Virtualization and Exchange faxing

I got an interesting question in my mailbox the other day, and I didn’t know the answer offhand, so I had to do some digging. I love it when that happens. The question:

I have a customer who’s using RightFax and wants to deploy Exchange 2010 in a completely virtualized environment. If they’re only using the UM role to serve as a fax gateway, i.e. no voice, then are the same processing requirements present?

Short answer: no. Although Microsoft doesn’t support virtualizing the UM server for any reason, Exchange 2010 fax should work OK in this configuration.

Longer answer: the UM server answers calls when they arrive from the gateway or IP PBX. If the call is to an extension that maps to a UM-enabled mailbox, the UM server will answer. If it hears a fax CNG tone, it will treat the call as a fax.

In Exchange 2007, this would have meant that the UM server accepted a T.38-over-RTP audio stream from the gateway/PBX and rendered it as a fax message. Exchange 2010, however, lacks inbound fax support, so instead it loads the external fax service URL (if one is defined on the UM mailbox policy) and sends a SIP redirect back to the gateway or PBX. That device then sends a SIP INVITE to the fax service, which accepts the INVITE, accepts the resulting audio stream, and generates a fax message.

If the call isn’t a fax, Exchange will record a voice message from the caller, transcribe it to generate the Voice Mail Preview data, and then send it on to the recipient’s mailbox.

Audio playback and recording are sensitive to CPU performance, and Voice Mail Preview transcription is CPU-bound; that’s why MS doesn’t support virtualized UM servers. Fax answering in Exchange 2007 requires audio recording and decoding, which is why it didn’t work well on virtualized servers. However, the overhead of sending a SIP redirect is really minimal, so this configuration should work well.

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