Overview of OSI Layer Model - Open Systems Interconnection

OSI LAYER MODEL

Transferring of data between different communication devices is divided into logical layers called OSI layers, these layers are seven in number. OSI model stands for Open Systems Interconnection, it was designed by Xerox Corporation and Intel in 1984 and later ISO (International Standard Organization) standardized it. OSI is logical framework for standards for computer networks and data communication. OSI model is used as set standards for computer networking world wide. Major networking protocols are specifically designed keeping OSI model in view.

These layers are as follow

Physical layer
Data Link layer
Network layer
Transport layer
Session layer
Presentation layer
Application layer

These layers helps dividing different tasks of communication and transferring data with in the network into subtasks, that’s how one completes cycle of communications between network(s) is completed. Each layer completes its tasks independently. These layers are also divided into upper and lower layers.

Physical Layer: Layer 1

As name is suggests, Physical layer handles all sorts of physical aspects including, networking cables, networking cards/adaptors and other physical devices require for networking. It defines bit steam on physical media. It is responsible for providing interfaces between networks and network communication devices. Physical layer also handles mechanical, electrical and optical aspects on the network. Following are the protocols which works with physical layers are IEEE 802, IEEE 802.2 and ISDN. For network devices are hubs, repeaters, amplifiers, and oscilloscope.

Data Link Layer: Layer 2

Communication links and its procedures are handled data link layers ,Frames packets
Handle detection and correction of packets transmitted errors.
Protocols: Logical Link Control
• Data flow control and error correction
• Handle SAPs & link control
802.1 OSI Model, 802.2 Logical Link Control
Media Access Control
• communicates with the network adapter card
• controls the type of media being used:
802.3 CSMA/CD (Ethernet), 802.4 Token Bus (ARCnet), 802.5 Token Ring, 802.12 , Demand Priority
Network Devices: Bridge / Switch, ISDN Router, Intelligent Hub, NIC ,Advanced Cable Tester

Network Layer: Layer 3

Network layer is of great importance when it comes to talk about communication between networking devices. This layer helps in determining the data being transferred between different network devices. Network layer also translate logical address into actual physical address for example, converting computer name in to MAC address. The layer is held responsible for controlling the core processes of networking, for example, routing, managing network and its problem, addressing etc. Router/Switches are the hardware which work on network layers, if sending devices are unable to break down data into packets similar to receiving devices then network layer comes in and breakdown the data into smaller unites and then re-assemble the data at receiving end.As we read earlier it handles the core processes of networking, network layer routes data in form of packets according to network address. It is responsible to send all the data to specific destination with in the network.Protocols used are RIP, ARP, ICMP,OSI, OSPE and IPX.
Network devices which are used at network layer are Routers, Frame Relay device and ATM switches.

Transport Layer: Layer 4

Transport layer handles the delivery of data packets at their respective ends and destinations. It is also responsible for identifying errors and duplication of data through out with in the network. Once data is transferred to their respective ends successfully it provides acknowledgement about it and resends the data on receiving errors during transferring. Transport layer keep the connection alive during the data transmission and also handles error during the course of transferring of data with in the network.Protocols which are used on this layer are SPX, NETBOIS, TCP, NELINK and ATPGateway and Cable tester etc are network devices.

Session Layer: Layer 5

Session layer is designed to manage established sessions between two different nodes (users) with in the network. Session layer is also responsible to establish connection then identifies who and how much data is to be sent how much time is required for it. Session layers keeps the connection alive, incase if it is disconnection during the transfer, it re connects and continues the session. Example of session layers can be Login system or time taking in file transferring between two different users is one session. Session layers also keep the reports and upper layer errors are also recorded.
Protocols in session layers Mail Slots, NetBIOS, Names Pipes and RPC where as gateways are used as network devices.

Presentation Layer: Layer 6

Presentation layers works as interfacing layer between different formats. It present data into similar format and update the difference of formats among two different systems. It also provide interface between application data to network data format. It is used for protocol conversions, encryptions, decryptions and data compressions. Cryptography is done with in presentation layer.
 
Application Layer: Layer 7

this is upfront layer that helps in interfacing for communication and data transfer with in the network. It also provides support services like transfer services, handles network access, e-mailing, error management and user application support.
Protocols that can be used in this layer are FTP ( file transfer protocol), DNS, SNMP, SMTP (emailing), FINGER, TELNET, TFTP, BOOTP SMB etc where as gateway are used for network devices.

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