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OSI Model Layer

10 April 2011 No Comment

OSI Model LayerThe Open Systems Interconnection model (OSI model) was developed by the International Organization for Standardization (ISO) in 1984. The Open Systems Interconnect (OSI) layer model is the foundation for data communications.

OSI reference model is a way of sub-diving network communication and grouping them into logical layers. On each layer, an instance provides the services the instances to the layer above it and requests services to the layer below it.

There are seven layers to the OSI model. It defines the stages that data must go through to travel from one device to another over a network. It describes how applications running on network-aware devices communicate with each other. The model is generic and applies to all network types.

A mnemonic is a memory aid designed to make it easier to remember lists of data. If you want to remember the sequence of the OSI layers model, you can use one of these mnemonics:

  • Please Do Not Throw Sausage Pizza Away
  • Please Do Not Touch Steve’s Pet Alligator
  • People Design Networks To Send Packets Accurately
  • Philys Did Networking Till She Passed Away
  • People Don’t Need To Study Protocol Analysis

If you want to go the other direction:

  • All People Seem To Need Data Processing
  • All People Standing Totally Naked Don’t Perspire
  • A Penguin Said That Nobody Drinks Pepsi
  • A Priest Saw Two Nuns Doing Pushups
  • All People Studying This Need Drastic Psychotheraphy

The Layers

Layer 7: Application

This layer interacts with the operating system or application whenever the user chooses to transfer files, read messages or perform other network-related activities. It defines the interface for communication and data transfer. Some examples this layer supports include file transfer, e-mail, and NetBIOS-based applications.
Purpose: User Interface
Network Components: Gateway network device

Layer 6: Presentation

The presentation layer takes data provided by the application layer and converts it to a standard format that the other layers can understand. It is responsible for protocol conversion, character conversion, data encryption / decryption, expanding graphics commands, and data compression.

Purpose: Translation
Protocols: N/A
Network Components: Gateway redirector

Layer 5: Session

Session layer manages the session between two users at different ends in a network. It manages who can transfer data in a certain amount of time and for how long. Some examples include interactive login and file transfer connections.

Purpose: Syncs and Sessions
Protocols: NetBIOS, names pipes, mail slots, RPC
Network Components: Gateway

Layer 4: Transport

Transport layer manages end to end message delivery in a network and provides the error checking. Hence, it guarantees that no duplication or errors are occurring in the data transfers across the network. It provides the acknowledgement of the successful data transmission and retransmits the data if no error free data was received.

Purpose: Packets, flow control, and error-handling
Network Components: Gateway, advanced cable tester, brouter

Layer 3: Network

The network layer determines how data transmits between the network devices. It translates the logical address into the physical address (e.g., computer name to MAC address). It is responsible for addressing, determining routes for sending, and managing network problems such as packet switching, data congestion and routing. Router works on the network layer.

Purpose: Addressing and routing
Network Components: Brouter, router, frame relay device, ATM switch, advanced cable tester

Layer 2: Data Link

Data link defines the format of data on the network. It handles the physical and logical connections to the packet’s destination using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world and a loopback interface to send packets to itself.

Purpose: Data frames to bits
Protocols: 802.1 OSI Model, 802.2 Logical Link Control, 802.3 CSMA/CD (Ethernet), 802.4 Token Bus (ARCnet), 802.5 Token Ring, 802.12 Demand Priority
Network Components: Bridge, switch, ISDN Router, intelligent hub, NIC, advanced cable tester

Layer 1: Physical

Physical layer defines and cables, network cards and physical aspects. It is the level of the actual hardware. It defines raw bit stream on the physical media.

Purpose: Hardware and raw bit stream
Protocols: IEEE 802, IEEE 802.2, ISO 2110, ISDN
Network Components: Repeater, multiplexer, hubs, TDR, oscilloscope, amplifier

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