The most effective way for controlling visitor access to a data center is to ensure that visitors are escorted by an authorized employee, as this prevents unauthorized or malicious actions by the visitors and provides accountability and supervision. Pre-approval of entry requests, visitors signing in at the front desk upon arrival, and closed-circuit television (CCTV) are also useful measures, but they are not as effective as escorting visitors, as they do not prevent or detect unauthorized or malicious actions by the visitors in real time. References: CISA Review Manual (Digital Version), Chapter 5: Protection of Information Assets, Section 5.1: Physical Access Controls1