ISO (International Standardization Organization) standard has its own architecture as the OSI (Open Systems Interconnection). The architecture ISO is the first to be defined, and so relatively parallel to the Internet. The distinction between the two is that the ISO formally defines the different layers architecture, while the Internet architecture is applied to achieve a practical environment.
The physical layer is complex. Numerous standards describe how to encode and transmit signals on a physical communication line. The frame layer provides the functional and procedural means necessary for the establishment, maintenance and release of connections between network entities and to the transfer of data link service units. It is ISO 8886 or CCITT X.212, which defines the service provided by the layer 2.
Other important ISO standards architecture is the following:
• ISO 3309 and 4335 for the standardization of the HDLC link protocol (High-level Data Link Control);
• ISO 3309, for the frame structure or LPDU;
• ISO 4335 and 7809, for procedural elements;
• ISO 8471, for a description of the class HDLC balanced way;
• ISO 7776 for the description of the CCITT LAP-B standard in an ISO context.
The role of the package layer (layer transfer) is, firstly, to provide the means to establish, maintain and release network connections between open systems, secondly, to provide the functional means and procedures necessary to exchange between transport entities, network service units.
The standardization of layer 3 includes the following standards:
• ISO 8348 or CCITT X.213, which defines the network service.
• ISO 8208 or CCITT X.25, which defines the protocol with mode network connection.
This protocol is often called X.25, and all major public networks world follow this recommendation.
• ISO 8473, which defines the network protocol connectionless, known as ISO Internet. This is a standard protocol developed by the Department of Defense under the name of IP (Internet Protocol).
• ISO 8878 or CCITT X.223, which describes the use of X.25 to get the service connection-oriented network.
• ISO 8648, which defines the internal organization of the network layer.
• ISO 8880, in four parts, which defines the various possible combinations of protocols to make a service level 3 according to standardization.
• ISO 8881, which allows the adaptation of the level 3 X.25 on a local network with a type 1 LLC link protocol.
The Message layer (transport level) must ensure the transfer of data between session entities. This transport must be transparent, that is to say independent of the sequence of characters and even transported bits. International standardization from ISO provides five classes of protocols able to meet the requirements of the user.
The different classes of level 4 range from very simple software, which only format the data from the upper level and unformat on arrival, to complex communication software, which include all the features of the three lower levels. One can find an error detection zone and error recovery algorithms. Restarts on message or packet loss reported by the lower layer are also among the tools available in the software.
The main standards of this layer are:
• ISO 8072 or CCITT X.214, which defines the transport service.
• ISO 8073 or CCITT X.224, which defines the connection-oriented transport protocol and which has, as we have seen, five underlying classes.
• ISO 8602, which defines a transport protocol connectionless.
The three upper layers correspond exactly to those described in the architecture of the reference model.