General A signalling data link is a bidirectional transmission path for signalling@ comprising two data channels operating together in opposite directions at the same data rate. It constitutes the lowest functional level (level 1) in the Signalling System No. 7 functional hierarchy. Functional configuration of a signalling data link is shown in Figure 1/Q.702. A digital signalling data link is made up of digital transmission channels1) and digital switches or their terminating equipment providing an interface to signalling terminals. The digital transmission channels may be derived from a digital multiplex signal at 1544@ 2048 or 8448 kbit/s having a frame structure as defined in Recommendation G.704 [1]@ or from digital multiplex streams having a frame structure specified for data circuits (Recommendations X.50 [4]@ X.51 [5]@ X.50 bis [6]@ X.51 bis [7]). An analogue signalling data link is made up of voice-frequency analogue transmission channels either 4 kHz or 3 kHz spaced@ and modems. Signalling System No. 7 is capable of operating over both terrestrial and satellite transmission links1). 1) The terms transmission channel and transmission link are used in Signalling System No. 7 instead of transfer channel and transfer link used in Signalling System No. 6.