OSI models | Open Systems Interconnection - OSI layers - Networking layers | Seven layers of network

The OSI Model is used to describe networks and network application. The OSI model is a technology standard maintained by the International Standards Organization (ISO). Although today's technologies do not fully conform to the standard, it remains a useful introduction to the study of network architecture.

• ISO(International Standards Organization) is a multinational bogy dedicated to worldwide agreement on international standards.
• Layers :--

 

Physical Layer

• Physical characteristics of interfaces and media: The physical layer defines the characteristics of the interface between  devices and the transmission media, including its type.
• Representation of the bits: the physical layer data consist of a stream of bits  without any interpretation. To be transmitted, bits must be encoded into signals –electrical or optical-. The physical layer defines the type of encoding.
• Data rate: The physical layer defines the transmission rate, the number of bits sent each second.
• Line configuration:  the physical layer is concerned with the connection of devices to the medium.
• Physical topology
• Transmission Mode

Data Link Layer

• Framing. The data link layer divides the stream of bits received from the network layer into data units called frames.
• Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame.
• Error control.  The data link layer  adds reliability to the physical layer by adding mechanisms to detect and retransmit  damaged or lost frames. Error control is normally achieved through a trailer to the end of the frame.
• Access Control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any time.

Network Layer

• Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally.
• The network layer adds a header to the packet coming from the upper layer, among other things, includes the logical address  of the sender and receiver.
• Routing. When independent networks or links are connected together to create an inter network (a network of networks) or a large network, the connecting devices (called routers or gateways) route or switch the packets to their final destination.

Transport Layer

• Port addressing: computer  often run several processes (running programs) at the same time.
• Process-to-process delivery means delivery from a specific process on one computer to a specific process on the other.
• The transport layer header include a type of address called port address.
• Connection control: The transport layer can be either connectionless or connection-oriented.
• A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.
• A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.
• Flow control:  the transport layer performs a flow control end to end. The data link layer performs flow control across a single link.
• Error control: the transport layer performs error control end to end. The data link layer performs control across a single link.

Session Layer

• The services provided by the first three layers are not sufficient for some processes.
• The session layer is the network DIALOG CONTROLLER.  It establishes, maintains, and synchronizes the interaction among communication systems.
• Dialog control:  The session layer allows two systems to enter into a dialog.  It allows the communication between two processes to take place in either half-duplex of full-duplex mode.
• Synchronization:  The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

Presentation Layer

• The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.
• Network process to application
• The presentation layer is responsible for translation, compression and encryption.
• Translation:  The process in two systems are usually exchanging information in the form of character strings, numbers, and so on.  The information must be changed to bit streams before being transmitted.  Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. 
• The presentation layer at the sender changes the information from its sender-dependent format into a common format.  The presentation layer at the receiving machine changes the common format into its receiver-dependent format.
• Encryption:  To carry sensitive information, a system must be able to ensure privacy.
• Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network.  Decryption reverses the original process to transform the message back to its original form.
• Compression:  It reduces the number of bits contained in the information.  In multimedia such as text, audio, and video.

Application Layer

• It is responsible for providing services to user.
• FTP, mail services, directory services.

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