1. Chapter 2 Dissecting the OSI Model
CompTIA Network +
Chapter 2
Dissecting the OSI Model

2. Objectives What is the purpose of a Network model?
What are the layers of the OSI model?
What are the characteristics of each layer of the OSI model?
How does the TCP/IP stack compare to the OSI model?
What are the well-known TCP and/or UDP port numbers for a given collection of common applications

3. The Purpose of Reference Models
It breaks network communication into smaller, simpler parts that are easier to develop.
It facilitates standardization of network components to allow multiple-vendor development and support.
It allows different types of network hardware and software to communicate with each other.
It prevents changes in one layer from affecting the other layers so that they can develop more quickly.
It breaks network communication into smaller parts to make learning it easier to understand.

4. The OSI seven-layer model
Application
Presentation
Session
Transport
Network
Data Link
Physical
Figure 2-2 OSI Stack

5. Protocol Data Unit (PDU)
Application
Presentation
Session
Transport
Segments
Network
Packets
Data Link
Frames
Physical
Bits
Figure 2-3 PDU Names

6. Physical Layer Application Presentation Session Transport Network
Data Link
Physical
How Bits are represented on the medium
Wring standards for connectors and jacks
Physical topology
Synchronizing bits
Bandwidth usage
Multiplexing strategy
Figure 2-4 Layer 1: Physical Layer

7. Data Link Layer Application Presentation Session Physical Addressing
Transport
Network
Physical
Physical Addressing
Logical topology
Method of transmitting on the media
MAC
Data Link
Connection Services
Synchronizing transmissions
LLC
Figure 2-8 Layer 2: The Data Link Layer

8. Network Layer Data Link Application Presentation Session Transport
Physical
Logical addressing
Switching
Route discovery and selection
Connection services
Bandwidth usage
Multiplexing strategy
Network
Figure 2-9 Layer 3: The Network Layer

9. Transport Layer Network Data Link Application Presentation Session
Physical
TCP/UDP
Windowing
Buffering
Transport
Figure 2-10 Layer 4: The Transport Layer

10. TCP Sliding Window
Figure 2-11 TCP Sliding Window

11. Session Layer Application Presentation Setting up a session
Transport
Network
Data Link
Application
Presentation
Physical
Setting up a session
Maintaining a session
Tearing down a session
Session
Figure 2-12 Layer 5: The Session Layer

12. Presentation Layer Application Data formatting Encryption Presentation
Session
Transport
Network
Data Link
Application
Physical
Data formatting
Encryption
Presentation
Figure 2-13 Layer 6: The Presentation Layer

13. Application Layer Application services Application
Service advertisement
Application
Presentation
Session
Transport
Network
Data Link
Physical
Figure 2-13 Layer 6: The Presentation Layer

14. The TCP/IP Stack
Application
Transport
Internet
Network Interface

15. The TCP/IP and OSI Models Compared
TCP/IP Stack
OSI Stack
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Internet
Network Interface
Figure 2-15 TCP/IP Stack

16. IP Ver4 Header
Figure 2-16 IP Version 4 Packet Format

17. TCP Header
Figure 2-17 TCP Segment Format

18. UDP Header
Figure 2-18 UDP Segment Format

19. TCP/IP Port Number Port numbers indicate the upper-layer
protocol that is
using the transport.

20. Port Types
Port numbers are assigned in various ways, based on three ranges:
System Ports (0-1023), System Ports are assigned by IETF process for standards-track protocols, as per RFC Also known as well-known-ports
User Ports ( ) ,User Ports are assigned by IANA using the "Expert Review" process, as per RFC6335
Dynamic and/or Private Ports ( ), Dynamic Ports are not assigned, they are dynamically created as your computer need them. Also known as ephemeral ports.

21. Communication Between Two Systems

22. Summary
The ISO’s OSI reference model cosists of seven layers: physical (Layer 1), data link (Layer 2), network (Layer 3), transport (Layer4), session (Layer 5), presentation (Layer 6), and application (Layer 7). The purpose of each layer presented, along with examples of technologies residing at the various layers.
The TCP/IP stack was presented as an alternate model to the OSI reference model. The TCP/IP stack consist of four layers: network interface, Internet, transport, and application. These layers were compared and contrasted with the seven layers of the OSI model.
The chapter discussed how port numbers are used to associate data at the transport layer with an appropriate application layer protocol. Examples of common application layer protocols in the TCP/IP suite were presented, along with their port numbers.