Download Chapter 8

OSI Physical Layer
Network Fundamentals
Chapter 8
Objectives
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Explain the role of physical layer protocols and
services in supporting communication across data
networks.
Describe the role of signals used to represent bits as
a frame as the frame is transported across the local
media.
Describe the purpose of physical layer signaling and
encoding as they are used in networks.
Identify the basic characteristics of copper, fiber and
wireless network media.
Describe common uses of copper, fiber and wireless
network media.
Outline
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Physical layer: Communication signals
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Physical signaling and encoding: Representing bits
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Purpose of the physical layer
Physical layer standards
Physical layer fundamental principles
Signaling bits for the media
Encoding: Grouping bits
Data-carrying capacity
Physical media: Connecting communication
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Types of physical media
Media connectors
Purpose of the Physical Layer
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The roles of the OSI physical layer:
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There are various types of physical media and they
carry different types of signals.
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To encode the binary digits that represent data link layer
frames into signals.
To transmit and receive these signals across the physical
media.
Copper cable – electrical voltage.
Fiber optic – light pulses.
Wireless – electromagnetic waves.
Physical layer will encode the binary data in a frame
to the proper type of signal depending on the
physical media used.
Purpose of the Physical Layer
Purpose of the Physical Layer
Physical Layer Standards
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The physical layer technologies are defined by
organizations such as:
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The International Organization for Standardization (ISO)
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
Physical Layer Standards
Physical Layer Standards
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The technologies defined by these
organizations include four areas of the
physical layer standards:
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Physical and electrical properties of the media.
Mechanical properties (materials, dimensions,
pinouts) of the connectors.
Bit representation by the signals (encoding).
Definition of control information signals.
Physical Layer Standards
Physical Layer Standards
Physical Layer Standards
Physical Layer Fundamental
Principles
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Three fundamental functions of the physical
layer:
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The physical components
Data encoding
Signaling
The physical components refer to the
physical media and its connector.
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Responsible for making sure that signals can
travels reliably from one device to another over
the physical media.
Physical Layer Fundamental
Principles
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Encoding refers to the method of converting a
stream of data bits into a predefined “code”.
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Codes are groupings of bits used to provide a
predictable pattern that can be recognized by
both the sender and the receiver.
Encoding is also used for control information such
as identifying the beginning and end of a frame.
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This is normally represented using specific patters of
0’s and 1’s.
Physical Layer Fundamental
Principles
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Signaling refers to the process of converting the
encoded bit streams into signals.
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The signals generated is dependant on the physical media.
The method of representing the bits is called the signaling
method.
The processes of encoding and signaling complete
the preparation of data for transmission over the
physical media.
The physical layer sends these bits out one at a
time onto the medium as a signal and those signals
get picked up and decoded at the receiving end.
Physical Layer Fundamental
Principles
Signaling Bits for the Media
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Bits are represented on the medium by changing
one or more of the following signal characteristics:
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Amplitude
Frequency
Phase
To make sure that the receiver reads the signals at
the right time, the timing for both senders and
receivers needs to be synchronized.
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Done by the use of a clock signal.
This ensures that they both have the same bit time (the
time that the signal for one bit stays on the media).
Signaling Bits for the Media
Signaling Method – Nonreturn
to Zero (NRZ)
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Bits are represented by voltage level:
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The simplest signaling method but only suitable for
slow speed data link.
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0 – low voltage value
1 – high voltage value
NRZ is used in communication over serial port.
Disadvantages:
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Uses bandwidth inefficiently.
Susceptible to electromagnetic interference.
No inherent clocking capability and therefore easy to lose
synchronization.
Signaling Method – Nonreturn
to Zero (NRZ)
Signaling Method –
Manchester Encoding
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Bits are represented by voltage transition:
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Better than NRZ and can be used in faster data
links.
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0 – change from high to low
1 – change from low to high
Provides inherent clocking capability which makes it
possible to transmit signals at faster speed without losing
synchronization.
Manchester encoding is used in 10 Mbps Ethernet LAN.
Disadvantage: the signal needs to be read twice
during each bit time.
Signaling Method –
Manchester Encoding
Encoding: Grouping Bits
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In transmitting bits across the transmission
media, the bits are normally not transmitted
as it is.
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If we have data bits 0011, normally we do not just
send signals that represent the bits 0011 into the
media.
Instead, the bits are first encoded to prepare
it for transmission.
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With encoding, the data bits 0011 may now be
represented by the bits 10101.
Encoding: Grouping Bits
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Although encoding may introduce more bits to
represent the data, it does provide several
advantages, for example:
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Specifies the start and end of data frame.
Provides better error detection.
Limiting effective energy transmitted into the media by
making sure that the number of +ve voltage produced is
equal to the number of –ve voltage produced.
There are two methods of encoding:
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Signal patterns
Code groups
Encoding – Signal Patterns
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Signal patterns can be used to identify the start and
end of a frame.
