ITU-T H.248.29 PDF

These packages are intended for use in analogue (one-way operation) or digital transmission systems (one-way or both-ways operations). The MGC should be unaware of the transmission details at the physical layer. The MG is therefore assumed to be provisioned with the actual signalling frequencies for interregister signalling (for example 2-out-of-n inband multifrequency code with forward and backward compelled signalling for R2) along with their properties such as amplitude, tone duration, cadence, etc. and also their logical significance. Any timers that dictate the interregister compelling actions are also assumed to be provisioned in the MG.

NOTE – Annex B specifies additional termination properties allowing the definition of some SSR2 timers. Support for any of these packages is optional.