Explanation: ISDN (Integrated Services Digital Network) is a connection oriented network system and uses Q.931 Protocol for managing its call states. Q.931 is a signaling protocol. It can be compared to SIP but SIP is not applicable to ISDN. SIP is a 3GPP signaling protocol. H.323 is a call signaling protocol that was initially implemented in LANs. ICMP is used in routers and helps in sending error messages.