Wednesday, 26 October 2011

Simplified File Transfer
A Three Layer Model

1• Network Access Layer
2• Transport Layer
3• Application Layer
Network Access Layer

• Exchange of data between the computer and
the network
• Sending computer provides address of
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
Transport Layer

• Reliable data exchange
• Independent of network being used
• Independent of application
Application Layer

Support for different user applications
• e.g. e-mail, file transfer

Protocol Architectures and
Addressing Requirements

• Two levels of addressing required
• Each computer needs unique network address
• Each application on a (multi-tasking) computer
needs a unique address within the computer
— The service access point or SAP
— The port on TCP/IP stacks

OSI Layers

— Transport of information
— Higher layers do not need to know about underlying technology
— Not needed on direct links
• Transport
— Exchange of data between end systems
— Error free
— In sequence
— No losses
— No duplicates
— Quality of service

• Session
— Control of dialogues between applications
— Dialogue discipline
— Grouping
— Recovery
• Presentation
— Data formats and coding
— Data compression
— Encryption
— Means for applications to access OSI environment

• Physical
— Physical interface between devices
• Mechanical
• Electrical
• Functional
• Procedural
• Data Link
— Means of activating, maintaining and deactivating a
reliable link
— Error detection and control
— Higher layers may assume error free transmission

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