Looking back to my ISDN – Integrated Services Digital Network – days.
ISDN was based on Q.931 (signalling) and introduced the notion of
2B+D for the Wireline local loop and 30B+D (Primary Rate Access (PRA)).
D channel was for signalling and the B channel was the “bearer”.
I.420 ws used for 2B+D, I.421 for I.421 (PRA)
Suspend/Resume is an ISDN service that allows calls to be “suspended” and “resumed”
Therefore, ISDN calls can be “suspended” (put on hold) to allow another call to use the B channel.
ISDN suspend/resume messages manage suspended calls.
For ISDN, ISUP (ISDN User Part) was meant to be the signalling protocol to be used.
TE1—I.420/421—-ISDN Switch———-ISUP————ISDN Switch—-I.420/421—-TE2