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Microsoft Unified Communications (UC) offers a host of benefits for organizations looking to create a more streamlined communications infrastructure. Within the Microsoft UC solution, the Office Communications Server platform enables organizations to take advantage of user presence, instant messaging, conferencing and Internet Protocol (IP)-based enterprise voice calling.

Any implementation of Microsoft UC that includes voice communications presents implementers with one very basic problem: How to connect Office Communication Server (OCS) with your public switched telephone network (PSTN) lines? On one side, OCS uses packet-based Internet Protocol (IP) to communicate with call equipment on your Ethernet network, while traditional phone lines use Time Division Multiplexing (TDM) to communicate on the phone network.

VX bridges the gap between the telephone network communications and Voice over Internet Protocol (VoIP) communication by translating between TDM and IP voice communication protocols and by providing intelligent call routing services between your IP and TDM call equipment. As an IP/TDM voice communications gateway, VX allows you to connect IP-based Microsoft Unified Communications with telephone service lines as well as your existing TDM and IP-based call equipment.

Whether you are planning keep your private branch extension (PBX) systems, desk phones and other phone equipment or just rip everything out and give everyone computer headsets, VX provides a powerful, flexible and reliable way to integrate Microsoft Unified Communications with your existing phone network resources.

Configuring VX for Unified Communications

Refer to these resources for more information on how to configure VX to work with Unified Communications:

Refer to these resources for more information on configuring VX to work with other phone network resources:

Solution Advantages

The VX solution for Microsoft Unified Communications offers some key advantages over other solutions.

Any-to-Any Protocol Conversion

VX provides true, any-to-any protocol translation and media interworking between H.323, SIP, TDM signaling, DTMF encoding while supporting a wide variety of codecs including Microsoft RT Audio. For more information, see VX Key Concepts.

Active Directory-Based Call Routing

The VX system provides advanced call routing functionality through integration with Active Directory and LDAP servers. For more information, see Active Directory Configuration Quick Start Guide.

Enterprise Class Intelligent Call Routing

Due to diverse set of call processing equipment in an enterprise, the dial plans may need to be modified. VX provides for digit manipulation to support any call processing equipment in the enterprise. For more information on calling number manipulation, see Phone Number Matching and Translation.


The VX gateway secures both signaling and media through encryption. It uses Transport Layer Security (TLS) protocol to secure signaling information and uses Secure Real Time Protocol (SRTP) for securing media traffic. Unlike the competition, the VX gateway provides very high call capacity even when encryption is used. For more information about using TLS with VX, see Managing General Settings. For more information on enabling SRTP, see Media Crypto Classes.

High Availability and Resiliency

A pair of VX gateways can be configured in Active-Active mode to provide enterprise grade network resiliency. In a UC environment, the VX gateways can also be used for load balancing across multiple UC servers. For more information, see Using the VX Configuration Wizard for Microsoft Unified Communications.

Branch Office Voice Availability

The VX gateway has several features that ensure no call is ever missed. When an IP network fails, the VX gateway can route the calls to PSTN. The VX administrator can create routes using the Active Directory integration such that if the user is not reachable on their UC client, the call can be routed to their mobile phone, home phone, or to a registered SIP phone. The VX gateway allows for over 5000 phones to be registered with it. For more information, see Active Directory-Based Call Routing.

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