The Public Switched Network (PSN) of the United States has undergone significant changes over the past decade as a result of new technologies, regulatory changes, and network consolidation. With the rash of new entrants, security measures applied to the infrastructure are more diverse and varied with respect to the technologies in use and the pervasiveness of their application. As a companion to that primer, this assessment guide offers guidelines and methodologies for conducting a security assessment for service providers. This guide describes a risk assessment procedure to identify high-value, high-risk components of a service provider's network and information assets. The descriptions have been designed to drive a comprehensive review of these important pieces of a service provider's security program. The OMNCS and Telcordia Technologies created this publication in an effort to provide practical security measures for the protection of communications networks, which are one of the Critical Infrastructures upon which the United States is dependent for its national security, as identified in Presidential Decision Directive 63 (PDD 63).