Computer Networks
Physical Layer
Lecture 6
The Physical layer
Connectivity-Networking Media
Dr:- Rania Abul Seoud
R-abulseoud@k-space.org
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LAN Physical Layer
The function of media is to
carry a flow of information
through a LAN.
Networking media is
considered Layer 1, or physical
layer, components of LANs.
Several symbols are used to
represent physical media types.
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LAN Physical Layer
Each media has advantages and disadvantages
Some of the comparisons concern:
• Cable length: how far can signal travel
before attenuation.
• Cost.
• Ease of installation.
• Capability to interference.
• Speed of bit transmission.
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Physical Media Types
Copper
Coaxial Cable - Thick or Thin
Unshielded Twisted Pair - CAT 3,4,5,5e&6
Optical Fiber
Multi mode
Single mode
Wireless
Short Range
Medium Range (Line of Sight)
Satellite
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Copper Media
Coaxial Cable
Coaxial cable consists of a copper conductor
surrounded by a layer of flexible insulation.
Over this insulating material is a woven copper
braid or metallic foil that acts as the second wire in
the circuit and as a shield for the inner conductor.
This second layer, or shield also reduces the
amount of outside electromagnetic interference.
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Copper Media
Coaxial Cable
 Poor shield connection is one of the biggest sources
of connection problems.
 Connection problems result in electrical noise that
interferes with signal transmission.
 Benefits include:
 Less expensive than fiber.
 Requires fewer repeaters than UTP (run longer
distances),and the technology is well known.
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Copper Media
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Coaxial Cable
Coaxial cable design
Coaxial cable connectors
N Type
F Type
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Cable Specifications
10BASE5
 10: refers to the speed of transmission is 10
Mbps.
 BASE: refers to the type of transmission is
baseband.
 The 5 indicates that a signal can travel for
approximately 500m (5x100m) before attenuation.
 It is often referred to as Thicknet.
 Thicknet is the type of the network 10BASE5 is
the cable used in that network.
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Cable Specifications
10BASE2
 10: refers to the speed of transmission is 10
Mbps.
 BASE: refers to the type of transmission is
baseband.
 The 2 indicates that a signal can travel for
approximately 200m (2x100m) before attenuation.
 The maximum segment length is actually 185 m.
 It is often referred to as Thinnet.
 Thinnet is the type of the network and 10BASE2 is
the cable used in that network.
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Shielded Twisted-Pair Cable
• STP combines the techniques of shielding, cancellation,
and twisting of wires.
• Each pair of wires is wrapped in metallic foil.
• The four pairs of wires are wrapped in an overall metallic
braid or foil.
• STP affords greater protection from all types of external
interference, but is more expensive and difficult to install
than UTP.
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Shielded Twisted-Pair Cable
STP design
Color-coded plastic insulation.
Twisted pair.
Pair shields / Foil shields.
Overall shield / Braided shield.
Outer jacket.
STP connector
STP cable is installed with RJ-45 connector.
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Unshielded Twisted Pair (UTP)
• It is used in a variety of networks.
• UTP consists of 4 pairs (8 wires) of physical
insulated copper wires typically about 1 mm thick.
• The wires are twisted together in a helical form.
• Each of the eight copper wires in the UTP cable is
covered by insulating material.
• UTP relies on the cancellation effect produced by
the twisted wire pairs to limit signal degradation
caused by Electromagnetic Interference (EMI) and
Radio Frequency Interference (RFI).
• Twisting of wires reduces the interference between
pairs of wires.
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Unshielded Twisted Pair (UTP)
Advantages of UTP
• It is a thin, flexible cable that is easy to string
between walls.
• It is easy to install
• It is small so it does not quickly fill up wiring ducts.
• UTP costs less per meter than any other type of
LAN cables.
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Unshielded Twisted Pair (UTP)
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Cable Specifications
10 BASE - T
 10: refers to the speed of transmission is 10 Mbps.
 BASE: refers to the type of transmission is baseband.
 T: stands for twisted pair (STP, UTP).
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10 BASE - T
50 m
Switch
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100 m
150 m
Workstation
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UTP Connector
UTP cable is installed with RJ-45 connector.
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UTP Implementation
• Inside the UTP cable there are 8 physical insulated
copper wires which are arranged in pairs .
• One pair is used to send information while the
other pair is used to receive information.
• On a PC, the pair on pins 1 and 2 of the connector
sends information and the pair on pins 3 and 6
receives information.
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Sources of Noise on Copper Media
Noise is any electrical energy on the transmission
cable that makes it difficult for a receiver to
interpret the data sent from the transmitter.
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Cables Types
1. Crossover Cable.
2. Straight-Through Cable.
3. Rollover Cable.
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Crossover Cable
• To connect 2 PCs we need to connect the “send” pair of
one PC to the “receive” pair of the other PC
and (vice-a-versa) so we need a cable with different ends.
• Crossover Cable is used to patch between similar types of
equipment (PC to PC or switch to switch).
• Crossover Cable is the cable that connects from one
switch port to another switch port.
• Crossover Cable has one end with the “Orange” set of
wires switched with the “Green” set of wires.
• Crossover Cable connects like devices.
• Notice that pin 1 on one side connects to pin 3 on the other
side and pin 2 connects to pin 6 on the opposite end.
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Crossover Cable Implementation
• One can compare the colors of the two RJ-45 connectors
ends of the same cable by placing them next to each other
with the clip placed into the hand and the top of both ends
of the cable pointing away from you.
• Looking at the RJ-45 connector with the clip facing away
from you, brown is always on the right and pin 1 is on the
left.