This is done by using a certain pattern of signals.
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When the receiver “sees” this pattern, it knows that a data
frame will follow afterwards.
Enables the receiver to get ready to read the frame.
Any signals that are not followed by the start frame
signal pattern will be ignored.
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This will help the receiver to know which signals to read
and which signals to ignore.
Encoding – Signal Patterns
Encoding – Code Groups
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Code group refers to a consecutive sequence of
code bits that are interpreted and mapped as data
bit patterns.
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Example: data bits 0011 can be represented by the code
bits 10101.
Code groups are normally used in higher speed LAN
technologies.
Example: 4B/5B (used in 100 Mbps Ethernet LAN).
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4 bits of data are turned into 5-bit code symbols.
These 5-bit code symbols may represent data or control
information such as symbols that indicate beginning / end
of transmission.
Encoding – Code Groups
Data Code / Control Information
0000
0001
0010
…
1110
1111
Idle
Start of stream
End of stream
Symbol
11110
01001
10100
…
11100
11101
11111
11000
00111
Encoding – Code Groups
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Advantages of using code groups include:
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Reducing bit level error
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The receiver read the bits by sampling the signal at
certain time interval.
It is important for timing between the sender and
receiver to be synchronized.
Timing can be synchronized by having the signal to
change its level every so often.
Code groups can help to achieve this by making sure
that there are not too many 0’s or 1’s used in a row.
Encoding – Code Groups
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Limiting the energy transmitted into the media
 It is important to balance the number of high and low signal
levels (this is called DC balancing).
 Otherwise, excessive energy may be injected into the
media and this may cause interference.
 Code groups can help to achieve this by balancing the
number of 0’s and 1’s.
Helping to distinguish data bits from control bits
 In addition to data bits, control bits must also be transmitted
to facilitate data transfer.
 Code groups specifies special bit sequences for control
information (so that it cannot be confused with data codes).
Encoding – Code Groups
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Better media error detection
 Code groups defined symbols for data and control
information.
 They are also invalid symbols which are not used to
represent data or control information.
 These invalid symbols will never be generated by the
sender.
 If the receiver receives any of the invalid symbols, then it
knows there must be some error in data reception.
 This will enable the receiver to take an appropriate
corrective action.
Data Carrying Capacity
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Different media support the transfer of bits at
different speed.
Data transfer can be measured in three ways:
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Bandwidth
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Refer to the capacity of a medium to carry data in a
given amount of time.
Commonly measured in kilobits per second (kbps) or
megabits per second (Mbps).
Depends on the physical properties of the medium
and the signaling method applied.
Data Carrying Capacity
Data Carrying Capacity
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Throughput
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Refer to the actual transfer rate over the medium in a
period of time.
Influence by multiple factors such as the amount of
traffic, the type of traffic and the number of devices on
the network.
Goodput
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Refer to the transfer rate of actual useable data bits.
Goodput = throughput – (overhead for connection
establishment, acknowledgement and packet header).
Data Carrying Capacity
Data Carrying Capacity
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Example:
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A 100BaseT Ethernet LAN has a bandwidth of
100 Mbps.
However, due to the number of hosts connected
to the LAN and the amount of traffic generated by
these hosts, the throughput may only be 60 Mbps.
Out of all the bits transmitted, 1/3 of them may
just be control bits. Only the other 2/3 are data
bits. Therefore, the goodput is only 40 Mbps.
Types of Physical Media
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The physical layer defines the standards for the
physical components of a network (copper, fiber
cables) and the connectors used on them.
It also defines how bits are represented (signaling
method and encoding to be used).
The standards vary depending on the type of
physical media used and its applications.
In general, there are three types of media:
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Copper media
Fiber media
Wireless media
Copper Media
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Copper media is the most widely used media in local
networks.
Data travels as small pulses of electrical voltages.
However, the voltage is quite low and easily
distorted by outside interference and signal
attenuation.
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Interference (also known as noise): unwanted signals that
can distort or corrupt data signals.
Attenuation: the loss of energy in the signal as it travels
longer distance.
Copper Media
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There are various types of copper media:
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Unshielded twisted-pair (UTP) cable
Coaxial cable
Shielded twisted-pair (STP) cable
For each type, there are standards that specify the
following characteristics:
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Bandwidth of the communication
Type of connecters to be used
Pinout and color codes of connection to the media
Maximum distance of the media
Copper Media
Copper Media – UTP Cable
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UTP is the cheapest and the most common type of
copper media used.
Consists of eight wires twisted into four color-coded
pairs.
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The colors are used to identify wires for proper connection
at the terminals.
These four pair of wires are then bundled together into a
cable jacket.
Applications of UTP cable:
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Telephone network
Local area network (LAN)
Copper Media – UTP Cable
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The twisting is done to reduce crosstalk
interference.
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When electric current travels a wire, it produces magnetic
fields around it.
