N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 2 - Data Communications Basics and Legacy Technologies                                                Slide 24 of 45 _________________                                                                                  __________

                                                 OSI Layers

_________________                                                                                  __________

OSI, a suite of protocols sponsored by the International Organization for Standardization, for data communications between incompatible computer systems has been a dominant standard. Recently the TCP / IP suite of protocols has gained more acceptance over the OSI suite of protocols by the communications industry. TCP / IP has been the engine for the Internet network. ISO defines a complete computer systems architecture having seven layers.

Physical Layer - Responsible for activating, maintaining and deactivating a physical circuit between communicating devices. This layer also provides specifications for physical signals, cabling / wiring and characteristics of the connectors.

Data Link Layer - Responsible for transfer of data over the channel, data flow control, error control and data recovery mechanism.

Network Layer - Responsible for the interface of the user Data Terminal Equipment into a network as well as the interface of two DTEs with each other through a network. It is also responsible for network routing and internetworking.

Transport Layer - Responsible for providing the interface between the data communications network and the upper three layers (generally located in the CPE).

Session Layer - Serves as a user interface into the transport service layer. It provides for an organized means to exchange data between users.

Presentation Layer - Provides for the syntax of data in the model. Its principal role is to accept data types (character, integer) for the application layer and then negotiate with its peer layer as to syntax representation (such as ASC II).

Application Layer - Contains service elements to support application processes such as job management, and financial data exchange.