Introduced in 1978, the Open Systems Interconnection (OSI) Reference Model (RM) is a long-term project of International Organisation for Standardisation (ISO) to promote compatible communications among a wide variety of systems. It specifies a communications structure of seven distinct levels or layers.
The seven levels of the OSI-RM:
Level 7 - Application Layer (highest level)
Level 6 - Presentation Layer
Level 5 - Session Layer
Level 4 - Transport Layer
Level 3 - Network Layer
Level 2 - Link Layer
Level 1 - Physical Layer (lowest level)
One advantage of OSI-RM is that its levels are modular. As a result, different people can work on individual levels with some assurance that the various levels will work together in a system. Modularity also permits more than one protocol per level. Each level must communicate with the level above and below and must follow agreed rules for these inter-level interfaces.
In order for distant systems to communicate, peer levels (for example, the link layer on one system and the link layer on the other) must exchange data in an agreed protocol.
Although the protocols of the OSI-RM are in various stages of development it has won international acceptance in the telecommunications industry. ISO 7498 and CCITT Recommendation X.200 are the primary documents for the OSI-RM.
The function of the physical layer, the lowest level in the OSI-RM, is to send and receive bits (binary ones and zeros - marks and spaces). Layer 1 defines the electrical and mechanical standards such as connector configuration and which pins are used for different signals. The physical layer is responsible for getting individual bits transferred from one piece of equipment through the communication medium to the other equipment.
This level is concerned with the following:
This level is the only level that maintains an actual electrical connection with its peers. The other levels communicate with their peers through logical or virtual connections.
The link layer performs the following functions:
The network layer arranges data into packets (frames with network-information added).
The transport layer organises data into a transport protocol data unit (TPDU) that is, a packet with transport-layer data added. This layer ensures that all data sent is received completely and in proper sequence at the destination.
The session layer organises data into session protocol data units (SPDUs). This layer is concerned with:
The presentation layer is responsible for terminal management and performs the following functions:
The application layer is a window between the OSI communication environment and the application process. It is the only level that does not interface with a higher one. The application level provides the following functions:
This model is not used in its totality for interfacing any external device to MoBIC, but it should be the preferred model for any interfacing activities.
There are a lot of different interfaces available. The enumeration below shows a small selection. These interfaces belong to the OSI-RM level 1.
Two interfaces are proposed:
These levels are left out because they only deal with network specific problems currently not relevant for the MoBIC project.