The Translation Service is the server component that is responsible for translating a dialed number into the E.164 format or another format, according to the normalization rules that are defined by the administrator. The Translation Service can translate to formats other than E.164 if your organization uses a private numbering system or uses a gateway or PBX that does not support E.164.

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 a Microsoft 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 Front End Servers.

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 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.

The Exchange UM routing component handles routing between Microsoft Communications Server 2010 and servers running Microsoft Exchange Unified Messaging (UM), to integrate Communications Server with Unified Messaging features.

The Exchange UM routing component also handles rerouting of voice mail. If you have Enterprise Voice users at branch sites that do not have a resilient WAN link to a central site, the Survivable Branch Appliance (SBA) that you deploy at the branch site provides voicemail survivability for branch users during a WAN outage. When the WAN link is unavailable, the SBA does the following:

• reroutes unanswered calls over the PSTN to the Exchange Unified Messaging server in the central site
  
  
• provides the ability for a user to retrieve voice mail messages over the PSTN
  
  
• queues missed call notifications, and then uploads them to the Exchange UM server when the WAN link is restored.
  
  

To enable voice mail rerouting, it is recommended that your Exchange administrator provisions an Exchange UM Message Taking Mode Auto Attendant (AA).

For details about these features, see Exchange Unified Messaging Integration and Branch-Site Voice Resiliency, respectively.

Communications Server 2010 supports Enterprise Voice resilience for central site and branch sites. The intercluster routing component routes all calls made by a user homed on one pool to a user homed on another pool.

Users in a branch connect and register with the Enhanced Registrar in the branch. When the WAN link is unavailable, these users remain connected to the Enhanced Registrar in the branch site. Any calls made by the branch users to users located at other sites are automatically rerouted over the PSTN to the other user’s phone number by the Intercluster Routing component running on the Enhanced Registrar in the branch. Similarly, for calls made by users located at other sites to the branch user, the Intercluster Routing component at the branch site will reroute those calls over the PSTN.

Users in a central site connect and register with the Communications Server Enterprise pool or Standard Edition server in the central site. This pool is also the user’s primary Enhanced Registrar pool. If this pool is unavailable, users can failover to a backup Enhanced Registrar pool which then handles user registration and routing of calls. If a user makes a call to a central site user that fails over to the backup Enhanced Registrar pool, the Intercluster routing component running on the callers Enhanced Registrar pool facilitates call establishment by first calling the central site user’s primary Enhanced Registrar pool, and if that is unavailable by calling the user’s backup Enhanced Registrar pool. If both the pools are unavailable, the Intercluster routing component reroutes the call over the PSTN network to the user’s telephone number.

Other components residing on the Front End Server 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 match that number to the SIP URI of the destination user. Using this information, the Inbound Routing component distributes the call to that user’s 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 Domain Services (AD DS) 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. Provides global address list information from Active Directory Domain Services to Communicator "14" clients. It also retrieves user and contact information from the RTC database, writes the information to the Address Book files, and then stores the files on a shared folder where they are downloaded by Communicator clients. The Address Book Server writes the information to the RTCAb database, which is used by the Address Book Web Query service to respond to user search queries from the 2007 R2 version of Communicator Mobile for Windows Mobile. It optionally normalizes enterprise user phone numbers that are written to the RTC database for the purpose of provisioning user contacts in Communicator. The Address Book Server is installed by default on all Front End Servers. The Address Book Web Query service is installed by default with the Web services on each Front End Server.