There are four conferencing servers in OCS:

 IM Conferencing Server:  Provides server-managed group IM. See how to install Front End Server.

 Web Conferencing Server:  Enables multiparty data collaboration. See how to install Web Conferencing Server.

 A/V Conferencing Server:  Enables audio and video peer-to-peer between communications and audio and video conferencing.The A/V Conferencing Server can be collocated with the pool Front End Server or deployed in the pool on a separate computer. When deployed on a separate computer, the Audio/Video Conferencing Server can support up to 250 participants and six presenters within a single session. See how to install Audio\Video Conferencing Server.

Telephony Conferencing Server: Enables audio conference integration with ACP (audio conferencing providers).

The IM Conferencing Server and Telephony Conferencing Server always run as separate processes on the Standard Edition server or Enterprise Edition Front End Server. The Web Conferencing Server and A/V Conferencing Server optionally may be deployed on separate computers within an Enterprise Pool. Unless an enterprise has extraordinary performance and availability requirements, the simpler, less expensive, and therefore recommended choice for most organizations is to collocate all conferencing servers on a Standard Edition server or on Enterprise pool Front End Servers.

Question: What is MCU?  

Multipoint Control Unit (MCU): It is a conferencing server which manages and co-ordinates use of a media type during the course of a meeting. The media type includes data collaboration, group IM, audio & video, & multiparty audio conferencing.

There are three types of edge servers as follows:

Access Edge Server: Validates and forwards IM traffic between internal and external users. The Live Communications Server 2005 Access Proxy with a new name. See how to install Access Edge Server.

Web Conferencing Edge Server: Enables data collaboration with external users. See how to install Web Conferencing Edge Server.

A/V Edge Server: Enables audio and video conferencing and A/V peer-to-peer communications with external Communicator 2007 users. See how to install A/V Edge Server.

These three edge servers can be installed together on a single computer or separately on three computers. The recommended deployment for most organizations is, for reasons of economy and simplicity, to collocate the Web Conferencing Edge Server with the Access Edge Server, but to install the A/V Edge Server, which requires greater bandwidth, on a separate computer. Group IM and data collaboration with external users also requires deploying an HTTP reverse proxy in the perimeter network

Followings are the four main types of multimedia conferencing:

Group IM : An IM conversation among three or more parties.Users can add Microsoft Exchange Server distribution lists as contacts. Microsoft Office Communicator 2007 client allows expansion of distribution lists through a Web service exposed on the server. This expansion allows users to invite one or more individual members of the group to an IM session. Although groups of up to 1000 members can be expanded, IM sessions can include no more than 100 members.

Data Collaboration: Data collaboration is managed by the Web Conferencing Server, which can be collocated with the Enterprise pool Front End Server or deployed in the same pool but on a separate computer. The data itself is stored in a file share created by an administrator. Office Communications Server 2007 also enables data collaboration with external users. The Web Conferencing Edge Server, which is deployed in the network perimeter, provides the bridge between the Web Conferencing Server and external users.

Audio/Video: Office Communications Server 2007 supports multiparty A/V (audio/video) conferencing. Users can specify A/V when scheduling a conference or can add audio or video to an existing IM conversation or conference call. Managing multiparty audio and video sessions is the job of the A/V Conferencing Server. Office Communications Server 2007 also extends audio and video conferencing to external users. The Audio/Video Edge Server acts as a media relay for the transmission of both audio and video signals across corporate firewalls. This makes it possible to share audio and video with external users. The Audio/Video Edge Server can be collocated with the Access Edge Server or installed on a separate computer in the perimeter network.

PSTN Connectivity: Office Communications Server 2007 supports connectivity with the Public Switched Telephone Network through integration with an external Audio Conferencing Provider (ACP). This provides PSTN conferencing over an external bridge, with no interaction with internal VoIP audio conferencing. ACP integration is managed by the Telephony Conferencing Server, which always runs as a separate process on either a Standard Edition server or Enterprise Edition Front End Server.

Meeting participants fall into three groups: organizers, presenters, or attendees:

Organizer. The user who creates and initiates the meeting.

Presenter. A user who is authorized to present information at a meeting, using whatever media is supported. A meeting organizer is by definition also a presenter and determines who else may be a presenter. A presenter can also promote an attendee to the role of presenter either before or during the meeting.

Attendee. A user who has been invited to attend a meeting but who is not authorized to act as a presenter.

Based on their locations and credentials the users can be divided broadly into internal and external users as follow:

Internal users: Who have Active Directory credentials within the enterprise and connect from locations inside the corporate firewall.

External users:  Those who either temporarily or permanently connect to an enterprise from locations outside the corporate firewall. They may or may not have Active Directory credentials. Office Communications Server 2007 provides conferencing support for the following types of external users:

Remote Users have a persistent Active Directory identity within the enterprise. They include employees working at home or on the road, and other remote workers, such as trusted vendors, who have been granted enterprise credentials for their terms of service. Remote users can create and join conferences and act as presenters

Federated Users possess valid credentials with federated partners and are therefore treated as authenticated by Office Communications Server 2007. Federated users can join conferences and act as presenters, but they cannot create conferences in federated enterprises.

Anonymous Users. Users without an Active Directory identity and who are not federated with the enterprise.