General Signalling System No. 6 can be used to control the switching of all types of international circuits to be used in a worldwide connection@ including TASI-derived circuits and satellite circuits. The system meets all requirements defined by the CCITT concerning the service features for world-wide international semi-automatic and automatic telephone traffic. It is designed for both-way operation of the speech circuits. The system can also be used for regional and national applications@ and a large part of the signal code capacity is reserved for this purpose. Moreover@ a large unused signal code capacity will allow the addition of new signals to cater for some unknown future requirements. This spare capacity may be used for increasing the number of telephone signals as well as for introducing other signals@ e.g. network-management signals and network-maintenance signals. The system features are obtained by entirely removing the signalling from the speech paths and introducing the concept of a separate common signalling link over which all signals for a number of speech circuits are transferred. A number of these common signalling links interconnected by a number of transit centres and signal transfer points will form a coherent signalling network which can transfer all signals for all speech circuit groups within that network area. Modes of operation The signalling system may be operated both in an associated mode and in a non-associated mode. In the associated mode of operation@ the signals are transferred between the two exchanges which are the end points of a group of speech circuits over a common signalling link terminating at the same exchanges. In the non-associated mode of operation@ the signals are transferred via two or more common signalling links in tandem associated with other groups of circuits@ the signals being processed and forwarded through one or more intermediate exchanges acting only as signal transfer point. The associated mode of operation is suited for use with large circuit groups@ while a non-associated mode makes the signalling system economically suitable for use with small circuit groups by sharing the capacity of a signalling link among several groups. A signalling link may be operated in the associated mode for one circuit group and in a non-associated mode for other circuit groups@ either under normal or under breakdown conditions. Common signalling link The separate common signalling link is capable of operation over both analogue and digital circuits. Signalling information is transmitted in the serial data mode on a link-by-link basis ?C i.e. the signals are transferred from one link to the next only after processing. Analogue signalling links are capable of operations over standard international voice bandwidth channels including the 3-kHz spaced telephone channels used for some intercontinental circuits. Over voice-frequency channels the stream of pulses is normally transmitted at a rate of 2400 bit/s using the four-phase modulation method. With respect to digital signalling links@ the 1544 kbit/s and 2048 kbit/s internationally standardized PCM primary multiplexes (Recommendation Q.47 and Recommendation Q.46) are treated differently. In the case of 1544 kbit/s a channel is derived over which the stream of pulses is transmitted at 4 kbit/s. Signalling information is also transmitted at 4 kbit/s. In the case of 2048 kbit/s@ a channel is derived over which the stream of pulses is transmitted at 64 kbit/s. Signalling information may be sent over such a channel at specified rates of either 4 kbit/s or 56 kbit/s. Other bit rates may have application in the future and other provisions for channel derivation may also prove useful@ but neither are included in the present specification. In both analogue and digital channels the pulse stream is divided into signal units of 28 bits each and into blocks of 12 signal units each. The error control necessary for a common signalling link is based on error detection by coding and on error correction by retransmission. Error detection is based on decoding of checking bits included in each signal unit and on data carrier failure detection. This provides the desired system reliability. Error-free signal messages are used without delay. Provision is made for automatic transfer to an alternative link in the event of failure caused by breakdown or excessive error rate. Signal messages Signal messages carry information to identify the telephone circuit concerned. Since the circuit identity@ i.e. the label@ requires a large proportion of the bits (11 out of the 20 available information bits)@ provision is made for sending multi-unit messages consisting of several signal units under one label. A single digit or a random telephone signal will normally be transferred in a one-unit message while several or even all digits may be transferred in a multi-unit message. Signal processing All signals are processed at each transit centre or signal transfer point which has to be passed. The processing of messages at a signal transfer point is minimal and includes label translation@ if necessary@ and the sending of signal messages within the proper priority category. In addition to the processing required at a signal point@ a transit centre examines sufficient signal information to perform proper switching action. Signalling equipment Because of the new technique based on a separate common signalling link@ on data type transmission and on central processing of the signalling information@ Signalling System No. 6 will be used in general between exchanges of the stored programme control type.