Examines underlying technical security issues, policies, standards, implementations, and technologies associated with large-scale commercial telecommunications systems. Reviews the infrastructure and components of carrier class networks to include transport multiplexers and multi-protocol switches. Discusses the public switched telephone network (PSTN) and public land mobile network (PLMN). Begins with a review of the need for Signaling System No. 7 (SS7) and how security is implemented in SS7 networks. Presents fundamental trust assignments in second generation (2G) cellular mobile networks and specifically analyzes trust relationships between core components of the PLMN subsystems. Specifically discusses air interface (Um, Gm) protection measures and presents case studies of systemic flaws. Presents evolutionary changes in security practices in third (3G) and fourth generation (4G) protocols and standards. Examines underlying principles of lawful intercept (LI) implementation and discusses the evolution of LI capability from the PSTN through 3G and 4G networks. Studies the protection of data services in the PLMN and IP Multimedia Subsystem(IMS). Specifically focuses on the General Packet Radio Service (GPRS) Tunneling Protocol (GTP) and Roaming Exchanges (GRX). Discusses future research and proposed security standards in next generation systems.