What is the role of SS7 in PSTN signaling and the function of the D-channel in ISDN BRI/PRI?

• A. SS7 handles call setup and control; ISDN D-channel carries signaling in BRI/PRI.
• B. SS7 handles data only; D-channel carries voice.
• C. SS7 handles encryption; D-channel carries video.
• D. SS7 is obsolete; D-channel not used in ISDN.

Answer: (A)

Signaling is the control plane that sets up, routes, and ends calls, separate from the actual voice path. In the PSTN, SS7 is the signaling system that exchanges messages between network elements (exchanges, databases, service platforms) to initiate calls, determine routing, perform number translation, manage charging, and support features. In ISDN, the D-channel serves as the dedicated signaling channel on the ISDN line, carrying the call-control information between the user terminal and the network. The B-channels carry the user data (the actual voice or data streams), while the D-channel handles the signaling required to establish and manage the call. So, the role is that signaling handles call setup and control, and the D-channel is the channel used for that signaling in ISDN BRI/PRI.