CCNA Routing and Switching

Introduction to the OSI Model

Olawunmi Omosanya - Sat, 18th May, 2019 @ 16:33: PM

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Introduction to the OSI Model

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Open Systems Interconnection (OSI):

A good knowledge of the Open Systems Interconnection (OSI) is fundamental for those willing to take up a career in networking. Networking engineers will look up to the OSI model as a reference when working on the different layers of their job.

The OSI model is a reference model that defines and describes the various protocols, concepts, terms and functionalities in networking

The OSI model clearly explains the process of networking in seven layers.

Below are the seven layers of the OSI model

  • LAYER 1 or Physical Layer
  • LAYER 2 or Data Link Layer
  • LAYER 3 or Network Layer
  • LAYER 4 or Transport Layer
  • LAYER 5 or Session Layer
  • LAYER 6 or Presentation Layer
  • LAYER 7 or Application Layer

Note: A similar networking model to the OSI model is the TCP/IP Model or TCP/IP Stack or Department of Defense Model (DoD Model).

With regards to modern day networking and internet, The TCP/IP model is the preferred choice. Computer networking today uses only the TCP/IP stack in its implementation. The term internet is actually derived from 2nd Layer of the TCP/IP stack. The TCP IP Stack has four layers unlike the OSI Model which has seven.

Although the OSI model of networking is no longer supported by computer hardware manufacturers, It’s still crucial you understand how each of its layer work since its knowledge is applicable in network troubleshooting and comprehension.

Below is a summary of the functionalities of each layer, beginning from Layer 7


Layer 7 or Application Layer

Layer seven or the application is where an end user begins from when they intend to get information from a web server located somewhere on the internet. For instance, you were able to read this article because the application layer had been put into use through your phone or computer. To prove this, take a look at the Uniform Resource Locator (URL), which happens to be the link of this page on your browser; and you will see the http or https protocols. This protocol is always present in the application layer regardless the website you visit; you will just happen to see either of both protocol because that’s how the internet had been designed to work at the application layer. Other protocols found at the application layer include Telnet, File Transfer Protocol (FTP), Secure Shell (SSH), Domain Name Service (DNS), Dynamic Host Configuration Protocol (DHCP), Trivial File Transfer Protocol (TFTP) and so on. HTTP stands for HyperText Transfer Protocol while the "s" in HTTPS means that the protocol is secure.

The application layer is also responsible for application services like email and remote file access. Application services require the internet to function. Please read our article on the internet here.


Layer 6 or Presentation Layer

Before you could read this article, its content (arriving from the web server where it’s located) had to be presented on your screen. If this article had been encrypted from the server, then you probably would have seen an encrypted gibberish, but all thanks to the presentation layer. It helps to encrypt and decrypt data while also formatting text or images like ASCII, EBCDIC, PNG, TIFF, GIF, PICT, JPG and so on.

The presentation layer presents data on your computer screen.


Layer 5 or Session Layer

The session layer is responsible for starting, managing and terminating the connection between two computers. Protocols found on layer 5 include the Network File System (NFS), Structured query Language (SQL), Remote Procedure Call (RPC) and NetBios names.

Note: Layers 5, 6 and 7 are expected to be handled by the computer programmer while the responsibility of layers 1 – 4 lies in the operations of the networking technician.


Layer 4 or Transport Layer

The function of the transport layer is achieved through two main protocols. They are:

  1. Transmission Control Protocol (TCP)
  2. User Datagram Protocol (UDP)

The transport layer is responsible or either the connection or connectionless 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 or connection setup before the communication began. As a result, there is no sequence number, acknowledgement number and flow control.

In a connectionless communication, one of the communicating computers sends data or information to the other computer without any prior notice to the other. If something happens to the sent data along its way (probably traffic was heavy on its path) and it was unable to reach the receiving computer, then such data is lost and there is no retransmission of the lost data. This is why we hear gabbled sounds in voice communication because the specific voice data could not reach its destination device. Voice data are transmitted in a connectionless manner because it happens in real-time. The UDP protocol is responsible for connectionless communication.

The TCP protocol is responsible for the connection oriented communication. There is an initial three way handshake between both computers before any communication proceeds.

For now, just think of a three way handshake as an agreement or negotiation of the communications parameters between two or more computers before information or data exchange can proceed.

Sequence numbers, acknowledgement numbers and flow control are all associated with the TCP’s connection oriented communication.

Note: The Protocol Data Unit at the transport layer are called Segments


Layer 3 or the Network Layer

The main protocol in the network layer is the Internet Protocol (IP). IP can be either IPv4 or IPv6. Other layer 3 protocols include Apple-Talk, DECnet and IPX.

Routers and Layer 3 switches are found in this layer

Below is a summary of the network layer functionality
  • Routing and Switching technologies are found here.
  • The term "packet" is officially used in this layer. (Packet is one of the Protocol Data Units or PDUs). Routing and packet forwarding occurs in the Network Layer.
  • Error handling, congestion control and packet sequencing work at this layer.
  • Virtual circuits are created across communicating hosts.


Layer 2 or Data Link Layer

Media Access Control (MAC) address and switches are associated with this layer

The data link layer is responsible for the following functionalities
  • The term "frame" is officially used in this layer. (Frame is one of the Protocol Data Units or PDUs).
  • Error checking occurs here. Bad/error frames are discarded without any retransmission.
  • This layer converts electrical signals from the physical layer into frames.
  • Data Link layer is divided into two: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.
  • The MAC address of every computer is made up of two parts. The Organizationally Unique Identifier (OUI) and Network Interface Controller Specific (NIC specific). All computers will most likely have a unique MAC address.
  • Frame relay networks work in layer 2.

The Ethernet functions at both the Data Link layer (layer 2) and the Physical Layer (Layer 1)


Layer 1 or Physical Layer

  • Layer 1 comprise of devices like hubs, repeaters, cables.
  • This layer is responsible for electrical signals, light signal and radio signals.
  • The Asynchronous Transfer Mode (ATM), Fibre Distributed Data Interface (FDDI) and RS232 operate at the physical layer.
  • The Protocol Data Units are call Bits in layer 1.

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