The ISO ( International Standards Organization ) decided to build a framework of standards that different providers can use to communicate over a network equipment and applications . This framework is now considered the standard for communication networks . The OSI has seven layers , which divides the task working groups easier to manage smaller . Each task or group of tasks assigned to each layer can be implemented independently. This limits the complications between the layers because the solutions offered by one layer does not affect the other layers.
The layers 7 may be logically divided into two subgroups . Layers 7-4 focus on end to end communication of data sources and destinations. Layers 3-1 are to provide constant communication between network devices. An easier way to watch the OSI model divides the upper layers (7 , 6, 5 ) lower layers (4, 3, 2, 1). The upper layers deal with issues of application and implementation in the software. The highest layer , the application layer , is closest to the end user. The lower layers are responsible for data transport . The physical layer and data link layer are implemented in hardware and software. The lowest layer , the physical layer , is closest to the physical network medium ( the son , etc.) and is responsible for placing data on the carrier .
The following is an explanation of top-down of the OSI model . It starts with the PC of the user and follows what happens in the user file during his visit that the different layers of the OSI model . The top-down approach was chosen specifically ( cons from the physical layer and work up to the application layer ) to facilitate understanding. It is used here to show how the user files are processed ( through the layers ) in a bit stream for transmission over the network .
LAYER 7 - APPLICATION
The application level provides services that directly support user applications , such as user interface, e -mail , file transfer , access database , etc. There are many protocols that layer are generally required such as HTTP, Web , FTP, TELNET , SMTP . It gives applications access to the network through the layers below. Another important function is to transfer files between computers. Some filenames computers store or represent lines of text differently. The application layer supports incompatibilities and allows a smooth transfer between systems.
Protocols: FTP1 , HTTP2 , SMTP3 , DNS4 , TFTP5 , NFS6 , TELNET7 .
LAYER 6 - PRESENTATION
The level of presentation is a translator between the application and the size of the network. Unlike lower layers , its concern is the syntax and semantics of the information transmitted . Most users do not exchange programs random strings of binary bits . They exchange data such as names , addresses , dates, etc. Different computers store data in a different way . In order to allow these computers to transmit data to the other of the presentation layer translates the data into a standard form to be used on the network. Another function is that the data compression can be used to reduce the number of bits required to transmit the information packet . Security is also added to this layer by using the data encryption and decryption . This prevents others from intercepting the data and be able to decipher the meaning of the bits.
Protocols: ASCII8 , EBCDIC9 , MIDI10 , MPEG11 , JPEG12 .
LAYER 5 - SESSION
This layer allows applications to link systems communicate using a session. It opens and closes using the communication link . It also acts as a control mechanism which controls the dialogue is able to transmit . Sessions can allow data to be sent in both directions at the same time or one direction. The session layer determines that the transfer capacity currently . Another possibility is useful for inserting control points during data transfers. For a large file transmission if the system crashes checkpoints allow the system to start downloading the last known point of control. An example of this is an interactive connection or a file transfer , the session would recognize the names in the session and save them in a story. It could then connect and reconnect in case of a system crash or two systems.
Protocols: SQL13 , RPC14 .
LAYER 4 - TRANSPORT
The basic function of the transport layer is to accept data from the session layer , breaking the data into smaller units , if necessary, and send data packets to manage the network layer. At the destination this layer is responsible for combining the packets in their original condition . This layer also checks if the layers are in good order upon receipt and not as duplicated. If an error occurs in one of the packages there is a request for retransmission of the packet . There are two protocols that are sitting at this layer. First, TCP connects the transmitter and receiver using a socket which is determined by the IP address and port number. TCP keeps track of the packet delivery order and which ones should be returned. UDP on the other hand is a connectionless communication and does not guarantee delivery of packets between the transmitter and the receiver . Because it has no connection to the sender sends the data to the network with an IP address of the receiver and hope it goes to its destination. As there is no means to ask the sender to retransmit due to an error there is little protection error if any .
Protocols: TCP15 or UDP16 .
LAYER 3 - NETWORK
The network layer handles essentially all questions address . This layer handles the packets determines the best path or route , and manage network problems such as congestion data . There are three ways in which packets are routed to their destinations. First, there could be a static route through the entire network that will never change. Second, there may be a static line is used only during a particular session between the sender and the receiver . Finally, the packets can be sent dynamically through the network using changing paths to avoid bottlenecks . Bottlenecks are formed when there are too many packages in a subnet having them interfere with each other . The network level is responsible for the conversion of the network address and names to the MAC addresses of the machines . One of the most important functions of this layer is to allow the possibility of two different networks using addressing schemes in conflict to be able to send data to each other. The network layer allows different protocols to "talk" to each other and understand where the packet's destination is . Routers operate at this level by sending packets along the network .
Protocols: IP17 , ICMP18 , ARP19 , PING20 , Traceroute21 .
LEVEL 2 - DATA LINK
The data link layer defines the data format on the network. All data sent across the network are made in a part which is made at this level . The setting is a uniform way of sending data and the address and error control capabilities information. CRC is used for error detection at this level . If the receiving end of the CRC fails at this level, it is a request to the sender for retransmission of the packet .
Protocols: IEEE 802.222 , 802.323 , 802.524 .
LAYER 1 - PHYSICS
The physical layer is responsible for the establishment, maintenance and end physical connections ( point to point) between computers. This layer is concerned with the actual interpretation of the bit stream into an electrical signal that can be transported across a physical medium . The protocols of this layer according binary transmission , voltage levels , and data rates . This layer would also specify the physical medium properties such as cables and network adapters .
Protocols: IEEE 802.323 , 802.524 The ISO (International Standards Organization) Decided to build a framework of standards That different providers can use to Communicate over a network equipment and applications. This framework is now Considered the standard for communication networks . The OSI layers Has Seven , All which Divides the task working groups Easier to manage smaller . Each task or group of tasks Assigned To Each layer can be Implemented Independently . This limits the complications Between the layers Because the solutions offert by one layer Does not affect the other layers.
The layers 7 May be Logically divided into two Subgroups . Layers 7-4 focus on end -to-end communication of data sources and destinations. Layers 3-1 are to Provide constant communication Between network devices. An Easier way to watch the OSI model Divides the upper layers (7 , 6, 5 ) lower layers (4, 3, 2, 1). The upper layers deal with issues of enforcement and implementation in the software . The Highest layer , the application layer , is closest to the end user . The lower layers are responsible for data transport. The physical layer and data link layer are Implemented in hardware and software . The Lowest layer , the physical layer , is closest to the physical network medium (the sound, etc. . ) And is responsible for Placing data on the carrier .
The Following is an explanation of top-down of the OSI model . It starts with the PC of the user and follows what happens in the user file falling on His Visit That The different layers of the OSI model . The top-down approach Chosen WAS SPECIFICALLY (cons from the physical layer and work up to the application layer ) to Facilitate understanding. It is used here to show how the user files are processed ( through the layers) in a bit stream for transmission over the network .
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