OSI Model Explained

The various stages in computer networking can be essentially compiled into the OSI model. Many protocols that correspond to the networking layers reside in the stages of the OSI layers model. A network administrator must know the functions of these protocols so as to have a better understanding of the subject of computer networking. This article on 'OSI model explained' will give a fresh perspective on the OSI layers and ethernet protocols.

History of the OSI Model

The work on the Open Standard Interface (OSI) reference model was started in 1977 by the International Organization for Standards. It was then decided that OSI will have two major components - a 7 layer model and a set of specific protocols. The various issues on OSI design have evolved from a networking model called CYCLADES. CYCLADES also influenced the design of internet architecture then. Since the inception of the OSI reference model, the working of internet technology has become very smooth.

OSI Model

Before the advent of the OSI reference model, communication with different entities and different vendors was extremely difficult. This was because every vendor would have a different mechanism to communicate. Therefore, to communicate with entities of different vendors, there arose a need to have a common platform. This need forced the International Organization for Standards to have a viable and universally accepted platform. Thus, the OSI reference model was born.

OSI Layers Explained

Physical Layer: The physical layer is at the bottom of this data networking model. It deals with crude data that is in the form of electrical signals. The data bits are sent as 0's and 1's. 0's correspond to low voltage signals and 1's correspond to high voltage signals. The mechanical aspects of communication, such as wires or connectors come under this layer. The physical layer also deals with how these wires, connectors, and voltage electrical signals work. Also, the process that is required for these physical aspects are taken into account in this layer itself. Read more on ethernet cable wiring.

The Data Link Layer: The transmission of the data over the communication medium is the responsibility of this layer. The 0's and 1's that are used in the communication are grouped into logical encapsulation. This encapsulation is called frames. The data is transported in frames. The responsibility of these frames is that of the data link layer.

Network Layer: All over the world, there are many different types of ethernets. These networks are connected to each other through various media. When a data packet wants to reach a particular destination, it has to traverse through these networks. Essentially, there are lot of operations that are taking place between the connected networks. Also, the packet data which is traversing, has to choose an optimum route, and the addressing of these packets has to be proper. The various operations between the networks, packet data issues, addressing and routing are handled by this network layer.

Transport Layer: The transport layer ensures quality and reliability of the communication. The data packet switching is entirely handled by the transport layer. There are basically two types of packet switching. They are connectionless packet switching and connection oriented packet switching. In connectionless packet switching, the packet data is allowed to choose the route in which it is going to reach the destination. Obviously, the packet in itself cant do this. Physical devices like routers are mainly responsible for the behavior of packets, but the packets formed from the same datum can reach their destination in different ways. Whereas, in connection oriented packet switching, once the route is decided, then all the packets have to follow the same route. Examples of connectionless packet switching are text messages in mobile phones, and the example of connection oriented switching is a direct voice call.

The Sessions Layer: The sessions layer is mainly responsible for creating, maintaining and destroying the communication link. PDU (Protocol Data Unit), in which various protocols are defined, that have to be followed during communication, are the responsibility of the sessions layer. The applications that use RPC's (remote procedure calls) are taken care of by the sessions layer.

Presentation Layer: There are various techniques of data compression which are used to send and receive the optimized data. For example, if certain data is repeating itself for a number of times, then it is logical to send the data only once, and specify the number of times it is repeated. This bundling of the repeated data is one of the techniques of compressions. The compression and decompression of the data is handled by the presentation layer. Also, encryption and decryption techniques used to thwart malicious attacks on data are handled by the presentation layer.

Application Layer: This is the topmost layer of the OSI reference model. This layer comes into picture when there is a process to process communication. Whenever a user invokes any application, all the associated processes are run. Many a times, when an application wants to communicate with another application, then there has to be communication between these associated processes. The application layer is responsible for this interprocess communication.

OSI Layers Protocols

OSI Layer	Protocols
Application Layer	DNS, FTP, TFTP, BOOTP, SNMP, SMTP
Presentation Layer	SMB, NCP
Sessions Layer	NETBIOS
Transport Layer	TCP, ARP, RARP, SPX, NWLINK, NETBIOS
Network Layer	IP, ARP, RARP, ICMP, IGMP
Data link Layer	LLC
Physical Layer	LLC

OSI Layer Model vs TCP/IP Reference Model

Sr. No.	OSI Reference Model	TCP/IP Reference Model
1.	Model was first defined before implementation takes place	Model was defined after protocols were implemented
2.	Reliable delivery of packet data	Service interface and protocols were not clearly distinguished before
3.	Internet working not supported	TCP/IP supports Internet working
4.	Strict Layered	Loosely layered

You can also read more on:

• TCP/IP Model: Layers of TCP/IP Protocol Stack
• Computer Networking Tutorial

Since the establishment of the OSI model, there has been a revolution in the field of communication. The entire industry of communication can find its backbone in the OSI reference model. Hope the above article on OSI model explained has been apt and informative for the user.