ore attractive, SO digital ex- include bit rates higher and lower than this, this also includes an changes became cheaper to implement and maintain. These digital element of packet switching. The network will contain intelligence exchanges are designed to handle the 64kbit/s transmission chan- for the purpose of providing service features, maintenance and nels transparently using both space and time division digital swit- network management functions. The I 100 series recommendations ches. These digital main network exchanges together with the therefore describes the general structure of the recommendations, a digital transmission form what is known as the Integrated Digital vocabulary and a description of ISDNs which include the frame- Network (IDN). In the U.K. the first digital exchanges were put into work for providing additional packet mode bearer services. the main network in 1980 and now all the main network exchanges The majority of the current I series recommendations are on what are digital and fully interconnected by digital transmission routes is called narrowband ISDN based upon 64kbit/s rates; however and by the middle of 1991 half of the local exchanges were digital. during the last few years a lot attention has been given to what is The introduction of digital transmission and digital exchanges called a Broadband ISDN. In Part I of the recommendations there is was complemented by the introduction of common channel mess- also a recommendation (1.121) which describes the Broadband age based signalling systems, where messages relating to different aspects of an ISDN, giving significant details of the main par- connections are statistically interleaved on a common channel. ameters of such a network which will be based upon asynchronous CCITT signalling system No 7 is the system defined for use between transfer mode (ATM) techniques. These aspects will be discussed in switching nodes. more detail later. The conversion from the analogue signal produced by the stand- ard telephone to a digital signal occ