N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 2 - Data Communications Basics and Legacy Technologies                                                Slide 25 of 45 _________________                                                                                  __________

                                                 Legacy Technologies

                                            Binary Synchronous (BiSync)

                                            Synchronous Data Link Control (SDLC)

                                            High Level Data Link Control (HDLC)

                                            X.25

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Data communication have gone through many evolution phases. Originally data terminals were teletype. The data communication between a terminal and a mainframe or small computer was interactive, character by character. Many proprietary protocols like BiSync and SDLC were developed for the synchronous data transmission for bulk data transmission. Eventually standard HDLC protocol evolved which became the backbone for the higher level protocols like X.25 and the rest.

BiSync - A character-orientated, synchronous, data link protocol developed by IBM. This protocol was used by IBM for data transmission from their terminals to mainframe computers.

SDLC - A bit orientated data link protocol from IBM, similar to HDLC, used by IBM's Systems Network Architecture (SNA) on synchronous data link. IBM used SDLC protocol for front end processor to poll the peripherals if they have any data to send.

HDLC - A bit-orientated , synchronous, link control protocol sponsored by the International Standards Organization (ISO). It has achieved wide use throughout the world. It is frequently use as a foundation for the other protocols.

X.25 - X.25 uses HDLC as a backbone. It is a data packet communications protocol. X.25 specifications were issued by CCITT in 1974. X.25 has been the dominant interface for the wide area packet networks. It uses the first three layers of the OSI model.