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Traffic Analysis for Voice over IP 
white paper—Provides background information on various traffic analysis concepts and features that are applicable to Voice over IP (VoIP). This document presents fundamental traffic theory, several statistical traffic models, application of traffic analysis to VoIP networks, and an end-to-end traffic analysis example.
•Cisco Unified Communications Manager Capacity Tool 
—Calculates the minimum number of active subscribers that are required to support a given installation. Input consists primarily of quantity and usage information on the various device types that are supported in a Cisco Unified Communications Manager system.
In addition, these third-party traffic engineering tools are provided for your reference: