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Chapter 2: OSI Specifications
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Chapter 2 Objectives
The Following CompTIA Network+ Exam
Objectives Are Covered in This Chapter:
• 5.0 Industry standards, practices, and network
theory
• 5.1 Analyze a scenario and determine the
corresponding OSI layer
• • Layer 1 – Physical
• • Layer 2 – Data link
• • Layer 3 – Network
• • Layer 4 – Transport
• • Layer 5 – Session
• • Layer 6 – Presentation
• • Layer 7 – Application
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Chapter 2 Objectives
The Following CompTIA Network+ Exam
Objectives Are Covered in This Chapter:
• 5.2 Explain the basics of network theory and
concepts
• • Encapsulation/de-encapsulation
• • Modulation techniques
• o Multiplexing
• o De-multiplexing
• o Analog and digital techniques
• o TDM
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Internetworking Models
• In the late 1970s, the Open Systems Interconnection
(OSI) reference model was created by the
International Organization for Standardization (ISO)
to break through this barrier.
• The OSI model was meant to help vendors create
interoperable network devices and software in the
form of protocols so that different vendor networks
could work with each other.
• The OSI model is the primary architectural model for
networks. It describes how data and network
information are communicated from an application
on one computer through the network media to an
application on another computer. The OSI reference
model breaks this approach into layers.
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Advantages of Reference
Models
Advantages of using the OSI layered model include, but
are not limited to, the following:
• It divides the network communication process into
smaller and simpler components, thus aiding
component development, design, and troubleshooting.
• It allows multiple-vendor development through
standardization of network components.
• It encourages industry standardization by defining
what functions occur at each layer of the model.
• It allows various types of network hardware and
software to communicate.
• It prevents changes in one layer from affecting other
layers, so it doesn’t hamper development and makes
application programming easier.
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The OSI Model
• The OSI model has seven layers:
– Application (Layer 7)
– Presentation (Layer 6)
– Session (Layer 5)
– Transport (Layer 4)
– Network (Layer 3)
– Data Link (Layer 2)
– Physical (Layer 1)
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OSI Layer Functions
Application
Presentation
Session
Transport
Network
Data Link
Physical
•
File, print, message, database, and application services
•
Data encryption, compression, and translation services
•
Dialog control
•
End-to-end connection
•
Routing
•
Framing
•
Physical topology
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The Upper Layers
Application
Presentation
Session
•
Provides a user interface
•
•
Presents data
Handles processing such as encryption
•
Keeps different applications’
data separate
Transport
Network
Data Link
Physical
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The Lower Layers
Application
Presentation
Session
•
•
Provides reliable or unreliable delivery
Performs error correction before retransmit
•
Provides logical addressing which routers use for path
determination
Data Link
•
•
•
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
•
•
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Transport
Network
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Reliability
Reliable data transport employs a connection-oriented
communications session between systems, and the
protocols involved ensure that the following will be
achieved:
• The segments delivered are acknowledged back to
the sender upon their reception.
• Any segments not acknowledged are retransmitted.
• Segments are sequenced back into their proper
order upon arrival at their destination.
• A manageable data flow is maintained in order to
avoid congestion, overloading, and data loss.
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A Connection Oriented
Session
Sender
Receiver
SYN
SYN/ACK
ACK
Connection Established
Data transfer
(Send bytes of Segments)
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Flow Control
Sender
Receiver
Transmit
Buffer full
No ready –
STOP!
Segments
processed
GO!
Transmit
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Connection Oriented Session
A service is considered connection-oriented if:
• Virtual circuit is setup (three-way handshake).
• Uses sequencing.
• Uses acknowledgments.
• Uses flow control.
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Windowing Flow Control
Sender
Receiver
Window size of 1
Send 1
Receive 1
Ack 1
Receive 2
Send 2
Ack 2
Window size of 3
Send 1
Send 2
Send 3
Ack 4
Send 4
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Acknowledgements
Sender
1
2
3
4
Receiver
5
6
1
2
3
4
5
6
Send 1
Send 2
Send 3
Ack 4
Send 4
Send 5
Connection lost!
Send 6
Ack 5
Send 5
Ack 7
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The Lower Layers
Application
Presentation
Session
•
•
Provides reliable or unreliable delivery
Performs error correction before retransmit
•
Provides logical addressing which routers use for path
determination
Data Link
•
•
•
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
•
•
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Transport
Network
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Routing at Layer 3
3.0
1.0
3.1
1.1
2.1
2.2
S0
S0
1.3
E0
3.3
E0
3.2
1.2
Routing table
Routing table
NET
INT
Metric
NET
INT
Metric
1
E0
0
1
S0
1
2
S0
0
2
S0
0
3
S0
1
3
E0
0
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Routers at Layer 3
FastEthernet0/0
Internet
Serial0
WAN Services
FastEthernet0/1
Each router interface is a broadcast domain.
Routers break up broadcast domains by
default and provide WAN services
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The Lower Layers
Application
Presentation
Session
•
•
Provides reliable or unreliable delivery
Performs error correction before retransmit
•
Provides logical addressing which routers use for path
determination
Data Link
•
•
•
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
•
•
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Transport
Network
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Data Link Layer (Layer 2)
Logical Link Control (LLC)
Media Access Control (MAC)
802.11
802.3
802.2
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The Lower Layers
Application
Presentation
Session
•
•
Provides reliable or unreliable delivery
Performs error correction before retransmit
•
Provides logical addressing which routers use for path
determination
Data Link
•
•
•
Combines packets into bytes and bytes into frames
Provides access to media using MAC address
Performs error detection not correction
Physical
•
•
Moves bits between devices
Specifies voltage, wire speed, and pin-out of cables
Transport
Network
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Data Encapsulation
PDU
Application
Presentation
Upper layer data
Session
TCP Header
Upper layer data
IP Header
Segment
LLC Header
Packet
FCS
MAC Header
Packet
FCS
0101110101001000010
Segment
Transport
Packet
Network
Frame
Data Link
Bits
Physical
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Modulation Techniques
Modulation is the process of varying one or more
properties of a waveform called a carrier signal
In current networks, modulation takes digital or analog
signals and puts in in another signal that can be
physically transmitted
Modems – perform both modulation and demodulation
operations
Analog and Digital modulation use Frequency-Division
multiplexing. Several low-pass signals transferred
simultaneously over same shared medium
Ethernet uses digital baseband modulation or line
coding to transfer digital bit stream
Time-division multiplexing (TDM) method of transmitting
and receiving many independent signals over
common signal path through synchronize devices23
Summary
•
•
•
•
Summary
Exam Essentials Section
Written Labs
Review Questions
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