Infrastructure Framework  «Prev  Next»
Lesson 2 OSI network model
Objective Explain the OSI network model.

OSI Network Model

Networking is the ability to exchange data between two or more applications across two or more computers. Because each computer on the network may be different (a Macintosh, a PC, a Sun workstation), networks must not only transport data, they must translate protocols between dissimilar computers to assure compatibility at the hardware and software level.
To conceptualize the many translation steps required by the network server to successfully link computers, we can use the OSI network model. The model's purpose is to outline the logical processes required to exchange data between potentially dissimilar computers on a network.

OSI Model

The OSI (or Open Systems Interconnection) model conceived by the International Standards Organization (ISO), a U.N. agency, is a seven-layer framework for implementing protocols or tasks. The following series of images shows the function of each layer.

1) The application layer provides network services to user applicaitons
1) The application layer provides network services to user applicaitons and determines the resources required for applications to communicate with each other.

2) The Presentation layer prepares data by converting it to a format acceptable to the destination computer.
2) The Presentation layer prepares data by converting it to a format acceptable to the destination computer.

3) The Session layer provides authentication and synchronization services to computers.
3) The Session layer provides authentication and synchronization services to computers.

4) The transport layer is responsible for reliable data
4) The transport layer is responsible for reliable data

5) The Network layer is responsible for the delivery of packets to their ultimate destinations
5) The Network layer is responsible for the delivery of packets to their ultimate destinations

6) The Data Link Layer handles tasks associated with transmitting data using physical addressing across a physical link.
6) The Data Link Layer handles tasks associated with transmitting data using physical addressing across a physical link.

7) The physical layer is responsible for activating and maintaining the physical links
7) The physical layer is responsible for activating and maintaining the physical links

AWS Certified Networking

OSI Reference Model
The link below discusses the layers in the OSI model.

OSI Network Model and Components

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

Layered Protocols

In the context of the OSI Network Model, "Layered Protocols" refer to the way in which different network protocols are organized into distinct layers. Each layer of the OSI model has its own set of protocols that operate independently of the protocols in other layers, but also work together to provide end-to-end communication between devices on a network. The OSI model consists of seven layers, each of which is responsible for a specific set of functions related to network communication. Each layer uses a set of protocols that are specific to that layer, and that define how data is transmitted, received, and processed at that layer. These protocols are often referred to as "layered protocols."
For example, the Application layer of the OSI model uses protocols such as HTTP, FTP, and SMTP to provide services such as email, file transfer, and web browsing. The Transport layer uses protocols such as TCP and UDP to provide reliable, end-to-end data transmission between applications. The Network layer uses protocols such as IP and ICMP to provide routing and addressing services between different networks. Each layer of the OSI model communicates with the layer above and below it, using standardized protocols and interfaces. This allows different devices on a network to communicate with each other using a common set of protocols and interfaces, regardless of the specific hardware or software they are using. Overall, the concept of layered protocols in the OSI model helps to ensure that network communication is standardized, efficient, and reliable, and that devices on a network can communicate with each other in a consistent and predictable way.
  1. Protocols connect entities on same level, within the layer
  2. Higher layers use services provided by lower layers
  3. Layers are independent and defined by interface provided to the higher layer and required from the lower layer

1) Entity provides an interface 2) Entity uses an interface
1) Entity provides an interface 2) Entity uses an interface

Entities communicate by exchanging Protocol Data Units (PDUs) with entities on the same level

Why Layers

  1. Layers may be exchanged as long as interfaces stay unchanged
    a) WWW was built on top of existing TCP/IP implementations, b) WWW does not have to care about the media (modem, Ethernet, radio link etc.),
    c) ATM can be used to transport IP packets without any changes to applications, d)IPv6 will replace the entire IP part of the TCP/IP layer
  2. Layering makes development easier and adds flexibility

In the next lesson, the infrastructure of the Internet will be discussed.