Two devices are connected by a transmission medium. Data is exchanged through this medium. This exchange between the communicating devices is called data communication. In a data communication system, five major components make up the system.
- Message : The information or data communicated between the devices via the medium. Information can take up any forms including text, numbers, pictures, audio and video.
- Sender : The device which sends the data message. It can be a computer, mobile or a camera.
- Receiver : The device which receives the message.
- Transmission medium : The physical path through which a message travels from the sender to the receiver. Some examples of the medium include twisted-pair wire, coaxial wire, fiber- optic cable and radio waves.
- Protocol : A set of rules which govern the data communications. It represent the agreement between the two communicating devices. Any devices can connect with each other. But, they cannot communicate without protocols.
Protocols and Standards on Data Communication
Protocols are the rules which define what, how and when to communicate the data. Standards are agreed upon rules by the government or by fact.
Many standards govern the network. The ISO standard which governs all the characteristics of network communications is the Open Systems Interconnection (OSI) model. It was introduced in the late 1970s. The OSI model is one of the major data communication models.
An open system is a set of protocols. These protocols allow two systems to communicate despite of their different architecture. The purpose of the OSI model is to allow communication between systems without changing the hardware or software components. The OSI model is not a protocol; it is a model that helps in understanding and designing a network system that is robust, flexible and inter operable. Thus, allowing communication between all types of computer systems.
The OSI model consists of seven separate layers which are related. Each layer defines a part of the process during the transferring of data over a medium or network.
- The physical layer is responsible for movements of individual bits over a medium from one point to the next.
- The data link layer is responsible for moving frames (organisation of bits) from one point to the next.
- The network layer is responsible for the delivery of individual packets from the source host to the destination host, providing internetworking.
- The transport layer is responsible for the delivery of a message from one process to another.
- The session layer is responsible for establishing, managing and terminating sessions.
- The presentation layer is responsible for translation, compression and encryption.
- The application layer is responsible for providing services to the user, allowing access to network resources.
Another major data communication model is TCP/IP(Transmission Control Protocol/Internet Protocol). When two network systems communicate, two addresses are included. These addresses are the source address and the destination address. Normally, four pairs of addresses are required, one pair per layer. The physical layer does not need any addresses as the unit for data exchange is a bit. The layer, its address and packet name are related.
- At the application layer, we normally use names to define the site that provides services, such as something.com, or email addresses such as [email protected]/mail.com.
- At the transport layer, port numbers are used as addresses. Port numbers are the local addresses which define the application layer programs at the source and destination. It helps in distinguishing between several running programs.
- At the network layer, the addresses are global. A network layer address defines the unique connection of a device with the Internet network.
- The link layer addresses are sometimes called the MAC. They are locally defined which specifies a host or router in a network.
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