Distributed data exchange model
The ISO defined standard communications (layered) protocols:
Layer 1 Physical Layer
Controls the electrical interface of the physical connection between DTE (Data Terminal Equipment) and DCE (Data Communications Equipment).
Layer 2 Data Link Layer
Manages the data traffic between two adjacent nodes in a network.
Layer 3 Network Layer
Describes the various transmission techniques used in data networks. The major types are HDLC (point-to-point), X.21 circuit switching (point-to-point or multi-point), and X.25 packet switching.
Layer 4 Transport Layer Manages the end-to-end transfer of all messages. Services involved are traffic optimization, logical connection maintenance, transmission error detection and correction, and message routing.
Layer 5 Session Layer
Controls the application access points and the logical connection between two such points on a network for the duration of an application session.
Layer 6 Presentation Layer Conducts the transformation of data so that the communicating terminals can understand each other.
Layer 7 Application Layer
User applications to which the lower layers (0 through 6) are transparent. User s are unaware of the transmission process. Described in the X.400 standard.