Transcript Physical Layer Part 3

Chapter 7
Transmission Media
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Figure 7.1
Transmission medium and physical layer
Transmission medium(channel): the physical
path between the transmitter and the receiver.
Located below the physical layer
Controlled by the physical layer
Figure 7.2
Classes of transmission media
7.1 Guided Media
Twisted-Pair Cable
Coaxial Cable
Fiber-Optic Cable
Figure 7.3
Twisted-pair cable
 A twisted
pair consists of: two insulated copper wires
in a regular spiral pattern
A wire pair acts as a single communication link
Twisted to reduce electrical interference from similar
pairs close by (more twists means better quality)
Used in:
Telephone network Between house and local exchange
(subscriber loop)
LAN
Twist length of 7.5 cm to 10 cm
Twist length 0.6 cm to 0.85 cm
Figure 7.4
UTP and STP
UTP: Unshielded Twisted Pair
STP: Shielded Twisted Pair
Unshielded Twisted Pair (UTP)
Used for telephone wire
Cheapest type of cable
Easiest type to be installed
Suffers from external
Electromagnetic (EM)
interference
Shielded Twisted Pair (STP)
Metal braid or covering that
reduces interference
More expensive
Harder to handle (thick,
heavy)
Table 7.1 Categories of unshielded twisted-pair cables
Figure 7.5
RJ=Registered Jack
UTP connector
Twisted pairs
White/Orange
2
White/Green
Orange
Green
3
1
8
The RJ-45
Connector
TD+ : Transmit Data Positive value
TD- : Transmit Data Negative value
RD+ : Receive Data Positive value
RD- : Receive Data Negative value
Brown
White/Orange
Orange
White/Green
Green
Brown
TD+ : Transmit
Data Positive value
TD- : Transmit
Data Negative
value
RD+ : Receive
Data Positive value
RD- : Receive
Data Negative
value
Ethernet Cabling
• Crossover Cabling
• Straight-Through Cabling
1
1
2
2
3
3
1
1
2
2
3
3
6
6
PC
to PC
Hub
6
6
PC
to Switch/Hub
Router
to Switch/Hub
to Hub
Switch
Hub
to Switch
to Switch
Router
to PC
Router
Switch
Hub
Coaxial Cable
– used in bus technology Copper wires (braid or foil)
conductor
External coat

insulating
Connectors
BNC
BNC-T
Terminator
Table 7.2 Categories of coaxial cables
Radio
Government
Category
Use
RG-59
Cable TV
Thin Ethernet
RG-58
10base2
Thick Ethernet
RG-11
10base5
Optical Fiber

consists of three concentric sections
plastic jacket glass or plastic
cover
Fiber core and
clading
 Core: consists of one or more very thin strands or fibers made of glass or
plastic
 Each fiber is surrounded by its own cladding, a glass or plastic coating that has
optical properties different from the core
 Jacket: a plastic or other material acts as a layer to protect against moisture,
crushing, and other environmental dangers.
Fiber Optic Cable
Optical Fiber
Optical Fiber - Advantages
• Greater capacity
– Data rates of hundreds of Gbps
• Smaller size & weight
• Lower attenuation (signal loss)
– Greater repeater spacing
– 10s of km at least
• No crosstalk (no light leaking)
• Electromagnetic isolation
• highly secure (no light leaking)
Optical Fiber - Disadvantages
• Not easy to install and maintain
• Unidirectional, two fibers are needed for
bidirectional
• Cost: more expensive interfaces than electrical
interfaces used with other types (twisted, coaxial)
Optical Fiber - Applications
• Long-distance trunks (1500 km)
• Subscriber loops (to replace twisted pair)
• LANs (100 Mbps – 10 Gbps)
7.2 Unguided Media: Wireless
Radio Waves
Microwaves
Infrared
Figure 7.19
Wireless transmission waves
Unguided media: signals are transmitted
through air and are available to everyone
who has a device that can receive them
The Electromagnetic Spectrum
Figure 7.17 Electromagnetic spectrum for wireless communication
Table 7.4 Bands
Figure 7.18 Propagation methods
Radio waves

30MHz to 1GHz




Omnidirectional (signal propagates in all directions)
 Easily interfere with other signals sent at the same
frequency range
Can penetrate walls and can be received in the
building
Does not require dish-shaped antennas
Broadcast radio (AM,FM) ,TV, cordless phone, Paging,
cellular phones
Note
Radio waves are used for multicast
communications, such as radio and
television, and paging systems.
Microwaves
 2GHz to 300GHz
 Highly directional (line-of-sight propagation = straight lines)
 Requires dish-shaped antennas
 Point to point (sending and receiving antennas need to be
aligned)
 Very high frequency microwaves, usually, cannot penetrate
walls (disadv. if receivers are inside buildings)
 Used in long distance telephone communications
 Used for short point-to-point transmission between buildings
to connect their LANs
 Used in Wireless networks, satellite communication
Note
Microwaves are used for unicast
communication such as cellular
telephones, satellite networks,
and wireless LANs.
Infrared
 300GHz to 400THz




Have a very large bandwidth that is not yet completely utilized
Local- short distance communication
Line-of-sight propagation (directional)
Used in local point-to-point Transmission or Multipoint within a
very limited area (single room)
 Used in Remote control, IrDA (Infrared Data Association) port
(wireless keyboard, mouse)
 Cannot be used under the sun because of the interference
with the sun infrared rays
 IrDA operates at 75kbps up to 8 meters, and 1.15Mbps – 4Mbps
over a distance of 1 meter
Note
Infrared signals can be used for shortrange communication in a closed area
using line-of-sight propagation.