Failure Scenarios

In the event of a WAN outage, users in branch sites will have continued voice services through the use of a Survivable Branch Server or Appliance because the primary registrar service is local to the site. The clients will enter “Limited Functionality Mode,” in which not all features are available to users while the WAN link is unavailable. Figure 32.5 shows how a branch office user remains connected to the local registrar service when a WAN link is unavailable.

During a WAN outage when branch users cannot contact a Front End pool, the following features are unavailable for users inside the branch:

• Cross-Site Communication—Any form of communication to other sites is unavailable.

• Conferencing—All conferencing is unavailable. This applies to instant messaging, meeting, and audio/video conferences involving more than two parties because the multipoint control units (MCUs) exist on the Front End pool and not the survivable branch component.

• Presence-Based Routing—Because presence data is unavailable, calls to users homed on the inaccessible pool across the WAN link do not have calls routed based on presence. For example, if a user sets the status to Do Not Disturb, instant message and phone calls are still delivered.

• Call Park—Call Park is provided by the Front End pool, which is inaccessible across the WAN link.

• Response Group Service—Any workflows and queues associated with the pool across the WAN link are unavailable.

• Modifying Call Forwarding Settings—Although call forwarding capabilities remain in effect during a failover, users are unable to update or change their call forwarding settings.

The features that are still available for users inside the branch during a WAN outage include these:

• PSTN Calls—Inbound and outbound calls are possible because the local media gateway is available and not dependent on the WAN link.

• Internal Calls—Internal voice calls between users within the branch site have no issues. Calls to users in another site must leverage PSTN rerouting out the local media gateway to be completed.

• Call Control—Users are able to use basic call features such as hold and transfer. Advanced features such as call forwarding, simultaneous ringing, and team call are also available in a failover scenario.

• Instant Messaging—Instant messaging services are available, but only between two parties within the branch site. No instant messaging conferencing services are available.

• Audio/Video Calls—Audio and video calls are available between two parties only within the branch site. Audio/video conferencing services are unavailable.

• Call Detail Records—Call detail records continue to queue on the primary registrar and are delivered after the WAN link is restored.

• Audio Conferencing—Audio conferencing is possible by the placing of a PSTN call to the dial-in conferencing bridge. Click-to-join for Online Meetings does not function.

• Voice Mail Retrieval—Assuming that the Exchange infrastructure exists in a primary datacenter, users can still dial the PSTN number for Outlook Voice Access to retrieve voice mail.

• Voice Mail Deposit—Assuming that the Exchange infrastructure exists in a primary datacenter, Survivable Branch Servers and Appliances can reroute voice mail delivery across the PSTN. This is an Exchange Unified Messaging directory auto attendant that only accepts voice mail and does not allow transfer to users.

In the event of the Survivable Branch Server or Survivable Branch Appliance in the branch site becoming unavailable, users begin to use the backup registrar service on the Front End pool located across the WAN link. In this scenario, users do not experience any loss in functionality. The Front End pool starts servicing the users as if they were homed to it and all features are available. Through the multiple trunk pairing possible in Lync Server 2013, the branch users can also continue to make and receive phone calls using the media gateway in their office.