 of the poten- multiplexer will rob, or share the least significant bit, i.e. bit 8 from tial bandwidth is lost (see also Section 21.4.3). the user data stream. Consequently, this bit alternatively carries information or signalling data. 21.4.2 The extended superframe format For five consecutive frames bit 8 will contain voice bits, and on the sixth, it will contain a signalling bit. This should not be confused The DS1 format described above was developed in the 1960s and is with the mechanism described above for the superframe and the based on technology then available. The basic challenge of DS1 193rd bit, though both mechanism and principle are similar. This bit transmission is that of preserving bit and word synchronism. To do robbing occurs totally within one voice sample, namely within one so, 24 eight-bit words (corresponding to samples for 24 telephone 21/8 Time division multiplexing conversations) are followed by one bit (the 193rd bit) for frame the VF signalling rule, remain available. No new bits from the tracking and management. There are 8000 repetitions of this pattern 1.544Mbit/s are taken away from the user. in one second, which results in an additional 8kbit/s bandwidth loss. Since twenty four frames have now to be examined by the equip- The extended superframe format, denoted as ESF, was announced ment to establish synchronisation and extract other channel infor- in 1981 and adds two enhancements. First, the number of frames in mation, it takes longer to regain lost synchronisation. However, with a superframe is increased to 24, rather than 12. This provides extra the newer sophisticated clocking mechanism this should only hap- signalling bits for telephone signalling, a C bit and a D bit. A 24 pen very rarely. frame pattern makes an additional bandwidth available on the fram- A 4000bit/s data link, also referred to as the Embedded Operations ing bit. This is used to send maintenance data between T1 interfaces. Channel (EOC) or Facilities Data Link (FDL), used for maintenance The ESF has 24 fra