layers of OSI model

Discuss in detail about the layers of OSI model with a neat diagram

The ISO defined a common way to connect computers, called the Open Systems Interconnection (OSI) architecture. (eg. public X.25 network)

It defines partitioning of network functionality into seven layers as shown.

The bottom three layers, i.e., physical, data link and network are implemented on all nodes on the network including switches.

 

Physical Layer

The physical layer coordinates the functions required to carry a bit stream over a physical medium.

Representation of bits—To be transmitted, bits must be encoded into signals, electrical or optical. The physical layer defines the type of encoding.

Data rate—It defines the transmission rate (number of bits sent per second).

Line configuration—The physical layer is concerned with the connection of devices to the media (point-to-point or multipoint configuration).

Physical topology—It defines how devices are connected (mesh, star, ring, bus or hybrid) to make a network.

Transmission mode—The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex

Data Link Layer

The data link layer transforms a raw transmission facility to a reliable link.

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

Physical addressing—The data link layer adds a header to the frame to define the sender and/or receiver of the frame.

Flow control—If the receiving rate is less than the transmission rate, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error control—The data link layer adds reliability to the physical layer by adding a trailer to detect and retransmit damaged/lost frames and to recognize duplicate frames.

Access control—When two or more devices are connected to the same link, data link layer protocols determines which device has control over the link at any given time.

Network Layer

The network layer is responsible for the source-to-destination delivery of a data unit called packet.

Logical addressing—The packet is identified across the network using the logical addressing system provided by network layer and is used to identify the end systems.

Routing—The connecting devices (routers or switches) prepare routing table to send packets to their destination.

Transport Layer

The transport layer is responsible for process-to-process delivery of the entire message.

Service-point addressing—It includes a service-point address or port address so that a process from one computer communicates to a specific process on the other computer.

Segmentation and reassembly—A message is divided into transmittable segments, each containing a sequence number. These numbers enable the transport layer to reassemble the message correctly at the destination and to identify/replace packets that were lost.

Connection control—The transport layer can be either connectionless or connection-oriented.

Flow control—The flow control at this layer is performed end to end.

Error control—The error control at this layer is performed process-to-process. Error correction is usually achieved through retransmission.

Session Layer

The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

Dialog control—It allows two systems to enter into a dialog and communication between two processes to take place in either half-duplex / full-duplex mode.

Synchronization—The session layer allows a process to add checkpoints, or synchronization points, to a stream of data. For example, when checkpoints are inserted for every 100 pages and if a crash happens during transmission of page 523, then only pages 501 to 523 need to be resent.

Binding—binds together the different streams that are part of a single application. For example, audio and video stream are combined in a teleconferencing application.

Presentation Layer

The presentation layer is concerned with the syntax and semantics of the information exchanged between peers.

Translation—Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these encoding methods.

Encryption—To carry sensitive information, a system ensures privacy by encrypting the message before sending and decrypting at the receiver end.

Compression—Data compression reduces the number of bits contained in the information. It is particularly important in multimedia transmission.

Application Layer

The application layer enables the user, whether human or software, to access the network.

It provides user interface and support for services such as electronic mail, remote file access and transfer, shared database management and several types of distributed information services.

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.

File transfer, access, and management—This application allows a user to access/retrieve files in a remote host, and to manage or control files in a remote computer locally.

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

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