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.
  
  Return to top