FDD (Frequency Division Duplex)

Frequency Division Duplex uses different frequency bands for uplink and downlink, allowing simultaneous bidirectional transmission. FDD has historically dominated cellular deployments because it is robust and well-suited to symmetric voice traffic. Its alternative TDD reuses one band by alternating in time and is more flexible for asymmetric data, with 5G mid-band largely TDD.

Categories: Radio Access NetworkCore NetworkIdentity and Subscriber

FDD in context

The radio access network is the layer where mobile devices attach to the operator's infrastructure. Attacks in this layer include IMSI catching, rogue base stations, downgrade attacks and jamming, and defenses hinge on mutual authentication, integrity-protected signaling and Open RAN supply-chain hygiene.

The mobile core is where subscriber sessions, mobility and policy live. In 4G it is the EPC (MME, HSS, S/PGW); in 5G it is the Service-Based Architecture with AMF, SMF, UPF, AUSF, UDM and the NRF. Core exposure is the difference between a single-subscriber leak and a fleet-wide breach.

To place FDD in the wider telecom-security picture, review Encryption, VoWiFi, ADSL, TMSI, Confidentiality and MNO.

Related terms

Related topic hubs

  • Diameter Security 4G/LTE Diameter threats across IPX/roaming and DEA defenses.
  • Diameter Attacks S6a IDR/PUR/CLR abuse, subscriber tracking, LTE signaling attacks.
  • 5G Security 5G SBA, SEPP, SUCI, 5G-AKA, N32 and service-based interface security.
  • 5G NEF Security Network Exposure Function security in the 5G core.

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.