Which of the following is the signaling protocol used by the Public System Telephone Network $($PSTN$)$ on its internal circuit$-$switched connections?

a$.$G$\mathrm{.}711$

b$.$H$\mathrm{.}245$

c$.$Signaling Initiation Protocol $($SIP$)$

d$.$H$\mathrm{.}323$

e$.$Standard System Seven $($SS$7)$