GSM-CTS offers two levels of security: network (by the operator) and local.

Network security
The operator must supervise the whole GSM-CTS, including terminal, SIM card, and fixed part. Security is based on the monitoring of :
• Radio parameters
• Subscriptions
• Equipment
• Information transfers between network components

Local security
GSM-CTS internal security is designed to :

• Prevent undesirable connections to the fixed part.
• Authenticate users wishing to register with a fixed part
• Ensure call confidentiality at the interface between the mobile and fixed part

More:
CTS Introduction
General CTS Technical Overview
CTS Air Interface
CTS Protocols
CTS System Architecture

Use the search engine below to see more on CTS

* Much of this information was sourced from the CTS MoU