Open System Interconnect (OSI) Reference Model

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Open system interconnection (OSI) Model explains how packet travels through various layers to other devices on a network, even if the sender and destination have different types of network media.

Layer 1(Physical layer): 

Function 

• To activate, maintain, deactivate the physical connection. 
• To define voltage and data rates needed for transmission. 
• To convert the digital bits into electrical signals. 
• To decide weather the transmission the transmission is simplex, half or full duplex. 
• Physical layer doesn’t perform the detection and correction of errors.

Layer 2 (Datalink layer): 

• Framing (stream of bits into manageable data units): 
• The datalink layer divides the stream of bits received from the network layer into frame manageable data units called frames.
•  Physical addressing (MAC Address): 
• Data link layer adds a header to header to the frame to define the sender and receiver of the frame.
• Flow Control (mechanism for overwhelming the receiver): 
• If the data rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender or vice-versa, the data link layer imposes a flow control mechanism to avoid over whelming the receiver.
• Error Control (trailer, retransmission):
• It has mechanism to detect and re-transmit damaged or losses frames.  
• Access Control (defining master device in the same link):
• When two or more devices are connected to the same link, datalink layer protocols are necessary to determine which devices has control over the link at any given time.

Layer 3 (Network layer): 

•  To the signals through various channels to the other end. 
• To act as network controller by deciding which routes data should take. 
• To divide the outgoing message into packets and to assemble incoming packets into message for higher layer. 

Layer 4 (Transport layer): 

• It decides if the data transmission should take place on parallel path or single path. 
• It does the functions such as multiplexing, splitting or segmentation on the data. 
• It guarantees transmission of data from one end to another. 
• Segmentation of a large data so that it can be easily handled by the network layer.

Layer 5 (Session layer): 

• This layer manages and synchronizes conversation between two different applications. This is the level at which the user will establish system to system connections. 
• It allows a process to add check points or synchronization points to a stream of data. 
• It allows communication between two processes to takes places in either half duplex or full duplex mode.

Layer 6(Presentation layer): 

• It makes it sure that the information is delivered in such a form that the receiving system will understand and use it. 
• The presentation layer at the sender side changes the information from its sender dependent format into a common format. The presentation layer at receiving machine change the common format into receiver dependent format. 
• Data encryption and compression.

Layer 7 (Application Layer): 

• It provides user interface and support for services such as email, remote file access and transfer shared DB management and other types of distributed information services.