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Chapter 2: OSI Specifications Click to edit Master subtitle style
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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 2
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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 3
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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. 4
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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. 5
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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) 6
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OSI Layer Functions Application
File, print, message, database, and application services Presentation Data encryption, compression, and translation services Session Dialog control Transport End-to-end connection Network Routing Data Link Framing Physical Physical topology 7
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The Upper Layers Application Provides a user interface Presentation
Presents data Handles processing such as encryption Session Keeps different applications’ data separate Transport Network Data Link Physical 8
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The Lower Layers Application Presentation Session Transport
Provides reliable or unreliable delivery Performs error correction before retransmit Network 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 9
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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. 10
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A Connection Oriented Session
Sender Receiver SYN SYN/ACK ACK Connection Established Data transfer (Send bytes of Segments) 11
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Flow Control 12 Sender Receiver Transmit Buffer full No ready – STOP!
Segments processed GO! Transmit 12
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Windowing Flow Control
Sender Receiver Window size of 1 Send 1 Receive 1 Ack 1 Send 2 Receive 2 Ack 2 Window size of 3 Send 1 Send 2 Send 3 Ack 4 Send 4 13
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Acknowledgements 1 2 3 4 5 6 1 2 3 4 5 6 14 Sender Receiver Send 1
Connection lost! Send 6 Ack 5 Send 5 Ack 7 14
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Routing at Layer 3 Routing table Routing table 15 3.0 1.0 3.1 1.1 2.1
2.2 1.3 3.3 E0 E0 S0 S0 3.2 1.2 Routing table NET INT Metric 1 E0 2 S0 3 Routing table NET INT Metric 1 S0 2 3 E0 15
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Routers at Layer 3 16 Internet FastEthernet0/0 Serial0 WAN Services
Each router interface is a broadcast domain. Routers break up broadcast domains by default and provide WAN services 16
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Data Link Layer (Layer 2)
Logical Link Control (LLC) Media Access Control (MAC) 802.11 802.3 802.2 17
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Data Encapsulation PDU Application Presentation Upper layer data
Session TCP Header Upper layer data Segment Transport IP Header Segment Packet Network LLC Header Packet FCS Frame Data Link MAC Header Packet FCS Bits Physical 18
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Summary Summary Exam Essentials Section Written Labs Review Questions
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