OSI Reference Model- Internetworking!!!

Open System Interconnect (OSI) Reference Model

Nitesh is now working in a telecom company and wanted to work over a remotely operated machine which has Linux Operating System. He is working over his laptop which is windows based and now he is doubted, whether he would be able to telnet the remote machine. So he asked the same question from his colleague which is senior to him. His colleague gave him a nod and asked that’s all possible because of OSI Reference model.

Nitesh is now very happy and his curiosity starts raises towards the Open System Interconnect (OSI) Reference Model.

Open System Interconnect (OSI) Reference Model is a standard defined by International Organization for Standardization which allows interoperability between different vendors and helpful in troubleshooting network problems. Various RFC’s has been defined to for the operation of devices used in various layers of OSI. It has been divided into seven different layers categorized as:

 

Upper layers are used to talk to end hosts, these layers performs  programmer specific tasks and responsible for developing an application. Emails, FTP, TFTP, Telnet, encryption, decryption of data and authentication of data are all basic functions of Upper Layers.

Lower Layers are responsible for Network Engineering, which deals in the delivery of data in a network.

Function of layers of OSI Reference model:

1.       Application Layer: Application layer is responsible for End host communications. This layer supports all applications like opening a web page from IE, viewing a video from a vlc media player, printing a file from a Epson Printer, mail to friend from Yahoo, gmail, chatting on gtalk or google plus, social media application like facebook, orkut, etc are all been accessed with the help of Applications layers.

Supported Applications are: Email, FTP, TFTP, Telnet, SSH, BGP, etc.

2.       Presentation Layer: This layer ensures data can be understood between end hosts. If we defined it presents the data to Application layer for representation into desired format. Like if someone send a “.jpeg image”, presentation layer converts the data of image into EBDIC or ASCII formation which is computer acceptable and can be transported it to other end of host which further decrypt with the help of same layer.

This layer is responsible for data compression, encryption, decryption, decompression functions.

3.       Session Layer: This layer tells how end hosts figure out whom to talk to, means various end host would be able to talk to connect the application of their choice. As this layer is responsible for dialogue control and session control. Let’s take an example of in router we various sessions has been established, with the help of these sessions we can control the device.

4.       Transport Layer: The layer develops connection between sender and receiver and is fully responsible for end to end connectivity. It divides the data coming from upper layer into smaller segments and transmits the same into lower layers. This process is called as Segmentation. This layer provides reliable delivery of data into various layers, provides error detection and correction. Typical Protocols for OSI layers are TCP and UDP.

Transmission Control Protocol (TCP) is a connection oriented protocol provide reliable data delivery .For example Database Replication from one host to another.

User Datagram Protocol UDP is connection less protocol provide non-reliable data delivery. For example: video call or voice progression.

5.       Network Layer: This layer provides connectivity and path selection, the main function of network layer is Layer3 IP addressing. The routing of data packets, shortest path selection could takes place over this layer. The logical addressing of data packets takes place over this layer.

Routers comes into picture at this layer, Layer3 switches which switches on the basis of IP address are also comes into this category. The routing protocols responsible are IPv4 and IPv6.

6.       Data Link Layer:  This layer defines data for retransmission and converse of bytes. It also control access to physical media like this layers tells which media will function first like if we need to access a voice call or data browsing, data link layer sets the priority and help in accessing the layers. This layer provides MAC based addressing means on the basis of MAC address data will travel. Higher order Mac will give lesser priority. This layer is also responsible for error detection and correction through Cyclic Redundance Check. Let’s take an example when I put the frame over a copper cable, if anything goes wrong or any discrepancy occureed at another end, this layer will publish error on the basis of CRC’s. The devices comes under this layers are layer 2 switches, bridges, etc. 

7.       Physical Layer: All media properties have been taken care by this layer. The conversion of bytes into computer accepted form i.e bits are taken place at this layer. This layer controls electrical, physical data rates, physical connectors, cable distances, optical wavelength in case of wired media, optical frequency in case of wireless media.  The various devices comes under this layer are Cat5 cable, Fiber Optic cable, Hubs, repeaters, UTP connector, cross connectors, etc.

To remember the various layers of OSI reference model, we can memorize as:

All People Seem  To be Need Data Processing.

Please Do not touch Steves Personal Allegator.

Why we use OSI Layers:

      Device need to be aware of their own layer, every layer is independent of other layer functions.

-          For example in application layer, We Server doesn’t care if Ethernet or wireless was used to reach it. It means if a client request a web page from the Web Server, it need not to take care about, how the connection has reached, it will output on the basis of standirds defined in an application layer.

-          Similarlarly Etherent layer switch wii not care about IP packets whether they are Ipv4 or IPv6 packets.

      Provides interoperability between devices and vendor.

-          Like cisco router at one end can talk to Juniper router at other end.

-          Cisco Switch at one end can communicate to Netgear Switch at other end.

-          Windows IE browser can connect to Linux Apache Server.

Encapsulation and Decapsulation Procedures:

Protocol Data Units (PDU’s) are the data units which are present at each layer after encapsulating or decapsulating the data units. It means the final data unit  present at each layer are called as PDU’s.

PDU’s at Layer 4 are called as Segment

At layer 3- Packets

At Layer 2 – Frame

At Layer1- Bits.

The encapsulation process starts from top layer to bottom layer, means when data units reach at transport layer a transport layer header and trailer is been added along with the data and it becomes segments, when going down at Network Layer IP header and trailer has been added to make it IP Packet. Once it reaches to Data Link layer, it becomes, frame after adding Ethernet header and trailer. The entire frame is then further converted into bits and travel over the cable or physical layer media.

The same process would be taken care during the process of decapsulation of data reaches moving from bottom to top..

Transmission Control Protocol/ Internet Protocol TCP/IP:

This reference model was introduced by Department of Defense (DoD) and basically a condensed model of OSI Reference Model. It consists of four layers which are equivalent to seven layer of OSI Reference model.

The four layer model is as under:

·         Application Layer-       Equivalent to layer 5-7 of OSI Model.

·         Transport Layer-          Equivalent to layer 4.

·         Internet Layer-            Equivalent to N/W layer and provide fragmentation.

·         Network Access Layer- Equivalent to layer 1&2.