Read these conceptual, overview topics for the background knowledge that you need to build an intelligent plan.
Cisco Unified Communications System Description—This document contains a high-level overview of the Cisco Unified Communications family of products.
Links to other key concept topics:
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IP Telephony Components and Applications
•Design for High Availability on the Design tab
Cisco Unified Communications System Description
The Cisco Unified Communications System Description document is an overview of the Cisco Unified Communications systems approach, in which individual Cisco hardware and software components are integrated for seamless interoperability. Download the Cisco Unified Communications System Description 
(right-click to download PDF), or view these topics:
•Cisco Unified Communications Features and Benefits Overview
•Cisco Unified Communications Component Overviews