1. Net 221D:Computer Networks Fundamentals
Behrouz A. Forouzan” Data communications and Networking
Lecture 4: OSI versus TCP/IP

2. What is the OSI Reference Model
The OSI Reference Model is a seven-layer conceptual model developed by ISO in 1984.
Describes the standards for inter-computer
communication.
The seven layers of the OSI model are:
Application
Presentation
Session
Transport
Network
Data link
Physical
Computer networks / Andrew S. Tanenbaum-- 5th ed

3. OSI Reference Model Layers
Application Layer
Provides access to the OSI environment for users.
Network applications
Mail
Web
File transfer
Datagrams are called Upper Layer Data.
Presentation Layer
Provides independence to the application processes from differences in data representation (syntax).
Encryption and Decryption
Compression
Computer networks / Andrew S. Tanenbaum-- 5th ed

4. OSI Reference Model Layers
Session Layer
Provides the control structure for communication between applications; establishes, manages, and terminates connections (sessions) between cooperating applications.
Controls dialogs between computers
Datagrams are called Upper Layer Data.
Transport Layer
Provides reliable, transfer of data between end points.
End to End connections
Reliability
Flow control
End-to-end error recovery
Datagrams are called segments
Behrouz A. Forouzan” Data communications and Networking

5. OSI Reference Model Layers
Network Layer
Provides connections between hosts on different networks
Routing of packets
Datagrams are called Packets.
Data Link Layer
Provides for the reliable transfer of information across the physical link; sends blocks (frames) with the necessary synchronization, error control, and flow control.
Provides Connection between hosts on the same network
Hop to hop communication
Datagrams are called Frames.
Computer networks / Andrew S. Tanenbaum-- 5th ed

6. OSI Reference Model Layers
Physical Layer
Concerned with transmission of unstructured bit stream over physical medium.
Describes electrical and physical specifications for devices
Reliability
Flow control
Data is called bits.
Computer networks / Andrew S. Tanenbaum-- 5th ed

7. Summary of the OSI Layers
Behrouz A. Forouzan” Data communications and Networking

8. The TCP/IP Protocol Architecture
The TCP/IP protocol architecture was developed by Defense Advanced Research Projects Agency (DARPA), and is generally referred to as the TCP/IP protocol suite.
This protocol suite consists of a large collection of protocols that have been issued as Internet standards by the Internet Activities Board (IAB).
Communications involve three agents:
Applications
Computers
Networks.
Computer networks / Andrew S. Tanenbaum-- 5th ed

9. The TCP/IP Protocol Architecture
Applications can be file transfer and electronic mail which execute on computers that can often support multiple simultaneous applications.
Computers are connected to networks, and the data to be exchanged are transferred by the network from one computer to another.
Behrouz A. Forouzan” Data communications and Networking

10. The TCP/IP Layers The TCP/IP protocol layers are:
Hardware devices/Physical
Network Interface/Data link layer
Internet layer/Network Layer
Host-to-host, or transport layer
Application layer
Computer networks / Andrew S. Tanenbaum-- 5th ed

11. Communication Over the Internet
Example
Computer A communicates with computer B.
Five communicating devices
Source host (computer A), the link-layer switch in link 1, the router, the link-layer switch in link 2,and the destination host (computer B).
Each device is involved with a set of layers depending on the role of the device in the internet.
Hosts: Five layers
Router: Three layers No transport or application layer in the router
Link layer switch: Data Link and Physical Layers
Behrouz A. Forouzan” Data communications and Networking

12. Logical Connections between TCP/IP Layers
Logical connections between layers help understand the duties of each layer.
Computer networks / Andrew S. Tanenbaum-- 5th ed

13. The TCP/IP Layers
From the figure, the duty of the application, transport, and network layers is end-to-end.
However, the duty of the data-link and physical layers is hop- to-hop, in which a hop is a host or router.
The domain of duty of the top three layers is the internet, and the domain of duty of the two lower layers is the link.
Computer networks / Andrew S. Tanenbaum-- 5th ed

