The recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lowest layer in this model.
|Layer||Protocol data unit (PDU)||Function|
|7. Application||Data||High-level APIs, including resource sharing, remote file access|
|6. Presentation||Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption|
|5. Session||Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes|
|4. Transport||Segment (TCP) / Datagram (UDP)||Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing|
|3. Network||Packet||Structuring and managing a multi-node network, including addressing, routing and traffic control|
|2. Data link||Frame||Reliable transmission of data frames between two nodes connected by a physical layer|
|1. Physical||Bit||Transmission and reception of raw bit streams over a physical medium|
At each level N, two entities at the communicating devices (layer N peers) exchange protocol data units (PDUs) by means of a layer N protocol. Each PDU contains a payload, called the service data unit (SDU), along with protocol-related headers or footers.
Data processing by two communicating OSI-compatible devices is done as such:
- The data to be transmitted is composed at the topmost layer of the transmitting device (layer N) into a protocol data unit (PDU).
- The PDU is passed to layer N-1, where it is known as the service data unit (SDU).
- At layer N-1 the SDU is concatenated with a header, a footer, or both, producing a layer N-1 PDU. It is then passed to layer N-2.
- The process continues until reaching the lowermost level, from which the data is transmitted to the receiving device.
- At the receiving device the data is passed from the lowest to the highest layer as a series of SDUs while being successively stripped from each layer's header or footer, until reaching the topmost layer, where the last of the data is consumed.
Some orthogonal aspects, such as management and security, involve all of the layers (See ITU-T X.800 Recommendation). These services are aimed at improving the CIA triad - confidentiality, integrity, and availability - of the transmitted data. In practice, the availability of a communication service is determined by the interaction between network design and network management protocols. Appropriate choices for both of these are needed to protect against denial of service.
Encapsulation of application data descending through the layers described in RFC 1122