Transport layer of the OSI model and its characteristics

len Alfred Ajibola - 31st October, 2021 @ 05:16 PM

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Transport Layer:

The transport layer is also referred to as layer 4 in the Open Systems Interconnections (OSI) model. The function of the transport layer is achieved through two main protocols. These are:

  1. Transmission Control Protocol (TCP)

  2. User Datagram Protocol (UDP)


The transport layer is solely responsible for connecting two devices before they can exchange information. As an instance, before you could read this article, your phone or computer device must first 'exchange a handshake' or connect to the server (another computer device) where this information (article) is stored. Importantly, this connection can be achieved via a connection or connectionless oriented data transfer.

When information is transferred in a connectionless manner, it only mean that both connecting computers did not partake in an initial three-way handshake (connection establishment or connection setup).

Please read an introduction to the transport layer here.


In a connectionless or stateless communication, one of the communicating computers sends data or information to the other computer without any prior notice to the latter. If something happens to the sent data along its way (probably traffic was heavy along its path or route) and it was unable to reach the destination computer, then such data will be lost (not delivered to the destination computer or device); and there won't be any form of notification from the receiving device, hence the retransmission of the lost data is impossible.

TCP is responsible for connection oriented communication while the UDP is connectionless.

Please read more on UDP's stateless connection here.

The Protocol Data Unit (PDU) at layer 4 are called segments.


Characteristics of the transport layer

The characteristics of the transport layer is often regarded as its functions or importance. Below are the characteristics of layer 4:

  1. The transport layer connects communicating devices through their respective port numbers.

  2. The transport layer brings about reliable communication among devices through the transmission control protocol (TCP).
    Please read more on TCP's three-way handshake here.

  3. The transport layer ensures that a single server can listen for various services through a process termed service port addressing. This is achieved because different services running on a server have got their respective port numbers. In this regard, a server can listen at port 80 and 21 simultaneously. Port 80 makes it a web server (http) while port 21 makes it a File Transfer Protocol (FTP) server. Transport layer ports - LenAcad

  4. The transport layer divides data into segment, with each containing a sequence number. The sequence number is utilized in tracking the delivery of sent segments. Upon arrival to the destination device, the transport layer correctly reassembles the segments through the sequence number.

  5. The transport layer brings about connection and flow control through the TCP. To do this, the sending device increases the number of segments sent (that is, it gradually increases the window size) when it consistently receive an acknowledgement for the previously sent segments.

  6. The transport layer ensures that different client machines can simultaneously request for services through random port numbers generated by each client. In this way, the client machines are able to receive and process appropriate replies from the server. Demultiplexing is a term used to describe this process.

  7. The transport layer is responsible for seamless voice communication through the lightweight and connectionless UDP.

  8. The transport layer handles real-time data like voice and video through the UDP. This process is achieved through a connectionless or stateless transfer since speed is of greater priority over reliability.

  9. The transport layer ensures the transmission of multiple data segments from diverse applications across a network through a dedicated control mechanism. This characteristics is referred to as multiplexing.
    You can read more on the functions of layer 4 here.

  10. The transport layer ensures that the all sent message arrives to both communicating devices without any error (that is, no duplication, loss or damage of segments). This is achieved through the SYN, ACK and NACK notifications alongside the retransmission of damaged or undelivered segments. This process is termed error control.

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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Physical parts of a computer are those parts that you can see and touch. It may also be referred to as the 'equipment of the computer'.

When the computer case is opened (with a screwdriver), one would see the different hardware components inside of it. It's important to state that some other hardware components are present outside the computer case; and these are termed peripherals.

Below are hardware components seen inside the computer case:

  • Motherboard

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  • Graphics Processing Unit (GPU)

  • Power supply Unit (PSU)

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Hardware components seen outside the computer case are called peripherals. The peripherals are grouped into the following:

  1. Input devices: They provide information to the computer.

  2. Output devices: Brings out information from the computer.

  3. Storage devices: They store information or data.

Below are examples of some peripherals:

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  • Monitor (output)

  • Mouse (input)

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  • Touch Pad (input)

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  • Hard disc drive (storage)

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  • Joystick(input)

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  • Speaker (output)

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