Physical Layer
B. Konkoth
The physical layer is responsible for movements of
individual bits from one node to the next.
Physical layer
Physical Links

How to make computers talk across a wire

How to share the wire
4
From Signals to Packets
Analog Signal
“Digital” Signal
Bit Stream
Packets
0 0 1 0 1 1 1 0 0 0 1
0100010101011100101010101011101110000001111010101110101010101101011010111001
Header/Body
Packet
Transmission
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Sender
Header/Body
Header/Body
Receiver
Analog


An Analog signal is any continuous signal for which the
time varying feature is a representation of some other
time varying quantity.
For example, in sound recording, fluctuations in air
pressure (sound) strike the diaphragm of a microphone
which induces corresponding fluctuations in the current
produced by a coil in an electromagnetic microphone, or
the voltage produced by a condenser microphone. The
voltage or the current is said to be an "analog" of the
sound.
Digital


A data technology that uses discrete (discontinuous)
values
The word digital is most commonly used in computing
and electronics, especially where real-world information is
converted to binary numeric form as in digital audio and
digital photography.
Transmission Media
Transmission medium: the physical path between
transmitter and receiver.
 Communication of electromagnetic waves is
guided or unguided.
Guided media: waves are guided along a physical path (eg,
twisted pair, coaxial cable and optical fiber).
Unguided media: means for transmitting but not guiding
electromagnetic waves (eg, the atmosphere and outer
space).

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Repeaters or amplifiers may be used to extend the
length of the medium.
Transmission Media Choices
 Twisted
pair
 Coaxial cable
 Optical fiber
 Wireless communications
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Cables

Coaxial

Twisted pair

Fiber optics
Twisted Pair




Two insulated wires arranged in a spiral pattern
Copper or steel coated with copper
The signal is transmitted through one wire and a
ground reference is transmitted in the other wire.
Typically twisted pair is installed in building telephone
wiring.
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Unshielded Twisted-Pair (UTP)

Typically wrapped inside a plastic cover (for
mechanical protection)

A sample UTP cable with 5 unshielded twisted pairs
of wires
Insulator
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Metal
Categories of UTP Cables
UTP cables are classified according to the quality:

Category 1 ― the lowest quality, only good for voice, mainly found in very
old buildings, not recommended now

Category 2 ― good for voice and low data rates (up to 4Mbps for low-speed
token ring networks)

Category 3 ― at least 3 twists per foot, for up to 10 Mbps (common in
phone networks in residential buildings)


Category 4 ― up to 16 Mbps (mainly for token rings)
Category 5 (or 5e) ― up to 100 Mbps (common for networks targeted for
high-speed data communications)

Category 6 ― more twists than Cat 5, up to 1 Gbps
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Shielded Twisted-Pair (STP)

14
STP cables are similar to UTP cables, except
there is a metal foil or braided-metal-mesh
cover that encases each pair of insulated
wires
Twisted Pair

Limited in distance, bandwidth and data rate
due to problems with attenuation, interference
and noise



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Issue: cross-talk due to interference from other
signals
“shielding” wire (shielded twisted pair (STP)) with
metallic braid or sheathing reduces interference.
“twisting” reduces low-frequency interference and
crosstalk.
Coaxial Cable
Center
conductor
Dielectric
material
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Braided
outer
conductor
Outer
cover
Coaxial Cable

Divided into two basic categories for coax used in
LANs:





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50-ohm cable [baseband]
75-ohm cable [broadband or single channel baseband]
In general, coax has better noise immunity for
higher frequencies than twisted pair.
Coaxial cable provides much higher bandwidth
than twisted pair.
However, cable is ‘bulky’.
Communication channel
Baseband – sends 1 signal or 1 channel at any
given time
 Broadband - enables a single wire to carry
multiple signals at the same time

Optical Fiber
Optical fiber: a thin flexible medium capable
of conducting optical rays.
 Optical fiber consists of a very fine cylinder
of glass surrounded by concentric layers of
glass.
 Attenuation in the fiber can be kept low by
controlling the impurities in the glass.

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Optical Fiber
(a) Geometry of optical fiber
light
cladding
core
(b) Reflection in optical fiber
c
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jacket
Wireless



Infrared - Infrared radiation (IR) is electromagnetic
radiation with a wavelength between 0.7 and 300
micrometers
Microwave - Microwaves are electromagnetic
waves with wavelengths ranging from as long as one
meter to as short as one millimeter
Radio - Radio waves transmit music, conversations,
pictures and data invisibly through the air, often over
millions of miles - wavelengths in the
electromagnetic spectrum longer than microwave
Electromagnetic spectrum