This magnetic field can cause interference to the data.
In a pair, each wire transmit signals in opposite direction.
This causes the magnetic fields generated by the two wires
to cancel each other.
The rate of twisting (the twist length) in each pair of wires is
different so that each pair self-cancels and reduces
crosstalk to a minimum.
Copper Media – UTP Cable
Copper Media – UTP Cable
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There are several categories of UTP cable:
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Category 3 (Cat 3)
 Used in telephone network and 10 Mbps Ethernet LAN.
Category 5 (Cat 5)
 Used in 100 Mbps Ethernet LAN.
Category 5e (Cat 5e)
 An improved version of Cat 5 cable with ability to perform
full-duplex transmission.
 Used in 1 Gbps (1000 Mbps) Ethernet LAN.
Category 6 (Cat 6)
 Has stricter manufacturing and termination standards.
 Has higher performance and less crosstalk.
 Used in 1 Gbps (1000 Mbps) Ethernet LAN.
Copper Media – UTP Cable
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The most common UTP cable connector in LAN
devices is an RJ-45 connector.
There are two standards that specify the cable
pinout (the order of wires in the connector):
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TIA/EIA 568A
TIA/EIA 568B
There are three types of UTP cable, each with
different pinout configuration.
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Straight-through cable
Crossover cable
Rollover cable
Copper Media – UTP Cable
Copper Media – Coaxial Cable
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Consists of a single, coated copper wire center and
an outer metal mesh.
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The outer metal mesh acts as both a grounding circuit and
an electromagnetic shield to reduce interference.
Applications of coaxial cable:
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Used in older Ethernet LAN standards such as 10Base2
and 10Base5.
Used in wireless implementations to connect antenna to
wireless devices.
Used to carry TV signals (cable TV).
Copper Media – Coaxial Cable
Copper Media – STP
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Consists of four pairs of wires that are wrapped in an
overall metallic braid or foil.
The entire bundle of wires as well as the individual
wire are shielded within the cable.
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For many years, STP was the cabling structure
specified for use in Token Ring network installations.
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STP provides better noise protection than UTP cabling,
however at a significantly higher price.
Token Ring is a LAN technology. It used to rival the Ethernet.
With the use of Token Ring declining, the demand for
shielded twisted-pair cabling has also waned.
However, STP is still useful in installations where
electromagnetic interference is an issue.
Copper Media – STP
Fiber Media
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In fiber optic cable, data bits are encoded as light
pulses generated using either laser or LED.
The cable consists of glass or plastic fibers that can
guide light pulses.
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Uses a property of glass called total internal reflection
where the light rays get reflected back and forth along the
medium.
Occurs when a ray of light strikes the boundary of a
medium that has a higher index of refraction at an angle
larger than the critical angle.
On the receiving end, a device called photodiode
interprets the light signal and decode it to bits.
Fiber Media
Fiber Media
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There are generally two types of fiber optic cable:
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Multimode
 Larger core: 50+ microns, can be glass or plastic
 Greater dispersion (loss of light)
 Shortest distance: up to 2 km
 Uses LEDs as light source for short distances
Single mode
 Small glass core: 8 – 10 microns
 Less dispersion of light
 Longer distance: up to about 100 km
 Uses lasers as light source
Fiber Media
Fiber Media
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Advantages of fiber optic cable (as compared to
copper cables):
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Disadvantages of fiber optic cable:
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Much greater capacity (bandwidth).
Lower attenuation – can run for longer distance.
Immunity to electromagnetic interference.
Cable has smaller size and weight.
More expensive.
More easily damaged.
Fiber optic cable is normally used in backbone
connections to connect between floors, buildings or
remote sites.
Wireless Media
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In wireless media, signal is carried using
electromagnetic waves.
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The main advantage is that devices no longer need
to use physical cables.
However, there are several disadvantages:
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Electromagnetic waves at different frequencies are called
with different names: radio wave, microwave, etc.
The speed is generally slower than cable connection.
More susceptible to interference.
More susceptible to security breach.
Wireless connections are best used in open areas.
Wireless Media
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Four common data communication standards that
apply to wireless media:
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IEEE 802.11 – A wireless LAN standard commonly known
as Wi-Fi.
IEEE 802.15 – A wireless personal area network (WPAN)
standard commonly known as Bluetooth.
IEEE 802.16 – A wireless WAN network commonly known
as WiMAX.
GSM (Global System for Mobile Communication), together
with GRPS, WCDMA or HSDPA – Provide data transfer
over mobile cellular network.
Media Connectors
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UTP cable
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Coaxial cable
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RJ-45 connector
BNC connector
N type connector
F type connector
STP cable
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D type connector
Media Connectors
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Fiber optic cable
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Straight Tip (ST) for multimode
Subscriber Connector (SC) for single mode
Lucent Connector (LC) for both multimode and
single mode
MT-RJ Connector for both multimode and single
mode