make possible the services that we expect from a network. Computer networks are created by different entities. Standards are needed so that these heterogeneous networks can communicate with one another. - An ISO standard that covers all aspects of network communications is Open Systems Interconnection (OSI) model - First introduced in late 1970s. - It is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture. - It is a model for understanding and designing a network architecture that is flexible, robust, and interoperable. - It consists of seven ordered, separate but related layers, each of which defines a part of the process of moving information across a network. - Each layer defines a family of functions distinct from those of the other layers. - Within a single machine, Layer 3 uses the services provided by layer 2 and provides services for layer 4. - Between machines, layer x on one machine communicates with layer x on another machine. - This communication is governed by an agreed-upon series of rules and conventions called protocols. - The processes on each machine that communicate at a given layer are called peer-to-peer processes.