IMPLEMENTATION OF DATA LINK LAYER PROCEDURE (LAPD) FOR ISDN USER-NETWORK INTERFACE

Abstract

ISDN is a new concept in public networks that proposes to use the same digital switches and routes to establish connection to a wide range of Telecommunication services. It may take a number of years for full ISDN to be put in place. However, the intent is to define a small set of standard interfaces which will permit access to all existing tele-communication services like telephone network, public data network, teletex, facsimile transmission and videotex etc. CCITT Recommendations published in 1984 provide specifications for ISDN user-network interface. But, these speci-fications are yet to be finalised based on implementation work in progress. Currently, the physical interface is the most defined of the set with the data link layer protocol LAPD in an advanced stage. The work presented in the dissertation deals with implementation of LAPD on personal computers. An algorithm has been developed which eliminates the need of hardware interrupts, timers and inter process communication facility as these are not easily available on PCs and micro-computers. Implementation of LAPD includes processing of U-frames for establishment and termination of multiple frame acknowledged information transfer state, processing of I-frames for data transfer and S-frames for flow control, error recovery and other data link control functions. Another facility provided in the work is the option of interactive data link control using which responses to various frames can be determined.