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Understanding the OSI Model

Exam: Microsoft 70-642 - Windows Server 2008 Network Infrastructure, Configuring

The OSI - Open Systems Interconnection was developed by the International Organization for Standardization (ISO) in the year 1977. Since its inception, the model has been accepted as a standard for understanding network communication and can be used for a comparative understanding the network implementations on various systems.
While communicating on computer networks as in the case of others there are two things that are required -

  • A communication language that is common and
  • A communication medium

The OSI model attempts to define rules covering various aspects of networks like how they contact each other in both situations of a common and a different language, methods that can be adopted for transmitting data, when not to transmit data, methods relevant to reception of data, arrangement of transmission media, maintaining the proper rate of data flow and representation of bits on the network media.

The OSI model has seven layers which are discussed in detail below:

The Physical Layer

This layer uses electric signaling for transmitting bits from one system to another. The components of the Physical layer do not concentrate on what the bits mean rather work to get the bits transmitted from one place to another. The physical layer caters to small details pertaining to the actual physical connection between the machine and the network medium. Types of network connections, physical topologies, signaling methods (digital as well as analog), multiplexing and termination are examples of information contained in the physical layer.

The Data-Link Layer

The Data Link layer is the 2nd layer and it has the function to transfer data on a single physical link between two systems. It receives information in the form of packets from the Network Layer and converts them into frames which are nothing but discrete data units. These packets are then sent to the Physical Layer for further processing, which in turn appends important control relation information to it.

Apart from the above function, this layer also makes it possible to transfer frames between two systems without any error. CRC or Cyclic Redundancy Check is used to check incoming frames for presence of any possible errors. Lost frames can also be detected and request be sent for resending these frames.

The IEEE has split the ISO Data Link Layer into two parts - LLC Sublayer and MAC sublayer. While the LLC sublayer provides for Service Access Points (SAPs), the MAC sublayer is responsible for providing shared access to the network adapter and direct communication with network interface cards.

The Network Layer

The Network Layer is responsible for handling packets that are moving between devices. The routing decisions are made here and packets are forwarded in order to assist them in the travel. In very large networks, intermediate devices and subnetworks may be available between two systems at different ends. The layer ensures that the Transport layer sends the packets without bothering about the placement of the end point system.

For carrying out its function, the Network layer churns out MAC/physical addresses from logical network addresses. This layer also affects the quality of service provided in message transmission, by ensuring that the message can get to its destination by at least one way or the other.

If data packets are too big in size as compared to what is required by the Data Link layer, the Network layer ensures that it chops these packets into smaller units for processing and then reassemble them at the receiving end.

This layer also performs multiple functions like Addressing, Switching (Circuit, packet and message), Route Discovery, Route Selection, connection Services and

The Transport Layer

The task of this layer is to ensure that the data is delivered in an error free condition and in a sequence with minimum loss or duplication. Large messages from the Session layer are broken into smaller parts to be handed over to the Network Layer and forwarded to the destination computer. An acknowledgement can be sent by Transport layer to the machine where the messages received originated. These services are optional and not required for implementation of all Transport layer protocols. One feature that is common to all protocols at the Transport layer is upper layer protocol multiplexing.

The Session Layer

This layer helps in establishing a connection between applications installed on different systems and this interconnection is known as a session, hence the name of this layer. It does so by providing a service so that the application on two or more systems can find and communicate with each other.

This layer also ensures that in case of any failure in the network there is no need to resend entire data but only packets which were lost in transit and this is termed as data synchronization. It also controls any communication taking place between two processes and determines which process can either transmit or receive.

The Presentation Layer

This last but one layer translates the data between the various formats required by the network. It performs functions like conversion of protocol conversion; translation, encryption and compression of data; conversion of character set and interpretation of graphics commands. The network redirector a component of Windows networking operating at the level of this layer. The redirector makes visible the file server to the client computer.

The Application Layer

This is the upper most layer in the OSI model and it is responsible for providing services which directly support user applications, which could range from anything between a simple file transfer to accessing a database. It allows for smooth communication between different applications installed on different systems, as if they were present on the same system.

Hence we have studied different layers of the OSI model and their roles and functionalities in the bigger picture.