Learn the OSI model – 7 layers made simple

Our latest blog comes from our Network Engineer, Dan Clark.

Dan breaks down every layer of the OSI model, explains its role in interoperability, and shares how it differs from the TCP/IP model.

This is one for the techies among us or anyone with a curious mind who likes to know how stuff works!

meet the author

We welcomed Dan to eve Networks back in 2022, when he joined as a Network Engineer.

Before joining ‘team eve’, Dan spent just under a decade supporting the team at Gloucestershire Health and Care NHS Foundation Trust.

Dan’s specialisms include VoIP telephone system configuration and support with a further background in networking infrastructure.

what does OSI stand for, and why was the OSI model developed?

OSI, or Open Systems Interconnection, is a framework that standardises the functions of a communication system or network into seven layers, with each layer being responsible for specific functions.

The requirement for a standardised approach became evident in the early days of computing when vendors produced proprietary systems which struggled to communicate with each other.

The overall goal of the model was to create a framework that would serve as a common language between networks and facilitate effective communication between systems.

ISO, the International Organisation for Standardisation, first proposed the framework in the 1970’s with the first OSI model being published way back in 1984.

what are the seven layers of the OSI model?

The seven layers of the OSI model are usually presented in descending order, from 7 to 1. They are: 7 – application, 6 – presentation, 5 – session, 4 – transport, 3 – network, 2 – data link, 1 – physical

Layer 7 – Application layer: This is the layer closest to the end-user. It provides network services to applications and is where user interfaces and application-specific protocols reside. Some of the main protocols used in this layer are HTTP (which web browsers and web servers use) and SMTP (which email applications and servers use).

Layer 6 – Presentation layer: This layer is responsible for data representation, encryption, compression, and formatting. It ensures that data exchanged between systems is readable by the receiving system.

Layer 5 – Session layer: Establishes, manages, and terminates connections (sessions) between applications. It allows synchronisation, checkpointing, and recovery of data exchange.

Layer 4 – Transport layer: Focuses on end-to-end communication. It ensures that data is delivered error-free, in sequence, and without loss or duplication. It also provides flow control and error-checking mechanisms.

Layer 3 – Network layer: This layer manages the addressing and routing of data packets to ensure they are delivered across networks. It deals with logical addressing and routing, allowing data to traverse multiple networks to reach its destination.

Layer 2 – Data link layer: Responsible for the reliable transmission of data frames between nodes on a network. It detects and possibly corrects errors that may occur in the physical layer.

Layer 1 – Physical layer: This layer deals with the physical connection between devices. It defines the electrical, mechanical, and functional specifications for activating, maintaining, and deactivating the physical link between devices.

how does the OSI model apply to eve Networks?

As a network service provider, we sit comfortably within layers 1 – 3 of ‘physical’, ‘data link’, and ‘network’.

• Layer 1 – Routers
• Layer 2 – Link between PE, CPE, and our core network
• Layer 3 – IP routing within our network and to our upstream peers

how do the OSI model layers interact during the transmission of data across a network?

When a user wants to request information across a network, for example, they open a web page or send an email, the following occurs:

Layer 7 – Application layer: generates the data for the request from the application to the network.

Layer 6 – Presentation layer: formats this request and encrypts or compresses data, if required.

Layer 5 – Session layer: establishes the session information to define how the data will be exchanged (TCP, SIP RTP, etc).

Layer 4 – Transport layer: data is divided into segments and transport protocols will add sequencing and error-checking.

Layer 3 – Network layer: logical addressing is added to the data, which allows it to find its way across the network.

Layer 2 – Data link layer: frames are created which also include the source and destination MAC addresses (on the same network) for the traffic.

Layer 1 – Physical layer: converts the frames into suitable signals to travel across the communication medium.

When the data is received by the destination the process is reversed and the data is reassembled.

in networking, the OSI model is often compared to the TCP/IP model. What’s the difference?

While both OSI and TCP/IP models are frameworks for understanding communication across a network, it is the TCP/IP model that has become the prevalent model in networks, primarily due to it being tied to the development of the TCP and IP protocols which are the basis for the modern internet.

The OSI model consists of the seven layers mentioned previously, however, the TCP/IP model is usually represented as only four layers. The four layers associated with the TCP/IP model are ‘link’, ‘internet’, ‘transport’, and ‘application’.

Commonly viewed side by side the ‘link’ layer in the TCP/IP model most resembles the functions of the ‘physical’ and ‘data link’ layers in the OSI model.

The TCP/IP ‘application’ layer resembles the ‘session’, ‘presentation’, and ‘application’ layers in the OSI model.

how does the OSI model facilitate interoperability among different network devices and technologies?

The OSI model acts like a set of rules and steps that devices can follow when they talk to each other. This helps different devices work together smoothly, like speaking the same language, ensuring they can understand and share information effectively.