A network consists of hardware and software. A network communication has many layers involved in the communication. Each layer has some protocols, policies and methods which facilitates data communication. There are two popular network models:
- OSI Model
- TCP/IP Model
OSI refers to Open Systems Interconnection model is an ISO standard that covers all aspects of network communications. It was first introduces in 1970. It must be noted that the OSI model is not a protocol, its purpose is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.
OSI is a layered architecture. It is composed of seven layers:
- Physical (layer 1)
- Data link (layer 2)
- Network (layer 3)
- Transport (layer 4)
- Session (layer 5)
- Presentation (layer 6)
- Application (layer 7).
Let’s have a one liner description of each layer.
The physical layer is concerned with the physical aspect of data transmission.it defines the type of transmission medium, encoding direction of transmission and duration of bits. It also defines the topology of network i.e. how devices are connected to make network.
Data Link Layer
The data link layer is responsible for moving frames from one hop (node) to the next.
The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks. It is also responsible for logical addressing and routing.
The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. The transport layer is also responsible for Service-point addressing, connection control, flow control, error control
The session layer is responsible for dialog control and synchronization. It establishes, maintains, and synchronizes the interaction among communicating systems.
The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. It is responsible for translation, encryption and compression of data.
The application layer enables the user to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.
TCP/IP model was developed prior to OSI model. TCP/IP defines four protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP)and Internet Protocol (IP). There are four layers in this model:
- Physical and Data Link layers
- Network layer
- Transport layer
- Application layer
Physical and Data Link Layer
In TCP/IP model, there is no definite physical and data link layer. A network in a TCPIIP internetwork can be a local-area network or a wide-area network.
At the network layer, TCP/IP supports the Internetworking Protocol (IP). The Internet Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. It is an unreliable and connectionless protocol-a best-effort delivery service. There is not error checking and tracking in IP. IP transports data in packets called datagrams. This IP supports four protocols: ARP, RARP, ICMP, and IGMP.
The transport layer represented in TCP/IP by two protocols: TCP and UDP.
The application layer in TCPIIP is equivalent to the combined session, presentation, and application layers in the OSI model.