ontrol information could be handled locally. 34368 44736 32064 40.3.3 Networking 139264 274176 97728 565148 397200 Only point to point multiplexers have been considered SO far, as they illustrate the basic principles of multiplexing. However, a major development in the mid 1970s was to give a user on a terminal the The underlying system employed is bit interleaved, although the facility to select more than one destination. The requirement grew framing structure is byte oriented. Analysis of the framing structures out of the development in computing power, which meant that many gives more detail on how they were used as general purpose data companies were expanding on the number of computers they used multiplexers. In general the input channels were constructed in and were perhaps dropping the mainframes in favour of distributed frames, each consisting of 12, 24 or 30 voice channels or 'timeslots' minis. To allow these to be accessible the users required the multi- depending on the system. This gave the first hierarchical grouping. plexers to become networked. In effect rather than let the computer Within this group the frames could be assembled together in super- switch the user to an application it was left to the network. frames to contain signalling and control information. The early US systems need to be examined separately as they used a technique that was completely alien to pure data use. The original High order multiplexing Bell D1 system had an input rate of 56kbit/s (7 times 8000 samples per second) because the timeslots used a 'bit robbing' technique 40.4.1 Standards whereby the 8th bit in every slot was used for maintaining timing information on the line. By always setting the 8th bit to zero and inverting the PCM data sample so that it was normally set to all 1's The term, high order multiplexing, is used to describe the technique i.e. no speech signal, the line signal could be guaranteed to have data for multiplexing multiplexers. By successively taking input chan- transitions which kept the timing 