The international organization for standardization (ISO) developed the Open Systems Interconnection (OSI) reference model to describe how information is transferred from one system to another system. When information is converted into electrical or light signals, it is transferred along a piece of wire or radio waves. It is very important to define a set of rules for their transmission. Various types of rules and regulations come under the ISO’s reference model. And it is very important to understand that why many network protocols, such as IP and IPX, fail to fit properly into the scheme explained in ISO’s model.

This model covers all aspects of network communication and is known as OSI model.


Physical layer is required for transmitting a bit stream over a physical medium. It deals with mechanical and electrical specification of the interface and transmission medium. It also defines the processor and the functions that the physical devices and interface have to perform for the transmission to occur, so in general physical layer is concerned with the following.

1. Physical characteristics of interface
2. It specifies data transmission rate.
3. It also defines the physical topologies of the network.
4. It deals with sender and receiver synchronization at bit level.
5. Representation of bits , that is, to convert digital bit into electrical signal and vice           versa.
6. It also defines the direction of the transmission of two devices whether it is simplex,        half-duplex or full duplex.

Bit stream Transmission through Physical Layer


The data link layer transforms the raw bit stream received from the physical layer into reliable information and is responsible for node to node delivery. It makes the physical layer appear error free to the upper layers.

Some of the responsibilities of data layer are as follows:

1. It deals with the physical address of both the source and the destination.

2. When two or more devices are connected to the same link, Data link layer protocols         determine which device will have control over the link at any given time.

3. This layer also deals with error control mechanism, that is, it detects and re-transmits     damaged or loss FRAME. It uses a mechanism to prevent duplication of frames.

4. It also deals with the rate at which the sender should send the data to the receiver       so that a fast sender does not overwhelm a slow receiver, this mechanism is called         FLOW CONTROL.

5. It divides the stream of bits received form the network layer into manageable data         units called FRAME.

6. At the receiver end, again it performs the function of assembling the raw bits received     from the physical layer into frames before passing them on to the network layer.

Data Link Layer


The network layer is responsible for source to destination delivery of packets across multiple networks. Whereas data link layer's delivery is hop to hop delivery. Network layer assures that each packet gets from its point of origin to its destination.

The specific responsibilities are as follows:

1. It deals with the logical addressing of sender and receiver.
2. It deals with the routing of data between different links and networks.
3. This layer also deals with the overall congestion control mechanism of the network.
4. It divides the outgoing message into packets and assembles the incoming packets          into message.
Network Layer


The transport layer is responsible for process to process delivery of the entire message, that is, this layer ensures that the whole message arrives intact and in order over error control and flow control process to process level.

Some responsibilities are as follows:

1. It deals with segmentation and re-assembly of the message with proper sequence           number in case the message is very large.

2. It deals with connection control, that is, whether an end to end connection should be     connectionless or connection oriented.

3. This layer also deals with flow control of the data but here the flow control is                 performed process to process rather than across a single link.

4. It deals with service point addressing, that is, the network layer gets each packet to       correct computer and the transport layer gets the entire message to the correct           process on the computer.

5. This layer also deals with error control so that the receiving transport layer receives       the message without any error or duplication; error correction is usually achieved            by re-transmission.

Transport Layer


The job of session layer is to establish, maintain and synchronize the interaction between the communicating systems.

Specific responsibilities of session layer are as follows:

1. It allows two systems to start a communication (dialog control) with each other. The       communication between two systems is either in HALF or FULL DUPLEX mode.

2. This layer allows addition of check points, that is, synchronization points into a stream     of data, so that in case of crash during the transmission of data, data can be                 re-transmitted from the check point last inserted, instead of re-transmitting from           the beginning.


The presentation layer is concerned with the syntax and semantics of information exchange between two communicating systems.

Some responsibilities are as follows:

1. It deals with protocol conversion.

2. It carries out data compression to reduce the bandwidth of the data to be                     transmitted.

3. It translates data between the format that the network requires and the format that       the computer expects.

4. For security and privacy purpose, it carries out encryption of data at the sender's end     and decryption at the receiver's end.


This layer enables the user, whether human or software, to access the network. It provides user interface and support for services such as,

  • Data base access
  • Email.
  • Remote file access and transfer.

  • Some specific functions are as follows:

    1. This layer allows a user to access file in a remote computer.

    2. This layer provides the bases of Email forwarding and storage.

    3. This layer provides distributed database sources and access for global information           about various services and objects.

    4. This layer creates software emulation of a terminal at remote host. This application is     known as network virtual terminal exchange.