Overview of OSI layer.- OSI stands for Open Systems Interconnection.
- The OSI reference model gives an overview of the layered communications and computer network protocol design.
The layers of the OSI are:
1. Application Layer:
- This layer serves as the window for users and application processes to access network services.
2. Presentation Layer:
- This layer formats the data to be presented to the application layer.
- It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.
3. Session Layer:
- This layer allows session establishment between processes running on different stations.
4. Transport Layer:
- This layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications.
- It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.
- The size and complexity of a transport protocol depends on the type of service it can get from the network layer.
- For a reliable network layer with virtual circuit capability, a minimal transport layer is required.
- If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.
5. Network Layer:
- This layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.
6. Data Link Layer:
- This layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.
7. Physical Layer :
- This is the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers.