content. When the D2 system was nels of fixed speed and creating a hierarchy of 'groups' of multi- introduced using HDB3 (high density bipolar 3) line encoding, this plexers, a final high speed output composite is produced, that is allowed 8 bits per slot. However, the system used part of the old logically structured to contain all the input channels in a defined standard in that it again allowed the same 'bit robbing' to take place sequence. in the same way but only every 6th frame and in this case it was used High order multiplexers were originally developed as described for passing control and signalling information. For speech purpose above to allow voice circuits to be multiplexed. This simple state- this did not impose an reduction in the quality of service as the ment is the underlying design principle for all the decisions that human ear will not detect this impairment, but for pure data it have been made on PTT digital speeds and frame structures throug- introduced an unacceptable error rate. hout the world in the past 20 years. Although today they can be used The speeds which became standard offerings were derived in the for any type of digitally encoded data, including fax and video, these following ways. US Bell system D2 (usually known as T1) used a are the result of data communications manufacturers, and lately frame made of 24 timeslots (or input channels) at 8 bits each i.e. 192 PTTs, 'highjacking' the circuits developed for digitised voice and bits: an additional bit was added on to each of the frames for using them for pure data. synchronisation purposes giving 193 bits per frame. The 'sweep' The lowest input speed for high order multiplexing contained rate or number of frames per second was 8000 (the voice encoding, within CCITT is 64kbit/s. This is the speed arrived at from encoding or codec, sampling rate) giving an overall speed of 1.544Mbit/s. voice, with a maximum frequency of 3.4kHz, at 8000 samples a In the version adopted by the CCITT the frame consisted of 32 second (which is above the