Unity 4.0(4) introduces support for a new device called PIMG (PBX/IP Media Gateway) that takes advantage of the Unity SIP interface. PIMG replaces the voice board as the means for Unity to integrate with legacy PBX systems. Rather than connecting lines from the legacy PBX system to a voice board installed in the Unity server, lines from the legacy PBX system are connected to the PIMG and Unity communicates with the PIMG over the network using SIP. With the PIMG integration, a voice board is no longer installed in the Unity server. Many of the PBX-specific parameters that were previously configured on the Unity server (mostly in the switch file) are now configured on the PIMG, albeit under different headings. As of Unity 4.0(4), one PIMG can support up to eight Unity voice ports. Multiple PIMGs can be configured with Unity to scale to a higher number of voice ports.
PIMG translates call-control commands from the PBX into SIP and sends these SIP messages to Unity. Likewise, PIMG translates SIP messages from Unity into call-control commands that the PBX can understand. Because the PIMG integration relies on Unity's SIP interface, much of the discussion in the previous section about SIP methods, digit generation and detection, and media-format negotiation apply to the PIMG integration as well. One SIP method that is not used with the PIMG integration is REGISTER. PIMG is configured with the IP address of the Unity server, so it knows where to send SIP messages destined for Unity. Because of this, there is no need for Unity to advertise its IP address using REGISTER. Figure 17-29 is an example call flow for a call setup between a phone on the legacy PBX system and Unity, via PIMG.
Notice that the SIP call flow between Unity and PIMG is the same as the call flow shown in the previous section, "SIP Methods used by Unity." Because the PIMG integration is new with Unity 4.0(4), Unity using PIMG does not yet support all of the legacy PBXs that Unity using a voice board can support. However, it is expected that the PIMG integration eventually will allow Unity to communicate with the entire currently supported legacy PBXs. In addition, PIMG opens the door to digital integrations with legacy PBXs that were previously not possible. In general, digital integrations to PBXs are considered more efficient and feature-rich than analog integrations to the same PBX. With PIMG, Unity will eventually support digital integrations to widely deployed PBXs like Nortel Meridian, Avaya Definity, and Siemens HiCom 300, among others. Another benefit of using PIMG with Unity instead of using voice boards is that Unity no longer needs to be collocated with the PBX. This allows for more easily managed systems, where several Unity servers could potentially be located at a central site, and each of these Unity servers could service a different legacy PBX at a remote site.
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