nal. for voice, data, video and image. The means to achieve these goals The foundation of digital conversion was therefore adopted, and were originally thought to be the new modern PBX, but this was its development was based on the earlier restrictions within the quickly thrown aside by a new generation of sophisticated multi- analogue network. The 64kbit building block was in turn used to plexer, the digital data multiplexer. (See Chapter 40.) form the basis of T1 (North American) and E1 (European) digital It was designed by new entrants who adopted their own proprie- services. T1 is 24 X 64kbit/s circuits combined to form 1.536kbit/s tary techniques of sending growing user data demands into T1 on service which is expanded by 8kbit/s to 1.544kbit/s. The 8kbit/s is E1 digital pipes. The multiplexers needed unstructured T1 or E1 used for PTT service information. (See Figure 56.6.) The signalling pipes to capitalise on their proprietary techniques of sending data information is either transported as a rob bit in each channel (called effectively through the network. Initially designed as point to point timeslot) for channel associated signalling or as a complete channel devices they effectively adapted into full networking platforms. (timeslot 24) for common channel signalling. (See Figure 56.8.) User's User's kbit/s 1.544kbit/s User's User's Timeslot PTT service information Channel (CRC Checking Synchronisation) Figure 56.6 T1 transmission frame User's E1 on the other hand is 30 (32) X 64kbit/s combined to form 2.048Kbit service. (See Figure 56.7.) In this arrangement timeslots 1 to 15 and 17 to 31 are the 30 user usable channels. Timeslot () is for PTT service information, whereas Figure 56.8 Use of multiplexers within a network: (a) timeslot 16 was preserved for signalling information either Com- point to point; (b) delta network mon Channel or Channel Associated. It was not until the mid 1980s that the user realised the weakness 2.048kbit/s of these 1st and 2nd generation multiplexers, whose design was principal