What is Communication Protocol?
Data networks are a combination of software and hardware components.
- The hardware includes transmission media, devices, and transmission equipments.
- The software allows the hardware to interact with one another and provide access to the network.
- The application programs that use the network do not interact with the hardware directly. The application programs interact with the protocol software, which follows the rules of the protocol while communicating.
- Protocol is a network term used to indicate the set of rules used by a network for communication.
- The International Standards Organization (ISO) has developed a seven-layer reference model for data networks, known as Open System Interconnection (OSI) model. The OSI model specifies the functions of each layer. It does not specify how the protocol needs to be implemented. It is independent of the underlying architecture of the system and is thus an open system.
The seven layers of the OSI model are—
(1) Physical layer, (2) Data link layer, (3) Network layer, (4) Transport layer, (5) Session layer, (6) Presentation layer, and (7) Application layer.
The functions of the different layers (Figure 9.25) are as follows:
qPhysical Layer—This layer specifies the basic network hardware. Some of the characteristics defined in the specification are—interface between transmission media and device, encoding of bits, bit rate, error detection parameters, network topology, and the mode of transmission (duplex, half-duplex or simplex).
q Data Link Layer—This layer specifies the functions required for node-to-node transmission without errors. It specifies the organization of data into frames, error detection in frames during transmission, and how to transmit frames over a network.
q Network Layer—The network layer specifies the assignment of addresses (address structure, length of address etc.) to the packets and forwarding of packets to the destination i.e. routing.
qTransport Layer—It specifies the details to handle reliable transfer of data. It handles end-to end error control and flow control, breaking up data into frames and reassembling the frames.
qSession Layer—The session layer maintains a session between the communicating devices. It includes specifications for password and authentication, and maintaining synchronization between the sender and the receiver.
qPresentation Layer—This layer specifies the presentation and representation of data. Its functions include translation of the representation of the data into an identifiable format at the receiver end, encryption, and decryption of data etc.
qApplication Layer—This layer specifies how an application uses a network. It deals with the services attached to the data. It contains the protocols used by users like HTTP, protocol for file transfer and electronic mail.
Data transfer in OSI model:
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