What is the difference between OSI model and TCP / IP model?

When we talk about layer 2 and layer 3 switches, we are actually referring to the layers of a generic protocol model: the Open Systems Interconnection (OSI) model, which is commonly used for the description of network communications. The communication of data between different networks would not be possible if there were no shared rules for its transmission and reception. These rules are known as protocols, among which the Transmission Control Protocol (TCP) / Internet Protocol (IP) is distinguished for being one of the most widely used. This is popularly used in the description of the network and is older than the OSI model, both with many layers. Next, we will explain what is the difference between them.
Layers of the OSI model
The seven-layer OSI model is a conceptual model that characterizes and standardizes the way in which the different software and hardware components involved in network communication must divide the workforce and interact with each other. In the following figure, you can see the names and basic functions of each of the layers.

The TCP / IP model has only four layers and is generally known as TCP / IP, as these are its two most important protocols.

Application layer:

The application layer of the TCP / IP model gives applications the ability to access the services of the other layers and defines the protocols that applications use to exchange data. The best-known application layer protocols are HTTP, FTP, SMTP, Telnet, DNS, SNMP, and Routing Information Protocol (RIP).

Transport layer:

The transport layer is responsible for providing session communication and datagram to the service application layer. The main protocols of this layer are TCP and UDP. TCP provides an individual, reliable, connection-oriented communications service. It is responsible for the sequencing and detection of sent packets and the recovery of lost packets in transmission. UDP provides an individual or group communications service, offline and unreliable. This is normally used when the amount of data to be transferred is small, such as when they fit in a single package.

Internet layer

The Internet layer is responsible for the routing, packaging, and routing functions of the host. The core protocols of the Internet layer are IP, Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP), and Internet Group Management Protocol (IGMP). At this layer, the IP adds the header to the packets, which is known as the IP address. Currently, there are both IPv4 (32 bit) and IPv6 (128 bit) IP addresses.

Network access layer

The network access layer (or link layer) is responsible for placing TCP / IP packets on the network data carrier and receiving TCP / IP packets located outside of it. The TCP / IP protocol is designed to be independent of the method of network access, the format of the network frame and the feeder. In other words, this protocol is independent of any specific network technology, which means that it can be used to connect different types of networks, such as Ethernet, Token Ring and Asynchronous Transfer Mode (ATM).

How is data processed during transmission?

In a layering system, devices in one layer exchange data in a different format, which is known as a protocol data unit (PDU). The following table shows the PDUs in the different layers.


For example, when a user requests to browse a website on their computer, the remote server software first delivers the requested data to the application layer, where it is processed from layer to layer with each layer performing its designated functions. The data is subsequently transmitted through the physical layer of the network until it is received by the destination server or another device. At this point, the data passes through the layers again, each layer performs its assigned operations until the receiving software finally uses the data.

During transmission, each layer adds a header, footer, or both to the PDU from the top layer, which directs and identifies the packet. This process is called encapsulation. The header (and footer) and body form the PDU for the next layer. The process continues until it reaches the lowest level layer (physical layer or network access layer), from which the data is transmitted to the receiving device. The receiving device reverses the process, encapsulating the data in each layer with the information in the header and footer that directs the operations. Finally, the application uses the data and the process continues until all the data is transmitted and received.
Because the operation of layer splitting is known, it is possible to diagnose the problem when a connection fails. The key is to test the operation from the lowest level, rather than from the highest level, as each layer caters to its immediately higher layer, making it easier to deal with lower-layer problems. For example, if your computer cannot connect to the Internet, the first thing you should do is check if the network cable is connected to it or if the wireless access point (WAP) is connected to the switch.

OSI model and TCP / IP model

The TCP / IP model is older than the OSI model. The following figure shows the correlation between its layers.

Photo

After making the comparison between the layers of the TCP / IP model and the OSI model, it is concluded that the application layer of the TCP / IP model is similar to the combined OSI layers 5, 6, 7, although the TCP / IP model does not have the so-called presentation or session layer. The TCP / IP transport layer encompasses the responsibilities of the OSI transport layer and some of the responsibilities of the OSI session layer. The TCP / IP network access layer encompasses the data link and the physical layers of the OSI model. Note that the TCP / IP Internet layer does not take advantage of the sequencing and recognition services that may be present in the data link layer of the OSI model. The responsibility lies with the transport layer in the TCP / IP model.

Importance of TCP / IP and OSI for troubleshooting

If we consider the meanings of the two reference models, the OSI model would only be a conceptual model; This is mainly used to describe, discuss, and understand individual network functions. However, TCP / IP is designed to solve a specific set of problems and not to function as a build description for all network communications, just as the OSI model does. The OSI model is generic and protocol independent, although most protocols and systems adapt to it; while the TCP / IP model is based on standard protocols developed by the Internet. Another factor to take into account in the OSI model is that, for the simplest applications, not all layers are used. While layers 1, 2, 3 are mandatory for any data communication, there are also applications that can use certain specific interface layers instead of the usual top layers of the model.

Summary

The TCP / IP model and the OSI model are conceptual models used for the description of all network communications, in turn, TCP / IP is also an important protocol that is used in all Internet operations. Generally, when we talk about layer 2, layer 3 or layer 7 in which a network device works, we refer to the OSI model. The TCP / IP model is used both to model the current architecture of the Internet and to provide a set of rules followed by all forms of transmission over the network.