ISUP (ISDN User Part)

ISDN User Part is the SS7 application protocol that controls the setup, supervision, and tear-down of circuit-switched calls in PSTN and legacy mobile networks. ISUP carries the calling and called party numbers, charging information, and call progress signalling between exchanges. Although largely replaced by SIP in modern networks, ISUP remains in service across interconnect trunks and is exposed to fraud and CLI spoofing when operators trust unverified incoming signalling. STIR/SHAKEN and IP-NNI migration are progressively reducing this exposure.

Categories: SignalingRoaming and InterconnectThreats and Attacks

ISUP in context

Signaling protocols carry the control-plane messages that set up calls, register subscribers and route SMS. SS7, Diameter, GTP-C and SIP are the four dominant families, and interconnect exposure of any of them is treated by GSMA as top-tier telecom risk.

Roaming and interconnect are where two operators exchange signaling and user-plane traffic. Trust boundaries here are the primary attack surface for location tracking, SMS interception and fraud, which is why GSMA now mandates SEPP with PRINS on 5G interconnect.

To place ISUP in the wider telecom-security picture, review IBCF, Interconnect Security Model, Inter-PLMN Security, NetFlow / IPFIX / sFlow, FASG and Firewall: each entry cross-references back to this page so you can walk the topic in either direction.

Related terms

Related topic hubs

  • SS7 Security SS7/MAP/CAP/SCCP attacks and defenses across 2G/3G interconnect.
  • Signaling Firewall Unified SS7/Diameter/GTP/SMS signaling firewall framework.
  • Roaming Security IPX/GRX interconnect risk, home-routed vs local-breakout, SEPP.
  • Telecom Fraud IRSF, Wangiri, SIM box, PBX hacking and CLI spoofing.

More from the TelcoSec Glossary

Browse the full TelcoSec Glossary, the Ultimate Guide to Mobile Network Security, or the P1 Arsenal of telecom-security tools.