Public Networks

The networks which is owned neither by Government or by private organization but by an individual network operator, provides communication services for the customers' hosts and terminals. Such a system is called public networks.

All of them use OSI model and the standard CCITT (Consultative Committee International for Telephony and Telegraphy, a French standard organization) or OSI protocols for all the layers. For the lowest three layers, CCITT has issued recommendations that have been universally adopted by public networks worldwide. These layers are always known collectively as X.25 standards.

They physical layer protocol, called X.25, specifies the physical, electrical and procedural interface between the host and the network. The data link layer standard has a number of variations. They all are designed to deal with transmission errors on the telephone line between the user's equipment and the network. The network layer protocol deals with addressing, flow control, delivery confirmation, interrupts and related issues.

ISO has developed standards for a connection-oriented transport layer service definition and a connection-oriented transport layer protocol. Also it has adopted standards for the connection-oriented session service, protocol and presentation service and protocol.

The Application layer contains following protocols:

1. FTAM (File transfer, access and management) protocol, which provides a way to transfer, access and generally manipulate remote files in a uniform way.
2. MOTIS (Message-Oriented Text Interchange Systems) protocol is used for electronic mail.
3. VTP (Virtual Terminal Protocol) provides a terminal-independent way for programs to access remote terminals.
4. JTM (Job Transfer and Manipulation) protocol is used for submitting jobs to remote mainframe computers for batch processing.