Architecture

Microsoft Office Communications Server 2007 and Microsoft Office Communications Server 2007 R2 will reach end of support on January 9, 2018. To stay supported, you will need to upgrade. For more information, see Resources to help you upgrade your Office 2007 servers and clients.

All Communications Server 2007 topologies support Enterprise Voice. The following figure illustrates a generic topology, showing how new Communications Server 2007 voice components work with other components supporting IM, enhanced presence, conferencing, and federation.

Figure 1. Office Communications Server 2007 Reference Architecture

The following figure shows the essential Enterprise Voice components extracted from the overall Communications Server 2007 architecture shown in Figure 1.

Figure 2. Enterprise Voice Components

When an on-site worker places a call from his Enterprise Voice client to a PSTN destination, the calls moves through the Enterprise Voice infrastructure as follows:

1. User places call from an Enterprise Voice client by dialing or clicking the name in the name or number of a contact in Communicator or Microsoft Outlook®.

2. Server normalizes number to E.164 format and invokes routing rules based on location profile and user policy, and directs call to appropriate Mediation Server.

3. Mediation Server performs any necessary media transcoding and routes call to IP-PSTN gateway.

4. Gateway, based on topology, applies local dialing rules or PBX dialing rules and passes call to the PSTN or PBX.

Protocols

Enterprise Voice is an implementation of IP telephony that uses SIP (Session Initiation Protocol) for signaling and RTP (Real-Time Transport Protocol) for voice. Figure 2 shows the path of SIP signaling and RTP media transport through the Communications Server 2007 infrastructure.

SIP is an industry standard, application layer signaling protocol for starting, controlling, and ending communication sessions in an IP-based network. SIP is formally described in the IETF (Internet Engineering Task Force) reference specification RFC 3261. Using SIP, one user can explicitly invite another to join a conversation or multimedia session. A SIP session begins when the second user accepts a SIP INVITE request.

In Communications Server 2007, SIP is used for IM, conferencing, presence subscriptions, video, and voice, enabling Enterprise Voice clients to provide a common user experience across all these communication modes. Signaling for phone calls coming from the PSTN is converted to SIP by the media (PSTN) gateway.

Although SIP sessions can include the sharing of real-time media, SIP itself does not handle the actual media data, such as audio, video, and application sharing. In practical terms, this separation means that SIP and various media protocols can evolve independently.

Enterprise Voice uses RTP for media. Like SIP, RTP is an IETF standard. It defines a packet format for carrying audio and video over IP networks.

Office Communications Server 2007 Server Voice Components

Communications Server 2007 is responsible for seamlessly routing all voice calls for users who are enabled for Enterprise Voice. Core routing components and logic for Enterprise Voice reside on the following Communications Server 2007 roles:

• Standard Edition Server in the role of Front End Server or Director

• Enterprise Edition Front End Server

The core routing components are the Translation Service, Inbound Routing Component, and Outbound Routing Component:

Translation Service

The Translation Service is the server component that is responsible for translating a dialed number into E.164 format, using normalization rules based on location profiles 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 to be forwarded or simply logged for notification. If call forwarding is enabled, users can then 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 Servers and 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. This guide is primarily devoted to explaining how to plan and configure data structures that the Outbound Routing Component uses to route outgoing calls. These data structures include phone usage records, voice policies, and routes.

Other Server Components

Other components residing on the Communications Server 2007 Front End or Director that provide essential support for voice but are not themselves voice components, include User Services, User Replicator, and ABS (Address Book Service).

The User Services component performs 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 forks the call to that user's registered SIP endpoints. User Services is a core component on all Front End Servers and Directors.

The User Replicator component 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 Service. The User Replicator is a core component on all Front End Servers and Directors.

The Address Book Service component normalizes enterprise user phone numbers written to the RTC database to E.164 format for the purpose of provisioning user Contacts in Communicator 2007. Address Book Service is installed by default on all Front End Servers and Directors.

Call Flow

The following figure illustrates the flow of a call through the Communications Server and how each server component in the path of the call responds to the SIP INVITE request.

Figure 3. Call flow through a Front End Server or Director

The following figure shows another way to visualize the call flow through the Enterprise Voice infrastructure:

Figure 4. Call flow through the Enterprise Voice Infrastructure

The call flows shown in Figure 4 include:

• PC to PC

• PC to PBX/PSTN phone

• PBX/PSTN phone to PC

The SIP INVITE requests for each type of call are shown in the column on the right.

