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SEMESTER 1 Chapter 8
Physical Layer
V 4.0
8.1.1
What does the physical
layer provide?
90 Points
provides the means to transport across the
network media the bits that make up a Data
Link layer frame
What are the four elements The physical media and associated connectors
of delivering frames across
A representation of bits on the media
the media?
Encoding of data and control information
Transmitter and receiver circuitry on the
network devices
8.1.2
What are the three basic
Copper cable
forms of media?
Fiber
Wireless
What are the three
the Physical layer encodes the bits into the
concerns when the physical signals for a particular medium, it must also
layer encodes bits?
distinguish where one frame ends and the next
frame begins
8.1.3
What does the physical
hardware, developed by engineers, in the form
layer consist of?
of electronic circuitry, media, and connectors
What are the organizations
The International Organization for
that are responsible for
Standardization (ISO)
physical layer standards?
The Institute of Electrical and Electronics
Engineers (IEEE)
The American National Standards Institute
(ANSI)
The International Telecommunication Union
(ITU)
The Electronics Industry
Alliance/Telecommunications Industry
Association (EIA/TIA)
National telecommunications authorities such
as the Federal Communication Commission
(FCC) in the USA.
8.1.3.2 The technologies defined by Physical and electrical properties of the media
these organizations include Mechanical properties (materials, dimensions,
what four areas of the
pinouts) of the connectors
Physical layer standards?
Bit representation by the signals (encoding)
Definition of control information signals
8.1.4
What are the three
The physical components
fundamental functions of the Data encoding
Physical layer?
Signaling
What are the two values
1 and 0
that the physical layer
encodes?
8.2.1
How are the bits of the
One bit at a time
frame transmitted by the
physical layer?
What is a bit time?
How are the signaling and
receiving ends clocks
synchronized in the LAN?
What characteristics can be
changed to represent bits
on the medium?
What will happen if there is
no agreement between
nodes on the signaling
method?
8.2.1.2 What are the three
characteristics of NRZ
signaling?
What type of transmission is
NRZ suited for?
8.2.1.3 What are represented by in
Manchester encoding?
How does the transition
help maintain clock
synchronization?
Which medium uses
Manchester encoding?
8.2.2
What method is used to
signal start and end of a
frame in higher speed
technologies?
8.2.2.2 What is a code group?
What are the advantages of
transmitting symbols?
What are the four
advantages of using code
groups?
What are the three types of
code groups?
specific amount of time each signal has to
occupy the media
Many signaling methods use predictable
transitions in the signal to provide
synchronization between the clocks of the
transmitting and the receiving devices
Amplitude
Frequency
Phase
The transmission will fail
Discrete impulses
Only two states
Voltage jumps between levels
for slow speed data links
voltage transitions
one voltage transition must occur in the middle
of each bit time
10BaseT Ethernet
Specific pattern of grouped bits
consecutive sequence of code bits that are
interpreted and mapped as data bit patterns
the error detection capabilities and timing
synchronization between transmitting and
receiving devices are enhanced
Reducing bit level error
Limiting the effective energy transmitted into
the media
Helping to distinguish data bits from control
bits
Better media error detection
Data symbols - Symbols that represent the
data of the frame as it is passed down to the
Physical layer.
Control symbols - Special codes injected by
the Physical layer used to control transmission.
These include end-of-frame and idle media
symbols.
Invalid symbols - Symbols that have patterns
not allowed on the media. The receipt of an
invalid symbol indicates a frame error.
8.2.2.3 How does 4B/5B represent
a nibble?
8.2.3
What are the three
measures for Data
Transfer?
8.2.3.2 What is the difference
between throughput and
goodput?
Which measure of data
transfer is most important to
the end user?
8.3.1
What are the standards for
copper media defined for?
8.3.2
What type of connector is
commonly used in copper
cabling?
8.3.2.2 What is done to the pairs of
wires to minimize signal
degradation due to
electronic noise?
What are the other methods
listed to limit the
susceptibility of copper
cables to noise?
