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Understanding Unified Messaging Availability

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Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2

In many organizations today, accessing e-mail and voice mail is critical to the success of daily operations. To provide continuous access to e-mail and voice mail, you have to correctly plan and implement a solution for Microsoft Exchange Server 2010 that will ensure the availability of the servers that provide these services. You need to understand how to implement a solution that will make your Unified Messaging servers highly available.

Unified Messaging Availability

Minimum uptime requirements may vary among organizations. But every organization wants to achieve a high level of up time, especially for their telephony system. An organization's telephony system is frequently business-critical and must be highly available to users. One of the factors you need to consider when you deploy a UM system is the ability for the system to provide services for users when a key component, for example, a UM server or an IP gateway, becomes unavailable.

To provide a highly available UM system, you must include additional key components to protect against hardware failures for the following:

  • UM servers   Unified Messaging runs as a service and a worker process. This means that if the service is using lots of system resources or has become unresponsive, the worker process can be recycled. The UM worker process is responsible for dealing with outages in Mailbox servers, Hub Transport servers, and domain controllers. If, for example, a Mailbox server for a user is unavailable, the UM server will continue to accept calls on behalf of the user. But the user's custom greeting won't be played. Instead, a standard greeting will be used for calls to that user. Additionally, if the Hub Transport server is unavailable, the UM server will continue to accept calls and queue the calls, depending on how you've set up the queuing limit, until the Hub Transport server is available. In a situation where all domain controllers are unavailable, the UM server will be unable to accept calls.

    UM deployments can be made more resilient and more available by adding UM servers to a single dial plan in an N+1 configuration. This means that, if you need two UM servers, you'd install and set up an additional UM server so you'd have a UM server to take the place of a UM server that's failing or has to be taken offline.

    When you add multiple servers to a single dial plan, the IP gateway will try to connect to a UM server that's listed in the configuration on the IP gateway by IP address, or by fully qualified domain name for TLS deployments. If the UM server is unavailable, the IP gateway will try to connect to the UM server again after 5 seconds. If there is no response from the UM server, the IP gateway will try to connect to the next UM server in the list that's set up on the IP gateway.

  • IP gateways If you want to create redundancy to provide for IP gateway availability, you should add multiple IP gateways in an N+1 configuration. This means that, if you need two IP gateways, you'd install and set up an additional IP gateway so that you'd have an IP gateway to take the place of an IP gateway that's failing or has to be taken offline.

    You have to set up your PBX to send the incoming calls it receives to different IP gateways. After you set up the PBX, the PBX will detect a failure or no signal or that a port isn't answering calls. This lets the PBX redirect calls to an IP gateway that can answer incoming calls.

    The IP gateways supported by Unified Messaging can be set up to route calls to UM servers in a round-robin manner. To enable an IP gateway, you need to set up each IP gateway with the IP addresses for your UM servers that will be answering calls from the IP gateway. These are the UM servers associated with the same dial plan as the UM IP gateway object that logically represents the IP gateway. This will let all the UM IP gateways to forward incoming calls to the UM servers associated with the same dial plan. Then, if an IP gateway fails, the PBX will send the call to an IP gateway that can answer the call. The IP gateway, in turn, will forward the call to a UM server within the same dial plan. If the call is sent to a UM server that isn't available, the IP gateway will try to contact the UM server again. If it's unsuccessful in contacting the UM server, it will then use the next UM server in the list that's set up on the IP gateway to answer the call. But not all supported IP gateways can be set up to support both load balancing and detecting when a server has been taken offline or is failing.

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Load Balancing in Unified Messaging

Unified Messaging deployments can be made more resilient by deploying multiple UM servers to a single dial plan to balance the load of incoming calls. The IP gateways supported by Unified Messaging can be set up to route calls in a round-robin manner to balance the load between multiple UM servers in a dial plan.

Round robin is a method for distributing the workload among multiple servers. However, round robin doesn't by itself enable an IP gateway to detect a server failure. If one of the UM servers fails and if the IP gateway can't detect that a UM server is unavailable, the IP gateway will continue to send incoming calls to the UM server until you detect the failure and remove the server from the dial plan. After you remove the UM server from the dial plan, you should also remove the IP address or FQDN for the UM server from the configuration on the IP gateway.

Unified Messaging doesn't use round-robin DNS or Network Load Balancing to distribute incoming calls. Round-robin DNS can be used on multi-homed computers and can be used to distribute the load for other services, but not for Unified Messaging. NLB is used with other services to distribute client requests and to automatically detect whether a server is unavailable, and then to redistribute other client requests to the remaining server. But it also can't be used with Unified Messaging. The only way to distribute or balance the load between Unified Messaging servers in a dial plan is for the IP gateway to be set up with the IP addresses or FQDNs of the UM servers in the dial plan. The IP gateway will use the list to distribute the load across all the UM servers in the dial plan and can also detect a server failure if the IP/VoIP supports this functionality.

Another way to load balance your UM deployment is to set up PBX hunt groups to connect to multiple IP gateways and then set up the hunt groups to load balance across the IP gateways.

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