Fax Machines

Fax machines are supported in a similar manner to other analog devices in Lync, but require a bit more planning and configuration. Specifically, there is a parameter on the New-CsAnalogDevice cmdlet called AnalogFax that must be set to $true for any fax machines. The reason for this is to ensure that the media for a fax call is never routed through Lync.

For an analog phone it’s not a big problem if the media stream runs through Lync. The Mediation Server can transcode G.711 audio from a gateway to RTAudio and back again if needed, but the issue is that the Mediation Server cannot handle the T.38 fax tones properly. So the point of identifying fax machines is to indicate to Lync Server that it must remain out of the media path and allow the analog device to leverage media bypass.

The media problem isn’t an issue if the fax machine is connected to the same media gateway from which the call came in, but in order to support calls from one gateway reaching a fax machine attached to a second gateway, the two gateways must be configured to route calls between one other. Assuming a call comes in through a media gateway and the fax machine is connected to a device referred to as the fax machine gateway, the call flow resembles the following process shown in Figure 17.16:

1. The PSTN or PBX caller dials a fax machine.

2. The call is routed through the media gateway and to Lync Server.

3. Lync determines that the DID belongs to an analog fax device.

Figure 17.16. Fax call.

4. Lync places a new, outbound call to the analog device with the calling party set to the PSTN or PBX phone number. The call is always sent directly back to the same gateway that originally sent the call, not the fax machine gateway.

5. Media gateway is now responsible for routing the new call from the Lync Server to the fax machine gateway.

6. The call is placed from the media gateway to the fax machine gateway, and media bypass is enabled.

7. The two gateways use the G.711 codec between them for the fax transmission.

The takeaway in this scenario is that the Mediation Server is out of the media path and a fax machine behaves like a Lync media bypass client. The difference is that the fax machine gateway is taking the place of the Lync client, but the concept is the same. This requires media bypass to be enabled on the trunk configuration for both gateways. It is also possible for a single physical gateway to have both T1 and FXS ports so that a dedicated analog gateway is not required.