14. TCP IP Layers
Physical Layer: The physical layer covers the physical interface between a data transmission device (e.g., workstation, computer) and a transmission medium or network.
Logical unit between two physical layers in two devices is a bit.
Data Link Layer: The data link layer is concerned with the exchange of data between an end system (server, workstation, etc.) and the network to which it is attached.
Behrouz A. Forouzan” Data communications and Networking

15. TCP/IP Layers
TCP/IP does not define any specific protocol for the data-link layer.
Supports all the standard and proprietary protocols.
Any protocol that can take the datagram and carry it through the link suffices for the network layer.
When two devices are attached to different networks, procedures are needed to allow data to traverse multiple interconnected networks. This is the function of the Internet layer/Network Layer.
Routing: There can be several routers from the source to the destination, the routers in the path are responsible for choosing the best route for each packet.
Behrouz A. Forouzan” Data communications and Networking

16. TCP/IP Layers The communication at the network layer is host-to-host.
The Internet Protocol (IP) is used at this layer to provide the routing function across multiple networks.
IP is a connectionless protocol that provides no flow control, no error control, and no congestion control services.
Defines the format of the packet, called a datagram at the network layer.
Defines the format and the structure of addresses used in this layer.
Responsible for routing a packet from its source to its destination, which is achieved by each router forwarding the datagram to the next router in its path.
Behrouz A. Forouzan” Data communications and Networking

17. The TCP/IP Layers
Transport/Host to Host Layer: There is usually a requirement that data be exchanged reliably. That is, assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. The layer providing this functionality is referred to as the host-to-host layer, or transport layer.
The logical connection at the transport layer is also end-to-end.
Behrouz A. Forouzan” Data communications and Networking

18. The TCP/IP Layers
TCP and UDP are the two most commonly used protocols at the transport layer
Transmission Control Protocol (TCP), is a connection-oriented protocol that first establishes a logical connection between transport layers at two hosts before transferring data.
TCP provides a reliable connection for the transfer of data between applications.
Provides flow control and error control
User Datagram Protocol (UDP), is a connectionless protocol that transmits user datagrams without first creating a logical connection.
The User Datagram Protocol (UDP) does not provide flow, error, or congestion control.
Computer networks / Andrew S. Tanenbaum-- 5th ed

19. The TCP/IP Layers
The logical connection between the two application layers is end to end.
Communication at the application layer is between two processes (two programs running at this layer).
The application layer in the Internet includes many predefined protocols.
Hypertext Transfer Protocol (HTTP): Accessing the World Wide Web
The Simple Mail Transfer Protocol (SMTP) is the main protocol used in electronic mail ( ) service.
The File Transfer Protocol (FTP) is used for transferring files from one host to another.
Behrouz A. Forouzan” Data communications and Networking

20. TCP-IP Protocol Suite Summary
Behrouz A. Forouzan” Data communications and Networking

21. Addressing
Any communication that involves two parties needs two addresses: source address and destination address.
Figure shows the addressing at each layer.
Application layer: uses names to define the site that provides services, such as someorg.com, or the address, such as
Behrouz A. Forouzan” Data communications and Networking

22. Transport layer address: addresses are called port numbers, and these define the application-layer programs at the source and destination. Port numbers are local addresses that distinguish between several programs running at the same time.
Network-layer address: the addresses are global, with the whole Internet as the scope. A network-layer address uniquely defines the connection of a device to the Internet.
Link-layer address: sometimes called MAC addresses, are locally defined addresses, each of which defines a specific host or router in a network (LAN or WAN).
Behrouz A. Forouzan” Data communications and Networking

23. The TCP/IP Versus OSI Layer
Comparison of the TCP/IP and OSI Model shows that two layers, session and presentation, are missing from the TCP/IP protocol suite.
The application layer in the suite is usually considered to be the combination of three layers in the OSI model, as shown in the Figure.
Computer networks / Andrew S. Tanenbaum-- 5th ed