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Crossover Cable
Hub or Switch
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UTP Crossover Cable
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Straight-Through Cable
• Straight-through cable has identical ends.
• Straight-through cable is the cable that connects
from one switch port to another switch port.
• Straight-through cable is used to patch between
different types of equipment (PC to hub or switch
to PC)
• The transmit bin of the source needs to connect
to the receiving bin of the destination.
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UTP Straight-Through Cable
Hub or Switch
Host or Router
Host or Router
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Rollover Cable
• Rollover Cable is typically flat and has a light blue
color to help distinguish it from other types of
network cabling.
• This cable gets the name rollover because the
pinouts on one end are reversed from the other
end as if the wire had been rolled over and you
were viewing it from the other side.
• One can configure the router by connecting a
computer terminal (computer port) to a router's
console port.
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Router
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Routers + Rollover Cable
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Rollover Cable
Old, The adaptor, was
sold the rolled cable,
The DB9 to RJ45 converter
is coming molded to the
console cable, like this:
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Routers + Rollover Cable
Router
Console
port
Rollover
cable
Com1 or Com2
serial port
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Terminal or
a PC with
terminal
emulation
software
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Straight-Through or Crossover
Use straight-through cables for the following
cabling:
• Switch to router.
• Switch to PC or server.
• Hub to PC or server.
Use crossover cables for the following cabling:
• Switch to switch.
• Switch to hub.
• Hub to hub.
• Router to router.
• PC to PC.
• Router (Ethernet port) to PC.
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Unshielded Twisted Pair (UTP)
Straight-through
Cross-over
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Rollover
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Cables Types
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Cabling – Show straight-through cables
and crossover cables
router
switch
hub
hub
switch
hub
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hub
hub
hub
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Cabling – Show the straight-through and crossover cables
Straight-through cable
Crossover cable
router
switch
switch
hub
hub
hub
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hub
hub
hub
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Optical Fiber
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Ray Model of Light
 In a vacuum light travels in a straight line.
 In an other material light travels at different slower
speeds.
 The incident ray crosses the boundary.
 Some of rays will be reflected back (reflection).
 Some of rays will enter the glass (refraction) depending
on the angle of incident.
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Reflection
•Air to Glass
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Reflection
•Glass to Air
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Total Internal Reflection
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Total Internal Reflection
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Optical Fiber Cable Structure
 The optical fiber is designed to guide the light waves through the
fiber with a minimum energy loss (total internal reflection).
 Core is the part through which the light rays travel and the light
transmission element at the center of the optical fiber cable.
 Light rays can only enter the core if their angle is inside the
numerical aperture of the fiber.
 Surrounding the core is the cladding that is made of silica.
 Surrounding the cladding is a buffer material that is usually
made of plastic.
 Buffer material helps shield the core from damage.
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Physical Media
Optical Fiber Cable Structure
 The strength material surrounds the buffer material (Kevlar
material) preventing the optical fiber cable from being stretched
and the same material is used to produce bulletproof vests.
 The outer jacket surrounds the optical fiber cable to protect the
fiber against abrasion, solvents, and other contaminants.
 the optical fiber cable contains one or several glass fibers
at its core.
 the optical fiber cable may have 1 to over 1000 fibers.
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Optical Fiber Cable Structure
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Optical Fiber Cable Structure
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Physical Media
Fiber Media Modes
 Modes are the paths which the light rays can follow when traveling
down a fiber.
 There are two types of Fiber media modes :
 Multimode fiber
 Single-mode fiber
 Multimode fiber : the diameter of the core is large enough so that
there are many paths which the light rays can take through the fiber
 Single-mode fiber : the diameter of the core is much smaller so that
there is only one path which the light rays can take through the
fiber so that there is only one path which allows the light rays to
travel along one mode inside the fiber.
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Multimode Fiber Optic Cable
• A standard
multimode fiberoptic cable uses an
optical fiber with
either a 62.5 or a
50-micron core and
a 125-micron
diameter cladding.
• A micron is one
millionth of a meter
(1µ).
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Single-Mode Fiber Optic Cable
• The single-mode
core is eight to ten
microns in
diameter.
• Nine-micron cores
are the most
common.
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3.2.6
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Single-Mode Fiber Optic Cable
 Small core.
 Less dispersion.
 Suited for long distance applications (up to 100 Km, 62 .14 mi).
 Uses lasers as the light source often within campus backbones for distance of
several thousand meters.
 Requires very straight path.
 Glass core (5-8 microns).
 Glass cladding 125 microns dia.
 Coating.
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Multimode Fiber Optic Cable
 Larger core than single-mode fiber optic cable (50 or 62 .5 microns or greater).
 Allows greater dispersion and therefore loss off signal.
 Used for long distance applications but shorter than single-mode fiber optic
cable (up to 2 Km, 6560 ft).
 Uses LEDs as the light source often within LANs or distance of couple hundred
meters within a campus network.
 Requires multiple path-sloppy.
 Glass core (50 or 62 .5 microns or greater).
 Glass cladding 125 microns dia.
 Coating.
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Optical Fiber Media
 Every optical fiber cable used for networking consists of two glass
fibers enclosed in separate sheaths.
 One fiber carries transmitted data from device A to device B.
 The second fiber carries data from device B to device A.
 This provides a full-duplex communication link.
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3.2.6
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Optical Fiber Connectors
 Connectors are attached to the optical fiber cable ends so that the
fibers can be connected to the ports on the transmitter and the
receiver.
 The type of connector most commonly used in multimode fiber
optic cable is the subscriber connector (SC connector) and the type
of connector most commonly used in single-mode fiber optic cable
is the Straight Tip connector (ST connector ).
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