Chapter 2 Physical Layer.docx

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Chapter 2: The Physical Layer
2.1 The Theoretical Basis for Data Communication
2.1.2: Bandwidth-Limited Signals
-no transmission facility can transmit signals without losing some power in the process.
Distortion: all transmission facilities diminish different Fourier components by different amounts
-for wires, the amplitudes are transmitted mostly undiminished from 0 up to some frequency (measured
in cycles/sec or Hertz) with all frequencies above this cutoff attenuated.
Bandwidth: the width of the frequency range transmitted without being strongly attenuated. The cutoff
is not really sharp, so the quoted bandwidth is usually from 0 to the frequency at which the received
power has fallen by half.
Bandwidth is a physical property of the transmission medium that depends on the construction,
thickness, and length of a wire or fiber.
Filters are used to further limit the bandwidth of a signal which allows more signals share a
given region of spectrum which improves the overall efficiency.
The frequency range for some signals will not start at 0 but this doesn’t matter because the
bandwidth is still the width of the band of frequencies that are passed, and the information
carried depends only on the width and not on the starting and ending frequencies.
Baseband: signals that run up from 0 to maximum frequency
Passband: signals that are shifted to occupy a higher range of frequencies (wireless transmissions)
Digital Transmission: the goal is to receive a signal with just enough fidelity to reconstruct the sequence
of bits that was sent
-limiting the bandwidth limits the data rate, even for perfect channels
Analog Bandwidth: a quantity measured in Hz
Digital Bandwidth: is the maximum data rate of a channel; a quantity measured in bits per second. That
data rate is the end result of using the analog bandwidth of a physical channel for digital transmission.
2.2 Guided Transmission Media
2.2.1: Magnetic Media
-common way to transport data from one compute to another is to write them on magnetic tape or
removable media (recordable DVD), physically transport the tape or disks to the destination machine,
and read them back.
-effective bandwidth of a 1000 tapes in a box equaling 800 terabytes being delivered in the US in 24
hours by express mail is 6400 terabits/86400 seconds
2.2.2: Twisted Pairs
-most common transmission media is twisted pair
Twister Pair: consists of two insulated copper wires, typically about 1mm thick. The wires are twisted
together in a helical form, in which the twisting is done because two parallel wires constitute a fine
antenna. When the wires are twisted, the waves from different twists cancel out, so the wire radiates
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less effectively. A signal is carried as the difference in voltage between the two wires in the pair which
provides a better immunity to external noise. (Telephone System common application)
-able to run several kilometers without amplification, for longer distances the signal becomes too
attenuated and repeaters are needed
-when many twisted pairs run in parallel for a substantial distance, they are bundled together and
encased in a protective sheath; the pairs in the bundles would have interfered with each other if it were
not for the twisting
-Twisted pairs can be used for transmitting analog or digital information. Bandwidth depends on the
thickness of the wire and the distance travelled but several megabits/sec
Full-Duplex Links: links that can be used in both directions at the same time
Half-Duplex Links: links that can be used in either direction but only one way at a time
Simplex Links: links that allow traffic in only one direction
Shielding: reduces the susceptibility to external interference and crosstalk with other nearby cables to
meet demanding performance specifications
-LAN: 100-Mbps Ethernet uses two (out of the four) pairs, one pair for each direction, 1-Gbps uses all
four pairs in both directions simultaneously to reach higher speeds
2.2.3: Coaxial Cable
Coaxial Cable: consists of a stiff copper wire as the core, surrounded by an insulating material. The
insulator is encased by a cylindrical conductor. The outer conductor is covered in protective plastic
sheath.
Two kinds:
50-ohm cable: commonly used when it is intended for digital transmission from the start
75-ohm cable: commonly used for analog transmission and cable television
-It has better shielding and greater bandwidth than unshielded twisted pairs so it spans longer distances
at higher speeds. It provides noise immunity and offer up to GHz bandwidth. (Widely used within
telephone systems for long distance but has now been replaced by fiber)
2.2.4: Power Lines
Power Lines: deliver electrical power to houses, and electrical wiring within houses distributes the
power to electrical outlets
-the data signal is superimposed on the low-frequency power signal (on the active or “hot” wire) as both
signals use the wiring at the same time.
-the difficulty with using household electrical wiring for a network is that it is designed to distribute
power signals which are different from data signals. The electrical properties of the wiring vary from
house to house and change as appliances are turned on and off, which cause data signals to bounce
around the wiring.
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-Practical to send at least 100 Mbps over typical household electrical wiring by using communication
schemes that resist impaired frequencies and burst of errors
2.2.5: Fiber Optics
Fiber Optics: optical transmission systems have three components: the light source, the transmission
medium, and the detector. A pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. The
transmission 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 transmission system that accepts an electrical signal, converts, and transmit it by
light pulses, and then reconverts the output to an electrical signal at the receiving end.
-are used for long haul transmission in network backbones, high speed LANs, and high speed Internet
such as FttH (Fiber to Home)
-the cost to install fiber over the last mile to reach consumers and bypass the low bandwidth of wires
and limited availability of spectrum is extremely expensive + cost more energy to move bits than to
compute.
Multimode Fiber: each light ray has a different mode (any light ray incident on the boundary above the
critical angle will be reflected internally, many different rays will be bounced around at different angles).
The core is typically 50 microns in diameters and about the thickness of human hair.
Single-Mode Fiber: if the fiber’s diameter is reduced to a few wavelengths of light the fiber acts like a
wave guide and the light can propagate only in a straight line, without bouncing (more expensive but
widely used for long distance). The core is 8 to 10 microns.
Transmission of Light through Fiber
-Optical fibers are made of glass which in turn is made from sand (inexpensive raw material)
Three wavelengths: 0.85, 0.30, 1.44 microns
Chromatic Dispersion: light pulses sent down a fiber spread out in length as they propagate. The
amount of it is wavelength dependent. One way to keep these spread out pulses from overlapping is to
increase the distance between them by reducing the signaling rate.
Fiber Cables
Fiber Optic Cables: at the center is the glass core through which the light propagates. The core is
surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the
core. Next is a thin plastic jacket to protect the cladding. They are typically grouped in bundles,
protected by an outer sheath.
Fibers Connected by 3 Ways:
1) They can terminate in connectors and be plugged in fiber sockets. Connectors lose about 10-20%
of the light, but they make it easy to reconfigure systems
2) They can be spliced mechanically. Mechanical splices just lay the two carefully cut ends next to
each other in a special sleeve and clamp them in place. Results in 10% light loss.
3) Two pieces of fiber can be fused (melted) to form a solid connection.
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