osi modal full decriptation, Lecture notes of Data Communication Systems and Computer Networks

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OSI Model

MIS 416 – Module II Spring 2002 Networking and Computer Security

• (^) The OSI reference model
• (^) Services in the OSI model

Topics

• (^) Layers 1-4 relate to communications technology.
• (^) Layers 5-7 relate to user applications.

7-Layer OSI Model

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

• (^) Level at which applications access network services.
• (^) Represents services that directly support software applications for file transfers, database access, and electronic mail etc.

Layer 7: Application

Layer

• (^) Allows two applications on different computers to establish, use, and end a session.
• (^) e.g. file transfer, remote login
• (^) Establishes dialog control
  • (^) Regulates which side transmits, plus when and how long it transmits.
• (^) Performs token management and synchronization.

Layer 5: Session Layer

• (^) Manages transmission packets
  • (^) Repackages long messages when necessary into small packets for transmission
  • (^) Reassembles packets in correct order to get the original message.
• (^) Handles error recognition and recovery.
  • (^) Transport layer at receiving acknowledges packet delivery.
  • (^) Resends missing packets

Layer 4: Transport

Layer

 (^) Packages raw bits from the Physical layer into frames (logical, structured packets for data).  (^) Provides reliable transmission of frames  (^) It waits for an acknowledgment from the receiving computer.  (^) Retransmits frames for which acknowledgement not received

Layer 2: Data Link

Layer

• (^) Transmits bits from one computer to another
• (^) Regulates the transmission of a stream of bits over a physical medium.
• (^) Defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable. Deals with issues like
• (^) The definition of 0 and 1, e.g. how many volts represents a 1, and how long a bit lasts?
• (^) Whether the channel is simplex or duplex?
• (^) How many pins a connector has, and what the function of each pin is?

Layer 1: Physical Layer

• (^) In OSI model, each layer provide services to layer above, and ‘consumes’ services provided by layer below.
• (^) Active elements in a layer called entities.
• (^) Entities in same layer in different machines called peer entities.

Services in the OSI

Model

• (^) Layer N provides service to layer N+

Layering Principles

(N+1) Entity Service User (N) Entity Service Provider (N+1) Entity Service User (N) Entity Service Provider Layer N Service Access Point (SAP) Layer N protocol N+ PDU Layer N+1 protocol SDU PDU - Protocol Data Unit SDU - Service Data Unit N PDU N PDU

• (^) Reliable services never lose/corrupt data.
• (^) Reliable service costs more.
• (^) Typical application for reliable service is file transfer.
• (^) Typical application not needing reliable service is voice traffic.
• (^) Not all applications need connections.

Reliability

• (^) Service = set of primitives provided by one layer to layer above.
• (^) Service defines what layer can do (but not how it does it).
• (^) Protocol = set of rules governing data communication between peer entities, i.e. format and meaning of frames/packets.
• (^) Service/protocol decoupling very important.

Topics