OSI Reference Model

OSI (Open Systems Interconnection) is a reference model to define application communication flow over a network and is also defined as conceptual framework for understanding relationships.

In other words, to divide the interrelated aspects of Network operation into less complex operations and to defines standard interfaces to achieve compatibility and multi vendor integration.

Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data Link
Layer 1 Physical

 

Application Layer:

  • Provides interface between OSI RM and end user application.
  • Provides network services to user client/server-based application.
  • Establishes and defines program-to-program communication.
  • Identifies availability of intended communication partner.
  • Examples include ftp, tftp, http, www browsers, DNS, SMTP, telnet.

Presentation Layer

  • Defines data format for transmission.
  • Ensures arriving data from the network can be transmitted by the application and information sent by the application can be transmitted on the network.
  • Performs encryption and decryption.
  • Example representation includes ASCII, JPEG, HTML etc.

Session layer

  • Defines how to strat, control and end session.
  • RPCs (Remote Procedure Call) operate at this layer.
  • Logon validation happens at this layer.
  • Named Pipes defined at this layer – Named Pipes use TCP to guarantee communication.
  • Session layer organizes communication through simple, half-duplex or full-duplex.
  • Example protocols include SQL, RPC, Named Pipes etc.

Transport Layer

  • Segmented data to be passed down to Network layer and reassembles data for session and upper layers.
  •  Provide the choice of connection-oriented and guaranteed (TCP) or connectionless and non-guaranteed(UDP) delivery of data .
  • Provides end to end transport services.
  • Provides flow control to overcome congestion in network.
  • Provides 3 flow control mechanism:
  1. Buffering: Each computer has enough buffer space to hold data before it os processed.
  2. Congestion Avoidance: Receiving computer notices its buffers are filling quickly and sends a stop message to the sending host to temporarily stop transmitting while it processes data already received. Then signals that it is ready for more data. Eg. Protocols – Synchronous Data Link Control (SDLC), Link Access Procedure, Balanced (LAPB), ICMP Source Quench (slows down rate instead of stopping it).
  3. Windowing: Defines maximum number of bytes it is willing to send at one time before an acknowledgement is expected.

Network Layer

  • Defines the network address.
  • Routers operate at this layer.
  • Segments from the transport layer are placed into and passed down to Data Link Layer.
  • Network layer routes data from one node to another.
  • Determines the best path/ route to destination device to use for routing data on the inter network- this is done using hop count or tick where 1 tick = 1/18th of a second.
  • Network layer maintains routing table.

Data Link Layer

  • Provides error- free link between 2 devices- CRC used for error checking.
  • Packets from the Network layer are placed into frames.
  • Data Link layer handles physical transmission of data from one node to another.
  • Handles error notification.
  • IEEE subdivided this layer into 2 sub layers
  1. Logical Link Control (LLC): Uses destination Service Access Point (DSAP) and Source Service Access Points (SSAP) to help lower protocols access Network layer protocols.
  2. Media Access Control (MAC) : build frames from bits performs CRC Handles MAC address- first digits of 12 hex defines vendor Id, next 6 defines serial no of that vendor Id.

Physical Layer

  • Defines connections – RJ-45, RJ-11, BNC etc.
  • Places Frames, represented as bits, onto media as electric signals or pulses of light.
  • Hubs are repeaters operate at this layer.

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