OSI Model
Initial computer networks had their own set of standards and conventions that were hardware dependent. Each manufacturer used to develop its own communication protocols for its networks. For example IBM launched SNA (Systems Network Architecture) in 1974. Similarly DEC (Digital Equipment Corporation ) launched DNA (Digital Network Architecture) in 1980 for use on the DEC range computers. Hence data communication protocols of one network were not compatible with any other network.
International Organization for Standardization (ISO) recognized this problem and established a subcommittee to develop an international standard for network architecture. The result of this subcommittee’s recommendations was the Open System Interconnection (OSI) model. Hence, standardized OSI protocols made it possible for any two heterogenous computer systems, located anywhere in the world, to communicate easily with each other.
OSI model provides following services:
i. Provides peer to peer logical services with layer physical implementation
ii. Provides standards for communication between system.
iii. Define point of interconnection for the exchange of information between systems.
iv. Each layer should perform a well defined function.
v. Narrows the options in order to increase the ability to communicate without expansive conversions and translation between products.
The OSI model is designed in a highly structured way. A separate set of protocols is defined for each layer in the seven layer architecture. Roles of these seven layers are described below.
In actual implementation of these seven layers, first three layers are likely to be hardware, next two layers in operating system, presentation layer in library subroutines in user’s address space, and application layer in user’s program.
1. Physical Layer
Physical layer is responsible for transmitting raw bit streams between two nodes. That is, it converts sequence of binary digits into electric signals, light signals, or electromagnetic signals, depending on whether the two nodes are on a ,cable circuit fiber-optic circuit, or microwave/radio circuit, respectively. Even electrical details, such as how many volts to use for 0 and 1, how many bits can be send per second, whether transmission can take place in one direction or both directions simultaneously are decided by physical layer protocols. In addition physical layer also deals with mechanical details such as size and shape of connection plugs, number of pins in plugs and function of each pin. RS232-C is a popular physical layer standard for serial communication lines.
2. Data link Layer
Physical layer simply transmits data from sender’s node to receiver’s node as raw bits. Data-link layer is responsible for detecting and correcting any error in transmitted data. Since physical layer is concerned only with a raw it stream , data-link layer partitions it into frames, so that error detection and correction can be performed independentaly for each frame. Data-link layer also performs flow control of frames between two sites to ensure that a sender does not flood a receiver with data by sending frames at a rate faster than the receiver can process.
3. Network Layer
The network layer is responsible for delivery of packets from the source to destination. A transmitting network layer accepts a message from the transport layer. During reception this layers accepts the data stream from data link layer and physical layer. A network layer interface can accept the data from a router. The message to the transport layer at the receiver’s end transmits the message after the appropriate changes have been made in accordance with the transport protocol. The message intended for transmission may be communicated in parts. Before communicating a message to the next data link layer or router, the data bytes of the message fragmented into packets.
A packet is of specific to all length that a particular network permits. Each packet has a header. This has the additional bits for the packet’s source and destination addresses, sequence and the acknowledge management, error control, flow control etc. The network layer has another important role. It allows an interconnection between the dissimilar networks. Two popular network layer protocols are the X.25 Protocol and the Internet Protocol (called IP).
4. Transport Layer
Transport layer accepts messages of arbitrary length from session layer, segments them into packets, submits them to network layer for transmission, and finally reassembles packets at destination. Some packets may be lost on the way from sender to receiver, and depending on routing algorithms used in network layer packets may arrive at destination in a sequence that is different from the order in which they were sent. Transport layer protocols include mechanisms for handling lost and out-of-sequence packets. For this, the transport layer assigns a sequence number to each packet, and uses sequence numbers for detecting lost packets and for ensuring that messages are reconstructed in correct sequence. Two most popular transport layer protocols are Transport Control Protocol (TCP) and User Datagram Protocol (UDP).
5. Session Layer
The session layer is network dialog controller i.e. it establishes and synchronizes the interaction between communication system. It allows communicating parties to authenticate each other before establishing a dialog session between them, it specifies dialog type – one-way, two-way alternate, or two-way simultaneous – and initiates a dialog session, if message is a connection request It also provides priority management service that is useful for giving priority to important and time-bound messages over normal less important messages.
6. Presentation Layer
Presentation layer deals with syntax and semantics of the information being exchanged. It provides facilities to covert message data into a form that is meaningful to communicating application layer entities, For this, it may perform such transformations as encoding and decoding, code conversion, compression and decompression, encryption and decryption, on message data depending on application’s requirements.
7. Application Layer
Application layer is responsible for accessing the network by user. It provides user interfaces and other supporting services such as e-mail, remote, file access, file transfer, sharing database, message handling (X.400), directory service (X.500).
