In addition to operating as a fully featured data modem, the modem also has the capability to handle the transmission and reception of facsimile information. To provide this functionality, it has to be able to communicate with a group 3 fax machine anywhere in the world.

Communication with the remote fax machine is performed in a similar fashion to that used for data transmission. However, the modulation schemes used are half duplex V.27ter, V.29 and V.17 unlike the full duplex modulations used for data transmission. V.29 is the more common, but the modem will automatically fall back to the slower V.27ter standard if the line quality is poor. V.17 is a higher speed standard which is becoming more common.

Also required in fax transmission is session management. This is an agreed procedure for exchanging information during the call. The T.30 standard that specifies V.21 channel 2 for negotiation and HDLC (High-level Data Link Control) for data transfer provides this function. There are five consecutive phases to the T.30 procedure. Phase A uses tone signalling to perform call establishment. Phase B is the parameter negotiation phase using V.21. The actual message transfer is performed with V.17, V.29 or V.27ter during Phase C. Phase D again uses V.21 but this time for message administration before disconnection in the final phase, E.

Another ITU-T standard, T.4, defines the format of the image data. It also defines the means of compressing the data prior to transmission.

In addition to communicating with the remote fax machine, the modem also has to have a means of communicating these images to and from the user of the modem. The class 1 commands describe the physical and data link layers for dialling, waiting, signalling and data formatting. It requires the host DTE software to handle the T.30 session management and T.4 image data handling.

+FCLASS? <Return>

A typical response will be 1 to indicate that fax Class 1 is enabled.

+FCLASS=? <Return>

The response would be 0, 1.