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


OSI Reference Model

  1. Physical (Layer 1) :This layer conveys the bit stream, light or radio signal, through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.
  2. Data Link (Layer 2): At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
  3. Network (Layer 3): This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
  4. Transport (Layer 4): This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
  5. Session (Layer 5): This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
  6. Presentation (Layer 6): This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
  7. Application (Layer 7): This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.


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.

SEMrush Software