The Open Systems Interconnection Open Systems Interconnection (better known as ISO / OSI) is a standard established in 1978 by the International Organization for Standardization (ISO, the greek word isos, "equal"), the main international standards organization, which establishes a protocol stack in 7 levels.
The organization felt the need to produce a set of standards for computer networks and started the project OSI (Open Systems Interconnection), a standard reference for the interconnection of open systems. The document shows that this activity is the Basic Reference Model of OSI, known as ISO 7498.
The ISO / OSI model consists of a stack (or stacks) of protocols through which reduces the complexity of implementing a communication system for networking. In particular, ISO / OSI model consists of layers (or levels), the so-called layer, containing one or more aspects associated with each of the communication between two nodes on a network. The layers are a total of 7 and range from the physical layer (the physical media, ie cable or radio waves) to the application layer, which is achieved through the provision of high quality.
levels or layers
Each layer identifies a communication protocol of the same level. ISO / OSI achieves a communication levels, ie, given two nodes A and B, the level n node A can exchange information with the level n node B but not with the other: this gives the system modularity and ease of deployment and redeployment . In addition, each level produces communication with the level corresponding to other nodes using the POS (point of service) in the level immediately below. So ISO / OSI layer encapsulates the messages in e n n-1 level. So if A has to send, for example, an e-mail to B, the application (level 7) To propagate the message of using the underlying layer (level 6) which in turn will use the PoS of the layer below until you get the notice on the physical medium.
In this way there is a multi-level communication that allows, for example, to implement different algorithms for routing in the network despite having connected transport protocols.
ISO / OSI is designed to allow communication in networks' switching package ', similar to the paradigm TCP-UDP/IP used in Unix and ARPANET, which later became the Internet. The key difference between TCP / IP and OSI is that the TCP / IP application layer is outside the protocol stack (that is a stand-alone application that 'use' TCP / IP to communicate with other applications), layers are therefore only 5 (application, transport, network, data link, physical) and the level session, presentations are missing because implemented (possibly) elsewhere, that is in stand-alone outside.
ISO / OSI protocol stack is an encapsulated, which is certainly more flexible than the paradigm of TCP / IP, but only because it is more abstract than that. In practical implementations there are 'complete' ISO / OSI, apart from proprietary ones (eg Digital's DECNET) and academic interest.
In order to obtain an improvement of the model may be of interest can benefit from an exchange of information within the same protocol stack, between non-adjacent layers. This possibility, which is gaining considerable interest in recent years, called Cross-Layer (CL), and it is interesting especially in radio communications for the variable nature of the medium. The basic idea of \u200b\u200bthe concept is to introduce cross-layer capacity, various communication protocols to exchange information to adapt to specific states of the grid connection. Unlike the classic OSI model, no longer want to just take countermeasures or preventive control mechanisms after a particular event occurring in the network, but trying to decide interactively with it using the information common to all layers.
List and function levels
Level 1: Physical
Objective: to transmit a stream of unstructured data through a physical connection, dealing with the shape and the voltage signal. It has to do with the procedures necessary to establish mechanical and electronic, and off to maintain a physical connection. Simply
: responsible to control the network, hardware and devices that make possible the connection.
In this level you decide:
· The voltages chosen to represent the logical values \u200b\u200b0 and 1
· The duration in microseconds of the electric signal that identifies a bit
· the simultaneous transmission in both directions
· The form and mechanics connectors used to connect the hardware to the transmission medium
Level 2 datalink
Objective: To enable the reliable transfer of data through the physical layer. Send data frames with the necessary synchronization and maintains control errors and signal loss. All this' you can bring up the upper level, the physical medium such as a transmission line error-free transmission.
This level aims to train the data to be sent through the physical layer, encapsulating the data in a package provided with header (header) and tail (tail), also used to control sequences. This fragmentation of data packets is called the specific and individual packages are framing the frame.
For each packet received, the recipient sends the sender an ACK (acknowledgment, confirmation) holds the state of the transmission: the sender has to resend packets transmitted ill and those who have not received a response. To optimize the transmission of the ACK, it uses a technique known as piggybacking, which is nell'accodare to outgoing messages, the ACK for an incoming connection, to optimize the use of the physical layer. The ACK packets can also be grouped and sent in blocks.
This level is also responsible for controlling the flow of data: in case of unbalance of transmission speed, slow down the work deals with the fastest car, agreed to it and minimizing losses due to overloading.
Its fundamental data unit is the plot.
Level 3: Network
Objective: To make the upper levels are independent of the mechanisms and transmission technologies used for the connection. He works to establish, maintain and terminate a connection, ensuring the correct and optimal operation of the communication subnet.
is responsible for:
· routing: Optimal choice of the route to be used to ensure the delivery of information
• Management of congestion: avoid too many packets arrive at the same router simultaneously
· Route
· Conversion of data in transit between a network and another with different characteristics. It must, therefore:
or translate addresses
or assess the need 'to fragment the data if the new network has a different maximum transmission unit
or assess the need' to manage different protocols through the use of gateway
Its unity is the fundamental data packet.
Level 4: transport
Objective: To allow a transparent and reliable data transfer (also implement error checking and losses) between two hosts. It is really the first level end-to-end, that 'from source to destination host.
Unlike the previous levels, dealing with connections between adjacent nodes of a network, the Transport (logic level) deals only with the point of departure and arrival.
He is also carrying out the fragmentation of data coming from the top level in packages, called 'segments' and send them in an efficient and reliably using the network layer and isolating it from the upper levels. In addition, she is concerned to optimize the use of network resources and prevent congestion.
Its fundamental data unit is the message.
Level 5: Session
Objective: To control the communication between applications. Establish, maintain and terminate connections (sessions) between cooperating applications.
It adds to services provided by the transport layer, more advanced services, such as management of the dialogue (mono or bi), the management of the token (to make mutual exclusion) or synchronization (inserting checkpoints in order to reduce the amount of data to be retransmitted in case of serious malfunctions).
He is also to include control points in the data stream: in case of an error in sending packet communication resumes from the last checkpoint was successful.
Level 6: Presentation
Objective: To transform the data provided by the applications in a standardized format and provide common communication services, such as encryption, text compression and reformatting.
It allows you to manage the syntax of information to be transferred. And there are three different syntaxes:
· Abstract (formal definition of the application data exchange),
· concrete local (how the data are represented locally)
, acting on the transfer (as the data is encrypted during transfer).
Level 7: Application
Objective: user interface and machine.
Provides a set of protocols that work closely with the applications. It is wrong to identify a user application as part of the application layer.
The Protocols of the typical applications of this level perform tasks such as:
· Transfer files
· Virtual Terminal • Mail
electronic
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