ming word Extended superframe Extended superframe framing word C bits = CRC-6 bits D bits = Data link bits (maintenance) 001011 = 6 bit framing pattern Figure 21.6 Extended superframe format The CEPT PCM-30 framing format 21/9 complement the A and B bits of the regular superframe format. Out in each frame to inject a 1. This bit is called the network control bit of these, 16 codes options are available for transparency (no robbed and is not made available to the user. While this resolves the ones signalling bits), two state signalling (A bit only), four state signall- density problem, it imposes the speed restriction alluded to above. ing (A and B bits), or sixteen state signalling (A, B, C, and D bits). The approach embodied in the current generation of equipment, In the transparent signalling option, all eight bits within a frame of substituting a 1 in the LSB position of an all zeros word, may be within the extended superframe are given to the user for data. When acceptable for voice, because the difference between a 00000000 the A signalling mode is employed, the LSB of every sixth signall- coded sample and a 00000001 coded sample is imperceptible to the ing frame is robbed to show the desired bit. When A/B signalling is ear. When the bits of a DS1 channel represent directly input digital used, the first and third signalling frames are robbed to carry the A data, such as DDS or ISDN, however, one is not at freedom to alter bits, while the second and fourth signalling frames will carry the B the data, and a technical solution must be achieved. bits. If the A/B/C/D signalling is employed, the desired bits are A possible solution has already been introduced. This concept, gained by robbing the first, second, third, and fourth frames, respec- also referred to as the primary rate interface or PRI, is one of the tively. central physical components of ISDN. Its implementation involves From the above we can conclude that ESF has economically taking all of the signalling information of the individual 23 T1 reallocated