What is OSI Architecture?

OSI Architecture

Open System Interconnect (OSI) Architecture

• International Standards Organization (ISO)
• International Telecommunications Union (ITU), formerly CCITT
• “X dot” series: X.25, X.400, X.500
• Primarily a reference model

DISCUSS TCP IP Model & Address in TCP IP.

TCP/IP PROTOCOL SUITE:

The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application.

Topics discussed in this section:

• Physical and Data Link Layers
• Network Layer
• Transport Layer
• Application Layer

TCP/IP and OSI model:

ADDRESSING:

Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific.

• Physical Addresses
• Logical Addresses
• Port Addresses
• Specific Addresses

Addresses in TCP/IP:

Relationship of layers and addresses in TCP/IP

Physical addresses

In the above diagram a node with a physical address, 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

IP addresses:

The above example shows a part of the internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection. 

Port addresses:

The Above Example shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process and in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. 

The physical addresses will change from hop to hop, but the logical addresses usually remain the same.

The physical addresses change from hop to hop, but the logical and port addresses usually remain the same.

Discuss OSI model layers in Details

Concept of Layers

We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

Example of Layers:

Tasks involved in sending a letter

THE OSI MODEL

Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to the worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. 

• ISO is the organization.

• OSI is the model.

OSI Model Layers

Seven layers of the OSI model

The interaction between layers in the OSI model:

An exchange using the OSI model:

LAYERS IN THE OSI MODEL:

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Physical layer:

The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Physical Layer Concerned About:

• Physical Characteristics: type of transmission medium
• Bits Representation
• Data Rate: no. of bits sent each second
• Synchronization of Bits
• Line Configuration: P2P, Multi-point
• Physical Topology
• Transmission Model

Data link layer:

The data link layer is responsible for moving frames from one hop (node) to the next.

Data Link Layer Concerned About:

• Framing
• Physical Addressing
• Flow Control
• Error Control
• Access Control

Network layer:

The network layer is responsible for the delivery of individual packets from
the source host to the destination host

Network Layer Concerned About

• Logical Addressing
• Routing

Example of Source-to-destination delivery:

Transport layer:

The transport layer is responsible for the delivery of a message from one process to another.

Transport Layer Concerned About:

• Service Point Addressing
• Segmentation and Reassembly 
• Connection Control
• Flow Control
• Error Control

Reliable process-to-process delivery of a message:

Session layer:

The session layer is responsible for dialog control and synchronization.

Presentation layer:

The presentation layer is responsible for translation, compression, and encryption.

Application layer:

The application layer is responsible for providing services to the user.

Summary of layers