Advance Computer Networks (Lecture # 01)

1.1 Definition and Structure of Computer Networks             

 Definition:
 "A computer network connects independent computers for the purpose of data exchange".
 
 Node : point of connection into the network such as computer, router, switch etc

                       Standard Organizations

 Standardization is absolutely necessary for networks of computers from different manufacturers.
  Examples of standardization organizations include
International Organization for Standardization (ISO)
International Telecommunications Union (ITU)
CEN/CENELEC/ETSI (European)
National Institute of Standards and Technology (NIST)
and many more.


               The Structure of Networks

                      Structuring the problem


                Protocol Architecture in Layers

 We structure the complex communication system into layers:
The lowest layer (layer 1) provides the physical connection.
Each higher layer adds more functionality, hiding details from the next higher layer.
“Horizontal” interfaces between layers are a local matter (there is no need for standardization). They are called “service interfaces”.
The rules of communication between two entities at the same layer are called “communication protocol”.
 

1.2 The ISO/OSI Reference Model for OSI


OSI = OPEN SYSTEMS INTERCONNECTION
In short: ISO/OSI Reference Model
ISO International Standard 7498
Introduces the layered architecture and basic terminology
Proposes seven layers and their functionality



       Seven Layers and Their Functionality


Clink the link below:

            Seven Layers of the ISO-Reference Model (1)


The physical layer provides the transmission of a digital data stream over a transmission line.
The data link layer provides error and flow control for the physical bit stream. In LANs it also implements medium access control.
The network layer provides routing algorithms to compute end-to-end paths through the network. It also implements packet queueing and forwarding in the “routers”.
The transport layer provides reliable end-to-end data connections from a process at the source node to a process at the destination node.
The session layer coordinates the cooperation (dialog) between applications.

Click the link bellow:

      Seven Layers of the ISO-Reference Model (2)


The presentation layer provides a common transfer syntax for all systems, and conversion rules from the local to the common data representation.
The application layer implements the application-specific protocols. 

          Headers and Trailers in the Layered Model


    Headers and Trailers in the Layered Model


        Conclusions:


 Network architectures consist of layers.
 A protocol is defined between two entities of the same layer.
 The ISO Reference Model for Open System Interconnection distinguishes seven layers.


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