The OSI Model
The OSI model provides a standard for different computer systems to communicate with each other. However, OSI is a theoretical model and works very well for teaching purposes, but it’s not practically used. IP is used practically.
The Layers of the OSI Model
Application Layer
- End users interact with the application layer
- Where most end-user applications such as web browsing and email live
- Where the outgoing message starts its journey so it provides data for the layer below
Presentation
- Presents data in a way that can be understood and displayed by the application layer
- Encoding is an example. The underlying layers might use different character encoding compared to the one used by the application layer.
- Encryption is also done at this layer
- End-to-end compression: the presentation layer might also implement end to end compression to reduce traffic in the network
Session
- Responsibility is to take services of the transport layer and build a service on top that manages user sessions
- A session is an exchange of information between local apps and remote services on the other end systems
Transport Layer
- Since the application, presentation and session layers may be handing off large chunks of data, the transport layer segments it into smaller chunks
- These are called Datagrams or segments depending on the protocol used
- Sometimes additional information is required to transmit the segment/datagram reliably.
- Checksum - ensure message is correctly delivered without corruption
- Header - Information at the start of the datagram
- Trailer - Information at the end of the datagram
Network Layer
- Network layer messages are termed as packets.
- Facilitate the transportation of packets from one end system to another
- Routing protocols are apps that run on the network and exchange messages with each other to develop information that helps them route transport layer messages
- Help determine the best routes that a message should take
- Load balancing
Data Link Layer
- Allows directly connected hosts to communicate
- Encapsulates packets for transmission across a single link
- Resolve transmission conflicts - when two end systems send a message at the same time across one singular link
- Handles addressing
- Multiplexing and Demultiplexing
- Multiple data links can be multiplexed into something that appears like one to integrate their bandwidths
Physical Layer
- Consists largely of hardware
- Provides medium to transmit data
- Transmits bits, not logical packets, datagrams or segments
Example
- Application layer writes out streams of data to presentation layer
- Presentation hands it off to session layer then to transport layer
- Transport layer segments this data into datagrams
- Network layer turns these datagrams/segments into packets
- Physical layer carries each packet as bits to other end system
- Reverse process happens from here to convert the bits back to application layer