OSI model

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1 Introduction

The OSI (Open Systems Interconnection) model is a conceptual model that standardises the functions of a communication system and locates them on multiple levels. This model has never been used in practice, but it is interesting because it constitutes the bases of the IP model.

This model divises the functions of a network in 7 levels depending on the services linked to each layer. Each layer needs a key fonction, and it produces a key function for the level above by using the functions produced by the level below. The OSI model is suitable for integrating new technologies into the communication network; in fact for inserting them into the model we only need to define their level of integration.


2 Layer

  • The “application” layer is the higher and it represents the languages used by the programs for communicating between them. This type of protocol is used for the interactions between a client and a server, where a program (client) asks another program (the server) for a service. Some systems entirely work on a client-server logic. Other programs work in a peer-to-peer logic, where all programs work as well as server as clients. In the client-server system, if the client wants to access a service it has to wait for the server, and if the server is busy the client has to wait for its turn.
  • The “presentation” layer is equivalent to alphabet in communication. It defines the individual elements of the language used for the application layer. For instance this level is in charge of the translation and coding of characters in a safe browsing protocol.
  • The “session” layer is in charge of maintaining the dialogue between programs. Here we can still identify individual "conversations". This level separates the trafic sent by the level below and it distributes it to the receivers.
  • The “transport” layer manages the communications between the source and the receiver computer, and it is responsible of the organization of multiple sessions and optimization of the bandwidth. It separates the data in "packages" that can travel independently. On the receiver computer, the "transport" layer collects data "packages" and reconstitutes data for transferring it to the "session" layer. The “transport” layer can detect problems in the data transmission and ask for the retransmission to the "transport" layer of source computer.
  • The “network” layer schedules the route taken by data. It identifies the most efficient way of transporting data from the "transport" layer of source machine to the "transport" layer of the receiving machine. It also joins data on the route to the data to be sent. In this way, data packages can travel in different ways depending on the network conditions or on the scheduled route. This layer also monitors the conditions of the network, in order to plan efficient routes.
  • The “data link” layer implements the route established by the "network" layer and it manages the communications between the network nodes. It is informed of the network's physical structure and it is able to use different addresses for identifying different parts of hardware (physical components of network); these addresses are called Medium Control Access address and they are generally established when the component is produced. This level is also in charge of re-packaging data from the "transport" level in suitable packages for physical transmission.
  • The “physical” layer transforms data packages into something transportable through the physical network, such as electrical impulses, light signals or radio signals.


3 Sources

Marshall, A. (2008). Digital Forensics. Digital Evidence in Criminal Investigation. Chichester: Wiley-Blackwell.