Transport Layer in OSI Model

This article will help you in getting a good understanding of the transport layer in the OSI model.

The transport layer is the fourth layer from the bottom in the OSI model. The transport layer is mainly responsible for faster and more efficient arrival and transport of messages across various networks.

The transport layer receives various messages and forwards these messages to layer 3 i.e., networking layer. The OSI Model is largely dependent on the Transport layer for establishing efficient communication between two hosts.

The transport layer receives the message from the upper layers and divides it into small units and sends these units to the network layer in the correct order. There are many types of transport connections like error-free point-to-point channel which ensures that the messages are sent in the correct order.

Functionality of Transport Layer

Below are a few key functionalities of the transport layer which are helpful for the efficient functioning of the OSI model.

1. Segmentation and Reassembling

As mentioned earlier the transport layer receives the message from the upper layers and divides it into small units and sends these units to the network layer in the correct order. These small units are called segments. It assigns a unique sequence number which helps in the easy identification of messages.

2. Connection control

Connection control is nothing to control the connection provided to various devices in simple words it allows you to limit/restrict the devices from connecting to SMTP Server. This connection control includes 2 types. They are

• Connectionless Transport Layer: In this type, each data segment and independent packet and transferred to the transport layer.
• Connection-Oriented Transport Layer: In this type, a connection is established with the transport layer at the destination.

3. Flow control

Flow control is the process of finding out all the data which is transferred to the receiver side without any acknowledgment. The transport layer is performed end to end instead of across a single link.

4. Service Point Addressing

Computers run several programs simultaneously due to this reason, the transmission of data from source to the destination not only from one computer to another computer but also from one process to another process.

The transport layer adds the header that contains the address known as a service-point address or port address. This layer receives the message to the correct process of the computer, unlike the network layer which receives each packet to the correct computer.

5. Error control

Error control is the process of identifying all the errors and problems and rectifying them for error-free transfer of messages across various networks. transport layer carries out end-to-end error control to make sure that the complete message is sent to the receiver. The retransmission method is used for Error correction.

6. Multiplexing and Demultiplexing

The transport layer provides the multiplexing facility to the sender’s side. Multiplexing is a process of combining multiple analog or digital signals into a single signal. And at the receiver side, it provides demultiplexing facility to gather all the data coming from various processes.

7. Data Integrity

The transport Layer helps in easier and faster detection of damaged and corrupted data packets and it even tracks such data packets and again transfers them by repairing them. It identifies all the redundant/repeated data and removes them. Transport control utilizes the sliding window concept for providing flow control.

Working of Transport layer

At the sender’s side:

At the sender’s side, the transport layer receives all the data from the upper layer i.e... application layer and carries out the segmentation process on these messages, After then, it assigns the port number of the source and assigns destination in header and forwards the message to network layer.

At Receiver’s Side:

At the receiver’s side transport layer gathers the data from its below layer called network layer and carries out the process of reassembling the segmented data and recognizes the port number by scanning the header to transfer the message to suitable port in the session layer.

Protocols of Transport Layer

Transport layer utilizes following 2 protocols:

1. TCP (Transmission Control Protocol)

2. UDP (User Datagram Protocol)

1. TCP

It is a standard networking protocol that helps in enabling communication between various devices over the network.

It creates and maintains the connection between the hosts. When the data is transmitted through this protocol then it divides the data into smaller segments. Each data segment travels over the internet in various routes and reaches destinations in different orders. This protocol rearranges the order of data segments at the receiver end.

2. UDP

It is a transporting protocol that is used to transfer the data in a non-sequential manner. UDP is also known as connectionless transport layer protocol.