2. 7.2
Figure 7.1 Transmission medium and physical layer
◼ Transmission media are located below and is controlled by the
physical layer
◼ Reminder: data is represented by signals that are transmitted
between devices in the form of an ElectroMagnetic (EM)
energy that propagates through the transmission media
◼ EM energy includes: power, radio wave, infrared light, visible
light, etc. They all are part of the EM Spectrum
6. 7.6
7-1 GUIDED MEDIA
Guided media, which are those that provide a
conduit from one device to another, include twisted-
pair cable, coaxial cable, and fiber-optic cable.
Twisted-Pair Cable
Coaxial Cable
Fiber-Optic Cable
Topics discussed in this section:
7. 7.7
Figure 7.3 Twisted-pair cable
◼ Consists of two conductors (usually copper)
◼ Each conductor has its own plastic insulation
◼ The two are twisted around each other
◼ One carries the signal, the other is used as a ground reference
◼ The signal used is the difference between the two
◼ Why are they twisted?
◼ The two are subject to noise and/or cross talk from another pair
◼ If the noise source is closer to one side, twisting make the effect
even
◼ When the signal difference is used, the noise cancels out
8. 7.8
Figure 7.4 UTP and STP cables
◼ Unshielded Twisted Pair (UTP) is the most commonly used cable
◼ Shielded Twisted Pair (STP) has a metal foil or braded-mesh covering the
shielded pair
◼ STP is more immune to noise and cross talk due to shielding, but is it bulkier
(i.e.; larger and heavier) and more expensive than UTP
◼ Electronic Industries Association (EIA) has standards that categorize UTP
◼ e.g. Cat 4, cat 5, cat 6, cat 7
◼ The categories are based on the cable quality (1 is the lowest and 7 is the
highest)
13. 7.13
Figure 7.5 UTP connector
◼ The most common UTP connector is the RJ-45
(Registered Jack-45)
◼ RJ-45 is a keyed connector
◼ Commonly used for Ethernet connections
◼ Other types are RJ-11 (phone lines) and RJ-10 (phone
16. 7.16
UTP Applications
◼ Twisted-pair cables are widely used in telephone lines
◼ The local loop (the line that connects the subscriber to the
central office) is mostly a UTP
◼ Digital Subscriber Lines (DSL) used to provide high-speed
data connection utilizes the high bandwidth of the UTP
◼ Local Area Networks (LANs) such as 10Base-T and 100Base-
T use twisted-pair cables
17. 7.17
Figure 7.7 Coaxial cable
◼ Coaxial cables have central core conductor (usually copper wire) enclosed
in an insulating sheath
◼ The sheath is also enclosed in an outer conductor (usually a metal foil
and/or braid)
◼ The outer metallic shield works as a protection against noise and as a
second conductor for ground reference
◼ The cable is protected by a plastics cover
◼ Has higher frequency range than twisted-pair cable
18. 7.18
Table 7.2 Categories of coaxial cables
◼ Coaxial cables are categorized by Radio Government (RG) ratings
◼ Each RG rating defines a unique set of physical specifications
such as:
◼ Gauge (the thickness of the wire)
◼ The thickness and type of the inner insulator
◼ The construction of the shield
◼ The size and type of the outer casting
◼ Each RG rating is usually specified for a certain function and
application (TV cables, LANs, Cameras Sys, ..etc)
20. 7.20
Figure 7.8 BNC connectors
◼ The most common type of coaxial connectors is the BNC
◼ The BNC Connector is used to connect a device
◼ The BNC T is used to branch out a cable
◼ The BNC Terminator is used to at the end of the cable to absorb the signal
and prevent signal reflection back into the cable
21. 7.21
Figure 7.9 Coaxial cable performance
◼ Much higher attenuation than the twisted-pair
◼ More frequent repeaters needed
22. 7.22
Coaxial Cable Applications
◼ Mostly replaced by fiber-optic cables
◼ Earlier used in:
◼ Analog telephone networks (10,000 voice signals per cable)
◼ Traditional cable TV networks for the infrastructure
◼ Traditional Ethernet LANs:
◼ 10Base-2 (Thin Ethernet) using RG-58 with BNC connector for 10
Mbps data rate for a range of 185 m
◼ 10Base-5 (Thick Ethernet) using RG-11 with specialized connectors
for 10 Mbps data rate over a range of 5,000 m
◼ Later used in:
◼ Digital telephone networks (600 Mbps per cable)
◼ Cable TV networks, where RG-59 is used to connect the
subscriber to the infrastructure (mostly fiber-optic now)
24. 7.24
Fiber-Optic Cable
◼ Made of glass or plastic
◼ Transmits signals in the form of light
◼ Uses the light ray reflection and refraction
laws of physics for signal propagation
25. 7.25
Figure 7.10 Bending of light ray
◼ Light travels in straight lines if traveling through a single uniform substance
◼ If the light moves from one substance to another with a different density, the
light changes its direction
◼ The angle of incidence is the angle the light makes with the line
perpendicular to the interface between the two substances
◼ The critical angle of incidence is the angle at which the light travels along
the interface between the two substances
◼ If the angle of incidence is greater than the critical, the light reflects back
◼ The critical angle is a property of the substance
27. 7.27
Figure 7.11 Optical fiber
◼ Optical fibers use reflections to guide a light through a channel
