Thursday, April 7, 2011

OSI 7 Layers Reference Model

These 7 layers further divide the tasks of moving the data across the network into subtask and hence complete one communication cycle between two computers or two network devices. Each layer is assigned a task and the task is completed independently. The OSI layers have the clear and independent characteristics and tasks.

Layer 7: Application Layer

The application layer defines the interfaces for communication and data transfer.
This layer also provides and support services such as job transfer, handles network access, e-mail, supports user applications and error recovery.

Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer.
Network Devices: Gateway network device is operated on the application layer.

Layer 6:Presentation Layer

The presentation layer presents the data into a uniform format and masks the difference of data format between two dissimilar systems. It also translates the data from application to the network format. Presentation layer is also responsible for the protocol conversion, encryption, decryption and data compression. Presentation layer is a best layer for cryptography.
Network Devices: Gateway Redirector is operates on the presentation layer.

Layer 5: Session Layer

Session layer establish and manages the session between the two users at different ends in a network. Session layer also manages who can transfer the data in a certain amount of time and for how long. The examples of session layers and the interactive logins and file transfer sessions. Session layer reconnect the session if it disconnects. It also reports and logs and upper layer errors.
Network Devices: Gateway

Layer 4: Transport Layer

Transport layer manages end to end message delivery in a network and also provides the error checking and hence guarantees that no duplication or errors are occurring in the data transfers across the network. Transport layer also provides the acknowledgement of the successful data transmission and retransmits the data if no error free data was transferred.
It also provides and error handling and connectionless oriented data deliver in the network.

Network Devices: The Brouter, Gateway and Cable tester work on the transport layer.

Layer 3: Network Layer

The network layer determines that how data transmits between the network devices. It also translates the logical address into the physical address e.g computer name into MAC address. It is also responsible for defining the route, managing the network problems and addressing. Router works on the network layer and if a sending device does not break the data into the similar packets as the receiving device then network layer split the data into the smaller units and at the receiving end the network layer reassemble the data.

Network layer routes the packets according to the unique network addresses. Router works as the post office and network layer stamps the letters (data) for the specific destinations.
Network Devices: Network devices including Router, Brouter, Frame Relay device and ATM switch devices work on the network layer.

Layer 2:Data Link Layer

Defines procedures for operating the communication links
Frames packets
Detects and corrects packets transmit errors
Protocols: Logical Link Control
• error correction and flow control
• manages link control and defines SAPs
802.1 OSI Model
802.2 Logical Link Control
Media Access Control
• communicates with the 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
ISDN Router
Intelligent Hub
Advanced Cable Tester

Layer 1: Physical Layer

Physical layer defines and cables, network cards and physical aspects. It defines raw bit stream on the physical media. It also provides the interface between network and network communication devices. It is also responsible for how many volts for 0 and how many for 1. Physical layer also checks the number of bits transmitted per second and two ways or one way transmission. Physical layer also dealing with the optical, mechanical and electrical features.
Protocols: Protocols that work on the physical layer are ISDN, IEEE 802 and IEEE 802.2
Network Devices: Hubs, Repeaters, Oscilloscope and Amplifier works on the network devices.

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