Internetworking models

When networks first came into being, computers to communicate only with computers of the same manufacturer. In the late 1970s, the open systems interconnection (OSI)  Difference model was created by the international organization for Standardization (ISO)  to break this barrier.

 

The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other. The OSI  model is the primary architectural models for networks. it describes how data and network information are communicated from an application on one computer, through the network media, to an application on another computer. The OSI  preferences break this approach into layers.

 

 

The layered approach

A reference model is a conceptual blueprint of how Communications should take place. It addresses all the processes required for effective communication and divides these processes into a logical grouping called layers. When a communication system FB sign in this manner, it's known as layered architecture.

 

 

Advantages of reference models

The OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. The primary purpose of all such models, especially the OSI model, is to allow different vendors networks to interoperate.

 

Advantages of using the OSI  layered model include, but not limited to, following.

  • Allows multiple- vendor developments through standardization of network components.
  • Allows various types of network hardware and software to communicate.
  • Prevent changes in one layer from affecting other layers, so it does not hamper development.

 

The principles that were applied to arrive at the 7 layers can be 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 to word 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 layout of necessity and small enough that the architecture does not become unwieldy.

 

 

ISO OSI Reference Model

One of the greatest functions of the Wi-Fi specification is it assists in data transfer between disparate hosts- meaning, for example, that they enable how to transfer data between a Unix host and a PC or a Mac.

 

The OSI Seven Layers

 

The seven different layers are divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with uses. the bottom four layers define how data is transmitted end-to-end.

 

The upper layers

 P14

The lower layers

 P15 

 

 

The Application Layer 

The application layer of the OSI model marks the spot where users actually communicate to the computer. The application layer contains a variety of protocols Betta commonly needed by users.

 

One widely used application protocol is HTTP, which is the basis for the WWW. when a browser wants a web page, it sends the name of the page it wants to the server using HTTP. The then the page back. Other application protocols used for file transfer (FTP), E-mail (SMTP) etc.

 

The Presentation Layer

The presentation layer gets its name from its purpose It presents data to the Application layer and is responsible for data translation in code formatting.

 

This layer is essentially a translator and provides coding and conversion functions. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading (for example, EBCDIC to ASCII). By providing translation services, the presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one.  The OSI has ‘ protocol standards that define how standard data should be formatted. Tasks like data compression, encryption, and decryption are associated with this layer.

 

The Session Layer

The Session layer is responsible for setting up, Managing, and then tearing down sessions between presentation layer entities.  this layer also provides dialogue control (keeping track of whose turn it is to transmit) between devices, of nodes. It coordinates communication between systems,  serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex. As another service that is offered as a part of the Session Layer might include data synchronization. 

 

The Transport Layer

The Transport layer segments and reassembles data into data steam.  Services located in the Transport layer both segment and reassemble data from upper-layer applications and unite it onto the same data stream. They provide an end-to-end date of transport services and can establish a logical connection between the sending host and destination host on an internetwork.

 

The Data Link Layer

The data link layer provides the physical transmission of the data and handle error notification, net topology, and flow control. This means the Data Link Layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and translates messages from the Network layer into bits for the physical layer to transmit.

 

The Data Link layer formats the message into pieces, each called a data frame and adds a customized header containing the hardware destination and source address. 

 

The IEEE Ethernet Data Link layer has two sublayers.

  1. Media Access Control (MAC) 802.3 Defines how packets are placed on the media. contention media access is “ first come/ First served”  access where everyone shares the same bandwidth.  Line discipline, error notification ( not correction),  ordered delivery of frames, optional flow control can also be used at this sublayer.
  2. Logical Link Control (LLC)  802.2 Responsible for identifying Network layer protocols and then encapsulation them. An LLC header tells the Data Link Layer what to do with a packet once a frame is received. It works like this: A host will receive a frame and look in the LLC header to find out where the packet is destined for-say, the IP protocol at the Network Layer. The LLC can also provide flow control and Sequencing of control bits.

 

The Physical Layer

Finally arriving at the bottom, we find that the Physical layer does two things: It sends bits and receives bits. Bits come only in values of 1 or 0. The physical layer communicates directly with the various types of actual communication media. Different kind of media represent these bit values in different ways. Some use audio tones, while others employ state transition-changes in voltage from high to low and low to high. A specific protocol is needed for each type of media t describe the proper bit pattern to be used, how data is encoded into media signals and the various qualities of the physical media’s attachment interface.

 

The physical layer specifies the electrical, mechanical, Procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end Systems. This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE).

 

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