8.3.3
What is crosstalk?
What is used to limit
crosstalk?
What changes in the twist
from one pair of wires to the
next pair of wires to improve
the limitation of crosstalk?
What are some of the
elements defined by the
TIA/EIA 568A standard?
Why is it wasteful to install
less expensive but lower
rated cabling?
8.3.3.2 What is UTP cable
terminated with?
as five-bit values known as symbols
Bandwidth
Throughput
Goodput
Throughput is the measure of bits over a given
time and goodput is the measure of usable
data transferred over a given period of time
Goodput
Type of copper cabling used
Bandwidth of the communication
Type of connectors used
Pinout and color codes of connections to the
media
Maximum distance of the media
RJ-45
Shielding and twisting of the wires
Selecting the cable type or category most
suited to protect the data signals in a given
networking environment
Designing a cable infrastructure to avoid
known and potential sources of interference in
the building structure
Using cabling techniques that include the
proper handling and termination of the cables
the interference caused by the magnetic field
around the adjacent pairs of wires in the cable
Twisting pairs of wires
The number of twists per meter changes
Cable types
Cable lengths
Connectors
Cable termination
Methods of testing cable
If the decision is later made to adopt a faster
LAN technology, total replacement of the
installed cable infrastructure may be required.
RJ-45 connectors
What are the three types of
Ethernet cables?
What is a straight-through
used for?
What is a crossover used
for?
What is a rollover used for?
8.3.4
8.3.6
What happens if you use
the wrong type of cable
when connecting devices?
What are the other two type
of copper cabling?
What has replaced the use
of coaxial cable in today’s
Ethernet installations?
What type types of
materials can be used for
fiber optic cabling?
What is the encoding
method for fiber optic?
What are the issues with
implementing fiber optic
cabling?
What areas usually
implement fiber optic
cabling?
What are the major
advantages of using fiber
optic between buildings?
8.3.6.2 How many cables are
needed for each fiber optic
connection? Why?
8.3.6.3 What are the two types of
fiber optic cables?
8.3.7
How do wireless media
carry electromagnetic
signals?
What is the major concern
for wireless signals?
8.3.7.2 What are the 4 standards
for wireless networks?
Ethernet Straight-through
Ethernet Crossover
Rollover
Host to network device such as a hub or switch
Connecting two hosts
Connecting to network intermediary devices
Connecting a computers serial port to the
console port of a router (with an adapter)
Communication will not occur
1. Coaxial
2. Shielded Twisted-Pair (STP)
UTP
Glass or plastic
Light pulses
More expensive (usually) than copper media
over the same distance (but for a higher
capacity)
Different skills and equipment required to
terminate and splice the cable infrastructure
More careful handling than copper media
Backbone and between buildings
optical fiber does not conduct electricity and
has low signal loss
2, they can only operate in one direction
Single mode and multimode
radio and microwave frequencies that
represent the binary digits of data
communications
Security
IEEE 802.11
IEEE 802.15
IEEE 802.16
Global System for Mobile Communications
(GSM)
8.3.7.3 What are the two network
devices necessary to
implement a wireless
network?
8.3.8
What are the WLAN
Ethernet standards?
What are some of the CAT5
termination that may
performed onsite?
8.3.8.2 What are the differences in
the termination of the two
cables pictured?
What could happen if wires
are not terminated
correctly?
8.3.8.3 What are the three main
splicing errors when using
fiber optic cabling?
Wireless Access Point (AP) - Concentrates the
wireless signals from users and connects,
usually through a copper cable, to the existing
copper-based network infrastructure such as
Ethernet.
Wireless NIC adapters - Provides wireless
communication capability to each network
host.
IEEE 802.11a, IEEE 802.11b, IEEE 802.11g,
IEEE 802.11n
crimped connections to terminate Cat5 media
with RJ-45 plugs to make patch cables, and
the use of punched down connections on 110
patch panels and RJ-45 jacks
Insulation cut back to far and wires untwisted
too much
damaging voltage levels may be applied
between interconnected devices
Misalignment - the fiber-optic media are not
precisely aligned to one another when joined.
