt
  1000 Mbps (1 Gbps) over distances of kilometers
  common. Error rates are so low they are almost
  negligible.
• Difficult to tap, which makes it hard for
  unauthorized taps as well
• can have multiwavelength transmission

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Transmission Media Optical Fiber

• Much thinner (per logical phone line) than
  existing copper circuits.
• Because of its thinness, phone companies can
  replace thick copper wiring with fibers having
  much more capacity for same volume. This means
  that aggregate phone capacity can be upgraded
  without the need for finding more physical space
  to hire the new cables.
• Not susceptible to electrical interference
  (lightning) or corrosion (rust).
• Greater repeater distance than coax.

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Transmission Media Optical Fiber

• Fiber Optics
• Disadvantages
• Difficult to tap. It really is point-to-point
  technology. In contrast, tapping into coax is
  trivial. No special training or expensive tools
  or parts are required.
• One way channel. Two fibers needed to get full
  duplex (both ways) communication.
• Optical Switching/routing is not trivial

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Transmission MediaWireless Transmission

• Radio omnidirectional, AM, FM Radio, TV, ALOHA
  data network
• Microwave directional
• Terrestrial Microwave, long-haul common carrier,
  government communications.
• Satellite Microwave
• A communication satellite is a microwave relay
  station.

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Transmission MediaWireless Transmission

• Infrared does not penetrate walls, for indoor
  wireless LANs.
• To prevent total chaos, there are national and
  international agreements about who gets to use
  which frequencies. Since everyone wants a higher
  data rate, everyone wants more spectrum.
• Therefore, we have to share.
• FDMA Frequency Division Multiple Access
• TDMA Time Division Multiple Access
• CDMA Code Division Multiple Access
• (using spread spectrum technique)

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Transmission Media-Wireless Transmission
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Transmission Media-Wireless Transmission

• Difficulties
• Weather interferes with signals. For instance,
  clouds, rain, lightning, etc. may adversely
  affect communication.
• Radio transmissions easy to tap. A big concern
  for companies worried about competitors stealing
  plans.
• Signals bouncing off of structures may lead to
  out-of-phase signals that the receiver must
  filter out.