OSI Layers Model
OSI layer model define that how a message is transmitted between two points in the telecommunication network. There are seven layers of the OSI layers model and each layers plays different role in the network communications. The seven layers include the Application, Presentation, Session, Transport, Network, Data link and Physical layer. OSI layers model was developed by the ISO for the telecommunication standardizations. The seven layers divide the tasks of the data communication into the subtasks and complete the data transmission cycle. OSI (Open System Interconnectivity) model divides the complex network communication process. In the communication process, the control is passed from top layer to the bottom and from the bottom to the top layers.
Application Layer
Application layers support the applications such as file transfers, email, telnet, SNMP, DNS, Finger, BOOTP, MIME, SMB, network software and NETBIOS based applications. Application layer allow access to the services that support applications. It handles the flow control, error recovery and network access. Gateway works on the Application layer.
Presentation Layer
Presentation layer is responsible for the protocol conversion, data encryption, decryption and data compression. Presentation layer formats the data to be presented over the network. It also encodes and decodes the data. Gateway and redirectors work on the Presentation layer.
Session Layer
Session layer establish, maintain and manages the session between two computers. It is responsible for the name recognition so that only the designated parties can participate in the session. It manages that who can transmit the data at a certain time and for how long. NETBIOS, RPC and Mail Slots work on the session layer. Interactive login and file transfers are the examples of the session layers. It also reports the upper layer errors.
Transport Layer
Transport layer provides the transfer of data between the end systems and the host computers and it also controls the error recovery and the flow control. It provides reliable and sequential packet delivery. TCP, ARP, NETBIOS, SPX, ATP and UDP works at the Transport layer. Gateway, Brouter and cable tester works on the Transport layer. It provides the acknowledgement of the successful transmission and the requests the retransmission if some packets don’t arrive at the destination.
Network Layer
Network layer is responsible for the addressing, routing, determines the shortest possible path for the data packets and translates the logical network address and names to their physical address. The protocols such as IP, ARP, ICMP, IPX, OSI, DDP, IGMP, RIP and OSPF works on the network layer. Brouter, router, frame relay device, ATM switch and cable tester works on the network layer. Routing, forwarding, internetworking, error handling and the congestion control are the main functions of this layer.
Data Link Layer
Data link layer defines the procedures for operating the data links. It detects and corrects the packets transmit errors. It defines the formats of the data on the network. It is responsible for the error free transfer of the frames to the other computers via the physical layer. Data link layer is further divided into the Logical Link Control and the Media Access Control. The devices such as Bridge, Switch, ISDN router, NIC, Intelligent Hub and advanced cable tester works on the Data Link Layer. This layer is also responsible for the error checking, frame control and the synchronization.
Physical Layer
Physical layer defines the physical medium such as network cables, network cards, hardware and the physical aspects/mediums of the data transmission. The devices such as Repeater, Multiplexer, TDR, Amplifier and Hubs work on the Physical layer. It defines the techniques to transfer the data over the physical cables.
