Reference Models | Class 12

Reference model: It is a conceptual design that describes how communication between devices occurs. One of the reference model’s primary benefits is that it defines the standards for building network components, allowing for the development of multiple vendors.

The most important reference models are:

1. OSI reference model

2. TCP/IP reference model

Introduction to OSI Model

The OSI Model, created in 1984 by ISO, is a reference framework that explains the data transmission process between computers. It is divided into seven layers, which cooperate to carry out certain network operations, enabling a more methodical approach to networking. The OSI model consists of seven abstraction layers:

Physical Layer: Layer 1

The physical layer is the lowest in the OSI reference model. It is in charge of establishing the devices’ actual physical connection. Information is stored in bits in the physical layer. It’s responsible for sending certain bits from one node to the next. This layer receives data, reads it as a signal, and sends the converted 0s and 1s to the Data Link layer, which reassembles the frame. Hub, repeater, modem, and cables are physical layer devices.

Data Link Layer (DLL): Layer 2

In a network, the Data Link Layer (DLL) controls error-free data transport between nodes. It consists of Logical Link Control (LLC) and Media Access Control (MAC) sublayers. To properly route data, DLL divides data into frames, adds MAC addresses for sender and recipient identification, and uses Address Resolution Protocol (ARP) to find the target device’s MAC address. The data link layer is handled by the NIC (Network Interface Card) and device drivers of host machines. Switches and bridges are data link layer devices.

Network Layer: Layer 3

The network layer enables the transfer of data between hosts that are connected to various networks. It also handles packet routing, i.e., the selection of the shortest way among the available routes to send the packet. The network layer inserts the IP addresses of the sender and receiver into the header. The network layer is implemented by networking devices such as routers and switches.

Transport Layer: Layer 4

The Transport Layer manages end-to-end message delivery by dividing data into segments. It ensures reliable transmission, provides acknowledgment for successful data transfer, and re-transmits data in case of errors. On the sender’s side, it segments and formats data, implements flow and error control, adds source and destination port numbers in its header, and sends the segmented data to the network layer. Device or Protocol Use: TCP, UDP, SCTP, etc.

Session Layer: Layer 5

The session layer manages connection establishment, maintenance, and termination between processes. It enables synchronization by establishing checkpoints in data, aiding in error identification, and preventing premature data loss. Additionally, it facilitates communication initiation between systems in either half-duplex or full-duplex modes. Device or Protocol Use: NetBIOS, PPTP

Presentation Layer: Layer 6

The presentation layer is also called the translation layer. The data from the application layer is extracted here and handled as per the required format to transmit over the network. Its functions include data translation (e.g., ASCII to EBCDIC), encryption and decryption for security, and data compression to minimize the number of transmitted bits. Device or Protocol Use:  JPEG, MPEG, GIF

Application Layer: Layer 7

At the very top of the OSI model layers, we find the application layer, which is implemented by the network applications. These applications generate the data, which has to be transferred over the network. This layer also serves as an interface for the application services to access the network and for displaying the received information to the user. Example: Application: Browsers, Skype Messenger, etc.

Introduction to TCP/IP Model

The TCP/IP reference model is a conceptual framework used to describe the functions of networking protocols and how they interact. It is the foundational architecture used for the Internet and most networks today. Layers in the TCP/IP model are as follows:

1. Application Layer: This is the topmost layer of the TCP/IP model and is responsible for providing network services directly to user applications. It includes protocols that enable functions such as email (SMTP), web browsing (HTTP), file transfer (FTP), remote access (SSH), and domain name resolution (DNS). This layer interacts directly with software applications and provides a means for them to communicate over the network.

2. Transport Layer: The transport layer manages end-to-end communication and ensures the reliable and orderly delivery of data between devices. It includes protocols like Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP provides reliable, connection-oriented communication by establishing and maintaining a connection between devices, ensuring data delivery, and handling retransmissions if data is lost or corrupted. UDP, on the other hand, provides connectionless communication without ensuring reliability, making it faster but less reliable than TCP.

3. Network Layer: The network layer is responsible for addressing, routing, and packaging data packets to be sent over the network. It includes the Internet Protocol (IP), which assigns logical addresses (IP addresses) to devices and enables them to communicate within and across networks. IP addresses are used to identify devices and ensure that data packets are routed to their correct destinations across different networks.

4. Network Access Layer (or Link Layer): This is the lowest layer of the TCP/IP model and deals with the physical connection between devices on the same network. It encompasses the protocols and hardware necessary for transmitting data over specific types of physical infrastructure, such as Ethernet or Wi-Fi. It handles the transmission of data frames over the network and deals with issues related to media access control and physical addressing.