OSI Model and TCP/IP Model

Layered Architecture

Layer architecture simplifies the network design. As it is easy to debug network applications in a layered architecture network application are separated into layers. Network layers follow a different set of rules, called protocol. There is basically two reference model:

OSI reference model

OSI stands for Open System Interconnection. The OSI reference model explains how packets travel through the various layers to another device on a network, even if the sender and destination have different types of network media. This model divides the network into seven layers, each of which illustrates a particular network function.

Physical Layer

This layer is concerned with data transmission. It specifies wire and connectors for the system to connect. It also assigns voltage level ((voltage at which the binary data is being assigned and transmitted) and data rate at which bit is transmitted.

Data Link Layer

Data Link Layer provides a direct link control on the network. It provides a reliable data transfer over the link. It also does physical addressing and controls the flow and error.

Network Layer

This Layer determines the best path for data transmission. It does network addressing and data transmission between the subnets.

Transport Layer

Transport Layer creates an end to end connection. It establishes a connection between the host. It does flow and congestion control and provides a reliable data transfer.

Session Layer

This layer maintains the session of the user. It does token management, tracks all the event through the use and establishes and terminates the session.

Presentation Layer

This layer selects data structure, provides data transfer syntax and semantics. It maintains the format of data and ensures the data is readable by the application.

Application Layer

Application Layer provides applications for the user for network use. It provides different network services to the users like email, www, FTP, HTTP, SMTP, etc.

TCP/IP Model

TCP/IP stands for Transmission Control Protocol and Internet Protocol. This network model is used in the current Internet architecture as well.
Similar to OSI model, TCP/IP defines how to use the network to transmit an IP datagram. In TCP/IP Network Access Layer protocols must know the details of the underlying network (its packet structure, addressing, etc.) to correctly format the data being transmitted to comply with the network constraints.

Layer 1: Host-to-network Layer

It is the lowest layer of all. It is also known as Network Access Layer. A suitable protocol is used to connect to the host so that the packets can be sent over it.

Layer 2: Internet layer

This layer provides services that are roughly equivalent to the OSI Network layer. This layer holds the whole architecture together. It helps the packet to travel independently from source to the destination. IP (Internet Protocol) is used in this layer.

Layer 3: Transport Layer

It is designed to allow devices or peer on the source and destination hosts to carry on a conversation, just as the OSI transport layer. It carries out functions such as multiplexing, segmenting or splitting into the data. It chooses data transmission medium either parallel path or a single path. It also adds header information to the data and breaks the message into small units so that they are handled more efficiently by the network layer.

Layer 4: Application Layer

This layer includes the OSI Session, Presentation and Application layers of OSI model. An application is any process that occurs above the Transport Layer. These applications can read and write to the transport layer. There are different application layer protocols in TCP/IP, including Simple Mail Transfer Protocol (SMTP) and Post Office Protocol (POP) used for e-mail, Hyper Text Transfer Protocol (HTTP) used for the World-Wide-Web, and File Transfer Protocol (FTP) as they are required.

References:

1. A.S. Tanenbaum, “Computer Networks”, 3rd Edition, Prentice Hall India, 1997.
2. W. Stallings, “Data and Computer Communication”, Macmillan Press, 1989.
3. Kurose Ross, “Computer Networking: A top-down approach”, 2nd Edition, Pearson Education
4. Larry L. Peterson, Bruce S. Davie, “Computer Networks: A Systems Approach”, 3rd Edition, Morgan Kaufmann Publishers