Media Gateways

An enterprise-grade IP telephony solution obviously 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 their perspective, a call between the Enterprise Voice infrastructure and the PSTN should seem like just another SIP session.

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

• Basic Media Gateway

A Basic Media Gateway requires the assistance of a new 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 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 Microsoft Windows Server® 2003 computer. This third-party device installation and management overhead compared to deploying a Basic Media Gateway and Mediation Server on separate computers.

Mediation Server

The Communications Server 2007, Mediation Server provides signaling and media translation between the Enterprise Voice infrastructure and a Basic Media Gateway. A Mediation Server is also used to link Communications Server and a PBX in both departmental deployment and PBX integration topologies.

The Mediation Server is deployed as a stand-alone application inside the corporate firewall. On the Enterprise Voice side, Mediation Server listens on one mutual TLS transport addresses, each with a unique port that is associated with a single media gateway. On the gateway side, Mediation Server can be configured to listen on either a TLS or TCP transport address.

The main functions of the Mediation Server are as follows:

• In deployments where the gateway link is configured for TCP, translating SIP over TCP (on the gateway side) to SIP over mutual TLS (on the Enterprise Voice side).

• Encrypting and decrypting SRTP on the Communications Server side.

• Translating media streams between Communications Server and the media gateway.

• Connecting clients that are outside the network to internal ICE (Interactive Connectivity Establishment) components, which enable media traversal 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.

The following figure shows the signaling and media protocols used by the Mediation Server when communicating with a Basic Media Gateway and the Enterprise Voice infrastructure.

Figure 5. Signaling and media protocols used by the Mediation Server

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. At such time as Advanced Media Gateways become available, enterprises that choose to deploy them will no longer have any need for Mediation Server. Meanwhile, the Basic Hybrid Media Gateway provides an interim solution for organizations that prefer to avoid deploying and managing gateway and Mediation Server separately.

The following figure shows the logical equivalence of an Advanced Media Gateway and the combination of a Basic Media Gateway and Mediation Server.

Figure 6. Equivalent Media Gateway Topologies

A typical organization will support 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.

Edge Servers

Outside callers using Communicator for individual or conference calls rely on the following edge servers 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 Communications Server 2007 Access Edge Server provides SIP signaling for calls from Communicator users who are outside the corporate firewall.

A/V Edge Server

The Communications Server 2007 A/V Edge Server enables media traversal of firewalls. A caller using 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 attempting to connect to the A/V Edge Server require an authentication token provided by the A/V Authentication Service before their calls can go through.

Voice Clients

Communications Server 2007 supports Microsoft Office Communicator 2007 as the primary voice client. For a more traditional phone-like experience, users may prefer the Microsoft Office Communicator Phone Edition.

Microsoft Office Communicator 2007

Communicator 2007 is the principal client for Enterprise Voice. Communicator supports the following types of calls:

• PC to PC.

• PC to PBX/PSTN phone.

• PBX/PSTN phone to PC.

• PC to audio conference.

• Video calls and conferencing.

• Call control features such as call forwarding and deflection, call transfer, and call hold and retrieve.

Communications Server 2007 provisions Communicator 2007 with user names and phone numbers, enriched presence information, and location profiles.

Users outside the enterprise can use Communicator 2007 to call users inside the enterprise regardless whether those users are enabled for Enterprise Voice.

Microsoft Office Communicator Phone Edition

Microsoft has developed the Microsoft Office Communicator Phone Edition telephone for organizations or users who are more comfortable making calls from a phone-like device than from the Communicator software client. For most purposes the Communicator Phone Edition phone is a physical version of Communicator, with a similar user interface and comparable functionality. Like Communicator, the Microsoft Office Communicator Phone Edition allows you to place a call either by using a numeric touchpad or by clicking one of your contacts. The Microsoft Office Communicator Phone Edition also supports enhanced presence, SIP signaling, and a user experience similar to that of Communicator, all in a desktop telephone. The Microsoft Office Communicator Phone Edition can be used to connect from home as well as in the office.

The Microsoft Office Communicator Phone Edition requires Communications Server 2007, with Media Gateways deployed to enable PSTN interoperation. Like Communicator, the Microsoft Office Communicator Phone Edition is provisioned for voice by the Communications Server.