Computer network, Reference models, in computer network , OSI model, OSI model layers, TCP/IP protocol , TCP/IP Models layers , ,OSI&TCP/IP Difference

Computer network, Reference models, in computer network , OSI model, OSI  model layers, TCP/IP protocol , TCP/IP Models layers ,OSI&TCP/IP Difference

Reference Models :

 There are two important network architectures.

1.  OSI reference model 
2.  TCP/IP reference model

The OSI Reference Model : 

The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarized as follows: 

1.  A layer should be created where a different abstraction is needed. 
2.  Each layer should perform a well-defined function. 
3.  The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the interfaces. 
5.  The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

The Physical Layer :

• The physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit it is received by the other side as a 1 bit, not as a 0 bit.

The Data Link Layer :

• The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors. The sender breaks up the input data into data frame and transmit the frames sequentially. 

•  If the service is reliable, the receiver confirms correct receipt of each frame by sending back an acknowledgement frame. The medium access control sublayer deals how to control access to the shared channel. 

The Network Layer :

•  The network layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are ‘‘wired into’’ the network and rarely changed, or more often they can be updated automatically to avoid failed components. Handling congestion is also the responsibility of the network layer

The Transport Layer :

•  The basic function of the transport layer is to accept data from above it, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of the network. The transport layer is a true end-to-end layer; it carries data all the way from the source to the destination.

The Session Layer :

• The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control , token management (preventing two parties from attempting the same critical operation simultaneously), and synchronization.

The Presentation Layer :

•  The presentation layer is concerned with the syntax and semantics of the information transmitted. The presentation layer manages these abstract data structures and allows higher-level data structures (e.g., banking records) to be defined and exchanged. 

The Application Layer :

The application layer contains a variety of protocols that are commonly needed by users. One widely used application protocol is HTTP (Hyper Text Transfer Protocol), 

The TCP/IP Reference Model :

The Link Layer :

• The link layer describes what links such as serial lines and classic Ethernet must do to meet the needs of this connectionless internet layer. 

The Internet Layer : 

The internet layer is the linchpin that holds the whole architecture together.

• Its job is to permit hosts to inject packets into any network and have them travel independently to the destination (potentially on a different network). They may even arrive in a completely different order than they were sent. 

•  The internet layer defines an official packet format and protocol called IP (Internet Protocol), plus a companion protocol called ICMP (Internet Control Message Protocol) that helps it function. 

The Transport Layer :

• The layer above the internet layer in the TCP/IP model is now usually called the transport layer. Two end to- end transport protocols have been defined here. The first one, TCP (Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. 

•  The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable, connectionless protocol for applications that do not want TCP’s sequencing or flow control and wish to provide their own.

The Application Layer :

 On top of the transport layer is the application layer. It contains all the higher- level protocols. 

A Comparison of OSI and TCP/IP Reference Models : 

A Critique of TCP/IP Reference Model :

1.   The model does not clearly distinguish the concepts of services, interfaces, and protocols. 
2.  The TCP/IP model is not at all general and is poorly suited to describing any protocol stack other than TCP/IP. 
3.  The link layer is not really a layer at all in the normal sense of the term as used in the context of layered protocols. 
4. The TCP/IP model does not distinguish between the physical and data link layers.