oose a framing pattern with low autocorrelation, SO signalling. In this system each digital channel arranges, besides that it is impossible to imitate by shifting and infringement on its octet, one or several signalling bits. These signalling bits of random neighbouring bits. Examples are the patterns 110 or the n channels of the frame can be distributed, i.e. juxtaposed 1110010 for for 3-bits or 7-bits grouped framing patterns re- at each octet as a supplementary bit per time slot (possible spectively. signalling rate being 8kbit/s per channel), or grouped in a time 2. One can block all the channels of the frame when the framing slot reserved for this purpose and of which the bits are assigned is lost at reception. The frame would be replaced by the trans- in turn, cyclically, to the n channels of the frame (possible mission of a resynchronisation signal. This necessitates an signalling rate being 64/n kbit/s per channel) announcement warning in the opposite direction, which is as- 3. Common signalling, where one time slot per frame is reserved sumed to be correctly aligned. for signalling and assigned according to need to one channel, 3. One could confirm correct framing by a criterion other than the then to another. Signalling is done using labelled messages, of presence of the framing pattern, e.g. absence of the pattern in which the label indicates which channel the message belongs one frame out of two. to. This technique allows an instantaneous signalling rate of 64kbit/s for one channel at a time. In order to avoid reacting to each transmission error, the reaction to an incorrect reception of the framing pattern in an aligned situ- ation, i.e. during normal data transfer, must be delayed. The solution to the framing problems of both T1 and E1 are given in the respec- 21.3 The digital hierarchy levels tive sections later. A range of digital systems with increasing capacity has been defined, the systems of each order being composed of four systems 21.2.3 Signalling of the immediately lower order. In America 