The OSI reference model

Open System Interconnect (OSI) Reference Model

Why Layer Model

•  Reduces complexity
•  Standardizes interfaces
•  Facilitates modular engineering
• Ensures interoperate technology Ensures interoperate technology
•  Accelerates the evolution of network technology.
• Simplifies teaching and learning

OSI Model – Physical Layer

• deals with mechanical, electrical and procedural interfacing.
• specifies cables, connectors, and other components.
• transmits raw information over communication channel communication channel.
• establishes, maintains, and disconnects physical links physical links.

OSI Model – Data Link Layer

•  provides reliable transfer of data  
• breaks data (packets) into frames
• adds bits for error detection/correction
• manages access to and use of the manages access to and use of the channel
• solve problems caused by lost, damaged, and duplicate frames
• sends acknowledgments
• adds flags to indicate beginning and end of message
• connectionless or connection oriented connectionless or connection oriented services
• IEEE MAC and LLC support

OSI Model – Network Layer

• establishes, maintains and terminates connections
• determines how packets are routed
• divides transport messages into packets divides transport messages into packets and reassembles them 
• performs congestion control flow control performs congestion control, flow control.
• provides virtual circuit or datagram services 
• recognizes message priorities
• sends messages in proper order
• handles internetworking

OSI Model – Transport layer

• establishes reliable end-to-end transport session (error detection and recovery), once path has been established
• fragmentation of message into packets (if not handled by layer 3)
• multiplexing of several sessions from multiplexing of several sessions from same source and all going to same destination

OSI Model – Session layer

• establishes and terminates connections
• accounting service
• authentication of users 
• controls dialogue, organizes and synchronizes

OSI Model – Presentation layer

• data encryption, security, compression and code conversion
• make sure data is encoded in standard form ( ASCII) 
• handles pass-through of services from session to application layer session to application layer

OSI Model – Application layer

•  login, password check
• agreement on semantics for information exchange
• file transfer access and management file transfer, access and management
• message handling, email 
• job transfer and manipulation
• directory service