hello

WHY A COMPUTER NETWORK?

The main reasons are:

• Distribute pieces of computation among computers (nodes)
  
• Coordination between processes running on different nodes
  
• Remote I/O Devices
  
• Remote Data/File Access
  
• Personal communications (like e-mail, chat, audio/video conferencing)
  
• World Wide Web
• ... and many other uses

WHY DO I NEED A STANDARD ?


• 
  

There are:


• Many types of connection media : telephone lines, optical fibers, cables, radios, etc.
  
• Many different types of machines and operating systems
  
• Many different network applications
• 
  
WHAT "STANDARD" MEANS ?



Agreements must be at many levels ...


• How many volts pulse is a 0 and 1?
  
• How to determine the end of a message?
  
• How to handle lost messages?
  
• How many bits for different data types? Integers/Strings, etc.
  
• Are characters coded in ASCII ?
  
• How machines are identified in a network? Names, numbers ?
  
• How to find the way to reach a machine ? How if there are more choices ?
  
• How different applications (and OSs) speaks together through the network ?
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THE ISO STANDARD MODEL FOR COMMUNICATIONS: OSI




ISO (the International Organization for Standardisation) has developed
a standard model for communications, called the


OSI (Open Systems Interface) Model


Model = it means that it's only theory! In fact the OSI model is not yet fully implemented in real networks.
Open System : It can communicate with any other system that follows the specified standards, formats, and semantics.
Protocols give rules that specifiy how the communication parties may communicate.
Supports two general types of protocols. Both are common.
Connection-Oriented :
Sender and receiver first establish a connection, possibly negotiate on a protocol. (virtual circuit)
Transmit the stream of data.
Release the connection when done.
E.g. Telephone connection.
Connectionless
No advance setup is needed.
Transmit the message ( datagrams ) when sender is ready.
E.g. surface mail