Traditionally, gateway is a term that describes a physical device that connects two incompatible networks. With Microsoft Exchange Server 2010 Unified Messaging and other Unified Messaging solutions, the IP gateway is used to translate between the Public Switched Telephone Network (PSTN)/Time Division Multiplex (TDM) or circuit-switched based telephony network and an IP or packet-switched data network.

Note:
A packet-switched network is a network in which packets (messages or fragments of messages) are individually routed between nodes that may be shared by many other nodes. This contrasts with a circuit-switched network that sets up a dedicated connection between the two nodes for their exclusive use for the duration of the communication.

Exchange 2010 Unified Messaging relies on the ability of the IP gateway to translate TDM or telephony circuit-switched based protocols, such as Integrated Services Digital Network (ISDN) or QSIG, from a Private Branch eXchange (PBX) to protocols based on Voice over IP (VoIP) or IP, such as Session Initiation Protocol (SIP), Realtime Transport Protocol (RTP), or T.38 for real-time facsimile transport.

Types of IP Gateways

The UM IP gateway is an Active Directory container object that contains one or more Active Directory UM hunt groups and other UM IP gateway configuration settings. UM IP gateways are created within Active Directory to logically represent a physical hardware device called an IP gateway. The UM IP gateway can represent either an IP gateway or an IP PBX. The combination of the UM IP Gateway object and a UM Hunt Group object establishes a logical link between an IP gateway hardware device and a UM dial plan.

After the UM IP gateway is created, the IP gateway can be linked to or associated with a single or multiple UM hunt groups and UM dial plans. The UM hunt group provides a link between the UM IP gateway and a UM dial plan. By creating multiple UM hunt groups, you can associate a single UM IP gateway with multiple UM dial plans.

After you create a UM IP Gateway object, the Unified Messaging server associated with the UM IP gateway will send a SIP OPTIONS request to the IP gateway to ensure that the IP gateway is responsive. If the IP gateway doesn't respond to the SIP OPTIONS request from the Unified Messaging server, the Unified Messaging server will log an event with ID 1400 stating that the request failed. To resolve this issue, ensure that the IP/VoIP is available and online and that the Unified Messaging configuration is correct.

A Unified Messaging server communicates only with IP gateways or IP PBXs listed as a trusted SIP peer. In some cases, if two IP gateways are configured to use the same IP address, an event with ID 1175 will be logged. Unified Messaging protects against unauthorized requests by retrieving the internal URL of the Unified Messaging Web services virtual directory located on the server that has the Client Access server role installed, and then uses the URL to build the list of FQDNs for the trusted SIP peers. When two FQDNs are resolved to the same IP address, this event will be logged.

Note:
Before an IP gateway can be used to process calls, a UM IP gateway must be associated with at least one UM dial plan. Also, at least one Unified Messaging server must be associated with at least one UM dial plan.

By default, IP gateways are left in an enabled state after they're created. However, the UM IP gateway can be enabled or disabled. If you disable a UM IP gateway, it can be in one of two disabled modes. The first disabled mode forces all associated Unified Messaging servers to drop existing calls. The second disabled mode forces the Unified Messaging server associated with the UM IP gateway to stop handling any new calls presented by the IP gateway.

Note:
If a UM IP gateway is deleted, the Unified Messaging servers associated with the IP gateway will no longer be able to accept or process new call requests from the IP gateway.