model is called open system Interconnection (OSI) and is usually referred as the OSI reference model . The OSI model has seven layers. These range from the lowest layer , where the physical and electrical connection to the network is made, up to the highest level , where the actual user of the data that has been passed resides.
The seven layers in the ISO model-
Level 7 (highest) Application
Level 6 Presentation
Level 5 Session
Level 4 Transport
Level 3 Network
Level 1 Physical
Not all applications need or use all seven layers for a successful, practical working system. The lowest three layers are enough for many applications. Let’s study the definition of each layer and its role.
Roles of each layer —
|Layer||Primary functions||Services provided to next layer|
|7||Application||Support the end user, LOGIN, password, file transfer||This is the highest layer and provides user oriented services|
|6||Presentation||Code and format conversion||Freedom from compatibility problems.|
|5||Session||Session management, synchronization||Dialogue management|
|4||Transport||Optimum utilization of the network resources||End-to-end transport connection of the required quality in a cost- effective manner|
|3||Network||Interaction with the subnetwork; Routing and relaying||Network connection linking the end systems|
|2||Data link||Error control; Flow control||Reliable transfer of bits across the physical connection|
|1||Physical||Conversion of bits into electrical signals of suitable characteristics and|
Transmission of bits
The physical Layer-
The physical layer is concerned with transmitting raw bits over a communication channel.
The Data link–
The main task of the link layer is to take a raw transmission facility and transform it into a line that appears free of undetected transmission errors to the network layer.
The Network Layer-
The network layer, acts to route the communications through the various communication resources to the other end
The Transport Layer-
The basic function of the transport layer is to accept data from the session layer, split it up into smaller units if need be,
The Session Layer-
The session Layer must manage and synchronize conversations between two different applications. Session can allow traffic to go in both direction at the same time, or in only one direction at a time.
The presentation Layer-
Makes sure that the information is delivered in a form that the receiving system can understand and use.
The Application Layer-
The top of the model is the application layer, layer 7, which provides for the manipulation of information in various ways.