OSI (Open System Interconnect ) Model

In layered architecture of Network Model, one whole network process is divided into small tasks. Each small task is then assigned to a particular layer which works dedicated to process the task only. Every layer does only specific work.

Open System Interconnect is an open standard for all communication systems. OSI model is established by International Standard Organization (ISO). This model has seven layers.

Layer 7 - Application Layer: This layer is responsible for providing interface to the application user. This layer encompasses protocols which directly interact with the user. The application layer is responsible for providing services to the user.

Specific services provided by the application layer include the following:

o Network virtual terminal. A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host.

o File transfer, access, and management. This application allows a user to access files in a remote host (to make changes or read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

o Mail services. This application provides the basis for e-mail forwarding and storage.

o Directory services. This application provides distributed database sources and access for global information about various objects and services.

Layer 6 - Presentation Layer: This layer defines how data in the native format of remote host should be presented in the native format of host. The presentation layer is responsible for translation, compression, and encryption.

Specific responsibilities of the presentation layer include the following:

o Translation. The processes (running programs) 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.

o 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.

o Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

Layer 5 - Session Layer: The services provided by the first three layers (physical, data link, and network) are not sufficient for some processes. This layer maintains sessions between remote hosts.   The session layer is responsible for dialog control and synchronization.

Specific responsibilities of the session layer include the following:

o 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 (one way at a time) or full-duplex (two ways at a time) mode.

o Synchronization. The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

Layer 4 - Transport Layer: This layer is responsible for end-to-end delivery between hosts. The transport layer is responsible for the delivery of a message from one process to another.

Other responsibilities of the transport layer include the following:

o Service-point addressing. Computers often run several programs at the same time. For this reason, source-to-destination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called

a service-point address (or port address). The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

o Segmentation and reassembly. A message is divided into transmittable segments, with each

segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

o 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.

o Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link.

o Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to-process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission.

Layer 3 - Network Layer: This layer is responsible for address assignment and uniquely addressing hosts in a network. The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Other responsibilities of the network layer include the following:

o Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

o Routing. When independent networks or links are connected to create intemetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

Layer 2 - Data Link Layer: The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer. The data link layer is responsible for moving frames from one hop (node) to the next.

Other responsibilities of the data link layer include the following:

o Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

o 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 sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

o Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

o Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and re transmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

o 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 given time.

Layer 1 - Physical Layer: The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur.

This layer defines the hardware, cabling, wiring, power output, pulse rate etc. The physical layer is responsible for movements of individual bits from one hop (node) to the next.