- Network Enviornment: concerned with protocols , exchange of messages, and
standards relating to different types of underlying data communication networks.
- OSI Enviornment: allows additional application-oriented protocols and standards
to allow end-systems to communicate with one another in an open way.
- Real Systems Enviornment: builds an OSI enviornment and is concerned with
the manufacturers own propietary software and services, which have been
developed to perform a particular distributed information processing task.
These envionments provide;
- well-defined interface between layers.
- function of layers , protocol defines the set of rules and conventions used by layer to communicate with a similar peer layer in another remote system.
- each layer provides a defined set of services to the layer immediately above it.
- uses those services immediately below to transport ,message units associated
with the protocol to the remote peer layer.