On the plus side, OCS can do call switching and routing, and the breadth of the unified communications software is all tightly integrated providing near plug-and-play between OCS and its Communicator client, presence, Exchange messaging, Office applications, data repositories and IP-based desktop and USB handsets built by Microsoft partners.

"What they are trying to do is make it easy for IT people to implement and deploy and they have made a platform that is really compelling for end-users," says E. Brent Kelly, an analyst with Wainhouse Research. "If you already have Outlook and Office, the integration is automatic."

Communicator uses Interactive Connectivity Establishment (ICE) standards which give external users the ability to use VoIP to call into the network, bypassing problems with NAT and firewall transversal that plague SIP-based PBXs.

Microsoft also has won praise for its Real-Time (RT) Audio Codec that improves sound quality, including in low-bandwidth situations.

In addition, the core of the unified communications platform will eventually come under Microsoft's System Center centralized management platform, which provides monitoring, software deployment and eventually automated load balancing and capacity management.

Not so fast

On the down side, Microsoft always has to answer reliability and security questions and it will be no different here. Viruses, worms or denial-of-service attacks could cripple phone and other communications.

"The typical way people deploy VoIP is they put phones on a virtual LAN separate from their data applications so if there is an attack on the data side the phones are not affected, but when you go to a fully software-based architecture it becomes next to impossible to do that isolation," says Irwin Lazar, an analyst with Nemertes Research.

Despite its all-software bravado, today OCS must be coupled with a PBX in many scenarios. The protocol stack in non-Microsoft SIP PBXs must be modified in order to integrate OCS's SIP extensions and the server does not support SIP trunking with a service provider.

Users will have to deploy OCS's Mediation Server to translate proprietary audio formats in standard formats, such as G.711. Larger deployments will need multiple Mediation Servers to handle large call volume.

Not surprisingly, the platform is Microsoft-centric, such as Communicator clients that won't work without modification on non-Microsoft SIP-based servers.

Users will have many components to upgrade, install and tweak including schema changes in Active Directory, creation and installation of certificates and certificate servers, deployment of edge servers for features such as connection to public IM networks, and rollout of a Quality of Experience Monitoring Server to watch voice and video quality.