End gap - the media do not completely touch
at the splice or connection.
End finish - the media ends are not well
polished or dirt is present at the termination.
* The 3 fundamental functions of the Physical layer are
– The physical components
– the electronic hardware devices, media and connectors that transmit and
carry the signals to represent the bits.
– Data encoding
– Encoding is a method of converting a stream of data bits into a predefined
code.
– Signaling
– The Physical layer must generate the electrical, optical, or wireless signals
that represent the "1" and "0" on the media.
* What is Optical Time Domain Reflectometer (OTDR)? What is TDR?
 An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to
characterize an optical fiber.
o An OTDR may be used for estimating the fiber's length and overall attenuation,
including splice and mated-connector losses. It may also be used to locate faults,
such as breaks, and to measure optical return loss.
 TDR is an electronic instrument used to characterize and locate faults in metallic cables
* Fiber media vs. metallic cable:
– Fibers are more expensive (usually) than copper media over the same distance
(but for a higher capacity)
– Optical fiber cables consist of a PVC jacket and a series of strengthening
materials that surround the optical fiber and its cladding.
– fiber can transmit optical pulses for long distances
– The fibers media is immune to electromagnetic interference and will not conduct
unwanted electrical currents.
– Optical fibers are thin and have relatively low signal loss, they can be operated at
much greater lengths than copper media.
– In most enterprise environments, optical fiber is primarily used as backbone
cabling for high-traffic connections and for the interconnection of buildings in
multi-building campuses.
* The following are main cable types:
– Ethernet Straight-through
– Ethernet Crossover
– Rollover
* Wireless is not restricted to conductors or pathways, as are copper and fiber media.
 Wireless media carry electromagnetic signals at radio and microwave
frequencies that represent the binary digits of data communications.
* Know the usage of the following types of connectors:
- RJ-45: the RJ-45 connector is used widely in LANs
- RJ11: often used for terminating telephone wires
- BNC: used for coaxial cable
- Type –F: used for cable modem
* Fiber optic cables can be broadly classified into two types:
– Single-mode optical fiber
• Carries a single ray of light, usually emitted from a laser.
• This type of fiber can transmit optical pulses for long distances.
– Multimode fiber
• Typically uses LED emitters that do not create a single light wave.
• LED light source used with it, are cheaper than single-mode fiber and
its laser-based emitter technology.
* Data transfer can be measured in three ways:
– Bandwidth
– Throughput
– Goodput
* This cancellation effect also helps avoid interference from internal sources called
crosstalk.
– Crosstalk is the interference caused by the magnetic field around the adjacent
pairs of wires in the cable.
* It is essential that all copper media terminations be of high quality to ensure optimum
performance with current and future network technologies.
 Each time copper cabling is terminated, there is the possibility of signal loss and
the introduction of noise to the communication circuit.
* Signle mode vs. multi-mode.
Full Duplex or Half Duplex
Unlike an RJ-45 port, a single fiber port is actually divided into two separate unidirectional ports. One port sends data and the other port receives data. Some fiber add-on
modules are capable of sending and receiving data at the same time. This is known as full
duplex (FDX) operation. Other add-on modules are only capable of either sending or
receiving data and cannot do both at the same time. This is known as half duplex (HDX).
Obviously, Fast Duplex or FDX is the faster of the two.
* The benefits of wireless data communications technologies
– The savings on costly premises wiring
– The convenience of host mobility.
* Unshielded twisted-pair (UTP) cabling, as it is used in Ethernet LANs, consists of 4
pairs of color-coded wires that have been twisted together and then encased in a
flexible plastic sheath.
– The twisting has the effect of canceling unwanted signals.
– Cable types with shielding or twisting of the pairs of wires are designed to
minimize signal degradation due to electronic noise. The susceptibility of
copper cables to electronic noise can also be limited by: