OSI reference model

OSI reference model

One  of  the  most  important  networking  concepts  to  understand  is  the  Open Systems  Interconnect  (OSI) reference  model.  This  conceptual  Model, created  by the  International  Organization  for  Standardization  (ISO) in  1978  and  revised  in 1984,  describes  a  network architecture  that  allows  data  to  be  passed  between computer systems. OSI model and describes how it relates to real-world  networking.  It  also  examines  how  common  network  devices  relate  to  the  OSI model. Even though the OSI model is conceptual, an appreciation of its purpose and function can help you better understand how protocol suites and network architectures work in practical applications.

OSI reference model is built, bottom to top, in the following  order:  physical,  data  link,  network,  transport,  session,  presentation, and application. The physical layer is classified as Layer 1, and the top layer of the model, the application layer, is Layer 7.



7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 - Data-link
1 - Physical

The physical layer of the OSI model identifies the network’s physical characteristics, including the following specifications:

   Hardware
   The type of media used on the network, such as type of cable, type of connector, and pinout format for cables.

   Topology
   The physical layer identifies the topology to be used in the network. Common topologies include ring, mesh, star, and bus.

In addition to these characteristics, the physical layer defines the voltage used on a given medium and the frequency at which the signals that carry the data operate.

The data link layer is responsible for getting data to the physical layer so that it can be transmitted over the network. The data link layer is also responsible for error  detection,  error  correction,  and  hardware  addressing.  The  term  frame is used to describe the logical grouping of data at the data link layer.

The data link layer has two distinct sublayers:

   Media Access Control (MAC) layer
   The MAC address is defined at this layer. The MAC address is the physical or hardware address burned into each network interface card (NIC). The MAC sublayer also controls access to network media. The MAC layer specification is included in the IEEE 802.1   standard.

   Logical Link Control (LLC) layer
   The LLC layer is responsible for the error and flow-control mechanisms of the data link layer. The LLC layer is specified in the IEEE 802.2 standard.

The  primary  responsibility  of  the  network  layer  is  routing providing  mechanisms  by  which  data  can  be  passed  from  one  network system  to  another.  The network  layer  does  not  specify  how  the  data  is  passed,  but  rather  provides  the mechanisms  to  do  so.  Functionality  at  the  network  layer  is  provided  through routing protocols, which are software components.

The basic function of the transport layer is to provide mechanisms to transport data between network devices. Primarily it does this in three ways:

   Error checking
   Protocols at the transport layer ensure that data is sent or received correctly.

   Service addressing
   Protocols such as TCP/IP support many network services. The transport layer makes sure that data is passed to the right service at the upper layers of the OSI model.

   Segmentation
   To traverse the network, blocks of data need to be broken into packets that are of a manageable size for the lower layers to handle. This process, called segmentation , is the responsibility of the transport layer.

The session layer is responsible for managing and controlling the synchronization  of  data  between  applications  on  two  devices.  It  does  this  by  establishing, maintaining, and breaking sessions.

The  presentation layer’s  basic  function  is  to  convert  the  data  intended  for  or received from the application layer into another format. Such conversion is necessary because of how data is formatted so that it can be transported across the network. 

In simple terms, the function of the application layer is to take requests and data from the users and pass them to the lower layers of the OSI model. Incoming information is passed to the application layer, which then displays the information to the users. Some of the most basic application-layer services include file and print capabilities.

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