OSI Reference Model

The Open System Interconnect (OSI)

The OSI reference model was developed by the International Standard Organization (ISO) to establish a framework of standards for computer-to-computer communication. The OSI model allows hardware and software companies to develop their products to work within certain parameters and guidelines of the model. This allows products to work with other products that operate within these guidelines.

The “Open” in OSI means that the model deals with systems that are open for communication with other systems. The OSI model is also called a layered protocol because of the seven (7) layers that its comprised of.

Benefits of the OSI Model

  • A network layer can be replaced by a layer from another network vendor.
  • Networks can be upgraded easily by replacing individual layers
  • The user and network designer are not restricted to using the product (hardware/ software) of a specific vendor.

Layer Function

Layer 1: Physical

This layer defines the the standards that provide guidelines on how to move data bits between modems. This layer also specifies the electrical connections between the transmission medium and the computer.

Layer 2: Data Link

This layer is responsible for ensuring that the data transmitted between two locations on the network is accurate. This layer also controls access to the network and breaks up data to be sent into frames. The data link layer solves competition problems using Media Access Control (MAC) and Carrier Sense Multiple Access/Collision Detection (CSMA/CD). This layer also solves lost, duplicated or destroyed frames.

Layer 3: Network

The network layer is concerned with determining how packets are routed from source to destination. Routes (paths) can be hard-wired into the LAN or they can be determined at the time a packet is sent. This layer solves congestion problems and ensures that the line is not overloaded with packets while other lines are under-utilized.

Layer 4: Transport

The basic function of the transport layer is to accept data from the session layer, break them into smaller pieces and pass them on to the network layer. This layer also ensures that pieces of data arrive at the other end correctly.

Layer 5: Session

The session layer allows users on different machines to establish and terminate a session between them. A session is used to allow a user to log into a remote time-sharing system or to transfer a file between two machines. The session layer manages communication and keeps track of whose turn it is to talk.

Layer 6: Presentation

This layer ensures that the receiving computer understands the information sent to it. Different computers represent data differently and it is the responsibility of this layer to provide a standard encoding (data representation) to be used by computers in the network. Data is formatted using this standard encoding before it is presented to destination machines. This layer is also responsible for data compression and encryption.

Layer 7: Application

This layer facilitates user functions (e.g. File Transfer, Email) and provides guidelines for network services.