OSI vs TCP/IP

Both the OSI and the TCP/IP models are the standard references for the communication that occurs between devices or networks when data is transferred. Different protocols must be used to break down the data being transferred from the sender's device, as well as to reassemble the data for the user on the receiving end; these two models offer a reference for this process.

Using protocols connected devices communicate together to transfer data, however in order to allow this communication, either the OSI or TCP/IP model must be followed.

What is the OSI reference model?

The OSI reference model is a 7 layer, the vertical standard for transferring data between devices or networks. Data flows down the layers of the model when sent from a connected device and rises back up the layers on the receiving device's end to ensure error-free and efficient data transfer.

The 7 Layers of the OSI Referencing model

Layer 7: The Application Layer

Layer 7 is the first layer that data will reach when being sent from one device to another, as well as the last layer data passes through on the receiving end. Also known as the Application layer, this is where data is prepared to be used by the user's software, such as web browser or email. It is also the layer in which communication is initiated with the network when data is sent from one device.

This layer is responsible for protocols such as HTTP, HTTPS, and SMTP. These protocols make the data received from another device meaningful for the software being used as well as beginning the data transfer process when sending information from a device.

Layer 6: Presentation Layer

The presentation layer makes the information presentable for the application layer. This is where encryption, translation and compressing occurs in the sending of data, as well as the reverse of these processes when receiving data.

It may be the case that two devices communicate in a different language, and so the presentation layer is responsible for translating what is received from one device, into a language that can be understood for the application layer. This layer also compresses data to help speed and efficiency during the transfer.

Layer 5: Session Layer

The session layer is responsible for opening and closing the connection between two devices. The time that a connection needs to remain open to transfer data between devices is known as a session and it must stay open long enough to transfer all data that is being transferred.

Disruptions to sessions may occur, however, the session layer is able to protect any data transfer through the use of checkpoints. For example, if a session was allowing the transfer of a 200MB file, the session layer may set checkpoints every 10MB. If a disruption occurs that closes the session, the download doesn't have to start from the very beginning but can continue on from the last checkpoint, offering a more efficient data transfer.

The session layer is also responsible for closing the session between two connected devices quickly after data transfer is complete. This ensures that no resources are wasted.

Layer 4: Transport Layer

The transport layer looks after end-to-end communication between two devices. It is responsible for breaking up data from the session layer into segments and sending this data on to layer 3. The transport layer on the receiving end, in turn, reassembles the segments into data that the session layer can understand.

The transport layer is also in charge of flow control and error control which help to ensure the efficient and successful transfer of data. Flow control chooses the best speed of transmission, this means that a user with a very fast connection won’t overwhelm a slower connection on the other end. Error control is undertaken on the reiving device and makes sure that all data has been transferred successfully; if this is not the case, retransmission is requested.

UDP and TCP are two of the most important protocols for the transport layer, as well as the OSI model as a whole. TCP stands for Transmission Control Protocol; it is used to provide a reliable connection between two devices. TCP transfers a data stream of bytes via an established connection. Through this protocol, data is sent and received in the same order and without errors.

UDP stands for User Datagram Protocol and unlike TCP, UDP is a connectionless protocol that transfers the data via link. It offers an unreliable datagram connection between two devices and the data can arrive out of order or even be lost in the transmission.

Layer 3: Network Layer

The network layer oversees routing, or choosing the best path for the data to take. It is worth noting that the networking layer is only needed if data is being transferred between devices on different networks.

The network layer breaks up the segments received from the transport layer even further into packets. The network layer in the receiving device will reassemble packets into segments.

Layer 2: Datalink Layer

The data link layer is very similar to the networking layer but helps the transport of data between devices that sit on the same network. The data link layer also breaks down the packets received from the network layer to even smaller fragments called frames.

The data link layer is responsible for flow and error control during inter-network communications.

Layer 1: Physical Layer

The physical layer is concerned with transmitting raw data over physical mediums, such as cables. With electronic and optical impulses, this layer transmits unstructured data in the form of 0s and 1s from one device to another.

The physical layer of both devices must agree on a signal convention to understand the 1s and 0s and communicate effectively.

What is the TCP/IP Referencing Model

The TCP/IP model, in comparison only consists of 4 layers and ultimately combines many layers of the OSI model to form a more condensed standard. Just as with the OSI model, the TCP/IP is a standard reference for networking, demonstrating how data is transferred between devices. However, unlike OSI, TCP/IP functions horizontally rather than vertically.

The name TCP/IP came from the most commonly used protocols used in this model, TCP (Transmission Control Protocol) from in the transport layer, and IP (Internet Protocol) and network layer.

Layer 1: Network Access Layer

This is the equivalent of both the physical layer and data link layer of the OSI model and focuses on the hardware involved in the data transfer.

Layer 2: Internet Layer

The internet layer corresponds with the Network layer in the OSI model. It addresses the protocols, such as IP and ARP, that allow for a logical transmission across the network.

Layer 3: Host-to-Host layer

This layer is equal to the transport layer of the OSI model and is responsible for error-free communication and end-to-end transfer of data.

Layer 4: Application Layer

The application layer combines the application, presentation and session layer of the OSI model and deals with node-to-node communication and user-interface specifications.

TCP/IP vs OSI Referencing Model

The TCP/IP model is a much newer and more reliable referencing standard than OSI. Developed in the 1970s, OSI can often be viewed as outdated in the networking industry and many have adopted the TCP/IP model in its place. The TCP/IP model is more flexible in what you can accomplish and does not feature the strict boundaries of the older OSI reference.

This having been said, it is a lot more difficult to introduce new protocols into the TCP/IP model and so the protocols of the OSI reference are easier to replace and develop upon as and when new technology is introduced. This means that, despite its age, the TCP/IP standard has the potential to become an outdated reference as new technologies are developed.

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