Transport layer: Layer 4 of the OSI model

len Alfred Ajibola - 17th October, 2021 @ 05:44 AM

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What is the first step towards disabling automatic image download on WhatsApp?

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The followings are ways to conserve data on android phones except _____.

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Transport Layer of the OSI model:

The transport layer (also called layer 4) of the Open Systems Interconnection (OSI) model of networking is solely responsible for connecting two computer devices before they can exchange information.

As an instance, this article is located on a server with an open port number of 443 (since it's an https protocol) as can be seen on the url of this webpage. Before you could read this article, your phone or computer device must first connect to the port number on the server where this information is stored.

This initial connection is a function of the transport layer or layer 4 of the OSI model. Importantly, this sharing of data between both communicating devices can be achieved via a connection (stateful) or connectionless (stateless) oriented data transfer.

The image below shows a client using the random port number '1025' to communicate with the server's unsecure web protocol (http) whose port number is 80.

Transport layer of osi model - Lenacad

Please read on the functions of the transport layer here.


The transport layer lies between the session layer (layer 5) and network layer (layer 3) of the OSI reference model. It therefore accepts data from the session layer (or the application layer in general), split it into smaller segments, and passes them to layer 3, the network layer. However, if the Transmission Control Protocol (TCP) of layer 4 is utilized, then such transfer will ensure that all the data segments arrive at the correct order in which they were sent from both connecting devices.

Understand that data integrity is always assured if the Transmission Control Protocol (TCP) is utilized as the layer 4 protocol.


The transport layer functions by initiating either a connection or connectionless data transfer between two communicating devices. In this regard, one of these devices functions as the client (seeks for information) while the other acts as the server (gives out information).

The connection and connectionless data transfer by the layer 4 of the OSI model is achieved through two main protocols. These are:

  1. Transmission Control Protocol (TCP)

  2. User Datagram Protocol (UDP


Recall that the transport layer utilizes a service port addressing (or port addressing) in order to appropriately deliver segment to the correct services on both computers. To achieve this, it controls and monitors the sent segments through flow control and error control.


The transport layer can also increases the rate at which segments are sent via the multiplexing and demultiplexing processes.

The term segments refers to the Protocol Data Unit (PDU) at the transport layer.

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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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Computer hardware is the collection of physical parts of a computer system.

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

  • Central Processing Unit (CPU)

  • Graphics Processing Unit (GPU)

  • Power supply Unit (PSU)

  • Northbridge

  • Southbridge

  • Peripheral Component Interconnect (PCI) slots

  • Network Interface Card (NIC)

  • Random Access Memory (RAM)

  • Ventilation fans

  • Heat sink

  • Hard Disc Drive (external present)
  • Solid State Drive (SSD)

  • CMOS battery

  • Optical Disc Drives (external present)

  • USB port, PS2 port, Ethernet port and other ports

  • Expansion Ports / Slots

Please read more on computer hardware here

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:

  • Keyboard (input)

  • Monitor (output)

  • Mouse (input)

  • Webcam (input)

  • Touch Pad (input)

  • Universal Serial Bus (USB) flash drive or memory card (storage)

  • Hard disc drive (storage)

  • Stylus (input)

  • Joystick(input)

  • Printer (output)

  • Speaker (output)

  • Headphones (output)