Open Systems Interconnection Model

OSI model provides framework dictating how all networked devices send, receive and interpret data. OSI model consists of seven layers.

7. Application

Application layer contains protocols and rules to determine how users should interact with the data. For example graphical user interfaces. Not much standardisation.

6. Presentation

Standardisation start to take place. Data needs to be handled in a same way, for example every email client have to display same content of message. HTTPS occurs at this layer. You could think this layer as translator.

5. Session

Sessions synchronizes two computers to ensure they are on same page before data is sent and received. Session layer will divide data to smaller chunks - packets - and send them.

4. Transport

Firewalls operate on levels 3 and 4.

TCP

Transmission Control Protocol reserves constant connection between the two devices for the amount of time it takes for the data to be sent and received. TCP is more reliable but much slower. Used for situations such as file sharing, internet browsing or sending email. If the data must be accurate use TCP.

UDP

User Datagram Protocol UDP is much faster than TCP but doesn’t care if data is received or not. Terrible for unstable connections. Used in situations such as APR, DHCP, streams, VOIP

3. Network

Determines what route the packets should use. Decided by reliability, speed and length by amount of devices. Routers and some switches are layer 3 devices. Firewalls operate on levels 3 and 4.

Terms

Focuses on the physical addressing of transmission. It receives IP address and adds physical MAC address to the receiving endpoint. Each network enabled device has Network Interface Card (NIC) that has the unique MAC address to identify it. When data is being sent over internet, the MAC address is the physical address that is used to identify where exactly to send the information. some switches are layer 2 devices.

1. Physical

Physical components of the hardware used in networking. It is the lowest level you will find. For example ethernet cables connecting devices.

0. Human Layer - a joke layer

Layer zero problems are caused by humans.