CCS信號系統是構築ISDN網之主要技術，故為達提供ISDN服務予全區用戶，其ISDN用戶／網路間第三層信號（ISDN DSS1 Layer 3）與ISDN局間信號方式（ISUP）之匹配技術至為重要；ISDN信號網接模型系統(ISIM)旨即在發此網接匹配技術。ISIM模型系統乃由ISDN訊息終端機（IMT）及ISDN信號網接單元（IWU）所構成，其分別提供信息終端機功能及信號網接功能。 本文主要是介紹ISDN訊息終瑞機（IMT）系統架構，各模組如何設計及實現ISDN服務之原則。期望經由此一系統之建立，能完全配合IWU展示其網接匹配技術，且讓ISDN使用者模擬ISDN各項服務之操作程序從而認識及熟悉ISDN服務。
The CCS siganlling system plays an important role constructing the areawide ISDN. It is important to interwrok ISDN DSS1 Layer 3 and ISUP for providing ISDN service to the areawide users. The purpose of developing ISDN Signalling Interworking Model (ISIM) is to develop the interworking technique. ISIM is composed of the ISDN Message Terminal (IMT) and the ISDN Interworking Unit (IWU), which simulate the functions of ISDN terminal and network interworking respectively. This paper introduces the architecture of IMT, the principle of designing and implementing ISDN services in each module. It is desired that the IMT system will completely cooperate with IWU to demonstrate the interworking technique. The IMT user will also be able to realize and familiarize with the ISDN services from simulating the operation of various ISDN services.