◼ The glass or plastic core is surrounded by a cladding of less
dense glass or plastic such that the light is reflected at the
required angle of incidence
29. 7.29
Optical Fiber Modes
◼ Multimode: multiple beams move through the core in different paths
1. Step-Index fiber:
◼ The density of the core is constant (the index of refraction is fixed)
◼ The light moves is straight lines until it hits the cladding where it
reflects suddenly
2. Graded-Index fiber:
◼ The density of the core decreases as the light moves from the center to the
cladding
◼ The light changes in curved line and reflects smoothly off of the cladding
◼ The received signal is less distorted compared to the step-index
30. 7.30
Optical Fiber Modes
◼ Single-Mode:
◼ Uses step-index fiber and highly focused light source with limited range of angles
◼ Smaller diameter and lower density fiber compared to multimode
◼ Therefore, critical angle ~90 degrees to force all beams to almost propagate
horizontally
◼ Propagation of different beams are almost identical with negligible delays
◼ The beams can be recombined with little distortion to the signal
31. 7.31
Table 7.3 Fiber types
◼ Defined by the ratio of the core to the cladding
diameters usually expressed in micrometers
32. 7.32
Figure 7.14 Fiber construction
◼ Outer jacket is made of PVC or Teflon
◼ Kelvar is used for bulletproof vests
33. 7.33
Figure 7.15 Fiber-optic cable connectors
◼ SC (Subscriber Channel) is used for cable TV
◼ ST (Straight Tip) is used for connecting cables to networking devices
◼ MT-RJ is a new type of connector that is similar in size to the RJ-45
connector
34. 7.34
Figure 7.16 Optical fiber performance
◼ The attenuation is flatter (slower rate of change) than
the twisted-pair and coaxial cables
◼ 10 time less repeaters compared to other guided media
35. 7.35
Optical Fiber Applications
◼ Mostly used for backbone networks where it is highly cost-effective
◼ Transfers at a rate of 1600 Gbps with WDM over SONET networks
◼ Backbone infrastructure for Cable TV networks
◼ LAN over 100Base-FX networks (fast Ethernet) and 1000Base-X
37. 7.37
Advantages and Disadvantages of Optical Fiber
◼ Advantages:
1. High bandwidth: its bandwidth is limited by signal
generation and reception; not by the medium
2. Low attenuation: 50 Km spaced repeaters
3. No EM interference
4. No corrosion
5. Light weight
6. No tapping
◼ Disadvantages
◼ Installation/maintenance
◼ Unidirectional
◼ Expensive
38. 7.38
7-2 UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves
without using a physical conductor. This type of
communication is often referred to as wireless
communication.
• Radio Waves
• Microwaves
• Infrared
Topics discussed in this section:
39. 7.39
Figure 7.17 Electromagnetic spectrum for wireless communication
◼ Transporting EM waves without a physical conductor (wireless)
◼ Signals are transmitted (broadcasted) over the air
◼ Frequency range: 3 KHz to 900 THz
◼ Types of unguided signal propagations:
1. Ground
2. Sky
3. Line-of-sight (LOS)
40. 7.40
Figure 7.18 Propagation methods
Ground
◼ Low frequency
◼ Travels in all
directions
◼ Follow curvature of
the earth
◼ Used navigation
systems
SKY
◼ Higher frequency
◼ Travels upward
◼ Reflects off of the
Ionosphere
◼ Greater distance with
lower power
◼ Used in: AM, FM, &
Line-of-Site
◼ Very high freq.
◼ Straight line
transmission
◼ Point-to-point
◼ High towers needed
◼ Used in: Radar &
Satellites & Cell
42. 7.42
Figure 7.19 Wireless transmission waves
◼ Radio Wave uses : 3 KHz to 1 GHz
◼ Microwaves uses: 1 to 300 GHz
◼ Infrared: 300 GHz to 400 THz
◼ Division is based on the wave behavior rather than the frequency ranges
◼ Radio waves are mostly omnidirectional (all directions)
◼ Microwave are mostly unidirectional (point-to-point)
43. 7.43
Figure 7.20 Omnidirectional antenna
◼ Signals propagate in all directions
◼ Suffer from co-channel interference (interference on the same channel)
◼ Sky mode radio waves travel very long distances (e.g. AM and FM)
◼ Low and medium frequencies can penetrate through objects
◼ No signal containing but good reception all over
◼ Relatively narrow bandwidth with limited data communications data rates
◼ Most bands are regulated (i.e.; licensed)
44. 7.44
Figure 7.21 Microwave and Unidirectional antennas
◼ Line-of-sight communications
◼ VHF cannot penetrate through objects (e.g. walls)
◼ Immune from interference
◼ Wide frequency band of about 299 GHz (1 to 300 GHz )
◼ Good potential for very high data rate transmission
◼ Mainly regulated except for the license-free or ISM (Industrial
Scientific, and Medical 2.4 GHz & 5.0 GHz)) bands
45. 7.45
Microwaves are used for unicast
communication such as cellular
telephones, satellite networks,
and wireless LANs.
Note
46. 7.46
Infrared
◼ 300 GHz to 400 THz
◼ Line-of-sight very short-range and very high data rate
communications
◼ Cannot penetrate through objects (e.g. walls)
◼ Immune from interference
◼ Use only for inside applications: such as remote control, PC
data transfer, etc.
◼ IrDA (Infrared Data Association)
◼ It is a Standard body for IR communications
◼ It Defines standards for communication between PC and
peripheral devices (e.g USB,
◼ Rate=75 kbps, range= up to 8 meters line-of-site
communications
