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

The mobile core carries subscriber sessions, mobility and policy. 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.

To place FDD in the wider telecom-security picture, review Confidentiality, Encryption, MNO, TMSI, UPF and VRAN — each entry cross-references back to this page so you can walk the topic in either direction.

Related terms

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.