VoIP Components

VoIP Components

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VoIP Components

The principal VoIP components on the Front End Server are as follows:

Translation Service
Inbound Routing
Outbound Routing

Translation Service

The Translation Service is the server component that is responsible for translating a dialed number into E.164 format according to the normalization rules that are defined by the administrator.

Inbound Routing Component

The Inbound Routing component handles incoming calls largely according to preferences that are specified by users on their Enterprise Voice clients. For example, users specify whether unanswered calls are forwarded or simply logged for notification. If call forwarding is enabled, users can specify whether unanswered calls should be forwarded to another number or to an Exchange 2007 Unified Messaging server that has been configured to provide call answering. The Inbound Routing component is installed by default on all Standard Edition servers and Enterprise Edition Front End Servers.

Outbound Routing Component

The Outbound Routing component routes calls to PBX or PSTN destinations. It applies call authorization rules to callers and determines the optimal media gateway for routing each call. The Outbound Routing component is installed by default on all Standard Edition and Enterprise Edition servers and on Enterprise Edition Front End Servers.

The routing logic that is used by the Outbound Routing Component is in large measure configured by network or telephony administrators according to the requirements of their organizations.

Other Server Components Required for VoIP

Other components residing on the Office Communications Server 2007 Front End or Director that provide essential support for VoIP, but are not themselves VoIP components, include the following:

User Services. Perform reverse number lookup on the target phone number of each incoming call and matches that number to the SIP URI of the destination user. Using this information, the Inbound Routing component distributes the call so that users registered SIP endpoints. User Services is a core component on all Front End Servers and Directors.
User Replicator. Extracts user phone numbers from Active Directory and writes them to tables in the RTC database, where they are available to User Services and Address Book Server. User Replicator is a core component on all Front End Servers.
Address Book Server. Normalizes enterprise user phone numbers that are written to the RTC database to E.164 format for the purpose of provisioning user contacts in Office Communicator 2007. The Address Book Server is installed by default on all Front End Servers.

Integration with the PSTN

An enterprise-grade VoIP solution must provide for calls to and from the PSTN without any decline in quality of service. In addition, users placing and receiving calls should not be aware of the underlying technology. From the user's perspective, a call between the Enterprise Voice infrastructure and the PSTN should seem like just another SIP session.

Media Gateways

Media gateways are third-party devices that translate signaling and media between the PSTN and the Enterprise Voice infrastructure. Office Communications Server 2007 supports three types of media gateways:

Basic Media Gateway. A basic media gateway requires the assistance of a new Office Communications Server 2007 server role, the Mediation Server, to present a PSTN call to an Enterprise Voice client connection. The Mediation Server also presents calls from Enterprise Voice clients to the basic media gateway for routing to the PSTN.
Advanced Media Gateway. An Advanced Media Gateway combines the functionality of a Basic Media Gateway and that of the Mediation Server. An Advanced Media Gateway does not require a separate Mediation Server to handle the signal and media translation that is necessary to present a PSTN call as an Enterprise Voice client connection.
Basic Hybrid Media Gateway. A basic hybrid media gateway collocates a basic media gateway with a Mediation Server on a single computer running Windows Server 2003. The Basic Hybrid Media Gateway is available from Microsoft partners for Office Communications Server 2007. It reduces installation and management overhead compared to deploying a Basic Media Gateway and Mediation Server on separate computers.

Mediation Server

The Office Communications Server 2007, Mediation Server, provides signaling and media translation between the VoIP infrastructure and a basic media gateway. A Mediation Server also links Office Communications Server with a PBX in both the departmental deployment and PBX integration topologies.

The Mediation Server is deployed as a stand-alone application inside the firewall. On the Office Communications Server side, Mediation Server listens on a single mutual TLS transport address. On the gateway side, Mediation Server listens on a single TCP or TLS transport address. TLS is recommended, but TCP is supported for gateways that do not support TLS.

The main functions of the Mediation Server are as follows:

Encrypting and decrypting SRTP (Secure Real-time Transport Protocol) on the Office Communications Server side
Translating SIP over TCP (for gateways that do not support TLS) to SIP over mutual TLS
Translating media streams between Office Communications Server and the media gateway
Connecting clients that are outside the network to internal ICE (Interactive Connectivity Establishment) components, which enable media traversal of NAT and firewalls
Acting as an intermediary for call flows that a gateway does not support, such as calls from remote workers on an Enterprise Voice client

Figure 12 shows the signaling and media protocols that are used by the Mediation Server when communicating with a basic media gateway and the Enterprise Voice infrastructure.

Figure 12. Signaling and media protocols used by the Mediation Server

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From the perspective of the Enterprise Voice infrastructure, the combination of basic media gateway and Mediation Server appear as a single entity. Together, they are the logical and functional equivalent of an advanced media gateway. When advanced media gateways become available, enterprises that deploy them no longer have any need for a dedicated Mediation Server. Meanwhile, the basic hybrid media gateway provides an interim solution for organizations that prefer to avoid deploying and managing a gateway and Mediation Server separately.

Figure 13 shows the logical equivalence of an Advanced Media Gateway and the combination of a Basic Media Gateway and Mediation Server.

Figure 13. Equivalent media gateway topologies

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A typical organization supports multiple gateway–Mediation Server combinations, depending on the number of office locations, the number and distribution of Enterprise Voice users, network traffic, and performance requirements.

Perimeter Network Configuration for VoIP

Outside callers who use Office Communicator for individual or conference calls rely on the Access Edge Server and the A/V Edge Server for voice communication with coworkers. For information about deploying edge servers, see the Microsoft Office Communications Server 2007 Edge Server Deployment Guide.

Access Edge Server

The Office Communications Server 2007 Access Edge Server provides SIP signaling for calls from Office Communicator users who are outside your organizations firewall.

Audio/Video Edge Server

The Office Communications Server 2007 A/V Edge Server enables media traversal of NAT and firewalls. A caller using Office Communicator 2007 from outside the corporate firewall relies on the A/V Edge Server for both individual and conference calls.

The A/V Authentication Service is collocated with, and provides authentication services for, the A/V Edge Server. Outside users who attempt to connect to the A/V Edge Server require an authentication token that is provided by the A/V Authentication Service before their calls can go through.